OCEAN/ATMOSPHERE VARIABILITY AND PREDICTABILITY
(IAPSO, IAMAS, IAG)
Location: Arts Building 120 LT
Location of Posters: Arts Building 101 LR4
Monday 26 July AM
Presiding Chair: M. Davey (Meteorological Office, London)
Concurrent Poster Session
OCEAN/ATMOSPHERIC VARIABILITY-ENSO EFFECTS, ENSO
DECADAL VARIABILITY
JSP25/W/24-B1 0850
THE EFFECTS OF ENSO ON INDIVIDUAL LARGE-SCALE WEATHER EVENTS
Joseph J. BARSUGLI, Jeffrey S. Whitaker, Prashant D. Sardeshmukh, Andy Loughe,(all at: NOAA-CIRES Climate Diagnostics Center, University of Colorado, Campus Box 449, Boulder, CO, 80302, USA, email: jjb@cdc.noaa.gov), and Zoltan Toth (NCEP/EMC, 5200 Auth Rd., Camp Springs, MD, 20746, USA, e-mail: zoltan.toth@noaa.gov)
What is the link between ENSO and individual weather events in the extratropics? This question is addressed quantitatively using ensembles of medium-range weather forecasts made with and without tropical sea surface temperature anomalies during the winters of 1997-8 (a record El Nino event) and 1998-9 (a substantial La Nina event).
We find statistically robust sub-seasonal variations in ENSO teleconnections over North America. These sub-seasonal effects can depart dramatically from the seasonally-averaged extratropical pattern classically associated with ENSO. For example, a large tropical influence was detected in the case of the devastating ice storm that hit Canada in January 1998, apparently involving an anomalous atmospheric wavetrain along the Atlantic Coast of North America. Some (though not all) of the variations in the heavy California rains of February 1998 also are associated with sub-seasonal variations in the strength of the ENSO teleconnections. Interesting cases for winter 1998-9 will also be shown.
Some mechanisms, including Rossby wave propagation on time-varying flows, and the effects of transient tropical convective forcing are discussed, as well as the implications for improving medium range forecasts. The inherent limitations of attributing individual weather events to specific causes in a chaotic system is also addressed.
JSP25/W/29-B1 0910
ENSO WESTERN PACIFIC OSCILLATOR PARADIGM AND THE 1997-98 EL NINO
Chunzai WANG and Robert H. Weisberg (both at Department of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA, email: wang@marine.usf.edu)
A western Pacific oscillator paradigm has been proposed for the El Nino-Southern Oscillation
(ENSO). This paradigm emphasizes the importance of western Pacific interannual anomaly patterns for the evolution of ENSO, in addition to eastern Pacific anomaly patterns. During the warm (cold) phase of ENSO, off-equatorial cold (warm) sea surface temperature (SST) and high (low) sea level pressure (SLP) anomalies in the western Pacific produce equatorial easterly (westerly) winds in the western Pacific. These winds force oceanic upwelling (downwelling) responses that evolve eastward tending to decrease (increase) SST in the east, providing a negative feedback for the coupled ocean-atmosphere system to oscillate. Thewestern Pacific oscillator paradigm may work in conjunction with the delayed oscillator paradigm that emphasizes wave reflection at the western boundary as a negative feedback. Observations show that evolution of the 1997-98 El Nino is consistent with the western Pacific oscillator paradigm. From November 1996 to January 1997, the eastern Pacific is characterized by equatorial cold SST and high SLP anomalies, while the western Pacific is marked by off-equatorial warm SST and low SLP anomalies. Corresponding to this distribution are high outgoing longwave radiation (OLR) anomalies in the equatorial central Pacific and low OLR anomalies in the off-equatorial far western Pacific. The off-equatorial low SLP anomalies in the western Pacific are associated with a switch in the equatorial winds over the western Pacific from easterly to westerly. These equatorial westerly anomalies then appear to initiate early SST warmings around the dateline in January/February 1997 and around the far eastern Pacific in March 1997. Subsequently, both the westerly wind and the warm SST anomalies, along with the low OLR anomalies, grow and progress eastward. The eastward propagating warm SST anomalies merge with the slower westward spreading warm SST anomalies from the far eastern Pacific to form large-scale warming in the equatorial eastern and central Pacific. The anomaly patterns in the eastern and central Pacific continue to develop reaching their peak values around December 1997
JSP25/W/30-B1 0930
A MODELLING STUDY OF THE INFLUENCE OF THE 1997/1998 ENSO ON THE
ATLANTIC OCEAN
Stephen JEWSON and Rowan Sutton (both at the Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading, RG6 6BB, UK, email:steve@met.reading.ac.uk)
The observational record shows us that one of the remote effects of ENSO can be to warm the waters of the northern sub-tropical Atlantic. The recent strong ENSO event was accompanied and followed by large sea surface temperature (SST) anomalies in other parts of the Atlantic as well. This raises the question as to whether these anomalies were also a response to ENSO, and hence whether they might have been predicted in advance. We address this question, and also evaluate the performance of a state of the art coupled model in simulating the whole of the Atlantic response to ENSO.
We have performed ensemble integrations of a global coupled model for the period 1997/1998. Over the tropical Pacific and Indian oceans the model SST is contrained with artificial heat fluxes to follow either the observed evolution for 1997/1998 or climatological SSTs. In the Atlantic, however, the ocean and atmosphere interact freely. In this way we can study the coupled response to a particular ENSO event. The model shows some deficiencies in simulating the observed evolution of Atlantic SST during 1997/1998, while some features are simulated correctly. This leads to insight into both the performance of the model and the role played by ENSO in creating SST anomalies in the Atlantic.
JSP25/W/60-B1 0950
MECHANISMS FOR THE INDIAN OCEAN WARMING DURING THE 1997-98 EL NINO
Lisan YU (Department of Meteorology, University of Maryland, College Park, MD 20742, USA,
email: lyu@atmos.umd.edu); Michele Rienecker (NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA, email: rienecke@mohawk.gsfc.nasa.gov)
Two major climate events took place in 1997-98: the greatest El Nino of the century in the tropical Pacific Ocean and a record-breaking broad-scale warming in the Indian Ocean. In this study, we examine the primary mechanisms that gave rise to the basin-wide change of the sea surface temperature (SST) in the Indian Ocean during this period and their relationship with the El Nino - Southern Oscillation (ENSO) event in the Pacific. The evolution of some key atmosphere-ocean variables, as revealed in our analyses of multi-source data sets, indicated that the changes of SST in the Indian Ocean were largely attributable to the ENSO impact on the large-scale atmospheric circulation. During June-December 1997, when the El Nino in the Pacific was maturing, the Indian Ocean experienced the reversal of the Walker Circulation and the prolonged equatorward displacement of the southeast trades. The resultant changes of surface wind influenced the SST through the following means. In the equatorial region, the easterly winds associated with the reversed Walker Circulation forced equatorial Kelvin/Rossby waves, which then affected the equatorial ocean heat balance (mainly through upwelling/downwelling) and eventually led to the reversal of the zonal SST gradient in the fall of 1997. The negative SST anomalies in the east and positive anomalies in the west in turn helped maintain and prolong the equatorial easterlies, indicating coupled atmosphere-ocean interactions in operation. Outside of the equatorial waveguide, the broad-scale warming was induced mainly by the changes of latent heat flux associated with the anomalies in wind speed. The most intensive surface warming occurred in the southern Indian Ocean during the summer/fall of 1997 when the southeasterly trade winds weakened significantly, leading to a dramatic reduction of the latent heat release and subsequently a rapid surface warming.
JSP25/W/14-B1 1010
MJO TRIGGERED 1997/98 ENSO
Tetsuo Nakazawa (Meteorological Research Institute, Tsukuba, Japan 305-0052,
Email:nakazawa@mri-jma.go.jp)
The onset phase of the 1997/98 ENSO is described by using several remote-sensing datasets including the ADEOS/NSCAT surface wind data, the NOAA/OLR data, the TOPEX/POSEIDON sea surface height data) as well as the TOGA/TAO buoy data. These data shows that the 1997/98 ENSO is triggered by the Madden-Julian Oscillation with a period of 30-60 days, which is originated in the equatorial Indian Ocean in February 1997 and propagated into the western Pacific in March 1997. Under the warmer condition of the upper ocean in the western Pacific, the strong and persistent westerlies assosiated with the MJO was favorable to generate the oceanic Kelvin wave in middle of March. This Kelvin wave propagated to the eastern Pacific end of April or early May when the ENSO was initiated.
Presiding Chair: S.Jewson (Dept of Meteorology, Uni of Reading, UK)
JSP25/W/26-B1 1050
DYNAMICS OF TROPICAL DROUGHTS DURING THE 1997-98 EL NINO
J. David NEELIN, Ning Zeng, Chia Chou and Hui Su (University of California at Los Angeles, Dept. of Atmospheric Sciences, UCLA, Los Angeles, CA 90095-1565 USA, e-mail: neelin@ucla.edu)
An intermediate complexity atmospheric model is used to analyse radiative-convective-dynamical interactions in the response to the 1997-98 El Nino sea surface temperature anomalies. The atmospheric model, QTCM1 (quasi-equilibrium tropical circulation model) makes use of properties of the quasi-equilibrium convective closure in the Betts-Miller convective scheme to produce efficient solutions that are accurate in convective regions and reasonable elsewhere. It is coupled to a one-layer land-surface model with interactive soil moisture, and simulates its own tropical climatology. The model is able to reproduce many tropical features in observed anomalies of precipitation, outgoing long-wave radiation, land surface temperature and winds associated with the recent El Nino. The radiative-convective-dynamical balances associated with the subsidence regions around ENSO convective heating regions are examined. We compare the feedbacks over land involved in the Amazon drought with those over the western Pacific/Indonesian ocean region. In both, cloud-radiative interaction affects the dynamics of relative descent in the convection zone. Over parts of the ocean region, ocean dynamical effects are inferred to have played some role. Over land regions adjacent to the SST anomaly, land feedbacks actually simplify the analysis via the surface energy balance condition. The surface temperature response is relatively passive from the dynamical point of view, adjusting to satisfy this condition, and is significantly associated with cloud feedbacks. The convective-radiative-dynamical feedbacks identified in this case likely play a role in most tropical teleconnections involving suppression of convection zones by remote SST anomalies.
JSP25/E/05-B1 1110
WHAT ROLE DID THE MJO AND ASSOCIATED WESTERLY WIND BURSTS PLAY IN THE DEVELOPMENT OF THE 1997/98 EL NINO
M.K. DAVEY, S. Ineson (both at Hadley Centre for Climate Prediction and Research, UK Met. Office, Bracknell, UK. Email: sineson@meto.gov.uk), J.M. Slingo and R. Brugge (both at Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, Reading, UK.)
The growth of the 1997/98 El Nino was dramatic and significantly faster than normal. It developed in association with strong atmospheric intraseasonal variability, characterised by a series of MJOs with strong Westerly Wind Bursts (WWB) embedded in the active phase of the MJO.
A series of experiments with the UK Met. Office Tropical Pacific OGCM, forced with ECMWF operational analyses for 1996/97, have been made to investigate the impact of the intraseasonal variability in the atmosphere and oceans on the evolaution of El Nino. The aim is to test the hypothesis that a series of MJOs and/or WWBs are required to precondition the ocean to El Nino.
JSP25/W/58-B1 1130
THE BMRC SEASONAL-TO-INTERANNUAL PREDICTION MODEL
Richard Kleeman, Guomin Wang and Neville SMITH (all from the Bureau of Meteorology Research Centre, Melbourne Vic. 3001, AUSTRALIA, email: N.Smith@BoM.GOV.AU)
A global coupled general circulation model has been developed for forecasts of the El Nino-Southern Oscillation. The atmospheric model is an R21L9 version of the BMRC atmospheric climate model. The ocean component is a global general circulation model with resolution focussed in the tropical region and 20 vertical levels. A univariate statistical interpolation method, with 10 day data ingestion windows, is used to assimilate temperature data and initialize the coupled model. Hindcasts have been carried out for the period 1981-1995 for each season (60 in all), for up to 12 months lead-time. This paper will describe these initial experiments and show that the skill comparable to, and in some cases exceeds, that of the present operational intermediate model, and is comparable to that of other published models. The skill of the model is focussed in the central Pacific, which suits seasonal forecasting in the Australian region, but is generally weak in the far eastern Pacific and outside the tropical Pacific. Some of the other attributes and weaknesses of the model will also be discussed. The hindcasts have been extended to the present with a view toward quasi-operational trials.
JSP25/W/66-B1 1150
A CONCEPTUAL EQUATORIAL OCEAN RECHARGE OSCILLATOR MODEL FOR ENSO
FEI-FEI JIN and Soon-Il An (School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, USA, e-mail:jff@soest.hawai.edu)
Through the dynamical coupling between ocean and atmosphere, the vertical advection of anomalous subsurface temperature by the mean upwelling and zonal advection of mean sea surface temperature (SST) by anomalous current constitute the so-called the thermocline and zonal advective feedbacks which are essential for El Niño / Southern Oscillation (ENSO) coupled dynamics. In this paper, we demonstrated that these two feedbacks are dynamically linked because of the semi-geostrophic balance the upper ocean zonal current and meridional gradient of thermocline. Both feedbacks thus play important and similar roles in the growth and phase transition of ENSO. We further proposed a new version of the conceptual recharge oscillator model for ENSO by including these two feedbacks. The new model retains the simplest possible form of a harmonic oscillator, yet presents a more complete description of slow physics for ENSO. Moreover, it also provides the reconciliation between the biased emphases in the ENSO theories on the thermocline and zonal advective feedbacks.
JSP25/W/36-B1 1210
ASSESSMENT OF THE DIABATIC HEATING DISTRIBUTION AND GLOBAL TELECONNECTION ERRORS IN THE UKMO UNIFIED MODEL DURING ENSO
RICHARD NEALE and Julia Slingo, (CGAM, Department of Meteorology, University of Reading PO BOX 243, Earley Gate, Reading, RG6 6BB)
Interannual variability in tropical sea surface temperatures (SSTs) is able to influence the atmospheric circulation in more remote regions. This is achieved due to a succession of processes, beginning with the generation of convection anomalies through to the impact of global teleconnection patterns. The major source of interannual variability in tropical SSTs is due to ENSO.
AMIP II integrations using the UKMO Unified Model (UM) (version HadAM3) reveal that the global response during ENSO conditions is exaggerated compared to observations. Analyses of these integrations are performed in order to determine the major sources of error in the model. In particular, the ability of the model to reproduce the correct magnitude and spatial distribution of the tropical diabatic heating anomalies in response to ENSO sea surface temperature (SST) distributions will be discussed.
ENSO also influences the more remote tropical climate through known global teleconnection patterns. In the tropics a remote response to ENSO is observed in the tropical Atlantic, primarily through changes in the Walker circulation. During El Nino conditions, increased descent is seen over the tropical Atlantic tending to result in a warming of SSTs. The intensity of this response gives rise to stronger than observed coupling between tropical Pacific and tropical Atlantic SSTs.
The impact of vertical resolution on model skill will also be assessed, with a comparison of 19 level and 58 level versions of the AMIP II UM integrations.
Monday 26 July PM
OCEAN/ATMOSPHERIC VARIABILITY-ENSO EFFECTS, ENSO
DECADAL VARIABILITY
JSP25/W/22-B1 1400
FLUCTUATIONS IN INDIAN OCEAN SST DIPOLE PATTERNS
Robert J. ALLAN (CSIRO Atmospheric Research, Aspendale, Victoria 3195, Australia, email: rob.allan@dar.csiro.au); Ian N. Smith (CSIRO Atmospheric Research, Aspendale, Victoria 3195, Australia, email: ins@dar.csiro.au); Chris J.C. Reason (School of Earth Sciences, University of Melbourne, Parkville, Vic., 3052, Australia, email: cjr@buster.unimelb.edu.au).
Indian Ocean sea surface temperature (SST) dipole patterns are examined in the context of historical, global mean sea level pressure (MSLP) and SST anomalies during the evolution of major El Niño and La Niña events. These SST dipoles occur at various stages during the life cycles of both the more ‘canonical’ and protracted of El Niño Southern Oscillation (ENSO) phases.
When seasonal MSLP and SST fields are filtered on interannual ENSO time scales, the resulting quasibiennial 2-2.5 year and ‘classical’ ENSO 2.5-7 year bands often both display SST dipoles, but they may not always be spatially aligned. Superposition of these bands can lead to the annulment of such SST signals. However, if either or both of the above bands is dominant, and carrying SST dipole signals, then an important modulation for rainfall producing systems in various regions of the Indian Ocean basin can emerge. In general, south west-north east oriented dipoles develop during the onset and cessation phases of El Niño or La Niña episodes. During the mature stage of strong ENSO phases a north-south oriented SST dipole structure is usually evident on both bands.
Filtering on lower frequency time scales reveals the presence of additional SST structures, particularly during protracted ENSO phases. This indicates that interannual SST dipoles are modulated further by decadal to interdecadal fluctuations in Indian Ocean SST dipole patterns.
JSP25/W/65-B1 1420
DECONSTRUCTING CORRELATIONS: USING PATH ANALYSIS TO DIAGNOSE OCEAN-ATMOSPHERE INTERACTIONS
NEVILLE NICHOLLS, Wasyl Drosdowsky, (both at Bureau of Meteorology Research Centre, PO Box 1189K, Melbourne 3001, Australia. E-mail:n.nicholls@bom.gov.au); Tahl Kestin, (CRC for Southern Hemisphere Meteorology, Monash University, Clayton 3168, Australia)
The number of tropical cyclones affecting the Australian region each year is related to the El Niño - Southern Oscillation. The correlation between year-to-year changes in tropical cyclone numbers and year-to-year changes in September-November NINO3.4 sea surface temperature (SST) is -0.62 (n = 48; significant at 1%). Year-to-year changes in tropical cyclone numbers are also correlated with SSTs around northern Australia (r = 0.71) and these SSTs, in turn, are correlated with NINO3.4 SSTs (r = -0.83). Does NINO3.4 SST have an effect on tropical cyclone numbers separate from that arising from acting in concert with local, north Australian SSTs? Path analysis allows us to deconstruct the observed correlations between SSTs and cyclone activity into unique and common effects, ie to separate the unique effect of NINO3.4 SSTs on tropical cyclones from the common effect involving both NINO3.4 and north Australian SSTs. This analysis indicates that the NINO3.4 SST only affects tropical cyclone activity through its common effect with the local, north Australian SSTs. Path analysis, and its more complex relation Structural Equation Modelling, allow us to investigate the underlying correlation structure in situations where there are several inter-related predictor variables all correlated with one or more predictand or response variables. These techniques appear not to have been used in the study of relationships between SSTs and climate variables, despite their obvious utility in such studies. They could, for example, allow the correlation between Indian Ocean SSTs and Indian rainfall to be separated into the contribution arising from the unique effect of Indian Ocean SSTs on rainfall, and the contribution arising from the common relationship of Pacific Ocean and Indian Ocean SSTs with rainfall (through the El Niño - Southern Oscillation). We will illustrate the use of these techniques in separating the effects of Indian and Pacific Ocean SSTs on Indian and Australian rainfall, as well as on tropical cyclone activity, and discuss their relationship to climate model experiments.
JSP25/E/23-B1 1440
THE CONNECTION BETWEEN ENSO AND PRECIPITATION IN ISRAEL
Colin PRICE, Pinhas Alpert and Yitzhak Carmona (all at Department of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, 69978, Israel, email: cprice@flash.tau.ac.il, pinhas@cyclone.tau.ac.il, carmona@cyclone.tau.ac.il); Lewi Stone and Amit Huppert (both at Department of Zoology, Tel Aviv University, Ramat Aviv, 69978, Israel, email:lewi@lanina.tau.ac.il, amit@lanina.tau.ac.il); Balaji Rajagopalan (Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000, USA, email: rbala@rosie.ldgo.columbia.edu)
Recent analysis of hydrological data in Israel has revealed some interesting connections between the ENSO phenomenon and weather in Israel. The precipitation season in Israel is during the winter months from October-April, with the maximum precipitation coinciding in phase with the maximum of the ENSO cycle. Hence the anomalous circulation patterns in the atmosphere due to ENSO coincide with the rainy period in Israel.
For this study we used rainfall data from nothern Israel, streamflow data from the Jordan river, and lake levels from the Sea of Galilee. The rainfall drains into the watershed of the Jordan river which then flows into the Sea of Galilee. Rainfall data were available from 1922, streamflow from 1967 and lake level data from 1927. We analysed the data on a seasonal basis, and compared the anomalies with both the NINO3 sea surface temperatures and the southern oscillation index, during the same winter months. A highly significant statistical correlation was found between all the hydrological parameters and the ENSO indices. During El Nino years there is generally above average rainfall in the northern parts of Israel, while during La Nina years the opposite is true. By analysing archived data related to global circulation patterns we have noticed that the subtropical jetstream shifts approximately 100-200 km southwards/northwards during El Nino/La Nina years.
It is interesting that the strong connection between ENSO and rainfall in Israel exists only since the 1970s, with little connection before this period. Whether this is due to a change in the teleconnection patterns since the 1970s is still uncertain. However, it is clear that there has been a change in the frequency and intensity of ENSO events since the 1970s.
JSP25/W/39-B1 1500
SEASONAL, DECADAL, AND ENSO RESPONSES OF THE UPPER LAYERS IN THE NORTHERN GULF OF ALASKA
Thomas C. ROYER (Center for Coastal Physical Oceanography, Department of Oceanography, Old Dominion University, Norfolk, VA 23529 USA, email: royer@ccpo.odu.edu)
Temperature and salinity versus depth to 250 m at the mouth of Resurrection Bay, Alaska (60 N, 149 W) (GAK 1) have been measured since December 1970 with various temporal sampling intervals, ranging from hours to months. The ocean climatology suggests that, seasonally, the temperature and salinity signals propagate downward from the surface to about 100 m.The influences of local upwelling, regional and global atmospheric circulation forcing can be seen in the temperature anomalies along with SOI (Southern Oscillation Index) influences. In contrast with temperature anomalies observed off the California coast, these are coherent with depth. This suggests a barotropic forcing rather than a vertical propagation or baroclinic influence. Amplitudes of the interdecadal temperature fluctuations decrease from more than 6 C at the surface to about 3.6 C at 250 m with those in the lower 150 m being coherent with their greatest variance at low frequencies (15-21 years).. Using a threshold level of one standard deviation, positive temperature anomalies at 150 m and deeper since 1974 have corresponded to ENSO events. The 1997-8 ENSO event was accompanied by the largest temperature anomaly yet seen at 250 m (1.43 C) (Feb. 1998), more than 3 SD (1 SD = 0.45 C) above normal. This signal first appeared in Jan. 1998 but by May 1998 had subsided to 0.29 C, below the ENSO threshold. Surface salinity anomalies have significant periods of a 4-5 years whereas the deeper waters (10-100 m) have a decadal periodicity (9-11 years) shifting to 12-15 years for 150-250 m.
JSP25/P/06-B1 1520
NUMERICAL SIMULATION RESEARCH ON THE CYCLIC RELATIONSHIP BETWEEN ANOMALOUS EAST-ASIAN WINTER MONSOON AND ENSO
MINGQUAN Mu and Chongyin Li(The Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, P.O.Box 2718, Beijing 10008, China E-mail: mmq@lasgsgi4.iap.ac.cn)
This study includes two parts. The first part gives general observational features about mutual relationship between ENSO and anomalous East Asian winter monsoon. On the one hand, stronger (weaker) East Asian winter monsoon can excite happening of El Nino (La Nina) through forced anomalous westerly (easterly) wind and stronger (weaker) convective activity over the tropical Pacific Ocean; on the other hand, the occurrence of El Nino (La Nina) weakens (enhances) East-Asian winter monsoon through the atmospheric teleconnection. Power spectral analyses imply that this cycle mainly locates in the 3-5 years; and quasi-biannual cycle is also clear. The second part mainly focuses on numerical simulations with tropical Pacific ocean model (OGCM) and coupled ocean-atmospheric general circulation model (CGCM). Both of the results reveal that stronger (weaker) winter monsoon is able to lead to El Nino (La Nina) event. The simulation results also indicate that the anomalous oceanic Kelvin waves and atmospheric intraseasonal oscillation, caused separately by anomalous westerly (easterly) and stronger (weaker) convective activity over the tropical Pacific, play an important role in the occurrence of ENSO (El Nino and La Nina). So, both of the observation and simulation have one consistent conclusion: The relationship between anomalous East Asian winter monsoon and ENSO is two-way interaction
Presiding Chair: R.Allan (CSIRO Atmospheric Research, Aspendale, Australia)
JSP25/W/37-B1 1600
ENSO VARIABILITY OF THE HYDROLOGICAL CYCLE DERIVED FROM RAINFALL ISOTOPE MEASUREMENTS
Ahmet S Kayaalp and John A T BYE (Faculty of Science and Engineering, Flinders University, GPO Box 2100, Adelaide Australia 5001, email John.Bye@flinders.edu.au)
Stable isotope measurements in precipitation have been used to obtain the ratio (f) of precipitation to evaporation for 104 stations in the International Atomic Energy Agency archive. The global annual oceanic precipitation fields predicted from published evaporation fields are found to be realistic in comparison with other methods, e.g. from coastal rainfall and MSU satellite data.
The results show that f is well correlated with the wet bulb temperature (Tw) which occurs during rainfall, and in particular that f becomes very large when Tw exceeds about 26oC. This diagnostic has been used to investigate changes in the global hydrological cycle which occur during ENSO.
It is found that the very high MSU derived oceanic rainfall in the central equatorial Pacific Ocean is consistent with the existence of positive Tw anomalies due to positive sea surface temperature anomalies. During the El Nino phase, a corresponding reduction in f occurs in the extra tropical regions. Associated changes in the global hydrological cycle will be discussed.
JSP25/P/07-B1 1620
A MODELING ENSO/DECADAL VARIATIONS IN THE NORTH DURING 1975 TO 1998
Tomonori MATSUURA and Satoshi Iizuka (both at National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai Tsukuba, Japan, email: matsuura&ess.bosai.go.jp)
'The El Niño and decadal scale simulation, which is long run from 1975 to 1998, is conducted using an ocean general circulation model (MOM 2). As surface forcings, we used daily fluxes obtained from Louis et al. (1982)’s formulae and NCEP data. We could successfully simulate the El Niño (1976 1979, 1982/83, 1986/87, 1991/92, 1997/98) and the decadal variation, the phase of which changes in 1982, 1989, and 1997. Both the El Niño and decadal signals appear in the North Equatorial Current region (145°E-180°, 10°N-20°N) and the decadal variability of heat content in the subpolar front (145°E-180°, 35°N-45°N) correlated inversely with the North Equatorial Current region. We show that the strong El Niño and La Niñas may play an important role in changing the phase of decadal variability.
JSP25/W/11-B1 1640
DECADAL MODULATION OF ENSO VARIABILITY IN THE ECHO-G MODEL
Keith RODGERS (Max Planck Institut fuer Meteorologie, Bundesstrasse 55, D-20146 Hamburg, Germany, Email: rodgers@dkrz.de); Mojib Latif (Max Planck Institut fuer Meteorologie, Bundesstrasse 55, D-20146 Hamburg, Germany, Email: latif@dkrz.de); Stefanie Legutke (DKRZ, Bundesstrasse 55, D-20146 Hamburg, Germany, Email: legutke@dkrz.de)
The global coupled ECHO-G (ECHAM4/HOPE) model is examined in order to identify those oceanic processes which modulate the decadal behavior of ENSO. The focus is on inter-gyre exchange of thermocline water between the subtropical Pacific and the upwelling regions of the eastern equatorial Pacific. Three processes which can contribute to changing the thermal structure of the equatorial pycnocline on decadal timescales are considered: Variability in ocean circulation fields, temperature anomalies which subduct in the extratropics and advect equatorward, and changes in diapycnal fluxes within the equatorial pycnocline. Special attention is given to the role of the South Pacific. The growing database of oceanic measurements strongly suggests that the South Pacific plays at least as important a role as the North Pacific in equatorial thermocline ventilation. The ocean model's performance is discussed within the context of hydrographic and tracer measurements.
JSP25/W/87-B1 1700
TIME SERIES OF SST, SEA-LEVEL PRESSURE, SURFACE WINDS, AND RAINFALL
Todd MITCHELL, (University of Washington, USA, Email: mitchell@atmos.washington.edu)
Time series of SST, sea-level pressure, surface winds, and rainfall are presented to document ENSO variability in the tropics from the 1850s through 1998. The series are derived from area-averages of very large regions and as such emphasize the tropics-scale nature of ENSO-related climate variability. This approach is a complement to the pioneering studies of Quinn and collaborators, which emphasized the historical record of El Nino at the Peru coast. Intercomparison of the series for the different variables will corroborate the identification of the major ENSO episodes and interdecadal variability in the tropics. The resultant series are useful to clarify the relationship between interdecadal variability and the frequency of ENSO episodes.
JSP25/W/41-B1 1720
WATER VAPOR TRANSPORT OVER THE INDIAN OCEAN AND WESTERN PACIFIC DURING THE WARM AND COLD EVENTS OF EL NINO
Nataly A. VYAZILOVA (Russian Research Institute of Hydrometeorological Information - World Data Center 6, Korolyov St., Obninsk, Kaluga region, 249020, Russia, e-mail: vjaz@meteo.ru)
Using mean monthly and climatic upper-air data of the objective analysis during the period 1982-97,made by the U.S. National Meteorological Center which are presented at the points of the 2.5 step regular grid, the following characteristics of the tropical atmosphere have been calculated for the centers of the 5*5 squares: mean monthly water content in the atmospheric column, water content anomalies for individual months (water content is expressed in percent relative to mean climatic values), mean monthly values of the zone, meridional and resulting water transfer, cross-equatorial water vapour flux. Peculiarities of a space - time distribution of water exchange characteristics are studied in the Tropical Indian ocean and Western Pacific during the summer monsoon in accordance with a type of the monsoon circulation. Four types of the monsoon circulation have been considered as compared to the mean climatic year:- Weakened monsoon circulation when an intensive development of El Nino is observed (warm events in 1982,1987 and 1997);- Weakened monsoon circulation when the El Nino development did not reach its peak (warm event in 1986 );- Developed monsoon circulation (cold events in 1988 and 1996);- Normal monsoon circulation (1989).
JSP25/P/04-B1 1740
THE ROLE THE INDIAN OCEAN SSTA TAKSE IN THE ENSO TO ASIAN CLIMATE
XIAO Ziniu and Sun Jihua (both at Yunnan Meteorological Observatory, 73 Xichang Road, 650034, Kunming, Yunnan, China); Li Chongyin (LASG, The Institute of Atmosphere Physics, Box 2718, Beijing 100080, China)
During the ENSO event period, the Indian Ocean SST will be anomalous almost simultaneously. But the space structure of the SSTA in the Indian Ocean is not same from time to time. The seesaw structure is a typical pattern of the Indian Ocean SSTA. Based on the IAP-GCM9L model, which is developed in Institute of Atmospheric Physics, the SSTA pattern with warm SST in the western Indian Ocean and cold SST in the eastern India Ocean is simulated during an ENSO period. And its effect to the Asian climate is studied. The results indicated that the Indian Ocean SSTA would mainly influence the climate in Bengal, Indo-China, Indonesia, India and China. During the ENSO period, the SSTA pattern with warm west and cold east SST in India Ocean will enhance the climate anomalous brought by the eastern Pacific Ocean SSTA. Especially, it makes Indo-China more dry and reduce the precipitation over northern China. But it increases the precipitation over the area from southwest China to southeast China obviously. So that, the precipitation will be concentrated in a zonal region around in Changjiang River drainage area and forms zonal drought and flood pattern which is similar to the fact. Therefore, during ENSO period, the Indian Ocean SSTA is important to Asian climate especially to the anomalous precipitation pattern in China.
Tuesday 27 July AM
Presiding Chair: Dr D.B. Stephenson (Meteo-France)
Concurrent Poster Session
SEASONAL-DECADAL VARIABILITY
Enso Related Variability
JSP25/E/27-B2 0930
IMPACT OF GLOBAL WARMING ON VARIABILITIES OF THE CLIMATE MODES IN A CGCM
Z.-Z. HU, L. Bengtsson, E. Roeckner, M. Christoph, A. Bacher (MPI for Meteorology, Bundesstrasse 55, D-20146 Hamburg, FRG, e-mail: hu@dkrz.de); J. M. Oberhuber (DKRZ, Bundesstrasse 55, D-20146 Hamburg, FRG)
In this study, we investigated the impact of global warming on the linear trend and standard deviation of geopotential height at 500 hPa (H500) in the NH winter, and on the variabilities of the large scale interannual and interdecadal climate modes and the teleconnection patterns with two long term integrations of the ECHAM4/OPYC3 CGCM. One is the control (CTRL) run with fixed present-day concentrations of greenhouse gases. Another experiment is a simulation of transient greenhouse warming, named GHG run.
In the global warming, the standard deviation of H500 over the tropics is enhanced (reduced) remarkably on the interdecadal (interannual) time scales. Except for the interdecadal mode related to the Southern Oscillation (SO) in the GHG run, the spatial variation patterns are similar for different (interannual+interdecadal, interannual, and interdecadal) time scales both in the GHG and CTRL runs. Spatial distributions of the teleconnection patterns in the GHG run are also similar to those in the CTRL run. But some teleconnection patterns manifest the linear trends and changes of the variances and the frequencies in the global warming scenario. The ENSO cycle has the most significant nonlinear response to the global warming. Besides the linear increasing trend of the SO, the interdecadal modulation to the ENSO cycle is enhanced during the GHG 2040~2099. This is the result of enhancement of the Walker circulation during that period. La Nina event will intensify and El Nino event relatively weaken during the GHG 2070~2090. It is also suggested that the growth of the greenhouse gas concentrations will trigger off the unstable correlation between the SO and the PNA pattern both on the interdecadal and interannual time scales.
JSP25/W/55-B2 0950
SEA SURFACE TEMPERATURES AND THE MEXICAN MONSOON: MECHANISTIC IMPLICATIONS
DAVID L. MITCHELL (Atmospheric Sciences Center, Desert Research Institute, P.O. Box 60220, Reno, NV 89506, USA; email: mitch@dri.edu); Dorothea Ivanova and Timothy J. Brown (Same address; dorothea@dri.edu, tbwrcc@dri.edu)
The North American or Mexican monsoon is responsible for roughly 40% of the annual precipitation in Arizona and New Mexico, USA, and about 60% for northern Mexico. It's regular occurance in summer also effects precipitation patterns in the mid-western and eastern United States, as well as the North American radiation budget. Recent studies indicate the heat content of the upper layer (e.g. 70 m) in the Gulf of California during spring/summer is primarily due to horizontal advection from the south, allowing sea surface temperatures (SST) to be higher than solar insolation alone would permit. Other studies show the Gulf of California (henceforth the gulf) to be the dominant moisture source for the monsoon. Our study is unique in that it quantiatively relates gulf SSTs to the timing, amount and regional extent of monsoon rainfall.
A detailed three season empirical study was conducted based on satellite SST data at 18 km and weekly resolution, along with satellite SSM/I pentad (5 day) precipitation data having a spatial resolution of 0.25 x 0.25 degrees. Four coastal ocean regions (three in the gulf) were evaluated for SSTs, and four land regions northwest of each ocean region (consistent with known moisture transport) were evaluated for rainfall amount. The main results were:1) Monsoon rainfall did not occur prior to the onset of gulf SSTs exceeding 26 C.2) The incremental advance of SSTs > 26 C up the mainland coast of Mexico appears necessary for the northward advance of the monsoon. 3) In the southern gulf region, an SST parameter was lag-correlated with rainfall amount in adjacent land regions (r = 0.78) occurring during the 5-15 day period after an SST increase. 4) Relatively heavy rainfall in Arizona and New Mexico depended on two factors: (1) northern gulf SSTs exceeding 29 C, and (2) a neutral or positive along-gulf SST gradient (positive means higher SSTs in the N. gulf). Periods meeting both criteria accounted for 75% of the monsoon rainfall in this region.
JSP25/W/15-B2 1010
INTERANNUAL VARIABILITY AND PREDICTABILITY IN THE MARITIME CONTINENT.
JOHN L MCBRIDE Bureau of Meteorology Research Centre Melbourne Australia and Paulus Winarso, Dodo Gunawan, and Soetamto Meteorological and Geophysical Agency, Jakarta, Indonesia, e-mail:jmb@bom.gov.au
Interannual variability is studied for the maritime continent through analysis of seasonal rainfall data for 102 seasonal forecast districts across Indonesia. These data have been assembled from the records of the Meteorological and geophysical Agency in Indonesia, and form the basis of their seasonal forecasts. Four parameters are available for the 102 districts over the period 1961 to 1998: Onset date for the wet season, onset date for the dry season, total wet season rainfall, total dry season rainfall. From these, two additional derived parameters can be constructed: length of wet season and length of dry season.
Linear correlations are calculated for each parameter with the Southern oscillation Index and with amplitudes of various rotated EOFs of global sea surface temperature data. The largest correlations are with the SOI, and these are in the magnitude range of 0.4 to 0.7 across most of the country during the dry months of June to November. Parameters relevant to those months include dry season rainfall and wet season onset, and based on 3-month lag correlations with the SOI, these parameters have a high degree of predictability. Conversely the relationships are quite small in the wet months of December to May; and the predictability is low for parameters whose timing occurs in that half-year, viz: wet season rainfall and dry season onset.
These relationships are studied further through calculating correlations with Sea surface temperature patterns as well as cross correlations between the 102 forecast districts. It turns out the parameters related to the SOI are also related to the SST patterns, with smaller but statistically significant correlations with amplitudes of several EOF's. On the other hand parameters occurring during the wet season are not well related to the SST patterns; and they also have close to zero coherence in interannual variations across the domain. The implication is that in this region association with the Southern Oscillation is related to a large scale for spatial coherence in interannual variations. It also suggests that the monsoon or wet season rainfall does not have large scale coherence and is inherently unpredictable.
Presiding Chair: Mitchell D.L
TROPICAL VARIABILITY
JSP25/W/08-B2 1110
THE INFLUENCE OF SST PERTURBATIONS ON INTRASEASONAL VARIABILITY OF TROPICAL CONVECTION.
Steven WOOLNOUGH and Julia Slingo (Centre for Global Atmospheric Modelling, Department of Meteorology, Univeristy of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK. email: s.j.woolnough@rdg.ac.uk); and Brian Hoskins (Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, RG6 6BB, UK)
Results from TOGA-COARE and other studies haveshown that convection associated with theintraseasonal oscillation can affect the upperlayers of the ocean through changes in thesalinity, mixed layer depth and SST.However, the ways in which these changes in SSTmay feedback on the the convection associated withthe intraseasonal oscillation are not wellunderstood. A better representation of theintraseasonal oscillation may lead to improvementsin seasonal forecasting and the prediction of theonset of El Nino.
Results from integrations of an aqua-planet GCMwill be presented to demonstrate the sensitivityof intraseasonal variability in convection to SSTperturbations. The implications for modelling ofthe intraseasonal oscillation will be discussed.
JSP25/E/03-B2 1130
A THREE-DIMENSIONAL SIMULATION OF THE SOUTH CHINA SEA CIRCULATION
QINYU LIU, Haijun Yang (Institute of Physical Oceanography, Ocean University of Qingdao, Qingdao, 266003, P.R.China Email: liuqy@ouqd.edu.cn)
A three-dimensional, sigma-coordinate primitive equation model(Princeton Ocean Model) with a free surface is used to simulate the monthly mean circulation in the South China Sea(SCS). The model has a resolution of 0.5° in the horizontal and 10 sigma layers in the vertical in a region from equator to 27°N and from 98°E to 125°E. There are four openings in the model region, where the inflow and outflow in the Kuroshio, through the Taiwan Strait and Karimata Strait are obtained from the model output of the Parallel Ocean Climate Model(POCM). At the sea surface, the model is forced by monthly averaged climatological wind stress, heat and salinity flux. Several important features are reproduced in the model results. First, for the vertical integrated circulation(VIC), there is a large cyclonic gyre dominated the whole basin in winter, in which a meso-scale cyclonic eddy named Luzon cold eddy(LCE) appears to the northwest of the Luzon island. In spring the large cyclonic gyre becomes weak due to weak wind stress forcing and the LCE moves northwestward. A weak anticyclonic gyre begins to appear in the central SCS(10°N-14°N) in March. In summer, the VIC is cyclonic in the north half basin and anticyclonic in the south half and the LCE disappears. In fall, the cyclonic gyre expands southward and then occupies the whole basin. Second, the surface current represents the Ekman transport, which direction is nearly perpendicular to that of surface wind. From the sea surface temperature (SST) distribution, there is upwelling between 11°N to 13°N off the Vietnam in summertime. Third, in the lower layer, the thermal-wind relation is nearly met between the temperature distribution and the circulation pattern due to relatively weak influence of the surface wind and solar radiation. For example, the LCE is corresponding to a strong upwelling whose center temperature is lower than that of the surroundings by about 2 to 3 degrees. In winter and spring, there is warm water appearing in the central SCS, which corresponds to an anticyclonic gyre there. While in the south of the SCS, there is a cycloni gyre corresponding to cold water. Finally, the LCE can reach at depth of 200m. Its generation and maintenance should be mainly due to the local wind stress curl. The Kuroshio can not flow directly into the SCS interior but forms an anticyclonic loop in the Luzon Strait, where a small part of Kuroshio water can flow out of the Taiwan Strait along the west coast of the Taiwan island.
JSP25/C/MI05/W/01-B2 1150
ONSET CHARACTERISTICS OF THE 1998 SOUTH CHINA SEA SUMMER MONSOON
Yongguang Wang, JOHNNY C. L. CHAN and Jianjun Xu (Dept. of Physics & Materials Science, City University of Hong Kong; Email: Johnny.Chan@cityu.edu.hk)
The South China Sea Monsoon Experiment (SCSMEX) was conducted in May and June 1998 to study the South China Sea Summer Monsoon (SCSSM). This paper presents the results of a preliminary study of the characteristics of the synoptic-scale conditions related to the SCSSM onset and its maintenance. The onset and break are defined using gridded zonal winds at 850 hPa and observed daily rainfall at Xisha (112.33°E, 16.83°N) and Yongshujiao (112.88°E, 9.53°N). Four sets of meteorological parameters are examined using the gridded data sets from the Beijing Data Center of SCSMEX: 1) surface and 850-hPa temperatures, 2) mean sea-level pressure (MSLP), 3) surface relative humidity, and 4) surface and upper-level winds. The results show that the summertime meridional temperature gradient and meridional wind circulation as well as high relative humidity are already established before May over the South China Sea (SCS). On the other hand, reversals of the meridional MSLP gradient and the zonal wind circulation occur only at the onset. An active tropical cyclone in the Bay of Bengal is apparently responsible for "leading" the strong southwesterly Somalia jet to the SCS. A vortex over the northern SCS causes a weakening of the subtropical high to change the meridional MSLP gradient over the SCS, and enhances the cross-equator-flow at 105°E. Westerly flow then sets in at the low levels. The enhanced Southern Asia high at 200 hPa and an upper tropospheric trough over northeastern SCS are related to the establishment of easterly flow at the upper levels.
JSP25/L/01-B2 1210
INTERANNUAL RAINFALL VARIABILITY
RICHARD WASHINGTON and Martin Todd (School of Geography, University of Oxford;1 Mansfield Rd, OX! 3TB UK)
The primary mode of interannual rainfall variability over southern Africa and the Southwest Indian Ocean during the austral summer and its relationship with oceanic and atmospheric fields are examined in this paper. Empirical Orthogonal Functions (EOFs) of the Xie-Arkin satellite derived rainfall data are used to determine modes of interannual variability for austral summer months between November and March. In all months it is shown that tropical-temperate troughs linking tropical convection with quasi stationary or propagating transients are the primary mode of rainfall variability. Further analysis of model rainfall from an AGCM (HADAM2A) forced with historical sea surface temperatures from 1904-1994 indicates a similar mode of variability on both interannual and interdecadal time scales.
Sea surface temperatures, moisture fluxes, sea level pressure and other fields are examined to determine the characteristics of oceanic and atmospheric anomalies accompanying extreme years of tropical-temperate trough activity. The NCEP re-analysis data is used in conjunction with the Xie-Arkin (observed) rainfall EOFs and the model data with the model rainfall analysis.
The analysis of both model and observed circulation data focuses on anomalous anticyclonic gyres in near surface atmosphere of the southwest Indian Ocean. Near surface water vapour flux anomalies associated with the anomalous gyres and with extremes in tropical-temperate trough variability are highlighted.
Tuesday 27 July PM
SEASONAL-DECADAL VARIABILITY
Asian Monsoon
JSP25/W/89-B2 1400
MONSOON-ENSO RELATIONSHIP ON INTERANNUAL AND INTERDECADAL TIME SCALES
B N GOSWAMI, Centre for Atmospheric and Oceanic Sciences Indian Institute of Science Bangalore 560012, India e-mail: goswamy@caos.iisc.ernet.in and V. Krishnamurthy Center for Ocean-Land-Atmosphere Studies Institute of Global Environment and Society, Inc. 4041 Powder Mill Road, Suite 302, Calverton, MD 20705, USA e-mail: krishna@cola.iges.org
Empirical evidence is presented to support a hypothesis that the interdecadal variation of the Indian summer monsoon and that of the tropical sea surface temperature (SST) are parts of a tropical coupled ocean-atmosphere mode. The interdecadal variation of the Indian monsoon rainfall (IMR) is strongly correlated with the interdecadal variations of various indices of El Nino and the Southern Oscillation (ENSO). We also show that the interannual variances of both IMR and ENSO indices vary in phase and follow a common interdecadal variation. However, the correlation between IMR and eastern Pacific SST or between IMR and Southern Oscillation Index (SOI) on the interannual time scale does not follow the interdecadal oscillation. The spatial patterns of SST and sea level pressure (SLP) associated with the interdecadal variation of IMR are nearly identical to those associated with the interdecadal variations of ENSO indices. As has been shown earlier inthe case of ENSO, the global patterns associated with the interdecadal and interannual variability of the Indian monsoon are quite similar. The physical link through which ENSO is related to decreased monsoon rainfall on both interannual and interdecadal time scales has been investigated using National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis products. The decrease in the Indian monsoon rainfall associated with the warm phases of ENSO is due to an anomalous regional Hadley circulation with descending motion over the Indian continent and ascending motion near the equator sustained by the ascending phase of the anomalous Walker circulation in the equatorial Indian Ocean. We show that, to a large extent, both the regional Hadley circulation anomalies and Walker circulation anomalies over the monsoon region associated with the strong (weak) phases of the interdecadal oscillation are similar to those associated with the strong (weak) phases of the interannual variability. However, within a particular phase of the interdecadal oscillation, there are several strong and weak phases of the interannual variation. During a warm eastern Pacific phase of the interdecadal variation, the regional Hadley circulation associated with El Nino reinforces the prevailing anomalous interdecadal Hadley circulation while that associated with La Nina opposes the prevailing interdecadal Hadley circulation. During the warm phase of the interdecadal oscillation, El Nino events are expected to be strongly related to monsoon droughts while La Nina events may not have significant relation. On the other hand, during the cold eastern Pacific phase of the interdecadal SST oscillation, La Nina events are more likely to be strongly related to monsoon floods while El Nino events are unlikely to have a significant relation with the Indian monsoon.
JSP25/E/18-B2 1420
POST-MONSOON SST ANOMALIES OVER INDIAN AND PACIFIC OCEANS
C. A. BABU and P. V. Joseph (Department of Atmospheric Sciences Cochin University of Science and Technology Cochin - 682 016, INDIA)
We have studied the Sea Surface Temperature (SST) anomalies over the Indian and Pacific Oceans in association with DRY Indian monsoons accompanied or not with El Nino during the period 1961-1985, over the following three seasons September-November (SON), Decemmber-February (DJF) and March-May (MAM). The domain studied is between latitudes 25 S and 25 N and longitudes 40 E to 160 W. In general, a DRY monsoon is followed by a warm SST anomaly over the tropical Indian Ocean and cold SST anomaly over tropical west Pacific ocean. Those DRY monsoons which are co-existing with El Nino are associated with larger SST anomalies.
Examining 3 monthly moving averages, it is found that warm SST anomaly first appear over the western Indian Ocean during JJA and expand in area northward and intensify during the Asian Summer Monsoon (JAS, ASO and SON). During OND this anomaly weakens and spreads to the east and south while a new and stronger SST anomaly appears over the eastern Bay of Bengal and South China Sea, apparently in association with the Asian Winter monsoon. This anomaly intensifies and spreads towards theAustralian waters over the following few months. By March to May (MAM) season the entire Indian Ocean about 10-15 degrees latitude either side of the equator, has a warm SST anomaly.
JSP25/W/73-B2 1440
A MODEL OF THE INTRASEASONAL FLUCTUATION OF THE INDIAN MONSOON
MR BRETT SCONCIA and Prof. Brian J. Hoskins, Department of Meteorology, University of Reading, 2 Earley Gate, Whiteknights, PO Box 239, Reading, UK, e-mail:swr96bcs@reading.ac.uk
The dominant mode of intraseasonal variability of the Indian summer monsoon is associated with an alternating position of the tropical convergence zone (TCZ). An "active" period of the monsoon occurs when the TCZ persists over the Indian sub-continent, contrasting with a "break" period that occurs when the TCZ is found in a more southerly position over the equatorial ocean. The heating structures of the atmosphere associated with a typical active and break period are produced from ERA data and are used as forcings to drive a simple numerical model. With the two heatings kept constant, resultant atmospheric flows are consistent with those seen in ERA data. The model is then made time-dependent with a linear combination of the active and break heatings used to drive the model. Characteristics of the low-level flow that are significantly different for the two regimes are used to determine the relative amounts of the two heatings. Results from a 1000-day integration show an apparently chaotic system with some evidence of bi-modality in the probability density function (PDF). Experiments with the time-dependent model to investigate the sensitivity of the PDF distribution to external forcing will be presented.
JSP25/P/03-B2 1500
IMPACT OF ENSO ON ASIAN MONSOON: NEW PERSPECTIVES
R.H.KRIPALANI and Ashwini Kulkarni (both at Indian Institute of Tropical Meteorology, Pashan, Pune 411008, India, email: rhksup@tropmet.emet.in)
El Nino Southern Oscillation (ENSO) phenomenon is now recognised as the single most important mode of the earth's year-to-year climatic variability. It has been well documented that majority of the ENSO warm (cold) extremes are associated with below (above) normal rainfall over the Asian domain.
Our recent analysis of the 128-year (1871-1998) data reveals that the Indian monsoon rainfall (IMR) exhibits epochal variability. The periods 1880-1895 and 1930-1963 are characterised by above normal rainfall while the periods 1895-1930 and 1963-1990 are characterised by below normal rainfall. Further the impact of El Nino (La Nina) on IMR is more severe during the below (above) normal rainfall epochs. Rainfall data analysis also reveals that the IMR is entering into an above normal epoch with a turning point around 1990 suggesting that the impact of El Nino (La Nina) on IMR may not (may) be severe. This may be a possible reason for India not experiencing a drought during the El Ninos after 1990s. In spite of the severe 1997 El Nino the IMR was 102 % of the long-term average. The equatorial Pacific entered into a La Nina phase during the Monsoon of 1998. The IMR for 1998 was 106 % of the normal.
Such phase-locking between the ENSO episodes and epochal variability is also seen over other regions of Asia. Implications of these results on the impact of ENSO during the coming decade will be discussed.
JSP25/E/36-B2 1520
THE OCEANS' ROLE IN LOW-FREQUENCY VARIABILITY OF THE INDIAN MONSOON
ROXANA C. WAJSOWICZ (Dept. of Meteorology/JCESS, University of Maryland, College Park, MD 20742, U.S.A., e-mail roxana@atmos.umd.edu); Paul S. Schopf (Institute of Computational Sciences and Informatics, George Mason University, Fairfax, VA 22030, U.S.A., e-mail schopf@cola.iges.org)
It is proposed that the southern tropical Indian Ocean (STIO) plays an important role in variability of the strength of the Indian summer monsoon. During years of a strong monsoon, its influence is either direct in providing more moisture to precipitate over India, or indirect in draining less heat from the Arabian Sea and so promoting a more energetic atmospheric circulation. Direct influence is supported by analysis of COADS, which shows that the evaporation rate over the STIO is increased as is the cross-equatorial moisture transport by the Findlater jet during boreal summer in strong years. Interestingly, the corresponding SST anomalies in the STIO are positive. In years of below average monsoon rainfall, the opposite is found. Indirect influence occurs because vigorous evaporation over the STIO is sustained during the boreal summer by heat transport from the Arabian Sea and from the western equatorial Pacific Ocean via the Indonesian throughflow. Whereas the heat exchange between the Arabian Sea and STIO has an annual period with a maximum southward transport in boreal summer, that between the western equatorial Pacific and the STIO has a strong semi-annual period with a maximum southwest transport over the upper thermocline during the monsoon transition seasons. Hence, the Indonesian throughflow preconditions the STIO for the ensuing monsoon season. The mutual interactions between these three components, namely the Arabian Sea, STIO and western equatorial Pacific, are illustrated with differently configured GCMs of the seasonal cycle and the last decade.
Presiding Chair: B.N. Goswami (Indian Institute of Science)
Tropical Atlantic and Connections
JSP25/W/17-B2 1620
CLIMATIC ANOMALIES DURING THE BOREAL WINTER OF 1997-98: THE ROLE OF ATLANTIC SST
R.T. SUTTON, S.P. Jewson, C. Jones, J. Slingo Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, PO Box 243, Earley Gate, Reading RG6 6BB, U.K.
During the boreal winter of 97/98 there were major climate anomalies in many parts of the world. Many of these anomalies were attributed to the El Nino conditions that prevailed at the time, implying a link to SST anomalies in the tropical Pacific. There were, however, significant SST anomalies elsewhere in the world: in the Indian and Atlantic oceans. There is considerable evidence that the Indian Ocean SST anomalies had a major impact on climate. We are investigating the less studied role of the Atlantic Ocean SST anomalies.
We have performed ensemble integrations of an atmospheric General Circulation Model with various SST fields used as lower boundary condition. Our results show that the Atlantic SST anomalies had a significant impact on both the tropical and extratropical atmospheric circulation during DJF 97/98. Our model suggests that this impact included a contribution to a predictable signal over Europe. The response to the Atlantic SST anomalies in the tropics involves intensification of the local Hadley Circulation. Over the North Atlantic an anomalous low is forced. An important question is whether this mid-latitude response is forced by the local Atlantic SST anomalies or is a remote response to the tropical Atlantic SST anomalies. We are presently addressing this question through further experimentation.
JSP24/W/45-B2 1640
TROPICAL-EXTRATROPICAL CONNECTION IN THE ATLANTIC ATMOSPHERE-OCEAN VARIABILITY
Masahiro WATANABE and Masahide Kimoto (Center for Climate System Research, University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8904, Japan, email:hiro@ccsr.u-tokyo.ac.jp)
Observations for the recent 40 years show two dominant modes of variability in the tropical Atlantic SST anomalies. One has a monopole structure associated with the El Ni\~no-like signal in the tropical Pacific SST anomalies, while another reveals a north-south dipole straddling the equator. These modes accompany 500-hPa height and SST anomalies in the North Atlantic.
To investigate the physical relationship between the tropical SST variations and the North Atlantic atmosphere-ocean system, a 60-yr long integration was conducted with an AGCM coupled to a 50-m-deep slab ocean except for the tropics where the SST is prescribed to observations (TOGA-ML run). The TOGA-ML run well reproduced the extratropical height and SST anomalies, suggesting an impact of tropical SST anomalies on the North Atlantic air-sea variations. However, linear responses of the dynamical operator of the AGCM to the heating in the tropical Atlantic corresponding to the two modes of SST variability are weak in the North Atlantic and moreover less similar to the observed height anomalies. The height responses resemble the anomalies when associated transient eddy forcing is included. This result implies that the tropical SST variations indirectly influence the North Atlantic atmosphere through changes in the eddy activity. A comparison of the TOGA-ML run with a GCM run coupled to the global slab ocean indicates that the tropical SST variability increases the variance in the extratropical atmosphere, selectively for the North Atlantic Oscillation.
JSP25/L/02-B2 1700
THE EQUATORIAL ATLANTIC OSCILLATION: A SELF-SUSTAINING CLIMATE OSCILLATION IN THE TROPICAL ATLANTIC
ITSUKI C. HANDOH (School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K., e-mail i.handoh@uea.ac.uk); Grant R. Bigg (School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K., e-mail g.bigg@uea.ac.uk)
The anomalous climate of the Atlantic of 1983-84 has been inferred as being triggered by the 1982/83 El Nino in the Pacific. Other such events have been observed in the tropical Atlantic, typically lagging El Nino events by 4-5 months, although there are not always clear links to such events, as in 1988. Here, using modelling, satellite altimetric and sea surface temperature data, we report an unusual equatorial Atlantic event, starting in autumn 1995 and continuing into the autumn of 1997, which does not seem to be connected to the El Nino ending in early 1995. This Atlantic event contained both a warm and a cold phase, with the latter seemingly being generated, through air-sea coupling, by the former. The propagating signal within each phase is identified in the sea surface height and temperature signatures as both equatorial and off-equatorial Rossby waves in the ocean, with coupling to atmospheric convection. The two-phased climate event appears to have been associated with internal atmospheric and oceanographic variability within the Atlantic basin and hence have been independent of the Pacific climate.
JSP25/W/83-B2 1720
A LINKAGE FOR DECADAL CLIMATE VARIATIONS IN THE LABRADOR SEA AND THE TROPICAL ATLANTIC OCEAN
Jiayan YANG (Dept. of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA, email: jyang@whoi.edu)
The Labrador Sea Water (LSW) thickness, according to observations, varies significantly on decadal time scales in response to atmospheric forcing and fresh-water flux from the Arctic Ocean. Sea surface temperature (SST) in the tropical Atlantic also exhibits considerably interannual to decadal changes. A distinct mode, as identified in previous studies, is the so-called tropical Atlantic SST dipole -- a cross-equator pattern. Analyses of observations and modeling results indicate that the SST dipole and variations of LSW thickness are linked through the Meridional Overturning Circulation (MOC). Southward transport of LSW along deep western boundary must be compensated by northward flow in the upper ocean. Once the LSW pulse enters the tropics, it affects the cross-equator heat transport and the upper ocean response generates a dipole pattern in the SST field. The correlation between observed LSW thickness and SST is significant with a lag of 5 years. The best correlation occurs in areas off the western boundary where the main cross-equator transport takes place. Results from an ocean model indicate that the time lag between LSW and tropical SST is set by coastal Kelvin waves that propagate from the Labrador Sea to the tropics along the western boundary.
Wednesday 28 July AM
Presiding Chair: M. Alexander (CDC-CIRES)
Concurrent Poster Session
SEASONAL-DECADAL VARIABILITY
NAO Related Variability
JSP25/E/12-B3 0830
NORTH ATLANTIC OCEAN VARIABILITY ON DECADAL TIMESCALES IN THE HADLEY CENTRE COUPLED MODEL
Claire COOPER and Chris Gordon (both at Hadley Centre for Climate Prediction and Research, Meteorological Office, London Road, Bracknell, Berkshire, RG12 2SY, UK.
Email: ccooper@meto.gov.uk)
The simulation of decadal variations in the North Atlantic Oscillation (NAO) and related ocean phenomena in a multi-century integration of the latest Hadley Centre coupled model which does not use flux adjustment will be described. Where possible the model simulation is compared with available observations. The ocean component of the model has a 1.25 deg resolution which allows a better representation of the North Atlantic Current (NAC) than in many of the earlier coarser resolution climate models. It has also enabled the relationship between the NAO, high latitude convection and the strength and path of the NAC to be explored in detail. It is shown that the model realistically simulates the NAO in both spatial pattern and time variability and, in addition, the ocean component confirms the observed relationship between convection in the Labrador and Greenland Sea and the phase of the NAO. The model also has propagating decadal timescale sea surface temperature anomalies, similar to those found in the historical observations. The mechanisms of propagation of these anomalies in the coupled model will be discussed.
JSP25/W/53-B3 0850
FLUCTUATIONS OF THE NAO AND NORTH ATLANTIC STORM TRACK ACTIVITY IN THE ECHAM4/OPYC3 COUPLED AOGCM
M. CHRISTOPH and U. Ulbrich, (Institut für Geophysik und Meteorologie der Universität zu Köln, Kerpener Straße 13, D-50923 Köln, Germany)
In a 300-year control integration of a coupled ocean_atmosphere GCM (ECHAM4 + OPYC3), a close relation of the North Atlantic Oscillation (NAO) and the eastern part of the Atlantic storm track is confirmed both for the interannual (< 10 years) and the decadal (> 10 years) time scales. Positive anomalies of the NAO are associated with a northward shift and intensification of the storm track, and increased freshwater fluxes into the Iceland and Norwegian Seas. On the other hand, the mean flow anomaly in the atmosphere induces Ekman pumping into the central parts of the subtropical North Atlantic during the positive NAO phase. This contributes to a spin up of the subtropical gyre circulation and thus to northward transports of warm and salty waters within the Gulf stream system. While the existence of such relations on the decadal time scale suggests that the dynamical ocean is important for the NAO and the associated storm track variability, an additional experiment using a 50 m fixed-depth mixed layer ocean proves the contrary. The NAO has very similar space-time behaviour as in the coupled run and amplitudes are only about 10% smaller than using the dynamical ocean. The second EOF of eddy activity over the Atlantic represents the variation of activity in the center of the Atlantic storm track, again both on the interannual and the decadal time scales. This variation exhibits a very weak connection to the NAO or to other coherent SLP variability patterns. Potential factors influencing this region are discussed.
JSP25/E/24-B3 0910
LONG-TERM CHANGES OF INTERANNUAL SEA LEVEL VARIABILITY IN THE BALTIC SEA AND RELATED CHANGES OF WINTER CLIMATE
Martin EKMAN (Summer Institute for Historical Geophysics Bomarsund Åland Islands)
The world's longest sea level series, that of Stockholm commencing 1774, has earlier been used by the author to study the eustatic rise of sea level. Here, the interannual sea level variability of the Baltic Sea and its relation to climate is studied using the same sea level series, together with equally long temperature and wind series. Long-term changes of the sea level variability are found to be closely related to changes of winter climate. The interannual sea level variabilty and the interannual winter temperature and winter wind variabilities have all decreased significantly from the end of the 1700s to the beginning of the 1900s; after that they have all increased significantly again. The common origin of these long-term changes turn out to be two consecutive wind processes over the North and Baltic Seas, especially the Baltic entrance: From the end of the 1700s to the beginning of the 1900s there is a rapidly decreasing number of dominating winter winds from northeast, after that there is an increasing number of dominating winter winds from southwest.
JSP25/E/06-B3 0930
IS THE NORTH ATLANTIC OSCILLATION A RANDOM WALK?
Dr. David B. STEPHENSON (Labo. de Statistiques et Probabilites, Universite Paul Sabatier, 118, route de Narbonne, 31062 Toulouse, France. E-mail: stephen@cict.fr); Dr. Valentina Pavan (CINECA, via Magnanelli 6/3, 40033 Casalecchio di Reno, Italy. E-mail: pavan@cineca.it)
A principal component analysis of the Jones gridded land/sea temperature data set reveals that year-to-year differences in global land/sea wintertime surface temperatures are dominated by a) the North Atlantic Oscillation (NAO) and b) the tropical biennial oscillation. The NAO indices exhibit long-memory fluctuations that can be modelled using a simple non-stationary random walk model. The model accounts for both year-to-year reversals and trends in the NAO, and has some slight yet encouraging skill at forecasting the NAO one year ahead. Decadal trends and cycles simulated by this non-stationary stochastic model resemble those in the observed NAO and offer a possible explanation for recent NAO variability.
JSP25/W/59-B3 0950
SEASONAL DEPENDENCE AND TIME EVOLUTION OF THE INTERANNUAL SEESAW BETWEEN THE ALEUTIAN AND ICELANDIC LOWS
HISASHI NAKAMURA (IGCR, Frontier Research System for Global Change, Tokyo/Dept. Earth, Planetary Physics, Univ. Tokyo, 113-0033, JAPAN; email: hisashi@geoph.s.u-tokyo.ac.jp), Meiji Honda (IGCR, Frontier Research System for Global Change, Tokyo; email: meiji@ frontier.esto.or.jp), and Jinro Ukita (IARC, Frontier Research System for Global Change, Tokyo;
email: qpee@frontier.esto.or.jp)
It is known that the surface Aleutian and Icelandic lows (AL and IL, respectively) exhibit a seesaw-like oscillation in winter. We examined seasonal dependence of this interannual seesaw by recording the minimum within each of the North Atlantic and North Pacific in the 31-day moving-averaged sea-level pressure (SLP) for each day over 1973-1994 based on the NMC analyses. The correlation coefficient between the 22-year timeseries of thus-defined AL and IL intensities was evaluated for each calendar days. The correlation was found to be strongly negative only in the late winter (February through mid-March). In the rest of the year it is weakly negative or even slightly positive. A similar seesaw is also evident in the SLP field from which ENSO- related anomalies as inferred from a linear regression analysis with SOI were removed.
We defined AI (AL-IL) index as the IL-center pressure subtracted from the AL-center pressure in the 45-day mean SLP for January 31-March 16. In the recognition of the equivalent barotropic structure of the seesaw, we investigated seasonal evolution of the seesaw in the linear lag regression maps between AI index and the mean of the 250-hPa height for each of the fourteen 45-day periods, whose central calendar days are mutually 15 days apart. In the following, we focus on a winter of the weaker AL with the stronger IL. After positive height anomalies gradually develop over the eastern North Pacific in December, a PNA-like wavetrain emanates from the positive anomalies in early January (Dec. 17-Jan. 30) with negative and positive anomalies appear over western Canada and the southeastern US, respectively. Then, another wavetrain emanates from the latter with negative and positive centers to the east of Newfoundland and over northern Europe, respectively. As these wavetrains gradually weaken by early February (Jan .16-Mar. 1), positive anomalies over the North Pacific fully mature and cover the entire North Pacific, which correspond to the weak AL. At the same time the negative anomalies over the North Atlantic further develop while shifting northward until they mature in the "peak period" (Jan. 31- Mar. 16), when the AL-IL seesaw is the most apparent.
JSP25/W/52-B3 1010
LONG-TERM CHANGES OF THE WINTER SEA LEVEL PRESSURE FIELDS AND RELATED SYNOPTIC ACTIVITY OVER THE NORTH ATLANTIC
Igor ZVERYAEV (IPRC, SOEST, University of Hawaii, MSB, Room 227, 1000 Pope Rd., Honolulu, HI 96822, USA, email: igorz@soest.hawaii.edu)
Long-term climatic changes of the winter synoptic activity over the North Atlantic are studied in relation to changes of the winter mean sea level pressure (SLP) fields. Analysis of linear trends has revealed a good agreement between long-term (interdecadal) changes of the intensity of synoptic processes and variations of the winter SLP. On the contrary, there was not such agreement found between detrended and low-pass filtered anomalies. There are periods, which are characterized by the enhanced (reduced) synoptic activity attributed to the low (high) index of the North Atlantic Oscillation (NAO). It appears that low-passed anomalies of intramonthly root mean square deviations of SLP are negatively correlated with NAO and East-Atlantic teleconnection patterns over the most of the North Atlantic.
The low-passed winter SLP anomalies demonstrate both propagating and standing patterns. The latter have a period of about 8 years. While meridional dipole-like structures formed by the winter SLP anomalies are shifted to the west (east) of the North Atlantic, related anomalies of synoptic activity tend to be located in the eastern (western) part of the region. When decadally averaged, anomalies in the intensity of synoptic activity are strongly linked to the North Atlantic storm track position. Exception is 1980-1990 decade, characterized by the very high NAO index. During this decade enhanced synoptic activity is observed to the south of the North Atlantic storm track.
Presiding Chair: M. Christoph (Institut für Geophysik und Meteorologie der
Universität zu Köln, Germany)
Predictability
JSP25/W/04-B3 1110
HYDROCLIMATOLOGICCAL PREDICTABILITY OVER THE AMERICAS
John ROADS (Scripps Institution of Oceanography, UCSD-0224, La Jolla, CA 92093, Email: jroads@ucsd.edu); Susan Marshall (Dept. Geog. & Earth Sci., UNC-Charlotte, Charlotte, NC 28233, Email: susanm@uncc.edu); Bob Oglesby (Purdue Univ., W. Lafayette IN 47907, Email: roglesby@purdue.edu); Franklin Robertson (NASA/Marshall Space Flight Center, Global Hydrology and Climate Center, Huntsville, AL 35806, Email: pete.robertson@msfc.nasa.gov)
Hydrologic variations over the Americas associated with interannual sea-surface temperature (SST) anomalies have been identified in long simulations made with the National Center for Atmospheric Centers (NCAR’s) community climate model (CCM3) and in the National Centers for Environmental Prediction (NCEP) reanalysis. Corresponding variations in global precipitation climatology project (GPCP) precipitation anomalies provide additional validation of the strength and magnitude of the anomalies. These hydrologic variations are also being compared to the variations associated with local soil moisture anomalies.
The extent to which the response of the CCM3 atmosphere to SST anomalies differs from the response to local soil moisture anomalies serves as the basis for climate predictability experiments using various combinations of SST and soil moisture anomalies. Preliminary results indicate that in the Mississippi River basin and in the Mackenzie River basin, both soil moisture and SST anomalies have significant influences on monthly to seasonal predictions. By contrast, hydroclimatological variations over the Amazon are governed much more strongly by SST anomalies.
JSP25/E/33-B3 1130
LONG-RANGE PREDICTION OF UK WINDSTORMS
S. E. GEORGE and M. A. Saunders (both at Department of Space and Climate Physics, Benfield Greig Hazard Research Centre, University College London, UK, Email: seg@mssl.ucl.ac.uk)
For the UK and NW Europe, winter storms and floods are the largest cause of economic damage and insurance loss. The skillful prediction of such events several months in advance would allow damage limitation procedures to be implemented. Current dynamical weather forecast models have a limit of predictability measured in weeks rather than months, and are therefore not suitable for such a task. We present a new statistical model for the long range prediction of UK windstorms. Initial results for winter storminess in the South East of England for the period 1958-1996, have shown 70% of the year-to -year variance in storminess can be predicted six months in advance. Physical mechanisms linking the climate predictors we use with UK windspeed several months later will be suggested.
JSP25/E/08-B3 1150
IMPACT OF THE ENSEMBLE SIZE ON MULTIMODEL SEASONAL FORECASTS
Francisco Javier DOBLAS-REYES and Michel Deque (CNRM, Meteo-France, 42, Av. G. Coriolis, 31057 Toulouse Cedex, France)
In the framework of the PROVOST project, the European seasonal prediction experiment, four long-lead ensemble forecast experiments with prescribed SST have been carried out. Each experiment has been run with a different GCM: the ECMWF, CNRM (with two different horizontal resolutions), and UKMO atmospheric models, all integrated from common initial conditions.
The performance of the combination of the different models in a whole ensemble has been compared with the performance of the individual models. The combination increases the ensemble size, providing a good test field for checking the feasibility of probabilistic forecasts in seasonal forecasting. Furthermore, the multimodel forecasts present the advantage of taking into account the role of the model in the forecast divergence. All the models exhibit some skill in the seasonal range in winter. This is also evidenced for the other seasons except for the European region. The hypothesis of a significant gain in skill for multimodel probabilistic forecasts independently of the ensemble size has been tested. Results show that multimodel represents a clear improvement over the tropics.
Wednesday 28 July PM
Presiding Chair: Dr D.B. Stephenson (Meteo-France)
SEASONAL-DECADAL VARIABILITY
Air-Sea Interaction and the Mixed-Layer
JSP25/E/32-B3 1400
THE INFLUENCE OF MIDLATITUDE AIR-SEA INTERACTION ON CLIMATE VARIABILITY
MICHAEL ALEXANDER and James Scott (CDC-CIRES, University of Colorado Boulder, CO 80309, USA e-mail: maa@cdc.noaa.gov)
A coupled atmosphere-ocean model is used to examine the role of air-sea interaction in climate variability. The coupled model consists of the Geophyscial Fluid Dynamics Lab (GFDL) atmopsheric general circulation model (AGCM) coupled to a grid of independent mixed layer ocean models which is nearly global in extent. The ocean model simulates vertical processes which makes it appropriate for studying aspects of the climate system which operate on interannual time scales in the extratropics. The coupled model has been integrated for 50 years and has been compared to a 50-year control AGCM simulation in which sea suurface temperatures (SSTs) corresponding to the long term mean in the coupled run are specified as boundary conditions.
Here we will focus on processes which create SST anomalies and how these anomalies influence the atmospheric circulation. The net surface heat flux is the dominant term in creating SST anomalies over most of the year except in fall when entrainment plays an equal or greater role over parts of the North Pacific and North Atlantic. This allows for the winter-to-winter recurrence of SST anomalies as thermal anomalies created by the net heat flux in late winter, remain below the mixed layer in summer and are then returned to the surface by entrainment in the following winter. Preliminary results indicate that air-sea interaction in the Atlantic influences the storm track and the circulation at 500 mb downstream over Europe.
JSP25/E/21-B3 1420
ERROR PROPAGATION ACROSS THE AIR-OCEAN INTERFACE
Peter C. CHU, (Naval Postgraduate School, Monetary, CA 93943, USA, Email: chu@nps.navy.mil) Shihua Lu, W. Timothy Liu, and Yuchun Chen
Error propagation across the air-ocean interface is an important factor to determine the ocean/atmosphere predictability. First, we use the Lorenz system (Lorenz, 1963) to show the propagation of boundary error. Second, we use the latest version of the NCAR Community Climate Model (CCM3) to study the propagation of tiny surface temperature error into the model atmosphere. The model was integrated from 1 September 1977 observational data over the globe for 16 months with and without SST disturbances. A time-scale of 20-day is found such that the response increases rapidly within this time-scale and then oscillates at high values. Third, we study the propagation of wind error into ocean model. Finally, some theoretical discussion will also be included.
JSP25/W/35-B3 1440
TESTING THE INFLUENCE OF THE SST ANOMALIES IN THE TROPICAL ATLANTIC UPON THE ATMOSPHERIC CIRCULATION AT 500 HP
AIleana Mares, C. MARES, Mihaela Mihailescu (National Institute of Meteorology and Hydrology, Bucuresti-Ploiesti 97, Bucharest 71552, Bucharest, RomaniaFax : 00-40-1-2303143, E-mail : imares@meteo.inmh.ro
We have started from the hypothesis that there is a dependence of Markov chain type of the main interaction types between the ocean state and the atmospheric state. The classification with four states has been used : blocking circulation conditioned by SST negative anomalies, blocking circulation conditioned by SST positive anomalies, zonal circulation conditioned by SST positive anomalies and zonal circulation conditioned by SST negative anomalies.
The probability matrix of the transformations from one state to another, is estimated from monthly mean values of the geopotential at 500 hPa and of the SST in the Atlantic Ocean area, during the period 1965-1987. The geopotential has been considered over the sector: 500W-400E and 350N-650N. The SST data in COADS in the same period have been considered in our processing for the sector: 370S-390N, 740W-100E. The limiting matrix of the Markov chain has become stabilised after 23 steps ( months), indicating a measure of the predictability of the supposed interaction. The statistical significance of the elements of the transition matrix has been established by the Monte Carlo simulation method.
JSP25/W/94-B3 1500
THE SEASONAL AND INTERANNUAL VARIATION OF THE HEAT BUDGET OF THE MIXED LAYER EVALUATED BY SATELLITE-DERIVED HEAT FLUX AND THE NUMERICAL MODEL VELOCITY FIELD
Masanori KONDA, Miho Toyoda, Taiyo Kobayashi, Norihisa Imasato (Department of Geophysics, Graduate School of Science, Kyoto University); Kitashirakawa-Oiwake, Sakyo, (Kyoto 606-8502, Japan, E-mail: konda@kugi.kyoto-u.ac.jp) , and Akira Shibata (NASDA/EORC, 14F, Roppongi-firt Buld., 1-9-9 Roppongi, Minato, Tokyo 106-0032, Japan, E-mail: ashibata@eorc.nasda.go.jp )
We evaluate the heat budget in the mixed layer in the global ocean from satellite-derived surface heat flux and the numerical model velocity field, and discussed the seasonal and the interannual variability of it. The surface heat flux data is the sum of the satellite-derived turbulent heat flux and the radiation flux from July 1987 to June 1991, and the seasonal cycle of the surface velocity field is computed by the robust diagnostic model (Kobayashi and Imasato 1998) without the interannual variation. The thermal response of the ocean to the surface heat exchange is very important in the point that the seasonal and the interannual climate changes largely depend on the thermal air-sea interaction.In careful consideration for the validity of using together different types of the data sources as imported from the numerical model computation and the satellite observation, the heat flux at the bottom of the mixed layer is computed as the residual of the surface and the horizontal heat flux and the change of the heat content in the surface layer with a constant depth in the global ocean. The distribution of the heat flux at the bottom of the mixed layer indicates the existence of the significant heat exchange between the surface and the subsurface layer. The seasonal change of the bottom heat flux suggests that the oceanic heat transport by the strong current has relatively greater effect during the season when the sea surface temperature (SST) is increasing, and the surface heat flux when the SST decreasing. Correlation analysis is done to examine how the result of the air-sea heat exchange can be induced to the subsurface ocean, and it is suggested that the change of the thermal condition in the remote area can teleconnectively contribute to the local heat budget through the horizontal heat transport, adding to the local effect of the surface heat flux.
JSP25/E/17-B3 1520
MIXED-LAYER HEAT BALANCE AND NET SURFACE HEAT FLUX ON A SEASONAL SCALE
S.A.B. KIM (Satellite Technology Research Center. 373-1, Kusung, Yusung, Taejon, S. KOREA 305-701. sbkim@krsc.kaist.ac.kr. Also at Korean Meteorological Administration)
Heat balance within an ocean mixed layer is important since it describes whether ocean-atmosphere interactions occur locally or remotely. The heat balance mechanism is analysed over the Kuroshio Extension region to identify the significance of the jet, seasonally from October 1992 to September 1995. The horizontal geostrophic heat advection is obtained using current velocities from satellite altimetry. The errors in the monthly and spatial mean values of the zonal and meridional geostrophic heat advection reach respectively 150 % (46 W/m**2) and 300 % (61 W/m**2) of the signal. The entrainment heat advection is estimated rigorously on a climatological monthly scale. The heat storage rate is computed using subsurface temperature records. The horizontal Ekman heat advection is derived using ECMWF model wind speeds. The seasonal heat advection by the Kuroshio Extension is <10 % of the heat storage rate in the basin-scale heat balance. The horizontal Ekman heat advection is also insignificant. It is concluded that the heat balance is achieved locally. Monthly net surface heat fluxes are derived from the sum of the heat balance terms. Errors in these fluxes are estimated thoroughly by developing formulae on how errors in the basic fields propagate into the flux. A value of 30 W/m**2 is obtained for the error in the three-year mean net surface heat flux over the basin. This is smaller in percentage terms than that achieved by the best bulk-formulae method. Horizontal heat advection (Ekman plus geostrophic) contributes 21 W/m**2 to the error in the three-year mean net surface heat flux.
Presiding Chair: Claire Cooper (Hadley Centre for Climate Prediction and Research, Meteorological Office, Berkshire,UK.)
JSP25/W/43-B3 1620
INTERNALLY GENERATED MODE WATER VARIABILITY
Wilco HAZELEGER (KNMI, Oceanogr. Res. Div., PO Box 201, 3730 AE De Bilt, The Netherlands, Email: hazelege@knmi.nl) Sybren Drijfhout (KNMI, Oceanogr. Res. Div., PO Box 201, 3730 AE De Bilt, The Netherlands, Email: drijfhou@knmi.nl)
Mode Water formation in the subtropical gyre of the North Atlantic exhibits substantial decadal variability. Here, we present results that indicate a large role of internal oceanic variability in generating this variability. The results are obtained with an isopycnic ocean model, with idealised geometry and forcing. The horizontal resolution is sufficiently high and the friction/diffusion sufficiently low for the flow to become unstable.
Two modes of low frequency variability are found. Both modes consist of westward propagating thickness anomalies. The anomalies have a first baroclinic modal structure. One mode has a time scale of 8 years and a basin wide spatial scale, the other has a time scale of 4.5 years and smaller spatial scale. The modes are damped when the diffusion is increased. In that case, the 8 yr mode can be excited by a spatially coherent stochastic wind stress, but the amplitude is much smaller. The modes are maintained by a release of energy of the mean flow in the thermocline. The instability of the mean flow is determined by the basic stratification, in particular the slope across the midlatitude jet. Finally, it is shown that coupling to the atmosphere and a parameterization of surface mixing are necessary for the low frequency variability to appear in the mixed layer.
JSP25/E/01-B3 1640
INTER-DECADAL VARIATIONS IN JAPANESE SARDINE AND THE KUROSHIO EXTENSION
ICHIRO YASUDA (Univ. of Tokyo/ Frontier Research System for Global Change, Hongo 7-3-1, Bunkyo, Tokyo 113-0033,Japan, E-mail ichiro@geoph.s.u-tokyo.ac.jp); Masayuki Noto (Hokkaido Univ., E-mail noto@geoph.s.u-tokyo.ac.jp); Tomoki Touzuka (Univ. of Tokyo, E-mail s72620@vsiron.geoph.s.u-tokyo.ac.jp)
Japanese sardine is known to greatly vary in inter-decadal time sacles. In 1988, the catch was over 4 million tons; it declined abruptly since 1989 and was below 1 million tons in 1995. We found the winter-SST in the Kuroshio Extension and its southern recirculation area (KESA: 30-35N, 145-180E) significantly correlates with the mortality coefficient of the Japanese sardine. The warming in the KESA since 1988 possibly causes the collapse of the sardine. The SST variations in the KESA and south of Japan were different from those in the Kuroshio-Oyashio frontal regions in the western/central Pacific (40-45N 145-180E) where a regime-shift around mid-1970s was known: the SSTs in the Kuroshio regions have a longer time scale of over 50 years than in the frontal regions (25 years), and the jump of the Kuroshio SSTs around late-1980s occurred two years earlier in 1988 than in the frontal regions. The shift around mid-1970s in the frontal regions was not observed in the Kuroshio and the KESA where the SST anomalies were already negative in early-1970s. These SST variations well corresponds to sardine catch records. Analyses of heat flux and wind before/after the late-1980s SST jump indicate that the warming since 1988 was caused partly by the decrease in the net heat flux and in the mixed layer depth due to the reduction of wind over the Kuroshio south of Japan and over the Kuroshio Extension regions.
JSP25/W/72-B3 1700
RECENT VACILLATIONS IN THE SURFACE PRESSURE SEMIANNUAL OSCILLATION
Ian SIMMONDS (School of Earth Sciences, The University of Melbourne, Parkville, Victoria, 3052, Australia, email ihs@met.unimelb.edu.au); David A. Jones (Bureau of Meteorology Research Centre, 150 Lonsdale Street, Melbourne, Victoria, 3001, Australia, email d.jones@bom.gov.au)
We present a climatology of the Southern Hemisphere semiannual oscillation of surface pressure and mid tropospheric baroclinicity compiled from 21 years of numerical analyses. In addition, long time series of this feature derived from pressure data from a number of mid and high latitude stations are presented. The results confirm that the oscillation is an important feature of the annual cycle of pressure and meridional temperature gradient. The pressure half-yearly wave attains its greatest amplitudes in the mid latitude ocean basins and on the Antarctic periphery, with a minimum near 55oS. The semiannual oscillation of the temperature gradient is strongest near 60oS, where it explains about 50% of the mean annual variance of monthly data, and the harmonic maxima (strongest gradients) occur during the transition seasons.
We document the extent to which the half-yearly wave exhibits variability on interannual to decadal time scales, and consider the extent to which recent changes may be seen as part of such overall variability. In both the mid and high latitudes the temporal variability of the semiannual oscillation of pressure has been found to be statistically related to the variability in the oscillation of the high latitude temperature gradient. Changes in the latter seem to be associated in the greater part with temperature changes at the higher latitudes.
JSP25/E/19-B3 1720
NATURE OF ATMOSPHERIC VARIABILITY OVER THE NORTH ATLANTIC OCEAN FROM INTERANNUAL TO DECADAL TIME SCALE
Christophe CASSOU, Laurent Terray, Sophie Valcke (CERFACS, Climate Modelling and Global Change Team, 42, Avenue Gustave Coriolis 31057 Toulouse, FRANCE, EMAIL: cassou@cerfacs.fr)
Predictability is considerably reduced at mid-latitudes especially over the Atlantic Ocean where the variability is mainly controlled by internal atmospheric processes which mask Sea Surface Temperature (SST) influences. In order to quantify and explain the response of the atmosphere to observed SST variability, the ARPEGE-Climat Meteo-France Atmospheric Global Circulation Model is used through 2 types of experiments : a 100-year simulation with a prescribed monthly climatological SST is carried out together with an ensemble of 8 50-year simulations forced by GISST month to month SST changes from 1948 to 1997 (GOGA simulations hereafter). Predictability is first investigated using ANOVA techniques applied on the ensemble experiment for different seasons. This statistical tool allows to separate in the total variability, the part forced by the SST from the part explained by internal dynamics. The North Atlantic Oscillation (NAO) is then more precisely studied in both experiments. The frequency analysis of the NAO related signal shows a weak 2.5-year peak in the climatological run whereas in GOGA simulations, the same analysis exhibits more energy concentration at longer time scale and a significant 3.7-year peak related to ENSO. This suggests important teleconnections and atmospheric linkages between the Atlantic and the Pacific. The SST impact is then examined using classification techniques in weather regimes; emphasis will be laid on differences between the climatological and the GOGA experiments in terms of spatial weather patterns, their occurences and transitions. Optimal filter methods finally allow to determine the spatial and time characteristics of the SST pattern which plays a role in setting up the simulated forced atmospheric response. Some preliminary results will also be presented on shorter sensitivity experiments using the detected SST signal superimposed on the climatological background as a new forcing.
Thursday 29 July AM
Presiding Chair: S. Minobe (Hokkaido University, Sapporo, Japan)
Concurrent Poster Session
GLOBAL VARIABILITY
JSP25/W/61-B4 Invited 0930
PATTERNS OF LARGE-SCALE CLIMATIC VARIABILITY IN THE INSTRUMENTAL ERA
Chris K. FOLLAND (Hadley Centre, Met Office, London Road, Bracknell, Berkshire, RG12 2SY, UK, Email: ckfolland@meto.gov.uk); Rob J. Allan (CSIRO Atmospheric Research, Aspendale, Victoria 3195, Australia, Email: rob.allan@dar.csiro.au); Michael E. Mann (Department of Geosciences Morrill Science Center, University of Massachusetts, Amherst, MA 01003, Email: mann@snow.geo.umass.edu); Scott B. Power (National Climate Centre, Melbourne, Victoria 3001, Australia, Email: s.power@bom.gov.au)
An empirical orthogonal function (EOF) analysis of ocean surface temperature, filtered to highlight periods greater than about 13 years, has been used to identify patterns of low frequency variability. These include a secular pattern of global warming, a 70-80 year interhemispheric pattern associated with interdecadal variations in Sahel rainfall, and a pattern reminiscent of El Nino but with a somewhat different structure that modulates high frequency ENSO influences on Australian climate. More insight can be obtained by carrying out joint EOF analyses of pressure at mean sea level (PMSL) and SST for different spectral bands, and multi-taper singular value decomposition analyses of joint SST and PMSL data. These highlight significant low frequency bands and the secular band, and provide patterns of variability that are mostly consistent between the analyses.
JSP25/W/49-B4 Invited 0950
QUASI-PERIODICITY AND GLOBAL SYMMETRIES IN INTERDECADAL UPPER OCEAN TEMPERATURE VARIABILITY
Warren B. WHITE and Daniel R. (Cayan1 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, e-mail:wbwhite@ucsd.edu)
Abstract Recent studies find interannual (i.e., 3 to 7 year), decadal (i.e., 9 to 13 year), and interdecadal (i.e., 18 to 23 year) periodicities, and a trend dominating global sea surface temperature (SST) and sea level pressure (SLP) variability over the past hundred years, with the interdecadal signal dominating sub-El Niño-Southern Oscillation (ENSO) frequencies. We isolate interdecadal frequencies in SST and SLP records by band passing with a window admitting 15 to 30 year periods. From 1900 to 1989, the rms of interdecadal-filtered SST and SLP anomalies is largest in the extratropics and eastern boundaries. First-mode empirical orthogonal functions (EOFs) explain about half the interdecadal variance in both variables, with the tropical warm phase peaking near 1900, 1920, 1940, 1960, and 1980. From 1955 to 1994, EOF spatial patterns of interdecadal SST, SLP, and 400m temperature (T400) anomalies reveals global reflection symmetries about the equator and global translation symmetries between ocean basins, with tropical and eastern ocean SSTs warmer (cooler) than normal, covarying with stronger (weaker) extratropical westerly winds, cooler (warmer) SSTs in western-central subarctic and subantarctic frontal zones (SAFZs), stronger (weaker) subtropic and subarctic gyre circulations in North Pacific and North Atlantic Oceans, and warmer (cooler) basin and global average SSTs of 0.1°C or so. Evolution of interdecadal variability from the tropical warm phase to the tropical cool phase is propagative, also characterized by reflection and translation symmetries. During the tropical warm phase, cool SST anomalies along western-central SAFZs are advected slowly eastward to the eastern boundaries and subsequently advected poleward and equatorward by the mean gyre circulation, the latter conducting extratropical SST anomalies into the tropics. A delayed action oscillation model is constructed that yields the quasiperiodicity of interdecadal variability in a manner consistent with these global symmetries in both pattern and evolution.
JSP25/E/02-B4 Invited 1010
INTERANNUAL-TO- DECADAL VARIABILITY OF THE TEMPERATURE-SALINITY STRUCTURE OF THE WORLD
SYDNEY LEVITUS (NODC/NOAA, E/OC5, 1315 East West Highway, Room 4362, Silver, Spring MD 20910-3282, Email:slevitus@nodc.noaa.gov)
Analysis of historical upper ocean temperature data suggests the existence of a global mode of upper ocean heat storage. Additional analysis suggests that in the North Atlantic Ocean, both the first (NAO) and second (EAO) modes of sea level pressure are correlated with upper ocean thermal structure demonstrating the existence of coupled modes of oscillation for this ocean. Variabiliy of the temperature-salinity structure of the intermediate and deep waters of various regions of the world ocean will also be described.
Presiding Chair: Chris K. Folland (Hadley Centre, UKMO, Bracknell, UK)
JSP25/W/71-B4 Invited 1050
RELATIONSHIPS AMONG RECENT ATMOSPHERIC CIRCULATION CHANGES, GLOBAL WARMING, AND SATELLITE TEMPERATURES
Jim HURRELL, (NCAR, USA, jwhurrell@meto.gov.uk)
There is much debate about our ability to detect a human influence on climate. Part of the problem is that the Earth's climate record includes both natural variability as well as human-induced effects, and the presence of the former makes it difficult to isolate possible anthropogenic signals. This is illustrated by changes in the atmospheric circulation over the past two decades which have contributed strongly to observed global warming patterns. Differences in the vertical structure of the temperature anomalies associated with the circulation changes largely account for discrepancies between temperature trends from surface and satellite-derived lower-atmospheric records. The circulation changes are related to variations in modes of natural variability over the North Atlantic and North Pacific basins. The question of the physical relationship between the variations in these two circulation modes on low frequency time scales, and furthermore their relation to secular changes in ENSO, will be discussed.
JSP25/W/09-B4 Invited 1110
GLOBAL AND REGIONAL TELECONNECTIONS IN ATLANTIC BASIN SST VARIATIONS
YOCHANAN KUSHNIR, Balaji Rajagopalan, Yves Tourre, and Jennifer Miller, (Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA)
To address the extent to which Atlantic climate variability (ACV) is governed by processes internal to the Atlantic Basin, we apply various multivariate analysis methods to a long record of sea surface temperature anomalies. We find evidence for both local and global influences on the leading modes of ACV. Tropical and South Atlantic SST variability appears to be mainly associated with global patterns of SST variability, namely ENSO and an almost-global SST warming trend, between 1930 or so and the present. North Atlantic SST variability appears to be regional with little evidence to other oceanic regions. The characteristic pattern of regional SST variability is its "tri-polar" structure where subpolar and subtropical SST are fluctuating in phase with one another, and out of phase with midlatitude SST. The spectrum of the regional SST fluctuations displays distinct decadal and multi-decadal peaks and a hint of propagating behavior. We compare these results with other recent findings on ACV and discuss their implication.
JSP25/W/31-B4 Invited 1130
DECADAL VARIABILITY IN THE EXTRATROPICAL OCEAN-ATMOSPHERE SYSTEM
Stephen P. MEACHAM, (Atmospheric and Environmental Research Inc., 840 Memorial Dr., Cambridge, MA 02139, USA. Email: smeacham@aer.com)
In the last four decades, a variety of mechanisms have been suggested as potential contributors to decadal variability of the extratropical atmosphere and ocean. It is difficult to unambiguously distinguish individual mechanisms in the complex behavior of a "full" climate model. We describe results from a pair of studies that focus on low frequency variability associated with redistribution of potential vorticity within the ocean. In the first, a simple baroclinic ocean with an adiabatic interior, forced with a specified double gyre wind stress, is shown to exhibit strong variability on decadal time scales. This variability is associated with the storage and release of potential vorticity in inertial recirculations embedded within the subtropical and subpolar gyres. A necessary condition for this mode of variability is the resolution of mesoscale eddies which are intimately connected with the episodic breakdown of the inertial recirculations. The inter-gyre eddy potential vorticity flux exhibits strong decadal variability. In the second study, a pair of mid-latitude ocean basins is coupled to a quasigeostrophic tropospheric model. The ocean is equipped with an active mixed layer that can store and transport heat. The interior of the ocean is essentially adiabatic. The oceans and atmosphere are coupled primarily through the transfer of potential vorticity: the atmosphere induces Ekman pumping in the ocean while the air-sea heat flux induces atmospheric heating and cooling anomalies which modify the potential vorticity budget of the atmosphere. The active atmosphere exhibits a moderately realistic planetary-scale and synoptic-scale wave regime, with quasistationary subtropical highs over the oceans, and drives subtropical and subpolar gyres in both oceans. Both the atmosphere and ocean exhibit significant low frequency variability. Part of this is the result of potential vorticity accumulation and discharge in the ocean gyres analogous to that seen in the ocean-only model. Another part of the variability can be attributed to coupled "modes" of the ocean-atmosphere-mixed layer system.
JSP25/E/35-B4 1150
INTERDECADAL OSCILLATIONS WITH A LOW DEGREE OF FREEDOM MODEL OF THE OCEAN-ATMOSPHERE
Alain COLIN DE VERDIERE ( Laboratoire de Physique des Oceans, Universite de Bretagne Occidentale,BP 809, 29285 Brest cedex France, email acolindv@univ-brest.fr); Thierry Huck ( Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, NJ 08542 USA, email tnh@GFDL.GOV)
Oceanic GCMs have been observed to oscillate on time scales in a large number of studies. Under the simplest situation of a temperature dependent equation of state, two conditions are necessary to produce the oscillations for a fixed strength of the overturning: a weak enough damping of SST anomalies (through constant heat flux for instance) and a low enough background eddy diffusivity. We suggest that baroclinic instability in western boundary current outflow regions governs the generation of the oscillations. In the complicated geometry of an oceanic GCM ( as compared to an atmospheric channel), the large scale instability is local, driving planetary waves in the interior that set the period of the oscillations. The situation is not different when the oceanic GCM is coupled to an Atmospheric Energy Balance Model. This is used to construct a two degree of freedom climate model that reproduces a number of features of the coupled Ocean-EBM model such as the Hopf bifurcation to the limit cycle and the damping effect of restoring boundary conditions. It quantifies the relative weight of the various dissipation processes against the instability that feeds on available potential energy. Significant differences appear, however, when the oscillations are in the finite amplitude regime.
JSP25/W/01-B4 Invited 1210
ON THE USE OF WAVELET COHERENCY FOR ANALYZING DECADAL CLIMATE VARIABILITY
TORRENCE, Christopher (National Center for Atmospheric Reasearch/Advanced Study Program, PO Box 3000, Boulder CO 80307-3000 USA, e-mail:torrence@ucar.edu)
The method of wavelet coherency is developed and applied to time series of climate variability. In particular, decadal changes in the El Nino-Southern Oscillation (ENSO) are linked to changes in Indian monsoon rainfall. The wavelet coherency provides a measure of the strength of the ENSO-monsoon connection and how the connection has varied over the last 125 years. Statistical significance tests are used to test the robustness of the results.
Thursday 29 July PM
Presiding Chair: Yochanan Kushnir (Columbia University, Palisades, USA)
VARIABILITY OVER THE ATLANTIC ocean
JSP25/W/47-B4 Invited 1400
SIMULATION OF QUASI-DECADAL AND INTERDECADAL VARIABILITY IN THE ATLANTIC OCEAN ASSOCIATED WITH THE NORTH ATLANTIC OSCILLATION
George Halliwell, (MPO/RSMAS, University of Miami, USA)
Over the last several decades, the North Atlantic Oscillation (NAO) index has varied with two distinct time scales, a quasi-decadal oscillation and an interdecadal oscillation with a period of about 50 years. Since high NAO index is associated with anomalously strong Westerlies and Trade Winds, NAO variability is expected to drive Atlantic Ocean climate variability. Properties of this ocean climate variability at both quasi-decadal and interdecadal periods are investigated through analysis of a primitive-equation ocean general curculation model driven by forcing fields derived from the NCEP/NCAR atmospheric reanalysis for 1946-1998. At both time scales, the strong Westerlies and Trade Winds associated with high NAO index generate an SST anomaly pattern characterized by anomalously cold water over the open ocean in both the Westerly and Trade Wind belts and anomalously warm water over the western Sargasso Sea. Model analysis reveals that anomalous SST is driven by local thermodynamical forcing in the two wind belts, and by anomalous heat advection associated with the spin-up of the North Atlantic subtropical gyre in the western Sargasso Sea. The opposite pattern is observed for low NAO index. A two-year time lag is observed in the western Sargasso Sea relative to the basin-wide atmospheric forcing resulting from the baroclinic adjustment of the subtropical gyre. Results suggest that at quasi-decadal periods, the atmosphere is predominantly driving the ocean. An additional time lag between SST anomaly patterns and the NAO index at interdecadal periods suggests that more complicated coupled variability between the atmosphere and ocean occurs at the longer time scales.
JSP25/W/07-B4 Invited 1420
THE FORMATION AND EVOLUTION OF NORTH ATLANTIC HEAT CONTENT ANOMALIES
Graham GLADMAN and Rowan Sutton (Department of Meteorology, University Of Reading, PO BOX 243, Earley Gate, Reading, RG6 6BB, Email: swr97gjg@reading.ac.uk)
Observations of the North Atlantic Ocean over the past century reveal decadal timescale fluctuations in heat content and related variables. Many aspects of these fluctuations are intriguing and require explanation. In particular, the mechanisms responsible for the formation and propagation of heat content anomalies are not understood. While such anomalies may be generated by processes internal to the ocean, forcing by the atmosphere is likely to play a key role.
We are investigating the role of atmospheric forcing in the formation and evolution of heat content anomalies by experimentation with an Atlantic ocean isopycnic GCM. In the first phase we have performed experiments to explore how heat content anomalies develop in response to forcing by idealised windstress anomalies. The interaction between the barotropic response and topography leads to very interesting, and sometimes surprising, behaviour. We will present analyses of both this short timescale response and also the longer timescale baroclinic response. In the second phase of our work we will be investigating the development of specific heat content anomalies observed in the North Atlantic this century.
JSP25/E/16-B4 Invited 1440
DECADAL VARIATIONS IN TROPICAL ATLANTIC CLIMATE IN OBSERVATIONS AND IN A GLOBAL OCEAN-ATMOSPHERE MODEL
VIKRAM M. MEHTA, (NASA-Univ. of Maryland Joint Center for Earth System Science, Department of Meteorology, Univ. of Maryland, College Park, Maryland 20742, U.S.A); Tom Delworth, (NOAA/Geophysical Fluid Dynamics Laboratory, P.O. Box 308, Princeton, New Jersey 08542, U.S.A.)
Decadal-multidecadal climate variations are receiving increased attention from the research community in the last one or two decades. It is very important to study decadal-multidecadal climate variability in its own right and also because of its potential to interact with interannual climate variability and anthropogenic climate change. Observed decadal climate variability in the tropical-subtropical Atlantic region and its statistical predictability will be described in this oral presentation. The mechanisms of such climate variability in the GFDL coupled ocean-atmosphere model will also be described.
JSP25/W/03-B4 1500
INTERDECADAL MODES OF VARIABILITY IN A SIMPLIFIED COUPLED MODEL
Thierry HUCK and Geoffrey Vallis (GFDL, Princeton University AOS, Forrestal campus, Route 1, Princeton NJ 08542, USA, email: tnh@gfdl.gov gkv@gfdl.gov); Alain Colin de Verdiere (Laboratoire de Physique des Oceans, Universite de Bretagne Occidentale, BP 809, 29285, Brest cedex, France, email: acolindv@univ-brest.fr)
A simplified coupled model is constructed including an ocean general circulation model (MOM3, GFDL) and an atmospheric Energy Balance Model with physical parameterizations for horizontal heat transport and zonal winds. The external forcing comes from constant insolation at the top of the atmosphere (no seasonal cycle). In various idealized and realistic configurations, the model produces interdecadal variability. One mode is clearly related to the thermohaline circulation in the North Atlantic and its sensitivity to parameters and parameterizations is analyzed: it compares very well with the ocean mode under constant surface heat flux. Other modes of variability, involving changes in the subtropical gyres intensity and mid-latitude winds, are investigated. Finally, comparisons are drawn with the observations.
JSP25/W/20-B4 1520
DECADAL VARIATIONS OF THE THERMOHALINE CIRCULATION IN MODELS OF THE ATLANTIC OCEAN FORCED BY OBSERVED SURFACE FLUXES
Carsten EDEN, Juergen Willebrand ( both at Institut fuer Meereskunde, Duesternbrooker Weg 20, 24105 Kiel, Germany, Email: ceden@ifm.uni-kiel.de, jwillebrand@ifm.uni-kiel.de) and Rene Redler ( Alfred Wegener Institut fuer Polar- und Meeresforschung, Bgm.-Smidt Str. 20, 27568 Bremerhaven, Germany, Email: rredler@AWI-Bremerhaven.DE)
The variation of the thermohaline circulation (THC) in the Atlantic Ocean on decadal time scales due to atmospheric surface flux variability is investigated.
A medium-resolution model (4/3 degree horizontal mesh) of the Atlantic Ocean, forced with observed surface fluxes taken from the NCEP-reanalysis dataset, was used to study the induced variations of the THC. Several experiments show that the model THC was sensitive to heat flux anomalies in the Labrador sea, where mixed layer depth variations are correlated with THC-anomalies, which propagate southwards into the subtropical gyre, resembling a first baroclinic mode boundary wave like structure.Atmospheric freshwater flux anomalies seem to be unimportanteven under mixed boundary conditions, whereas reshwater fluxanomalies originating from sea-ice variability do.
Experiments with a eddy-permitting model (1/3 degree horizontal mesh) of the North Atlantic Ocean were performed to study the impact of the resolution on the variation of the circulation. First results suggest that increased horizontal resolution does not significantly change the oceanic response on the flux variability.
Presiding Chair: Shang-Ping Xie (Hokkaido University, Sapporo, Japan)
JSP25/E/10-B4 1620
THE INFLUENCE OF SEA SURFACE TEMPERATURES ON THE NORTH ATLANTIC OSCILLATION AND EUROPEAN CLIMATE IN WINTER
Mark RODWELL, Chris Folland and David Rowell (Hadley Centre, UK Met Office, London Road, Bracknell, RG12 2SY, UK, Email: mjrodwell@meto.gov.uk)
Observational studies have shown that there is a relationship between the North Atlantic Oscillation (NAO) and North Atlantic sea surface temperatures (SSTs). Some of the mechanisms by which the NAO can influence these SSTs are fairly well understood but the opposite link, from SSTs to the NAO, has been unclear. We show for the first time that an atmospheric general circulation model (AGCM), forced with real SSTs and sea-ice extents, can reproduce much of the observed multiannual to multidecadal variations of the NAO over the last half century. The direct effects of greenhouse gases on the atmosphere were not required to achieve this. The SSTs highlighted as important for this NAO simulation are confined to the North Atlantic region and form a tripole pattern, similar to observed correlation patterns between the NAO and SST. We demonstrate that a realistic magnitude of this SST pattern results in local changes to evaporation and atmospheric convective heating that tend to enhance the thermal structure of the NAO. In agreement with observations, there is also a strong modulation of the modelled North Atlantic storm track. NAO-forced Ekman transport appears to provide a positive feedback onto the SST anomaly pattern and this may slow down its rate of damping. Significant multiannual predictability of North Atlantic SSTs has recently been proposed. Our results are therefore encouraging for the prediction of European winter climate features, such as temperature, precipitation and storminess, up to several years ahead.
JSP25/W/57-B4 Invited 1640
ARCTIC SEA ICE VARIABILITY AND ITS RELATION TO THE NAO
CLARA DESER (National Center for Atmospheric Research, P.O. Box 3000, Boulder, Colorado, USA, e-mail:cdeser@ncar.ucar.edu)John Walsh (University of Illinois, Champaign-Urbana, IL, USA)Michael Alexander (University of Colorado, Boulder, Colorado, USA)Michael Timlin (University of Colorado, Boulder, Colorado, USA)
Forty years (1958-1997) of gridded monthly sea ice concentration data are used to document variability of Arctic sea ice coverage and its association with atmospheric circulation changes. The dominant pattern of wintertime sea ice variability exhibits out-of-phase fluctuations between the western and eastern North Atlantic, together with a weaker dipole in the North Pacific. This mode is dominated by decadal-scale variations and a longer-term trend of diminishing ice cover east of Greenland and increasing ice cover west of Greenland. The changing sea ice margin, which we suggest is driven by recent trends in the North Atlantic Oscillation (NAO), induces large perturbations (several hundred Watts per square meter) in the surface energy exchange between the atmosphere and ocean. The enhanced upward surface energy flux in the area of ice retreat east of Greenland may have contributed to the observed increase in the number of cyclones over the Greenland Sea during the last 40 years. An ensemble of atmospheric GCM experiments forced by observed trends in sea ice coverage are used to investigate the sensitivity of the circulation to sea ice anomalies.
JSP25/W/16-B4 Invited 1700
INTER-HEMISPHERE RELATION OF DECADAL CLIMATE VARIATIONS IN ATLANTIC
Youichi TANIMOTO (Institute for Global Change Research, FRSGC, Shibaura 1-2-1, Seavans N Bldg.7F, Minato-ku, Tokyo 105-6791, Japan, email: tanimoto@frontier.esto.or.jp); Shang-Ping, XIE (Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan, email: xie@ees.hokudai.ac.jp
Sea surface temperature (SST) and Sea Level Pressure (SLP) anomaly fields with an equatorial anti-symmetric dipole structure in the tropical Atlantic affects on precipitation in surrounding areas. But zonal mean tropical SST anomalies have decorrelated scatters between 10-20 degree latitudes. The same observation applies to SLP anomalies. Therefore, it seems that there are, at least, two modes in the air-sea coupled field in the tropical Atlantic. The purpose of the present study provides dominant modes, which are physically reasonable, from statistical analyses by taking advantage of the spectral gap between decadal and interannual SST variability in the tropical Atlantic. After we divide the Atlantic basin into two independent parts, north and south of the equator, we compute empirical orthogonal functions (EOFs) for SST and SLP anomalies. Then anomalies are averaged for the boreal winter and filtered through the decadal band (8-16 year). These EOFs of SST (SLP) anomalies explain 44.2% and 43.2% (36.4% and 50.2%) of the band-passed variance on the Northern and Southern Hemispheres, respectively.Two EOFs (spatial patterns) of SST anomalies present meridionally lined up five centers of action. In two EOF of SLP anomalies, we realize subtropical centers of action with opposing polarities in either side of equator. The PCs (time series) of these four EOFs are highly correlated. In the present study, the two subdomains and the two fields are statistically independent and there is no a priori reason for them to be correlated. This indicates that tropical Atlantic SST and SLP anomalies are not always independent in either side of equator, at least on decadal variations, and further that the tropical dipolar oscillation is associated with the inter-hemispheric decadal variability in the Atlantic.
JSP25/E/14-B4 Invited 1720
RECONSTRUCTIONS OF NORTH ATLANTIC OSCILLATION INDICES
ROSANNE D'ARRIGO, Edward Cook, Heidi Cullen (all at: Lamont-Doherty Earth Observatory, Palisades, NY 10964, TEL 914-365-8617, FAX 914-365-8152, email druidrd@ldeo.columbia.edu) and Michael Mann (Dept. of Geosciences, Morrill Science Center, University of Massachusetts, Amherst, MA 01003, TEL 413-545-9573, FAX 413-545-1200, email mann@snow.geo.umass.edu)
The North Atlantic Oscillation (NAO) is one of the dominant features of climate for the North Atlantic and surrounding land areas (Van Loon and Rogers 1978, Rogers and Van Loon 1979, Rogers 1990). Longer records are needed to provide a long-term context for the variability of the recent instrumental record of the NAO (e.g. the persistently high index values of recent decades). Tree-ring width records from North America and Europe have been used to reconstruct the NAO and related time series of North Atlantic climate for the past several hundred years. These include the first reconstruction of the winter (DJF) NAO sea-level pressure (SLP) index (Cook et al. 1998).
We have also produced a reconstruction of a winter (DJFM) NAO sea-surface temperature (SST) index in order to derive a more complete picture of the NAO system. This index is based on SST averaged over five Atlantic regions which are best correlated with NAO SLP, from the Kaplan et al. (1998) analysis. This reconstruction extends from AD 1701 to 1979 and is based on North American and European tree-ring data as well as the long (335 year) Central England instrumental temperature series (CET) (Plaut et al. 1995). The NAO SST reconstruction accounts for 47% of the variance over the calibration period and is well verified using statistical tests typically used to evaluate dendroclimatic reconstructions (Cook and Kairiustis 1990). We have compared the NAO SLP (Cook)and SST (D'Arrigo) reconstructions to several other relevant time series, including an NAO reconstruction (Appenzeller) based on a Greenland ice core record (Appenzeller et al.1998), and an NAO SST reconstruction based on a network of tree-ring data and other proxies as well as long instrumental temperature records (Mann) Principal components analysis (PCA) scores based on the 4 NAO reconstructions covering 1750-1979 and 3 NAO reconstructions covering the period 1701-1979 (excluding Mann) show verification results that are improved relative to the individual series. The PCA scores of these reconstructions, when compared to global SST, SLP and surface air temperature fields, reflect the characteristic patterns of the NAO.
JSP25/W/90-B4 1740
THE PURPOSE OF THIS STUDY IS TO INVESTIGATE THE TEMPORAL STRUCTURE OF TEMPERATURE VARIABILITY
Marina V. SHABALOVA, (Royal Netherlands Meteorological Institute, P.O.Box 201, 3730 AE De Bilt, the Netherlands, email: shabalov@knmi.nl)
The purpose of this study is to investigate the temporal structure of temperature variability on timescales longer than 50 yr, as reflected in a number of long paleo records located in Europe and North America, using a combination of statistical techniques including uni- and multivariate singular spectrum analysis. Two statistically significant modes of temperature variability are identified, one on multidecadal and one on centennial timescales. The first mode is oscillatory, with a timescale ~60-80 yr. Its geographic pattern suggests a connection to the North Atlantic Oscillation. The second mode, confined to high latitudes, describes temperature variations on timescales longer than 100 yr; in the longest records a weak ~120 yr oscillation can be identified with confidence. We show that the temporal pattern of centennial mode is season-dependent in Europe. We also show that paleo records representing the same season and region (e.g. N.Urals versus Fennoscandia) may exhibit a striking difference in their temporal structures on centennial timescales, which is likely to indicate unreliability of centennial timescales in one/both record(s). In a number of seasonally homogeneous records the centennial components of variability are coherent, showing a multiple-phase 'Little Ice Age' and a 'Medieval Warm Period'. We assume that only those centennial signals which are coherent in a number of independent paleo series and are statistically si