JPM3C

JPM3c

The Western Boundary Currents and Their Role in the Southward Intrusion of NPIW Along the Philippine Coast

Tangdong Qu, Humio Mitsudera, Toshio Yamagata,
Takashi Kagimoto and Dunxin Hu

Japan Marine Science and Technology Centre, Yokisuka, Japan
Department of Earth and Planetary Physics, University of Tokyo, Japan
Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China

There is increasing evidence that much of the shallow, meridional, wind- and buoyancy flux-driven circulation that maintains the oceanic thermal structure and determines the water exchange between the subtropical and tropical North Pacific occurs at the Philippine coast. To gain a reliable picture of that circulation, this study describes the mean western boundary current in the Philippine Seas, using the repeated hydrographic sections conducted by the Chinese Academy of Sciences and the P. R. China - United States joint program combined with the model results from Kagimoto and Yamagata (1996). In barotropic sense, the observed longshore pressure gradients at the western boundary are consistent with those predicted by linear Sverdrup model (Godfrey, 1989), thus suggesting that the basin scale wind stress over the Pacific is more effective than local process in determining the annual mean western boundary currents in the region. Further investigation of vertical structure shows that the subsurface countercurrents, i.e., the Mindanao Undercurrent (MUC) and the Luzon Undercurrent (LUC), are persistent during the period of observations. On the annual average, the MUC is about 50 km offshore, with its maximum velocity of more than 10 cm s at about 900m. The LUC, mainly originating at about 18’N, appears to go shallower and stronger southward along the eastern coast of Luzon. The mechanism operative in the gradient, which changes its direction at about the same depth as the current does and provides the pressure head driving the currents against friction. Another possible phenomenon we suggest as a result of longshore pressure gradient is the near-shore convergence in the MUC and divergence in the Kuroshio. This probably explains why the MUC extends deeper than the Kuroshio on the inshore edge, despite the fact that the NEC shifts to the north with increasing depth, and why the LUC is more narrowly confined than the MUC on the shelf slope. Also examined in this study is the possible role of the western boundary currents in the southward intrusion of North Pacific Intermediate Water (NPIW). Because the low salinity (<34.4 psu) water does not appear to go further south of 12ƒN in the open ocean, we may come westward from the NEC, turn southward with the LUC, and finally leak into the tropics via the MC.

JPM3d

A Numerical Investigation of the Source Region of the Equatorial Subsurface Water

Masami Nonaka and Kensuke Takeuchi

Institute of Low Temperature Science, Hokkaido university, Sapporo, Hokkaido,, JAPAN

Observed tritium and salinity fields suggest that there is a connection between the subtropical-gyre and the equatorial region. Using an OGCM, we investigate the source regions of the equatorial subsurface water subducted from the subtropical-gyre. The OGCM (GFDL MOM v.1.1) covers the portion of the Pacific from 30ƒS to 30ƒN. The model ocean is forced by the annual mean climatological wind and thermal conditions until the solution reaches the steady state. We identify two kinds of the pathways from the subtropical-gyre to the equatorial region. In one pathway, the subducted water first flows to the western boundary region (WB) and then continues to flow to the equatorial region through the WB. In another pathway, the subducted water flows to the equatorial region through the interior region. Thus, there are two regions that connect to the equatorial region through these two pathways, the WB exchange region and the interior exchange region. In both hemispheres, the zonal extent of these exchange regions decreases with latitude to near 20ƒN/20ƒS. At these latitudes, the zonal extent of these exchange regions in the northern hemisphere is larger than the counterpart in the southern hemisphere. In the northern hemisphere, the width of the WB exchange region increases with latitude to near 20ƒN, while that of the interior exchange region decreases with latitude. In the southern hemisphere, the WB exchange region is extremely small.

Using a simple layer model of Liu (1994), we evaluate the factor governing the distribution of the exchange regions. Unlike in Liu (1994), the model is forced by the annual mean climatological wind. The distribution is similar to that in the OGCM qualitatively. This simple model indicates that the distribution of the exchange regions can be explained by the distribution of the Ekman pumping.

JPM3e

Decadal Variablity of the Main Thermocline Excited by Stochastic Wind Stress Forcing

Martina Junge, J.-S. von Storch and J. Oberhuber

Meteorologisches Institut Universitaet Hamburg, Hamburg, Germany

Deutsches Klimarechenzentrum GmbH, Hamburg, Germany

There is some evidence that modes of decadal time scale exist in the main thermocline. It was suggested that stochastic wind stress forcing might explain a substantial part of this variability. To investigate this further an isopycnic OGCM is used. The model domain is the Pacific Ocean. The questions posed are: 1. Is it possible to generate large-scale variations in the model thermocline system solely by daily variations of the weather? 2. Are spatially coherent patterns required in the forcing? 3. What does the spectrum of the response look like? 4. Which mechanisms might be responsible for the evolution of the structures? In order to answer these questions, two experiments have been designed. For both, daily anomalies of wind stress are superimposed on the climatological wind stress forcing. In the first experiment, the daily wind stress anomalies reveal the spatial characteristics as derived from ECMWF analysis. In the second one the wind stress anomalies do not have any spatial coherence. The results suggest that variations can be excited on the decadal to centennial time scale, the patterns in the forcing play no role and that the spectrum is red down to the centennial time scale without any significant peak.

JPM3f

Recent changes in the structure of the Oyashio and its fresh core eddies

Costa Rogachev

Pacific Oceanological Institute, Vladivostok, RUSSIA

Recent interannual variations of the western subarctic boundary currents structures are examined. The data were collected in the Oyashio area and in the Sea of Okhotsk in the period of 1990-1996, during the implementation of the INPOC (International North Pacific Ocean Climate) Study. This program was conducted as a joint Russia-Canada project in 1990-1992, and continued as POI project in 1993-1996. The prominent climate shift of the Oyashio and Kamchatka currents, occurred during recent years, is described.

Detailed long-term CTD surveys of the Oyashio and its anticyclonic fresh-core eddies off Kuril Islands are used for analysis of recent interananual variations. These observations show that there were significant changes of the Oyashio structure and its fresh-core anticyclonic eddies after 1990. These changes were pronounced in variations of the basic Oyashio features and its anticyclonic eddies, changes of their dimensions, depth of penetration and stratification. In 1990 the Boussole anticyclonic eddy (there was so-called WCR86B near the Strait of Boussole at that year) had warm and salt core in the upper layer. WCR86B was last eddy with the warm core during 7 years of observations in the area. During 1991-1996 gradual decrease of minimum temperature and salinity in the eddy cores has been observed.

Geostrophic transport by the First Oyashio Branch (Coastal Oyashio) and Second Oyashio Branch (Offshore Oyashio) substantially altered during 1990-1996. Fresh water budget of the western subarctic and Sea of Okhotsk is considered with the main goal to describe dramatic changes of salinity in the western subarctic. Some of these observations are summarised, and some mechanisms, responsible for such changes, are explored.

JPM3h

A Numerical Investigation of the Subduction Process in Response to Decadal and ENSO Scale Atmospheric Variability

Tomoko Inui, Kensuke Takeuchi, Atsushi Kubokawa
and Kimio Hanawa

Institute of Low Temperature Science, Hokkaido University, JAPAN
Graduate School of Environmental Earth Science, Hokkaido University, JAPAN
Department of Geophysics, Graduate School of Science, Tohoku University, JAPAN

An OGCM (MOM ver.1.1), extending from the equator to 60ƒN and 110ƒ in width, is driven by zonally averaged climatological winds to examine the oceanic response to variability of the Westerly winds. After the model reaches a steady state, the Westerlies are intensified and weakened with a period of 10 years for a decadal study (DC), and three years for an ENSO study (EN).

In both experiments, the SST in the subtropical north decreases/increases as the wind stress increases/decreases due to the enhanced/reduced Ekman transport. In the DC experiment the subsurface temperature in the subtropical south decrease/increase with a five years phase delay to the increase/decrease of the wind stress. No significant subsurface signal appears in the EN. The change in the isopycnal layer thickness in both experiments is examined to help explain the subsurface signal, seen in the DC experiment, in a region where the wind stress does not change locally. We consider the thickness between isopycnals corresponding to the bottom of the mixed layer and the top of the thermocline. Both isopycnals outcrop in the westerly region. It is found that a water column with lower/higher potential vorticity is formed when the wind stress increases/decreases. This water column penetrates southwestward by the subduction process from the point where the mixed layer front and the outcrop line intersect. (Similar results were obtained in another experiment with an abrupt intensification of the Westerlies.) In the DC experiment, the water column has enough time to reach the western boundary region, while in the EN the wind changes before the water can penetrate substantially from the outcrop region.

JPM3i

Decadal and Interdecadal Variations in the North Pacific Marginal Seas Adjacent to the Subtropical and Subarctic Gyres

Vladimir I. Ponomarev and Anatoly N. Salyuk

Pacific Oceanological Institute, Vladivostok, Russia

The climate variability in the North Pacific is displayed in different time scale oscillations of the oceanic, sea ice and atmospheric characteristics. The North Pacific ocean response to the El Nino with periods between 2-5 years is described by using hydrodynamic model (Johnson and O’Brien, 1990). The sea level data analysis and the coupled ocean-atmosphere circulation modeling indicate a 20 year cycle of the North Pacific gyre intensity coherent with weakening and strengthening of the Aleutian Low Pressure (Latiff and Barnett, 1994; Polovina, 1996). The oscillation of period about 50-70 years in winter-spring SLP and spring-summer SST over the North Pacific associated also with Aleutian Low variability is traced by Minobe, 1996.

In our study we reveal the dominant signals of the climate variations in different layers of the Sea of Japan and Sea of Okhotsk which are adjacent to the subtropical and subarctic ocean gyres in the North West Pacific correspondingly. The dominant decadal or interdecadal variations of potential temperature and total heat in the certain layers of these seas are found using all available historical OS (oceanographic station) and CTD profiles collected at the Pacific Oceanological Institute, Pacific Research Institute of Fisheries & Oceanography, World Data Center for Oceanography in Russia and at the NODC in USA.

The weak stratified Sea of Japan is connected to the subtropical gyre by shallow straits. This sea plays a role of passage in the upper layer of the North West Pacific. Effectively transformed warm water of the Kuroshio branch is directly transported through the Sea of Japan to the oceanic transitional area between subtropical and subarctic gyres and to the Sea of Okhotsk. Deep basin of the Japan Sea is a horizontally isolated marginal box, where according to our results the heat is accumulated in all layers during climatic periods of warming. We detect the periods of climate regime shifts and 12-16 years intervals of significant heat accumulation or some heat discharge in the Japan Sea deep basin. The pronounced 4-7 years variations are manifested in the upper sea layer temperature and in the annual sea ice production in the northern part of the Japan Sea. We show that these variations accompany the El Nino/La Nino oscillations in the equatorial and tropical Pacific ocean as a rule. During the whole period of observations from the beginning of 20th century the Japan Sea accumulates the heat. The vertical stability in the upper and intermediate layers significantly increases.

The strong stratified Sea of Okhotsk is connected to the subarctic gyre by deep Kuril straits where the heat exchange between the sea and the ocean takes place in the subsurface and deep layers. The dominant 3-7 years variations of the mean temperature and total heat in the Okhotsk Sea cold intermediate layer are revealed using historical oceanographic profiles along standard sections on the northern shelf. It is shown that the mentioned variations are in agreement with the El Nino oscillations in the equatorial and tropical Pacific ocean for the period from 1965 to 1987. The maximal warm anomaly of the Okhotsk Sea cold intermediate layer in 1983 is associated with a substantial signal in the North West Pacific. These phenomena accompany the major equatorial El Nino observed during late 1982 and early 1983. The high negative anomaly of the ice extent in the Okhotsk Sea has become in December,1983 - March,1984, which is one year later than the equatorial El Nino event. In this case the negative ice extend anomaly follows the warm anomaly in the Okhotsk Sea cold waters.

jpm3j

interdecadal climate signal along the north pacific subtropical gyre

Yves Tourre, Yohanan Kushnir and Warren B. White

LDEO, University of Columbia, USA

The characteristics of the interdecadal-like variability in the northern Pacific Ocean are explored. To that purpose Complex EOF (CEOF) methodology is applied to seasonal and annual values of surface atmospheric parameters (ie., sea-level pressure of SLP, wind-stress, sensible and latent heat fluxes) and of surface and sub-surface oceanic parameters (ie., sea surface temperature or SST, temperature at standard depths and heat storage in the upper 400 m or HS). The analysed period is from 1955 until 1994 for surface parameters and from 1970 to 1994 for the sub-surface parameters. While the first CEOF modes for different parameters display variability on ENSO time scales as expected the second CEOF modes display interdecadal time scale properties (approximately 20 years for a full cycle). Maximum spatial amplitudes associated with the second CEOF modes are found within the north Pacific baroclinic subtropical gyre. The space and time evolution of HS 20-year full cycle is obtained after linear adjustment of phase change and assuming constant amplitude. Insights of the physical mechanisms involved are obtained from regressing the SST (and temperature at standard depths) and SLP time-series against HS CEOF second mode. In the 30ƒ N to 40ƒ N zonal band the sub-surface temperature anomalies are in-phase all the way down to 400 m indicative of thorough wind stress forcing and mixing at these latitudes. Further south, along the mean circulation attaining 10ƒ N in the northwest Pacific Ocean and associated with strong subduction processes around 150ƒ W.

jpm3k

a theory of interdecadal climate variability of the north pacific ocean-atmosphere system

Fei-Fei Jin

Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, USA

A linear coupled model for the atmosphere-upper ocean system is proposed to highlight the mechanisms of decadal to interdecadal climate variability of the North Pacific. In this model, wind stress anomalies over the North Pacific are related to anomalies in the meridional temperature gradient of the upper ocean. The latter depends on air-sea thermodynamical feedbacks and meridional heat transport by upper ocean currents. Slow adjustment of the oceanic gyre circulation to the change in wind stress is accomplished by the forced baroclinic oceanic Rossby waves, which carry out the meridional heat transport. Uncoupled ocean dynamic adjustment can produce a weak decadal to interdecadal peak in the power spectrum of the meridional transport under temporally white-noise wind-stress forcing with organised spatial structure. Coupled dynamics produce a basin-scale interdecadal oscillatory mode. This mode arises from the dynamical coupling and the memory of the system, which resides in the slow gyre circulation adjustment. Its stability is heavily controlled by the ocean thermal damping and its period is about one and one half to three times the decadal ocean dynamic-adjustment time. In the relevant parameter regime, this coupled mode produces a robust and pronounced interdecadal spectral peak in upper-ocean temperature and the Sverdrup transport of the gyre circulation. The interdecadal oscillations reproduced in the simple model provide insights into the main physical mechanisms of the North Pacific decadal/interdecadal variability observed in nature and simulated in coupled general circulation models.

JPM3L

Interannual-Interdecadal Variations in Meteorological and Oceanographic Conditions Affecting Plankton Biomass in the Subarctic Pacific and Bering Sea

Takashige Sugimoto and Kazuaki Tadokoro

Ocean Research Institute, University of Tokyo, Tokyo, Japan

Interannual, decadal and interdecadal variations in summer plankton biomass during 1954-94 in the whole Subarctic Pacific and Bering Sea were compared among regions as well as with climate and oceanographic conditions. The zooplankton biomass and chlorophyll concentration during the mid 1960’s-early 1970s in the central and western Subarctic Pacific was a few times higher than those in its preceding and following decades. The values in the eastern Bering Sea and eastern Subarctic Pacific also increased in the mid 1960s, but remained at a higher level until the end of 1980s. The decade of this higher plankton biomass during the mid 1960s-early 1970s corresponds to a period of reduced density stratification in the upper layer in summer, is associated with less precipitation in the northern and eastern Subarctic Pacific, and positive NHZI values. However, in the decadal time scale, one can see a significant positive correlation between the summer plankton biommass and the wind speed during winters.

JPM3n

Variability of the Tropical Pacific Ocean Surface Temperatures at Decadal-multidecadal Time Scales

Vikram M. Mehta

NASA-University of Maryland Joint Center for Earth System Science, Department of Meteorology, University of Maryland, USA

Grided time series from the Global Ocean Surface Temperature Atlas were analysed with a variety of techniques to identify oscillation periods and spatial structures of the tropical Pacific sea surface temperature (SST) variability at decadal-multidecadal time scales. Each time series was 60 years (1932-1991) long. The tropical Pacific SST variability was compared with decadal-multidecadal variability in the 60 years (1931-1990) long Florida State University (FSU) pseudo-wind stress data set. The tropical Pacific SST variability was also compared with decadal variability in the extra-tropical Pacific SST.

A variety of analyses revealed the dominant spectral peak at approximately 20 years time scale, but the sixty years long time series are too short to attach much physical significance to the 20-year spectral peak even though it is as energetic as the ENSO peak. Much of the decadal-multidecadal SST variance during the analysis period was away from the Equator, especially in the eastern and central Pacific. Independent analysis of the FSU pseudo-wind stress data also showed the presence of decadal-multidecadal variability.

Evolution of decadal-multidecadal SST and wind stress anomalies was studied by low-pass filtering the anomalies. Maps of the filtered anomalies show that, at decadal time scales, SST anomalies travelled into the tropical Pacific from the extra-tropical Pacific along the eastern boundaries of the basin. The anomalies strengthened and resided in the tropical Pacific for several years. The tropical north Pacific SST anomalies frequently travelled northward into the mid-high latitude north Pacific along the western boundary of the basin. This behaviour of decadal SST anomalies was not a permanent feature of the tropical Pacific SST variability. The filtered wind stress anomalies were physically consistent with the filtered SST anomalies.

The results suggest that the climate of the tropical Pacific has experienced decadal-multidecadal variability during the 60 years analysis period. In this talk, the above-mentioned results and interaction between interannual and decadal-multidecadal variabilities will be described.

Long-range predictability of the decadal-multidecadal variability and its influence on predictability of interannual variability will also be described.

JPM3O

DECADAL CHANGES IN THE NORTH PACIFIC OCEAN: SUBDUCTION, ADVECTION AND WIND FORCING

Arthur J. Miller

Scripps Institution of Oceanography, La Jolla, USA

Observed decadal scale changes in the North Pacific upper-ocean (0-400m) temperature field will be discussed for the 1970-1994 time interval. A retrospective simulation will be used to diagnose physical processes of these observed thermal changes. In the central North Pacific, surface layer temperature changes are dominated by atmospheric heat flux forcing combined with vertical mixing and Ekman current advection, while deeper in the water column, anomalous subduction towards the equator appears to influence the temperature field. The magnitude of this subducted signal may be obscured by interannual propagating waves and direct isotherm displacement by atmospheric Ekman pumping. Near the western boundary, in the Kuroshio/Oyashio Extension, gyre-scale thermocline response to basin-scale Ekman pumping is clearly evident and geostrophic current advection appears to contribute to surface temperature changes. The relation of these observed changes to mechanisms implicit in the North Pacific Latif-Barnett model oscillation will be discussed.

jpm3p

Water mass changes in the North Pacific Ocean between the 1960s and the 1980s

Annie Wong, Nathaniel Bindoff and John Church

Institute of Antarctic and Southern Ocean Studies,
University of Tasmania, AUSTRALIA

Antarctic Co-operative Research Centre,
University of Tasmania, AUSTRALIA

CSIRO Division of Oceanography, Hobart, Tasmania, AUSTRALIA

Objective mapping of a historical hydrographic data set has been carried out for the North Pacific Ocean along three modern deep ocean zonal transects: P1 (47ƒN), P3 (24.5ƒN) and P4 (9.5ƒN). The objectively mapped historical data were then compared with the modern data for the purpose of investigating changes in temperature and salinity in the interior of the North Pacific Ocean between the 1960s and the 1980s. Differences along isopycnals reveal that changes in T-S correlations have occurred in water masses in the top 1000 dbar of the water column along all three latitudes.

Along P1, basin-wide averages indicate that between neutral densities of 27.0 and 27.4 (ª300 to 650 dbar), the water has cooled and freshened on isopycnals. The differences are more pronounced west of 155ƒE and are accompanied by cooling on isobars. Along P3, water between neutral densities of 26.5 and 27.5 (ª450 to 1000 dbar) also shows basin-wide cooling and freshening on isopycnals, accompanied by warming and freshening on isobars. The maximum differences occur along the 27.0 neutral surface, which is the bottom layer of North Pacific Intermediate Water (NPIW). Along P4, cooling and freshening on isopycnals are again observed on neutral surfaces between 26.5 and 27.5 (ª200 to 950 dbar). These are accompanied by basin-wide warming on isobars. The maximum differences are found along the 27.1 neutral surface, which represents the base of the 13ƒC Water: an intermediate salinity maximum layer that originated from the South Pacific subtropical gyre.

Using a diagnostic model that analyses subsurface oceanic changes in terms of surface processes, it is found that the changes observed in the subarctic water along P1 can best be explained by surface cooling in its source region, while changes in the 13ƒC Water along P4 can best be explained by surface warming in its source region. The nature of NPIW change along P3 is difficult to characterise: it is likely that surface warming and increased precipitation in the source region of NPIW play an approximately equal role in causing the observed T-S change.

JPM3q

An observational study on decadal variations of the Atmosphere-Ocean System in the Pacific

Misako Kachi and Tsuyoshi Nitta

Earth Observation Research Center, National Space Development Agency of Japan, Tokyo, JAPAN
Center for Climate System Research, University of Tokyo, Tokyo, JAPAN

Large-scale decadal variations over the Pacific are investigated and compared with interannual variations to obtain the characteristic spatial and temporal distributions of the atmospheric and oceanic variables as well as their interactions on such a time scale.

The empirical orthogonal function (EOF) and composite analyses for the global sea surface temperature (SST), sea level pressure, surface winds and surface precipitation demonstrate the existence of large atmospheric and oceanic decadal variations over the entire Pacific from the mid 1970s to the early 1980s. After around 1978, the SST spatial pattern shows large positive anomalies in the equatorial central Pacific and the Eastern Pacific, and negative anomalies in the mid-latitudes over the North and the South Pacific. The sea level pressure in the tropics shows negative anomalies in the Eastern Pacific, and positive anomalies in the Western Pacific and the Indian Oceans in the same period. There also exist westerly anomalies in the equatorial central and Eastern Pacific indicating the weakening of the tropical Walker Circulation.

The surface precipitation increases in the tropical central and Eastern Pacific where large positive SST anomalies are observed.

Singular value decomposition (SVD) analysis for the global SST and Northern Hemisphere 500hPa geopotential height fields shows that there also exists a decadal variation of the 500hPa height field with the strengthened Pacific/North American (PNA) teleconnection pattern coupled with the decadal variation of the global SST described above. The enhanced tropical convective activity may be due to the increase of SST over the tropical central and Eastern Pacific, which may affect the strengthening of the PNA pattern in the 500hPa height field.

To clarify the role of the upper ocean on a decadal time scale, we constructed a new objectively analyzed dataset of the oceanic subsurface temperature and investigated the decadal variations of the upper ocean horizontally and vertically using this dataset.

JPM3t

SST/Australian Rainfall relations in a multi-decadal OAGCM integration

J–rg-Olaf Wolff and Mojib Latif

Antarctic CRC, Hobart, AUSTRALIA
Max-Planck-Institut f¸r Meteorologie, Hamburg, GERMANY

In this study we investigate the interannual and decadal sea surface temperature (SST) variability in the global coupled ocean/atmosphere model ECHO-T42. The special area of interest is the ocean around Australia. The main goals of this study are firstly to identify the mechanisms by which SST variability in this region is generated, secondly the impact of SST variability on the atmospheric circulation over Australia and thirdly the variability of Australian rainfall and its relation to SST variability in the region.

The atmospheric component of ECHO is ECHAM, which is based on the European Centre atmospheric forecast model and was modified in Hamburg for long-term climate integrations. We use this global low-order spectral model here in triangular truncation at wavenumber 42 and 19 levels in the vertical. The oceanic component of ECHO is HOPE (Hamburg Ocean Primitive Equation Model) a state of the art primitive equation global ocean model (Wolff et al., 1996). The meridional resolution is variable, with 0.5 degree between 10ƒN and 10ƒS and increasing polewards to match the T42-resolution of the atmosphere model. Zonally the resolution is constant at the T42-resolution. The vertical is resolved by 20 irregularly spaced levels, with ten levels in the upper 300 m of the water column. Preliminary results of a 20 year integration have been discussed by Latif et al. (1994). The model has been integrated now for about 120 years.

We will discuss the climate variability in this model in different frequency ranges and focus on the correlation of the Indonesian/Indian Ocean SST Dipole with Australian Rainfall (Nicholls, 1989).

Latif, M., T. Stockdale, J.-O. Wolff, G. Burgers, E. Maier-Reimer, M.M. Junge, K. Arpe and L. Bengtsson, (1994), "Climatology and variability in the ECHO coupled GCM", Tellus, 46A(4), 351-366.

Nicholls, N., (1989) "Sea surface temperatures and Australian winter rainfall", J. Climate, 2, 965-973.

Wolff, J.-O., E. Maier-Reimer and S. Legutke, (1996), "The Hamburg Ocean Primitive Equation Model - HOPE", Technical Report No 13, German Climate Computer Center, 103pp.

jpm3u

Quasi-Periodicity and Global Symmetries in Interdecadal Upper Ocean Temperature Variability

Warren B. White and Daniel R. Cayan

Scripps Institution of Oceanography, University of California,
San Diego, USA

Multi-decadal records of sea surface temperature (SST), sea level pressure (SLP) 400 m temperature (T400), and wind stress curl (WSC) anomalies are analyzed over most of the global ocean for 90 years from 1900-1994 and for 40 years from 1955-1994. All records are band-pass filtered (i.e., 15-30 years), isolating interdecadal variability from ENSO and secular variability. From the 90-year record, the RMS of interdecadal SST and SLP anomalies have maximum magnitudes of 0.15ƒ-0.25ƒC and 0.5-1.5 hPa, respectively, in the extra-tropics. First modes from empirical orthogonal function (EOF) analysis explain half the interdecadal variance in both SST and SLP, with amplitudes peaking near 1900, 1920, 1940, 1960, and 1980, yielding quasi-cyclic behavior with periods ranging from 18-23 years. The EOF spatial patterns for both SST and SLP contains a reflection symmetry about the ITCZ and a translation symmetry across ocean basins in each hemisphere, with the entire global tropical strip and eastern boundaries of each ocean basin warmer (colder) than normal, associated with an equatorward (poleward) displacement of the Aleutian and Icelandic Low pressure systems and an increase (decrease) in Westerly Winds between 20ƒ-45ƒN. In the North Pacific from 1970-1989, these anomalous Westerly Winds intensified (weakened) the subtropic and subarctic gyre circulation, the latter in Sverdrup balance with associated WSC anomalies (Miller et al., 1996). These same reflection and translation symmetries can be seen in the evolution of interdecadal SST, SLP, T400 and WSC anomalies during the 40 year record, with all variables in the western-central extra-tropics of each ocean basin propagated equatorward toward the ITCZ at 1-2 cm sec^-1, similar to that observed in lag sequences of extended EOF analyses over the entire 40-year record, indicative of its quasi-cyclic nature over the 40 years.

JPM3v

Large decadal variability in North Pacific as simulated by a hybrid coupled model

W. Xu, T.P. Barnett and M. Latif

Center for Cliamtic Research, University Wisconsin-Madison, Madison, USA
Climate Research Division, Scripps Institute of Oceanography,
La Jolla, USA
Max Planck Institute for Meteorology, Hamburg, GERMANY

A hybrid coupled ocean-atmosphere model was recently developed jointly by the Scripps Institution of Oceanography and the Max-Planck-Institut fur Meteorologie, in order to study the dynamics and predictability of decadal climate variability over the North Pacific.

The hybrid coupled model consists of the oceanic general circulation model HOPE and a statistical atmosphere model which was derived from a 125-year integration with the couple general circulation model ECHO (Latif and Barnett, 1994). The domain of the hybrid coupled model covers the North Pacific from 10ƒN to 60ƒN.

In this study, the hybrid coupled model (HCM) is used to investigate the physics of decadal variability in North Pacific. This aids in an understanding of the inherent properties of the coupled system in the absence of stochastic forcing by noncoupled variability. It is shown that the HCM simulates decadal variability similar to that found in both the observation and coupled GCMs. In particular, the hybrid coupled model simulates a self-sustained decadal oscillation, with a period of about 20 years.

Sensitivity experiments are carried out to determine the relative importance of wind stresses, net surface heat flux, and fresh water flux on the initiation and maintenance of the decadal oscillation in the North Pacific. It is found that decadal variability is a mode of the coupled system and involves interaction of sea surface temperature, oceanic heat content and wind stress. This interaction can be strongly modified by the surface heat flux. The effect of salinity is relatively small, and it is not necessary in exciting the model decadal oscillation in the North Pacific.

JPM3w

A REEXAMINATION OF THE QUASIBIENNIAL AND QUASI-QUADRENNIAL ENSO COMPONENTS IN GLOBAL SSTs

Per Gloersen, Norden Huang

Oceans and Ice Branch, Laboratory for Hydrospheric Processes, NASA/Goddard Space Flight Center, USA

In recent papers, spectral components appearing in the ENSO record at periods of about 2.4 years (quasibiennial, QB) and 4.2 years (quasiquadrennial, QQ) have also been observed in the hemispheric sea ice and in the global sea surface temperature (SST) data obtained from the 9-year record of the Nimbus 7 Scanning Multichannel Microwave Radiometer. The SST QQ component has a large amplitude along the Equator in the east Pacific, and off the coast of Columbia and Ecuador, following the familiar pattern of sea surface temperature (SST) anomalies reported by others. Even larger amplitudes are observed in the immediate vicinity of the Colombian and Ecuadorian coasts. However, in terms of spatial extent, the QQ component in the Southern Ocean is the most prominent, extending as far as 200 km equatorward from the maximum sea ice extent in the Bellingshausen, Amundsen, and Ross Seas, and reaching the southern Australian coastline in the Indian Ocean. The earlier time series analyses encountered a number of difficulties, including broad spectral lines with peak frequencies varying from one location to the next, and phase shifts greater than 2 pi globally, making it troublesome to determine the origins of the signals. The first listed difficulty is an indication that the SST time series is non-stationary and therefore its Fourier spectrum is not easily interpreted. In this paper, we show the results of applying a new analysis technique appropriate for non-stationary time series: an empirical decomposition into intrinsic mode functions, followed by a spectral analysis using the Hilbert transform on the extracted modes. This technique permits examining the variation of the central frequency of the QB and QQ ENSO components with time and location, and facilitates examining the phase differences of a given component at different global locations.

jpm3x

A global primitive equation ocean model coupled to a quasi-geostrophic atmospheric model for climate studies

Charles A. Lin, Richard J. Greatbatch, John Kozlowski
and Hai Lin

Department of Atmospheric and Oceanic Sciences, and Centre for Climate and Global Change Research, McGill University, Montreal, Canada

Department of Physics and Physical Oceanography, Memorial University, St. John's Newfoundland, CANADA

Centre de recherche en calcul applique, Montreal, Canada

We have formulated a coupled ocean-atmosphere model of intermediate complexity for climate studies. The atmospheric component is a quasi-geostrophic global spectral model with a T21 horizontal resolution and 3 levels in the vertical. The oceanic component is a primitive equation model. The atmospheric model shows realistic eddy heat and momentum fluxes, and low and high frequency variability when run in the uncoupled mode. Because of the relative simplicity of the atmospheric model, the coupled model can be run for long periods on a workstation. The coupled model has been run for over 1,000 years. It shows a realistic variability of the air-sea heat flux on the synoptic and interannual time scales. An analysis of the variability of the thermohaline circulation will also be performed.

JPM3Z

Decadal Variability of Surface heat flux over the pacific

Masahisa Kubota

School of Marine Science and Technology, Tokai University, Shimizu, Shizuoka 424, JAPAN

We estimated the global heat flux between ocean and atmosphere during 1960-1992 using in-situ observation data. Comprehensive Ocean Atmosphere Data Sets (COADS). Seasonal variation is basically noticeable for all components for heat flux except tropical regions. However, decadal variation of heat flux was also found for a global region. In particular, it should be noted that the heat transfer from the ocean to the atmosphere remarkably increases during the late 1970 and the 1980’s. The increase is mainly caused by decadal variation of the latent heat flux. The increase of heat flux is strongly connected with the decadal variation of other physical values such as wind speeds and sea surface temperature as pointed out before. However, the decadal variation of heat flux is gradual rather than a climatological jump. Whether the recent increase of the latent heat flux can be detected by satellite data or not is investigated in this study.

JPM3aa

A 50-70 year climatic oscillation over the North Pacific and North America

Shoshiro Minobe

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

The existence of interdecadal climatic regime shifts and their periodicity are examined, using instrumental data over the North Pacific, North America and the tropical oceans, and reconstructed climate records for North America. In the North Pacific and North America, climatic regime shifts similar to the shifts in the 1940s and 1970s are detected around 1890 and in the 1920s with alternating polarities. Sea-surface temperatures in the tropical Indian Ocean-maritime continent region exhibit changes corresponding to these four shifts. Spectra obtained by the Multi-Taper-Method indicate these climate changes have a periodicity of 50-70 years.

The leading mode of the empirical orthogonal functions of the air-temperature reconstructed from tree-rings in North America exhibits a spatial distribution that is reminiscent of instrumentally observed air-temperature differences associated with the regime shifts. The temporal evolution of this mode is characterized by a 50-70 year oscillation in the eighteenth and nineteenth centuries. This result, combined with the results of the analyses of the instrumental data, indicates that the 50-70 year oscillation is prevalent from the eighteenth century to the present in North America.

JPM3bb

Decadal Pacific Variability from High-Resolution Global Ocean Models

Albert J. Semtner and Julie L. McClean

Oceanography Dept., Naval Postgraduate School, Monterey, California, USA

Pacific decadal variability derives from diverse causes such as advective changes in narrow boundary currents and broad interior flows, variations in mixed-layer and thermocline heat storage, propagation of Rossby waves excited by coastal Kelvin waves, decadal modulation of El Nino, and low-frequency changes in meridional heat transport related to instabilities. Many of these processes will be misrepresented in models unless there is sufficiently high spatial resolution to allow strong mean flows and instabilities, as well as low enough diffusion to preserve phenomena of interest. Two ocean models are used to simulate variabilities that arise in the Pacific when a full set of mechanisms are considered. The global models have mercator grids with average grid spacings of 1/4 and 1/6 degrees and 20 levels. ECMWF winds of 1985-94 have been applied with monthly and daily variations. Surface buoyancy fluxes have progressed from Levitus restoring terms to monthly ECMWF climatological fluxes, with high-frequency decacal forcing in progress. To this end, reanalyzed ECMWF data of 1979-94 and added data for 1995-96 are now forcing the models on the decadal time scale. In addition, a prognostic-depth mixed layer has been added to the 1/4-degree model; and its high-latitude variables are no longer constrained to climatology but instead interact with an overlying ice model. Representative results will be shown from the simulations, including trans-Pacific Rossby waves generated by El Ninos and wind-and buoyancy-induced changes in Pacific circulation. The ability to reproduce natural variability improves as forcing and boundary processes are improved, judging by comparisons with TOPEX data, Pacific tidal records, and WOCE hydrography. Examination of the suite of results shows how various mechanisms influence variability.

JPM3dd

Remotely Driven Long-term Changes in the Coastal Pacific Waters off California

Allan J. Clarke and Anna Lebedev

Oceanography Department, Florida State University,
Tallahassee, Florida, USA
Flinders Institute for Atmospheric and Marine Sciences,
Flinders University of South Australia, AUSTRALIA

The zooplankton biomass in the California current has suffered a massive decrease of about 75% since the early 1970s and this probably has caused the marked decrease of zooplankton feeders like rockfish and birds. The decline in zooplankton is likely due to a decline in the supply of cold, nutrient rich water to the well-lit surface layer, since the production of phytoplankton, which provides plant food for the zooplankton, is nutrient limited. Consistent with a decreased supply of cold water, near surface temperatures increased during the 1970s and 1980s. Long coastal California sea surface temperature records indicate that decadal and longer warming and cooling has also occurred earlier this century as part of a 'natural' decadal and longer varying climate signal. A comparison of equatorial and California records, together with some basic theoretical arguments suggest that these changes are driven remotely by decadal and longer weakening and strengthening of the equatorial Pacific trade winds which lower and raise the thermocline and likely affect ecosystems along thousands of kilometers off the western coastline of the Americas.

JPM3ff

Decadal Changes in the Structure of the Upper Ocean in the North Pacific

Tomowo Watanabe and Keisuke Mizuno

National Research Institute of Far Seas Fisheries, Shimizu-shi, Shizuoka, JAPAN

Interannual variations of the upper ocean in the North Pacific are investigated by using the temperature and salinity profile data base and the sea surface temperature data set. The 10-year mean state for 1976-85 is compared with that for 1966-75 to detect decadal changes occurred in the mid-1970's. In the mid-latitude region of the western North Pacific, the Kuroshio bifurcation front shifted southward and strong cold temperature anomalies appeared in the latter decade. Cold temperature anomalies were also formed in the surface mixed layer in the transition zone between subtropical gyre and subarctic gyre in the eastern North Pacific. On the other hand, in the subtropical gyre region, warm temperature anomalies were detected along the main thermocline. This suggests the intensification of the subtropical gyre in the latter decade. These decadal changes in the upper ocean are thought to be caused by changes in the atmospheric conditions of the North Pacific in the mid-1970's.

The propagation of temperature anomalies from the surface mixed layer at the transition zone to the subsurface at the subtropics was found in the eastern North Pacific on the decadal time scale. Ventilation is a possible mechanism for the propagation and the process may play an important role in the evolution of the decadal variation in the Pacific.

Jpm3ii

Interdecadal Hydrographic Variability in the Gulf of Alaska, 1970-1996

Thomas C Royer

Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, USA

Different time scales of temperature and salinity anomalies in the northern Gulf of Alaska suggest that the water column responds to at least two different decadal mechanisms. Temperature and salinity versus depth at the mouth of Resurrection Bay, Alaska (60ƒN, 149ƒW) (GAK 1) have been measured since December 1970 with various temporal sampling intervals, ranging from hours to months.

Interdecadal fluctuations of temperature are seen throughout the 263m water column. The largest amplitude changes, more than 1ƒC from 1972 to 1986, are in the temperatures at 200-250m. In contrast with temperature anomalies observed off California, these are coherent with depth. This suggests a barotropic forcing rather than a vertical propagation or baroclinic influence. Sea surface temperatures anomalies for the Gulf of Alaska and Bering Sea imply that the interdecadal ocean temperature fluctuations at GAK 1 are representative of changes in upper layer thermal structure over this broad zonal region. Using a proxy temperature data set, up to 30 percent of the low frequency temperature variance can be accounted for with the 18.6 year nodal tide signal. In contrast with the temperature signals, the salinity variations are of shorter period (9-11 years) and might be related to changes in the large scale cyclonic behaviour of the atmosphere over the Arctic. Decadal variations of the mixed layer depth caused by these salinity and temperature fluctuations should influence biomass concentrations here.

JPM3kk

Quasi-Decadal Variability in the North Pacific during recent decades

Hisashi Nakamura, Guoyao Lin and Toshio Yamagata

Department of Earth and Planetary Physics, Graduate School of Science, University of Tokyo, Tokyo, Japan

Low-frequency variability in the North Pacific wintertime SST and associated variability in the atmospheric circulation are investigated based on the COADS and NMC analyses, respectively. A harmonic analysis was applied to separate the quasi-decadal variability with periods of 7 years or longer from the variability associated with ENSO signal. We focus on the 25 recent years of 1968-1992, during which more ship observations are available and the quasi-decadal variability was more pronounced than before in the low latitudes. During that period the quasi-decadal SST variability within the North Pacific basin was concentrated around the two major oceanic fronts. The strongest variability is found around the subarctic front, the most intense front extending zonally at ~ 42ƒ N. The second maximum of the variability coincides with the subtropical front, the NE-SW oriented front to the north of Hawaii.

An EOF analysis applied to the North Pacific basin [20ƒ N-50ƒ N, 150ƒ E-140ƒ W] reveals that the quasi-decadal SST variability around the two major fronts are not varying coherently. The first mode, which explains 48% of the variability within the domain, represents SST fluctuations along the subarctic front. The fluctuations tend to be out of phase with those in the Gulf of Alaska, but they exhibit no significant simultaneous correlation with the tropical and subtropical variability. The second mode, which accounts for 26% of the variability, represents SST fluctuations along the subtropical front. They exhibit a strong seesaw with the tropical Pacific SST. The second mode also captures a nearly in-phase relationship between the SST variability in the Kuroshio extension and that in the tropics. The principal components (PCs) and the unsmoothed SST anomalies averaged over the two frontal regions both indicate a striking fact that in the 1970’s cooling around the subarctic front occurred 2 years in advance of warming in the topical Pacific and concomitant cooling around the subtropical front.

The first mode of the quasi-decadal SST variability in the North Pacific is associated with significant SLP anomalies near the center of the Aleutian low and anomalies in the surface westerlies over the extratropical North Pacific. These anomalies tend to occur in conjunction with the classical PNA pattern aloft. SLP anomalies associated with the second SST mode are characterized by a pair of meridional seesaws. The stronger one consists of anomalies that overlap the eastern Pacific subtropical high and weaker anomalies over western Canada. This seesaw is also apparent in the middle troposphere. The weaker seesaw resides in the western Pacific, indicative of associated changes in the wintertime Asian monsoon.

The results presented here is consistent with many of the previous results in the sense that the tropical decadal variability instantaneously influences the extratropics up to ~ 35ƒ N via the atmosphere. Yet, the weak simultaneous linkage between the SST fluctuations around the subarctic front and those in the tropics is indicative of self-maintaining mechanisms inherent to the northern North Pacific climate system of its quasi-decadal variability, as suggested in recent coupled GCM experiments. In fact, the intensification of the Aleutian low concomitant with the cooling around the subarctic front acts to reinforce that cooling, by augmenting the surface heat fluxes from the ocean, the southward Ekman transport across the frontal region and the consequent cold advection, and the wind forcing to the Pacific subpolar gyre.

JPM3nn

Can we Measure Decadal Changes in the Pacific-Indian Throughflow?

Susan Wijffels, Ann Gronell and Gary Meyers

CSIRO Division of Marine Research, Hobart, Australia

The Pacific-Indian Throughflow has been monitored since 1983 by volunteer merchant ships using XBT's along the IX1 line between western Australia and Java. This 12 year data set provides good statistics of the mean thermal structure and the geostrophic shear that supports the Throughflow as it enters the Indian Ocean. This data has also shown that the Throughflow is sensitive to changes in the Pacific Trades at interannual timescales. A decadal modulation of the Throughflow might be expected from the observed decrease in the Pacific Trades around 1975 observed by Clarke et al. (1996). Here, we report on an exploratory study of historical XBT and sealevel data along the IX1 line which will compare the pre-1975 with the post-1983 state, and assess the implications for Throughflow transport changes.

Jpm3oo

Decadal-to-interdecadal variability of upper and intermediate water masses in the North Pacific

Toshio Suga

Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan

Several recent studies indicate that interdecadal changes occurred in the mid 1970s in the atmospheric general circulation pattern, the SST fields and the upper thermal conditions in the midlatitude North Pacific. The present study further examines the decadal-to-interdecadal variations by using an extensive hydrographic dataset to produce mean property fields on isopycnal surfaces for consecutive time periods spanning several years to a decade. Preliminary comparison of upper and intermediate isopycnal properties are made between the two decades: 1966-75 (D67) and 1976-85 (D78). Substantial differences are found in the western part of the subtropical gyre. The isopycnals are deeper during D78, suggesting the gyre being relatively spun up, which is consistent with the previous analysis of the thermal conditions and wind stress fields. Isopycnal distribution of potential vorticity indicates that the North Pacific subtropical mode water is more prominent during D78. Since the salinity at the intermediate isopycnal in the western gyre is lower during D78, interdecadal changes in the intermediate water formation and/or redistribution processes are also suggested. Interdecadal differences in the central part of the subtropical gyre is also apparent. One of the characteristic features of the isopycnal properties in the central gyre is a low potential vorticity pool around the 26.2 sigma-theta surface, which corresponds to the core of the North Pacific central mode water (NPCMW). The low potential vorticity pool is more intense during D78, suggesting more intense ventilation associated with the NPCMW formation. Further analysis will be made with trying to resolve time evolutions of these changes for each isopycnal.

JPM3pp

Coral Sr/Ca Thermometry and ¹⁸O/¹⁶O Reconstructions of Ocean-Atmopshere Freshwater Exchange

M.K. Gagan, L.K. Ayliffe, S. Anker, D. Hopley, M.T. McCulloch, J.M.A. Chappell

Research School of Earth Sciences, The Australian National University, Canberra, AUSTRALIA
Sir George Fisher Centre, James Cook University, Townsville, AUSTRALIA
Research School of Pacific and Asian Studies, The Australian National University, Canberra, AUSTRALIA

Extension of the instrumental record of tropical climate backwards in time is a high priority in the face of climate change. Massive corals growing in the reef ecosystems of Indo-Australia provide some of the richest palaeoclimatic archives in the world. The purpose of this paper is to demonstrate that corals can provide palaeoclimate data available in no other way which, in many oceanic settings, approach the quality of instrumental climate records.

One of the most promising new developments in tropical palaeoceanography involves the precise measurement of Sr/Ca and ¹⁸O/¹⁶O in coral skeletons to uniquely reconstruct both SSTs and the surface-ocean hydrologic balance. We have recently completed calibration experiments demonstrating that the coral Sr/Ca thermometer is accurate to 0.4ƒ C. Coupled measurements of coral Sr/Ca and ¹⁸O/¹⁶O make it possible to reconstruct surface ¹⁸O/¹⁶O, as a proxy for sea-surface salinity, but removal of the temperature component of the ¹⁸O/¹⁶O signal. We are now at the point where it is possible to routinely extract weekly resolved records of sea-surface temperature (SST), precipitation, evaporation and in some coastal settings, wind velocity. Such records are important because they provide a way to examine the response of the hydrological cycle to subtle changes in tropical SSTs of the distant past.

To demonstrate the utility of this technique, we will present coral records for three calibration sites: (1) the Great Barrier Reef; (2) the eastern Indian ocean and (3) southern Java. The climatic manifestations of the prolonged 1991-94 ENSO), including drought, increased evaporation, and reduced trade wind velocity, are well recorded by the three coral sites. The Great Barrier Reef coral documents the cooling of winter SSTs and low runoff which accompanied the ENSO-induced drought that gripped northeast Australia. The timing and magnitude of seasonal evaporation are accurately recorded by the eastern Indian ocean coral; there is a clear increase in the magnitude of evaporation during the prolonged ENSO. The coral from southern Java records a period of intense coastal upwelling in the austral winter of 1994. The timing and magnitude of this "cold spot" is of considerable interest because it may have served to enhance and prolong the ENSO drought that was already underway in Australia.

We have applied this same approach to an ancient coral which gives a 100-year-long, high resolution record of climate in the Great Barrier Reef at 5,800 year ago. The calibrated coral Sr/Ca-SSTs for the ancient coral were 1ƒ C warmer than those of the last two decades. Interestingly, summer SSTs are closely confined to a mean temperature of about 29.5ƒ C, and rarely exceeding 30ƒ C. This is the situation today further north throughout the Warm Pool region where persistent clouds are thought to prevent further increases in SST. Examination of the coral UV fluorescence and ¹⁸O/¹⁶O, marking the intensity of runoff from mainland rivers, suggest that monsoon rainfall was much less variable than at present. Furthermore, there is no evidence for drought or the cool SST anomalies in winter that are indicative of the ENSO. Taken together, the evidence suggests that this period in the history of the Great Barrier Reef was marked by warmer SSTs, more dependable monsoonal rainfall, and possibly a weaker ENSO.

JPm3QQ

Decadal-scale Variations of the North Pacific Subtropical Gyre

Roxana C. Wajsowicz

Department of Meteorology/JCESS, University of Maryland, College Park, USA

Integrations from 1954 to 1988 of a 4-D data assimilation system, based on the Pacific Ocean system used at NOAA/NCEP, and an equivalent GCM simulation, have been analysed in view of the demonstrated contrasting behaviour of decadal-scale upper ocean heat content (UHC) anomalies and salinity anomalies over the latitudes of the North Pacific subtropical gyre. The UHC anomaly field can be interpreted as an oscillation of the subtropical gyre; the gyre is tilted about a horizontal axis and rotates clockwise. However, the salinity anomalies show a distinct eastward phase propagation. The need to prescribe actual sea-surface salinities or net fresh-water fluxes instead of climatological values is examined.

Also at issue is the effect of closed boundaries on the analyses and simulation; closed boundaries are necessitated by the expense of such long integrations at fine resolution. On decadal timescales, there can be significant interaction between ocean basins at all latitudes. Investigation of several wind-stress data sets shows a shift in the Pacific wind-stress curl pattern over the last thirty-five years. Meteorologically, the shift is slight, but oceanographically, the degree or two of latitude is sufficient to change the closure of the gyres in the tropical west Pacific such that the flux of heat and net fresh water, which escapes through the Indonesian archipelago, switches from predominantly fed by the cold, fresh Mindanao Current to being fed by the warm, salty South Equatorial Current. The impact of this switch, and the lack of its explicit inclusion in the analyses and simulation, are examined.

JPM3aa

A 50-70 year climatic oscillation over the North Pacific and North America

Shoshiro Minobe

Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan

JPM3bb

Decadal Pacific Variability from High-Resolution Global Ocean Models

Albert J. Semtner and Julie L. McClean

Oceanography Dept., Naval Postgraduate School, Monterey, California, USA

JPM3cc

Two distinct modes of interdecadal variability in the Pacific with a coupled GCM

S. Yukimoto, M. Endoh, Y. Kitamura, T. Motoi, S. Nakagawa and A. Noda

Meteorological Research Institute, Tsukuba, Ibaraki, Japan

Interdecadal variability in the Pacific is investigated from a 150-year-integration of an MRI ocean-atmosphere coupled GCM. In order to extract interdecadal transient response of the ocean subsurface to an atmospheric forcing, vertically averaged ocean temperature (VAT) from surface to 600 m deep are examined by using a complex empirical orthogonal function (CEOF) analysis after band-pass (15-32 years) filtering.

Reconstructed spatial pattern of the first CEOF mode shows an El NiÒo-like anomaly pattern. The equatorial variability is dominated by a quasi-standing wave pattern of east-west seesaw. In the North Pacific, on the other hand, westward propagating signals are apparent. Associated variability of the SST and the atmospheric circulation shows a coherent spatial structure with the VAT anomaly pattern. The easterly (westerly) wind stress anomaly in the central equatorial Pacific is dominant consistently with higher (lower) VAT in the western equatorial Pacific. The westward propagating VAT signal is enhanced in the central North Pacific where the wind stress exhibits anomalous vorticity consistently with the VAT changes through Ekman pumping. It is noted that these features resemble to those in the ENSO timescale.

The second CEOF mode of VAT is characterized by rotating behavior of VAT anomaly along the subtropical gyre in the North Pacific. It seems to have two centers of action. The northern action is accompanied by variations of the Aleutian Low and the SST around the Kuroshio extension. The southern action is in southern part of the subtropical gyre in the North Pacific. Increase of the subtropical VAT anomaly is associated with a spin up of the subtropical gyre circulation which follows an enhancement of the subtropical high pressure field over the North Pacific.

JPM3dd

Remotely Driven Long-term Changes in the Coastal Pacific Waters off California

Allan J. Clarke and Anna Lebedev

Oceanography Department, Florida State University,
Tallahassee, Florida, USA
Flinders Institute for Atmospheric and Marine Sciences,
Flinders University of South Australia, AUSTRALIA

JPM3EE

Verification of Long-term Changes in Surface wind/wind stress by ships-of-opportunity data

Kunio Kutsuswada

School of Marine Science and Technology, Tokai University, Shimizu, Shizuoka 424, JAPAN

Time series of wind data on the sea surface in the North Pacific measured on ships-or-opportunity during 1961-90 supplied by Japan Meteorological Agency (JMA) are used to detect variabilities of decadal to interdecadal time scales. Long-term linear trend is calculated for the low-pass filtered time series of scalar wind and wind components by the method of least squares. The trend for the scalar wind has large positive in a large portion of the North Pacific, and a maximum of about 2.0 m/s per 30 years in the central portion of the westerly wind region.

It can be considered that these enhancements in the wind field derived from ships-of-opportunity data are questionable due to errors in wind measurements on ships, because examinations of the anemometer height shows that it increases with time from 26 m in 1970 to 36-37 m in 1990. We construct a new data set of the wind/wind-stress which are corrected into 10-meter level ones using information of anemometer height on ships based on the assumption of the logarithmic profile, and check whether the enhancements in the wind field are real phenomena or not. In time series of the height-corrected zonal wind in the westerly wind region, we can still find an enhancement with almost the same magnitude as in the uncorrected time series. On the other hand, the corrected scalar wind exhibits the enhancement with less magnitude than the uncorrected one, which suggests that calculations of the heat flux on the sea surface should be made using the height-corrected wind data. Similar examinations are made for the time series of the wind-stress and the Sverdrup transport. Also in these time series, significant discrepancies are found between the corrected and uncorrected ones.

JPM3ff

Decadal Changes in the Structure of the Upper Ocean in the North Pacific

Tomowo Watanabe and Keisuke Mizuno

National Research Institute of Far Seas Fisheries, Shimizu-shi, Shizuoka, JAPAN

No JPM3gg

No JPM3hh

Jpm3ii

Interdecadal Hydrographic Variability in the Gulf of Alaska, 1970-1996

Thomas C Royer

Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, USA

JPM3JJ

A pacific interdecadal climate oscillation with impacts on salmon production

N.J. Mantua, S.R. Hare, Y. Zhang, J.M. Wallace and R.C. Francis

JISAO, University of Washington, Box 354235, Seattle, WA 98195, USA, International Pacific Halibut Commission, University of Washington, Box 357951, Seattle, WA 98195, USA, Fisheries Reserach Institute, University of Washington, Box 357980, Seattle, WA 98195, USA

Evidence gleaned from the instrumental record of climate data identifies a robust, recurring pattern of ocean-atmosphere climate variability centred over the mid-latitude North Pacific basin. Over the past century, the amplitude of this climate pattern has varied irregularly at interannual-to-interdecadal time scales. There is evidence of reversals in the prevailing polarity of the oscillation occurring around 1925, 1947 and 1977; the last two reversals correspond with dramatic shifts in salmon production regimes in the North Pacific Ocean. This climate pattern also affects coastal sea and continental surface air temperatures, as well as streamflow in major west coast river systems, from Alaska to California.

JPM3kk

Quasi-Decadal Variability in the North Pacific during recent decades

Hisashi Nakamura, Guoyao Lin and Toshio Yamagata

Department of Earth and Planetary Physics, Graduate School of Science, University of Tokyo, Tokyo, Japan

JpM3ll

Interannual and Interdecadal Variability of Pacific SST

Xuebin Zhang, Amir Shabbar and Jian Sheng

Atmospheric Environment Service, Environment Canada 4905 Dufferin Street, Downview Ontario, Canada, Canadian Centre for Climate Modelling and Analysis, Victoria, B.C. Canada

The statistical technique of Multi-channel Singular Spectrum Analysis has been used to investigate interannual and interdecadal variability in the sea surface temperature field over the Pacific ocean from 20S to 58N. Using COADS data from 1951 to 1993, statistically significant quasi-quadrennial oscillation (QQO) with a period of 51 months and quasi-biennial oscillation (QBO) with a period of 20-26 months are identified. The QQO mode explains nearly 20% of the total variance, while QBO accounts for 11.5% of the total variance. Additionally, two significant decadal modes are also found. In the QQO mode, SST anomalies are found to propogate eastward over the North Pacific, but behave like standing waves over the tropical Pacific. A striking feature of the QBO mode is the westward propogation of SST anomalies near the equator. The spatial-temporal structures of the interannual modes in the tropics are very similar to those reported in the litrature where only tropical SSTs are used, thus indicating that these modes have their origin in the ENSO phenomenon. The decadal mode, which has been related to a cooling trend in the North Pacific and a warming trend in the tropics since 1976, shows anomalies of opposite sign in the North Pacific from those indicated in the tropics. The distribution of SST anomalies at some phases of QQO and QBO resembles that of a decadal mode. The two decadal modes explain more variance in the North Pacific than in the tropics, while the variance contributed by QQO and QBO are the largest in the tropics. This suggests that the decadal scale variability dominates in the North Pacific and have their orginss in the mid-latitudes while the interannual variability is more prevelant in the tropics.

JPM3mm

Stratospheric Ozone and UV-B Observations in Korea

Hi-Ku Cho, Joon Kim, and Jeon-Woo Kim

Global Environment Laboratory, Department of Atmospheric Sciences, Yonsei University, Seoul, Korea

Measurements of the total ozone and the vertical profile have been carried out in Korea since 1984. The total ozone has been monitored daily with a Dobson spectrophotometer located in Seoul (37ƒ02’N, 129ƒ23’E, 6m above m.s.l.) and with q Lidar in Suwon (37ƒ16’N, 126ƒ59’E, 37 m above m.s.l.) have been added to our program recently. The observation of the harmful solar ultraviolet (UV-B) radiation also has been reformed since 1933. In band signal processing capability, which will be used in out national network of field stations. In this paper, we present a description of the activities carried out since 1984 and a summary of the results from out data analyses.