DYNAMICS OF SEA ICE AND OCEAN IN POLAR SEAS (IAPSO)
Location: Arts Building 125 LR1
Location of Posters: Arts Building 126 LR2
Monday 26 July AM
Presiding Chairs: P. Wadhams (Scott Polar Res. Institute, Univ. of Cambridge, UK);
A.J. Willmott (Dept. of Mathematics, Keele Univ., UK)
P13/E/01-B1 Invited 0830
LATERAL OCEAN TRANSPORT PROCESSES IN THE ARCTIC BASIN
Robin D. MUENCH (Earth & Space Research, 1910 Fairview East., Suite 102, Seattle, WA 98102-3620, USA, email rmuench@esr.org)
The relative importance of mechanisms that laterally transport water, heat and dissolved materials differs in the Arctic from that at lower latitudes because of the low energy levels and small baroclinicities that typify high latitude oceans. The primary mechanism for advective transport in the Arctic Ocean, below the upper mixed layer which is frictionally coupled to a wind-driven ice cover, is by way of strongly barotropic boundary currents that overlie the steep continental slopes around the peripheries of the basins. These currents provide the Arctic Ocean with a cyclonic overall circulation and bifurcate into branches that overlie the steep flanks of the several mid-ocean ridge systems to comprise an interlocking family of closed or partially closed gyres. There are regions, such as surrounding the Chukchi Rise and along the Alpha-Mendeleyev and Arctic Mid-Ocean ridges, where significant gaps exist in our understanding of the circulation. The central basins remote from steep bottom slopes and the associated currents are low in energy. Small-scale structures observed across the Makarov, Amundsen and Nansen basins suggest that molecular processes such as double diffusion play a significant if uncertain role in lateral transport there. Our understanding of these structures remains at the level of hypothesis development. Isolated observations of both warm and cold-core eddies in the central basins suggests that these, too, are important to central basin property transports. The role of fronts remains uncertain, as does the importance of dynamic interaction between the peripheral currents and complex bathymetric features. The balance of transport mechanisms results in relative isolation of the central basin waters when compared to the peripheral or ridge-oriented systems. More quantitative understanding of this balance awaits improved field and theoretically-based analyses of the dominant mechanisms.
P13/E/12-B1 0910
A TWO-DIMENSIONAL TIME-DEPENDENT MODEL OF A WIND-DRIVEN COASTAL POLYNYA: APPLICATION TO THE ST. LAWRENCE ISLAND POLYNYA
M. A. MORALES MAQUEDA, A. J. Willmott (both at Department of Mathematics, Keele University, Keele, Staffordshire ST5 5BG, UK, email: m.a.morales.maqueda@maths.keele.ac.uk)
We present a two-dimensional model of the temporal evolution of a wind-driven coastal polynya. Given the time-varying surface winds and heat fluxes, the model calculates the growth rate, distribution and motion of frazil ice within the polynya, and the mass fluxes of frazil ice and consolidated new ice at the polynya edge. The difference between these two mass fluxes determines the velocity of advance/retreat of the polynya edge. Analytical solutions of the model are found for the case when the coastline is a straight line segment of length D (an idealised representation of an island). Two time-scales and two spatial scales are shown to be important in characterising the polynya: the consolidated new ice and frazil ice time-scales, tc and tf, respectively, and the offshore and alongshore adjustment length-scales, Ro and Ra, respectively. The time-scale tc is the time required for the polynya to grow ice of thickness equal to the collection thickness of frazil at the polynya edge. The time-scale tf is the time it takes frazil to cross the equilibrium width of the polynya, which is, in turn, determined by the length-scale Ro. In combination, tc and tf control the time-scale for the polynya to respond to variations in the atmospheric forcing. The length-scale Ra measures the sensitivity of the polynya edge to alongshore variations in the coastline geometry. It is shown that if Ra is comparable to D then the offshore dimension of the polynya and the time-scale for the polynya to reach equilibrium can be very different from those obtained from a one-dimensional formulation. The model is applied to the study of seasonal and short-term variability of the St. Lawrence Island Polynya.
P13/E/04-B1 0930
THE PAST ANALOGIES OF RECENT CHANGES OF THE ARCTIC BASIN T,S CHARACTERISTICS.
PISAREV S.V. ( P.P. Shirshov Institute of Oceanology, RAS, 36 Nachimovsky ave.,117218, Moscow, Russia, sergey@pisarev.msk.ru)
Large and widespread warming of the Atlantic layer, salting of the upper layer of the Eurasian Basin, substantially retreating of the cold halocline from the Eurasian Basin were selected among others recent remarkable changes of the Arctic Ocean to try to reveal the similar phenomena in the past.
All available unclassified observed T,S data of the Arctic Basin, including WODB98, MOODS, and data from old Russian collections, were used to describe the incomplete picture of the past variability. To characterise the recent changes, the results of well known measurements of "Rossiya"-90, "Oden"-91, "Henry Larsen and Polar Star"-93, "Polarstern"-93,96, "SCICEXs"-93,95,96, "TAPEX-94", "Louis S. St.-Laurent"- 94 were collected. The published results of these expeditions and, partly, observed data were used. The data of "Lance"-93,94,95, "Logachev"-94, "Yasnogorsk"- 95, and "Ak.Fedorov"-98, that not widely discussed before, were also added to represent modern changes. The winter and summer T,S fields of EWG (1997,98) were applied as climatology to compare scarce observed stations with each other. The analogies in the past were found, in one or another degree, for all recent changes.
P13/E/17-B1 0950
TRANSFORMATION OF ATLANTIC WATERS IN THE BARENTS SEA AND THEIR FLOW TO THE EURASIAN BASIN THROUGH ST. ANNA TROUGH
Ursula SCHAUER (Alfred-Wegener-Institut fuer Polar- und Meeresforschung, D-27515 Bremerhaven, Germany, email:uschauer@awi-bremerhaven.de), Bert Rudels (Finnish Institute of Marine Research, FIN-00931 Helsinki Finland) Harald Loeng (Institute of Marine Research, N-5024 Bergen-Nordnes, Norway), Robin Muench (Earth & Space Research, Seattle, WA 98102-3699 USA) Jim Swift (UCSD Scripps Institution of Oceanography, La Jolla, CA92093-0214, USA)
The ventilation of the Arctic Ocean basins is largely associated with the input of water from the Barents and Kara Seas. Using hydrographic observations, made in the Barents Sea and along the shelf edge of the Kara Sea between 1991 and 1996, and time series of current, temperature and salinity from moorings in the eastern Barents Sea, we discuss the flow and the modification of the water masses between the western Barents Sea and the St. Anna Trough. The inflow of warm, saline Atlantic-derived water from the Norwegian Sea and the low salinity Norwegian Coastal Current feed a permanent eastward flow. An additional input of fresh water is melted ice which is advected from the central Arctic Ocean and the Kara Sea. These three water masses are modified through cooling and the freezing/melting cycle. Two distinct modes are formed, which leave the Barents Sea eastward and descend down the St. Anna Trough. The lighter mode is winter water of low salinity, thus confined to the upper layers, which leaves the Barents Sea seasonally at temperatures close to freezing point. The largest and most dense contribution consists in more saline but only moderately cold bottom water which is formed in the polynya west of Novaya Zemlya through highly brine-enriched water. Subsequent lateral mixing with warmer Atlantic water feeds a bottom-intensified flow throughout the year. At present conditions, both modes form a low salinity input of about 2 Sv to intermediate depths of the Arctic Ocean.
P13/W/26-B1 1010
INTERANNUAL VARIABILITY OF SUMMER SEA-ICE THICKNESS IN THE LAPTEV SEA AND THE TRANSPOLAR DRIFT
Hajo EICKEN (Geophysical Institute, University of Alaska, Fairbanks, AK 99775-7320, USA, Email: hajo.eicken@gi.alaska.edu); Christian Haas (Alfred Wegener Institute, D-27515 Bremerhaven, Germany)
The dominant sea-ice circulation pattern in the Eurasian sector of the Arctic Ocean is the Transpolar Drift (TPD), exporting ice from the Siberian shelves across the central Arctic Ocean into the Greenland and Barents Seas. Large-scale sea-ice models indicate that the highest net ice production rates in the interior Arctic (on the order of 2-3 m/yr) are found in the Laptev Sea in the upper reaches of the TPD. The present study focusses on the interannual variability of sea-ice thickness in the source area of the TPD and how contrasting atmospheric circulation regimes impact ice thickening in the Laptev Sea and adjacent sectors of the Arctic.Ice-thickness data sets were collected during icebreaker expeditions in the summers of 1991, 1993, 1995 and 1996 through electromagnetic induction measurements and drilling. Level-ice thickness varied considerably in the Laptev Sea, with modal thicknesses at the end of the melt season ranging between 1.2 and 1.9 m. Higher mean and modal thicknesses in 1993 and 1996 are associated with recirculation of second-year ice over the Laptev Shelf and severely reduced summer melt, both strongly influenced by the dominant spring and summer atmospheric circulation patterns with low pressure dominating over the central Arctic. Low ice thicknesses in August of 1995, a year of a record minimum summer ice extent, are mostly the result of excessive surface melt, associated with advection of warm air from the Siberian continent. A Lagrangian thickness-evolution study and an analysis of the thickness and composition of level ice downstream in the TPD indicates that (level-)ice thickness anomalies tend to decay as they are advected across the Arctic Basin due to both dynamic and thermodynamic processes.
P13/W/22-B1 1050
ADVANCED MODELING OF RECENT VARIABILITIES IN THE ARCTIC OCEAN
Wieslaw MASLOWSKI, Albert Semtner (both at Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943, USA, email: maslowsk@ucar.edu), Bob Newton, Peter Schlosser, Douglas Martinson (all at Lamont-Doherty Earth Observatory, Palisades, NY 10964, USA, email: bnewton@ldeo.columbia.edu)
Recent observations of the Arctic Ice Pack and the upper ocean circulation and water mass structure indicate dramatic changes taking place in this region, as compared to known climatologies. Submarine observations, sea ice buoy data, and various hydrographic sections made in the 1990s show large scale and large magnitude anomalies in distribution and in characteristics of sea ice, fresh water, and Atlantic Water in the Arctic Ocean. Those changes seem to be strongly associated with variability of the atmospheric circulation in the northern high latitudes. Changes in the atmospheric circulation manifested by prolonged summer cyclonic activity in the central Arctic and weakening of the high pressure system over Greenland have been known to occur in that region since the late 1980s.
In order to understand the Arctic Ocean system response to such variable atmospheric forcing, a high resolution, basin-scale, coupled ice-ocean model of the Arctic is used. This model is forced with ECMWF re-analyzed atmospheric forcing for 1979-93 to simulate sea ice and ocean conditions before and during the 1990s. Results from the 15-year simulation show changes in large scale ice and ocean circulation, including the distribution of arctic river runoff and Pacific Water. Comparison of model results for the 1980s and 1990s shows basin-scale changes in the distribution of sea ice and water masses in the upper ocean. Circulation of fresh water on the arctic shelves (as modeled by multiple river tracers), communication with deep basins and its export through Fram Strait and through the Canadian Archipelago vary significantly throughout the time of simulation. Relationships of sea ice and oceanic fluxes to the large scale atmospheric systems as represented by Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) are investigated.
P13/W/25-B1 1110
SIMULATION OF THE INTERANNUAL VARIABILITY OF THE WIND DRIVEN ARCTIC SEA ICE COVER DURING 1958-98
Gilles ARFEUILLE, (National Institute for Earth Physics, PO Box M.G 2 Bucharest, Romania Email: ardel@infp.ifa.ro) Lawrence A. Mysak, Louis-Bruno Tremblay
A thermodynamic-dynamic sea ice model based on a granular material rheology developed by Tremblay and Mysak is used to study the interannual variability of the Arctic sea ice cover during the 41-year period 1958-98.
Monthly wind stress forcing derived from the National Centers for Environmental Prediction (NCEP) Reanalysis data is used to produce the year-to-year variations in the sea ice circulation and thickness. We focus on analyzing the variability of the sea ice volume in the Arctic Basin and the subsequent changes in sea ice export into the Greenland Sea via Fram Strait. The relative contributions of the Fram Strait sea ice thickness and velocity anomalies to the sea ice export anomalies are first investigated, and the former is shown to be particularly important during several large export events.
The sea ice export anomalies for these events are next related to prior sea ice volume anomalies in the Arctic Basin. The origin and evolution of the sea ice volume anomalies are then related to the sea ice circulation and atmospheric forcing patterns in the Arctic. Large sea ice export anomalies are generally preceded by large volume anomalies formed along the East Siberian coast due to anomalous winds which occur when the Arctic High is centered closer than usual to this coastal area. When the center of this High relocates over the Beaufort Sea and the Icelandic Low extends far into the Arctic Basin, the above ice volume anomalies are transported to the Fram Strait region via the Transpolar Drift Stream. Finally, the link between the sea ice export and the North Atlantic Oscillation (NAO) index is briefly discussed. The overall results from this study show that the Arctic Basin and its ice volume anomalies must be considered in order to fully understand the export through Fram Strait.
P13/W/24-B1 1130
PARAMETRIZATION OF ICE CATEGORIES IN A COUPLED ICE-OCEAN MODEL
Hitoshi Shinkai, Motoyoshi IKEDA (Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan, shinkai@ees.hokudai.ac.jp); Tatsuro Watanabe (Japan Sea National Fisheries Research Institute, Fisheries Agnecy, Niigata, Japan)
Parametrization of ice categories in a sea ice model for the simulation of a seasonal ice cover is investigated. In various sea ice models, only two idealized thickness levels (two category model) are used : i.e., thick and thin, which is taken to have zero thickness and treated as open water. Hence, new ice freezing over open water is forced to merge immediately with the thick ice. However, in the situation of low ice concentration at lower latitudes, such as the Okhotsk Sea, the two category ice model may overestimate air-sea heat flux through thin ice (open water). We propose three ice categories : thick ice, open water and thin ice which is produced over open water and stays to be thin until it is deformed mechanically. The thin ice among the thick ice has much weaker effects on ice pressure, and a higher ice concentration is preserved than the two category model. The thin ice in the three category model prevents air-sea heat flux resulting in less ice formation. The consequences during a decay period is faster melting, because smaller ice volume overcomes the albedo effects of the thin ice (vs. open water). The parametrization of thin ice is found to be important for simulation of the seasonal ice cover in the Sea of Okhotsk, and a step-by-step toward a multicategory ice model.
P13/W/01-B1 1150
CONVECTION IN THE STRATIFIED OCEAN WITH BACKGROUND GEOSTROPHIC CURRENT
Yutaka YOSHIKAWA, Kazunori Akitomo, and Toshiyuki Awaji (Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan. email:yosikawa@kugi.kyoto-u.ac.jp)
Numerical experiments are performed using a 3D nonhydrostatic model to understand convective and water formation processes in the stratified ocean with background geostrophic current. The model ocean is a rectangular one (50km x 50km x 1km) on an f-plane. Background current is initially given, geostrophically balancing with upward- domed density structure.
With the beginning of uniform surface cooling, convective plumes of about 1-km scale develop and vertically homogenize the water column in the upper layer. At the same time, they accelerate the growth of baroclinic instability; unstable waves of 5-km wavelength develop at the periphery of the dome more rapidly than unstable waves due to pure baroclinic instability. Another effect of convection is to induce significant vertical motion in the preferred place provided by baroclinic unstable waves, upward motion on the ridge and downward on the trough. These up- and downward motions extend to greater depths along isopycnal surfaces, compared with those due solely to either convective or baroclinic instability. As a result, the upper layer water is effectively subducted to form patches with anticyclonic eddy at around the base of pycnocline. This is a possible formation process of eddies containing newly ventilated water as observed south of the Labrador Sea
P13/W/27-B1 1210
POLYNYA SIMULATIONS: A COMPARISON OF A FLUX MODEL AND A HIGH RESOLUTION DYNAMIC-THERMODYNAMIC SEA ICE MODEL
H. BJORNSSON, L.A. Mysak (Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Canada), Andrew J. Willmott and Miquel M. Morales (Department of Mathematics, Keele University, Keele, Staffordshire, UK)
The current theoretical understanding of wind-driven polynyas is based on flux models, which are generalizations of the steady state models of Lebedev and Pease. In these flux models, a polynya will evolve according to the balance of the nearshore ice formation and the offshore wind-driven ice transport. In these models, two types of ice are considered: the newly formed frazil ice, and the consolidated frazil ice pack. The boundary between the two ice types is the polynya edge. The influence of internal ice stresses is not considered, and thus the flux models are limited in that they employ no ice rheology.
Ice rheology is employed in traditional dynamic-thermodynamic sea ice models. However they do not consider different ice types (i.e. frazil and consolidated ice), which means that even though they correctly model the same thermodynamic processes as the flux models do, a clear polynya edge is not guaranteed. We apply a recently developed sea ice model to simulate a polynya edge in a semi-enclosed channel for several different wind directions, and compared the results with those obtained with a flux model.
We show that in the cases where the ice flow is not affected by internal ice stresses (eg, along-channel winds), the results from the two types of model compare extremely well, although the definition of the polynya edge in the ice rheology model becomes arbitrary. For the cases where there is an onshore wind component, the ice rheology model gives a clear polynya region and an ice pack region with a polynya edge separating the two. In this case, however, there were greater differences in the results for the two types of models, unless special care was taken to ensure that the effects of rheology were implicitly accounted for in the flux model.
We performed sensitivity experiments to see if the flux model results depended on the spatial non-uniformity of ice production within the polynya and the along-channel motion of the ice pack. We found only a slight sensitivity to these variables. The flux model proved to be more sensitive to the spatial variability in the cross-channel velocity component of the ice pack. This is the variable in the ice model that is most affected by rheology. Using the same cross-channel velocity component in both models gave results that compared quite favorably.
Monday 26 July PM
Presiding Chairs: P. Wadhams (Scott Polar Res. Institute, Univ. of Cambridge, UK);
A.J. Willmott (Dept. of Mathematics, Keele Univ., UK)
DYNAMICS OF OCEAN IN POLAR SEAS
P13/E/10-B1 1400
ON THE SENSITIVITY OF SOUTHERN OCEAN SEA-ICE TO THE SURFACE FRESH WATER FLUX: A MODEL STUDY
S. J. MARSLAND (Max-Planck-Institut fuer Meteorologie, Hamburg, Germany,
Email: simon.marsland@utas.edu.au) J.-O. Wolff (ICBM, University of Oldenburg, Oldenburg, Germany, Email: wolff@icbm.de)
An investigation of the ocean/sea-ice interaction in the Southern Ocean has been carried out using the Hamburg Ocean Primitive Equation Model (HOPE). Coupled to this is a thermodynamic model of sea-ice growth and melt, and a dynamic model of the sea-ice momentum balance employing a viscous-plastic rheology. Two versions of the model have been formulated: a high resolution re-entrant channel model along the East Antarctic coastline; and a medium resolution model of the entire southern hemisphere. Both models are found to be very sensitive to the magnitude of the surface fresh water flux (SFWF). The high resolution model has been tested for a range of values of a space and time constant precipitation minus evaporation (P-E). The time mean sea-ice extent is found to be linearly correlated to the magnitude of the P-E, and for the case of zero P-E there is a complete breakdown of the seasonal cycle of sea-ice advance and retreat, characterised by large scale oceanic convection and above freezing sea surface temperature. In the medium resolution version the occurrence of a large scale convective polynya in the Weddell Sea is found to depend critically on the magnitude of SFWF. There the polynya forms for two of the three P-E climatologies considered when no glacial meltwater is added to the SFWF, but does not form for any of the P-E climatologies when a contribution (10 cm/annum) due to glacial melt water is added to the region south of 60 degrees South. It is concluded that the Weddell Sea is the most marginally stable region of the Southern Ocean, and given this, that a negative anomaly in the P-E to that region in the early 1970's would be sufficient cause to explain the observed Weddell Polynya.
P13/E/07-B1 1420
STRUCTURE AND CHARACTERISTICS OF DICOTHERMAL LAYER IN THE ANTARCTIC OCEAN
BENNY N. PETER (Department of Physical Oceanography, Cochin University of Science & Technology, Fine Arts Ave, Cochin, India 682016, email: benny@md2.vsnl.net.in)
Dicothermal layer, a peculier temperature structure of cold water, sandwiched between the warm waters above and below is observed in the surface layers of high latitude oceans, and is conspicuous only during summer. From the vertical section of temperature along various longitudes in the Antarctic Ocean the dicothermal layer has been identified. This layer occupies between 50 and 150m depth and extends upto about 52 degree south from the Antarctic Coast. Characteristics such as salinity, temperature, thermosteric anomaly, oxyty and sound velocity in the dicothermal layer have been analysed. The dicothermal layer is associated with relatively high gradient of temperature, salinity, density and oxyty. The sound velocity decreases with depth in the dicothermal layer and attains a minimum at the core of the layer and then increases downwards. Thus, the sound wave is less attenuated along this channel. The dicothermal layer shows much spatial variation in the Antarctic Ocean. The occurrence and extent of this layer mainly depends on mixing processes.
P13/E/11-B1 1440
A SIMPLE ICE-OCEAN COUPLED MODEL IN THE ANTARCTIC MELTING SEASON
Kay I. OHSHIMA (Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo 060-0819, Japan, email: ohshima@soya.lowtem.hokudai.ac.jp)
In the Antarctic Ocean, sea ice melts mostly on the bottom and lateral faces by the heat input (mainly solar radiation) through the open water area. We propose a simple ice-upper ocean coupled model in which sea ice melts only by the ocean heat supplied from the air. The model shows that the relation between ice concentration and upper ocean temperature (CT-relation) converges to a certain asymptotic curve with time, which explains the relation observed in the Antarctic summer by Ohshima et al. (1998). The model also shows that the ratio of the heat used for the melting to the heat input into the open water from the air is determined by the ice concentration at that time. When the model is applied to the summer condition in the Antarctic Ocean, it roughly reproduces meridional retreat of sea ice, suggesting that the melting of sea ice be determined by the local balance to the first approximation. When the model is extended to two-dimensional in the meridional direction with the inclusion of the wind-drift effects, reproduction of the sea ice retreat has further progressed. This two-dimensional model can describe the following ice albedo feedback effect; once the ice concentration is decreased by the divergent wind field, the heat input to the upper ocean is enhanced, leading to further decrease of ice concentration. This mechanism can partly explain the year to year variation of the sea ice retreat in the Antarctic Ocean. Best-fit value of heat transfer coefficient, the key parameter in this model, becomes almost the same value both from CT-relation and meridional retreat of sea ice, and the value is also consistent with the result of McPhee (1992).
P13/E/14-B1 1500
THE DYNAMICS OF THE ANTARCTIC CIRCUMPOLAR CURRENT
A. GREZIO (School of Ocean and Earth Sciences, University of Southampton, Southampton Oceanography Centre, Southampton S0 14 3ZH, U.K., E-mail: a.grezio@soc.soton.ac.uk); N. C. Wells (Department of Oceanography, University of Southampton, Southampton Oceanography Centre, Southampton S0 14 3ZH, U.K., E-mail, n.c.wells@soc.soton.ac.uk)
The Antarctic Circumpolar Current (ACC) is the only current which flows around the globe without interruptions and connects all oceans to the South Polar regions. The ACC current is driven eastward by a strong wind field, and eddies together with topographic obstacles are crucial in the ACC dynamics. Eddies transfer heat from the lower latitudes to the higher latitudes. They also transfer momentum downward from the surface to the lower layers in the ACC. It is already known that the ACC undergoes interannual changes of the volume transport. Studies on the ACC transport are important for understanding of how rapidly these transfers take place. The dynamics of the ACC is investigated by two partial eddy-resolving models (Fine Resolution Antarctic Model -FRAM- and Ocean Circulation and Climate Advanced Model -OCCAM-). Those models are used in order to understand the role that eddies play in the main balance of the mean circulation in the dynamics of the ACC. Results from the vorticity budget in the OCCAM will be presented and compared with previous studies in the FRAM. Significant differences arise in the two models because of the wind forcing, the rigid-lid condition (in FRAM) and the free-surface condition (in OCCAM). Seasonal and interannual transport variability are going to be evaluated in OCCAM at the Drake Passage in order to analyse the mechanisms that control temporal variability in the Antarctic Circumpolar Current.
P13/E/05-B1 1520
OPEN OCEAN CONVECTION AND POLYNYA FORMATION IN A LARGE-SCALE ICEPOCEAN MODEL
H. Goosse, T. FICHEFET (both at Institut d'Astronomie et de Geophysique G. Lemaitre, Universite catholique de Louvain, Chemin du Cyclotron, 2, 1348 Louvain-la-Neuve, Belgium, email: hgs@astr.ucl.ac.be)
The formation of an offshore polynya located in the Southern Ocean near the Greenwich meridian is analysed in a large-scale ice-ocean model. This area, which receives at depth an inflow of relatively warm water from the Antarctic Circumpolar Current (ACC), is preconditioned for convection in the model by a wind-driven upwelling and by the convection that has occurred the preceding year. In autumn and early winter, the brine release during ice formation, which is enhanced due to ice divergence, is sufficient to remove the fresh layer caused by summer ice melting and to induce a deepening of the mixed layer. This incorporates warm water in the surface layer which, in a first step, slows down ice formation followed by ice melting in a second step. Then, the freshwater forcing associated with ice melting tends to stabilise the water column, but the destabilising effect of the oceanic cooling has a large magnitude, even though ice transport convergence prevails now. As a result, convection continues until the total disappearance of the ice at the end of September. Convection stops only when the atmosphere warms up in November. This mechanism of polynya formation seems fairly realistic and the polynya is located in a region where such features have been frequently observed. Nevertheless, the polynya is too wide and persistent in the model, probably because of a too low density at depth and because of the coarse resolution of the model. A passive tracer released in the polynya area shows that the polynya area contributes significantly to the renewal of deep water in the Weddell gyre and that it is major component of the Antarctic Bottom Water (AABW) inflow to the Atlantic.
P13/W/16-B1 1600
NUMERICAL MODELING OF THE CIRCULATION AND SEA ICE IN THE REGION OF PRYDZ BAY, ANTARCTICA
Le KENTANG, Shi Jiuxin (Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PRC, email: ktle@ms.qdio.ac.cn)
A coupled sea ice-mixed layer-isopycnal general circulation model formulated on isopycnal coordinates is used to simulate annual variations of the circulation and sea ice in the region of Prydz Bay, Antarctica. Both dynamic and thermodynamic processes are included in this model. The initial condition and forcing fields were calculated from the monthly-mean observed data. To obtain a resolution sufficient enough for the study region, the grid space was to be designed as a varying one.
The simulated circulation pattern and sea ice cover agree reasonably well with the observations. Some new phenomena were found. The computed circulation show that there is a clockwise gyre with annual and inter-annual variation in Prydz Bay. The water exchange between both inside and outside the day is weak, which mainly occurs in the mouth of the bay, where vertical mixing probably exists. The eastward Antarctic Circumpolar Current in the northern deep region is not really zonal due to topography. The ACC is changeable with seasons. The amplitude of the current is widest in spring. The divergence zone exists between the westward and eastward currents. Many eddies appear in this region. The sea ice cover in this region has a obvious asymmetry annual cycle (the freezing period is longer than the thawing period, which is well consistent with the observation one.
P13/W/08-B1 1620
ON THE COMMUNICATION BETWEEN THE WEDDELL AND SCOTIA SEAS
M.P. SCHODLOK, H.H. Hellmer (Alfred-Wegener Institute for Polar and Marine Research, D-27568 Bremerhaven, F.R. Germany, e-mail: mschodlok@awi-bremerhaven.de)
The Scotia Sea is filled with newly ventilated Weddell Sea Deep Water (WSDW) which in turn is a vital part for the ventilation of the World Ocean abyss. Depending on the formation process and/or its location along the Weddell Sea periphery, deep and bottom water masses follow different routes to fill the Scotia Sea. Observations along the South Scotia Ridge, i.e. including the Weddell-Scotia confluence region, are limited in space and time possibly leaving escape-routes undiscovered. As a part of the BRIOS (Bremerhaven Regional Ice-Ocean Simulations) project a stand alone ocean model (SPEM) is focused on the Weddell-Scotia confluence region with a resolution of ~20 km. The model is initialized with hydrographic data from Southern Ocean Atlas. The surface forcing data are averaged monthly means from a stand alone sea ice-mixed layer model, based on 6-hourly ECMWF (1985-1993) re-analysis data. Lagrangian floats deployed at different locations served as a tool to examine various pathways for deep and bottom water renewal within the Scotia Sea. Modelled characteristics of the WSDW throughflow into the Scotia Sea through Orkney Passage (~40W) are shown and compared to observations.
P13/W/05-B1 1640
EAST ANTARCTIC SEA-ICE DRIFT: OBSERVATIONS AND MODELLING
Petra HEIL (Antarctic CRC, Hobart, Tasmania, Australia, email: petra.heil@utas.edu.au); Xingren Wu, Ian Allison (Antarctic CRC and Australian Antarctic Division, Hobart, Tasmania, Australia, email: x.wu@utas.edu.au, ian.allison@utas.edu.au)
Many of the characteristics of polar sea ice that are important to global climate processes are influenced by the ice drift and deformation. This work presents an overview of the buoy-derived sea-ice drift off East Antarctica (20 - 160E) and compares these data with modelled ice drift fields. Possible improvements in the simulation of sea-ice dynamics over this region are suggested.
The buoy-derived velocity field confirms the broad-scale feature of East Antarctic sea-ice drift. This consists of a westward drift in the coastal current, an eastward current north of the Antarctic Divergence, and a broad region of northward ice transport in Prydz Bay. Sea-ice velocities in the coastal current (0.22 m/s) are generally higher than elsewhere (0.17 m/s), and the flow in the coastal current appears to be steered by the shelf break. In the region between 20 and 152.5E the average meridional component of the coastal current is southward, but small. This average is strongly influenced by a southward component in the flow west of Cape Anne (45E). In contrast, persistent northward ice outlets from the coastal current are found in the region east of Prydz Bay. Although these northward transport rates are smaller than in Prydz Bay itself, they contribute considerably to the divergent character of the region. The drift pattern appears to be largely robust with time, with some variance in the meridional flow. There is some evidence of a seasonal cycle in the sea ice drift speed. The length of the buoy-data record (1985 - 1998) is not sufficient to derive long-term changes, however interannual variability has been identified in the region east of Prydz Bay.
A coupled atmosphere-sea ice model is used to simulate the sea ice over this region. A simple scheme for sea-ice dynamics is currently applied in the model with only the compressive stresses effective in the sea ice. Ice of low concentration moves in free drift from atmospheric wind forcing. At high ice concentration the resistance of sea ice is considered. Preliminary results suggest that the model can simulate the broad structure for the sea-ice drift over this region. However, comparison with the buoy-derived sea-ice field suggests that there is a general tendency for northward transport in the model; also the magnitude of the sea-ice speed appears to be underestimated. Improvement of the ice rheology should remedy the simulations. Some experiments with modified sea-ice dynamics are underway. We will present results from these simulations.
P13/W/10-B1 1700
OBSERVED WATER MASS PROPERTIES AND OUTFLOW ACROSS THE WEDDELL-SCOTIA CONFLUENCE: A SPANISH CONTRIBUTION TO THE DOVETAIL PROJECT
Ileana BLADE, Marc A. Garcia (both at Laboratori d'Enginyeria Marítima. Universitat Politècnica de Catalunya. c/ Jordi Girona 1-3, Edifici D-1. 08034 Barcelona, Spain. email: garcial@etseccpb.upc.es)
As part of the international DOVETAIL project, a series of CTD stations were obtained in the Weddell-Scotia Confluence (WSC) region in January-February 1998, in an attempt to characterize the source waters for the WSC and quantify the Weddell Sea outflow into the Scotia Sea through the Confluence. Due to adverse ice conditions, the two projected meridional transects across the Confluence, to the west and east of the South Orkney Islands, could not extend poleward of 61S. Nevertheless, it is clear that the waters in the WSC are characterized by weakened CDW temperature and salinity maxima compared to those found in the adjacent Scotia Sea waters to the north and at a station farther southwest in the NW Weddell Sea -- though nowhere was the water colder than 0ºC, in contrast to previous summer observations in that area. On the basis of this one Weddell Sea station available, it appears that the WSC waters, at densities of 27.8 kg/m**3, cannot result from an isopycnal mixture between waters to the north and south, which supports earlier results calling for a shelf water contribution to the mixture. As for the Weddell Sea outflow into the Scotia Sea, geostrophic transports computed across a zonal section along the WSC and the ridge separating Powell Basin from the Scotia Sea reveal inflow into the Scotia Sea of both Warm Deep Water (1.7 Sv) and Weddell Sea Deep Water (1 Sv). Neutral density surfaces show sinking of Weddell Warm Deep Water from depths of 1000-1500 m on the southern side of the ridge to depths below 2500 m in the northern Scotia Sea (58S). On the other hand, the meridional flow across the gap to the east of the South Orkneys between Bruce Bank and Laurie Island is directed poleward. These results confirm that waters from the NW Weddell Sea, presumably mixed with (colder, fresher) waters from the eastern shelf off the Antarctic Peninsula, contribute to the deep ventilation of the Scotia basin.
P13/W/03-B1 1720
THE INFLUENCE OF MAUD RISE ON OCEAN CIRCULATION AND SEA ICE DISTRIBUTION IN THE WEDDELL SEA
Ralph TIMMERMANN and Aike Beckmann (both at Alfred-Wegener-Institute for Polar and Marine Research, D-27568 Bremerhaven, Germany, email: rtimmerm@awi-bremerhaven.de)
Water mass circulation and sea ice distribution in the Southern Ocean are generally assumed to be strongly affected by the bottom topography in the eastern Weddell Sea. Maud Rise is the most prominent bathymetric feature in this area. As a part of the BRIOS (Bremerhaven Regional Ice-Ocean Simulations) project a coupled ice-ocean model for the the Weddell Sea has been developed. A sigma-coordinate primitive equation ocean model (SPEM) is coupled to a dynamic-thermodynamic sea ice-model with viscous-plastic rheology (based on Hibler/Lemke). Model runs are initialized with data from the Hydrographic Atlas of the Southern Ocean and forced with wind, cloudiness and temperature fields of the 6h-reanalyses of the European Centre for Medium Range Weather Forecasts (ECMWF). In the reference integration, the simulated circulation in the Weddell Sector of the Southern Ocean features a double cell structure of the Weddell Gyre. Its vertical structure is revealed by trajectories of (numerical) floats which are deployed at different depth levels along the Greenwich meridian. Sensitivity studies using a modified bottom topography in the eastern Weddell Sea indicate that the double cell structure of the Weddell Gyre is strongly controled by the presence of Maud Rise while the effect on the sea ice distribution seems to be less pronounced.
P13/L/02-B1 1740
VARIOUS NATURE OF INTERNAL WAVES GENERATION BY AN ICEBERG IN SHELF ZONE
Lyudmila Pisarevskaya (Arctic and Antarctic Research Institute, 38 Bering st., 199397, St.-Petersburg, Russia)Olga SHISHKINA (Institute of Applied Physics, Russian Academy of Sciences,46 Ul'janov st., Nizhny Novgorod, 603600, Russia, email: ols@hydro.appl.sci-nnov.ru)Valeriya Vasilieva (State Marine Technical University,3 Lotsmanskaya st., 190008, St.-Petersburg, Russia)
One of the typical features of the Arctic zone of the ocean is a sharp pycnocline presence in summer time. Such a natural condition provides internal waves induction in the near-surface water layers. Previous theoretical and experimental investigations proved that under some conditions drifting icebergs may act as sources of short-period internal waves.
In the present work both theoretical and experimental study of both stationary and nonstationary internal waves generation by an iceberg drifting under water flow and wind influence was fulfilled. Some experimental data on plane regular internal waves interaction with the wave system induced by the drifting iceberg will be presented. Conditions of nonlinearity influence on internal waves formation were analysed. Experimental results on lee internal waves propagation in a fluid with the pycnocline - type stratification in the presence of regular plane internal waves are presented. Internal waves generation by the agrounded iceberg in the shelf zone taking into account its melting followed by intensive air bubbles injection (up to 10% of melting water volume) was considered as well. It occurs due to the pycnocline upvelling following bubbles emerge. Natural observations of the 6-meters pycnocline shift near the agrounded iceberg are presented.
Tuesday 27 July AM
Presiding Chairs: P. Wadhams (Univ. of Cambridge, Scott Polar Res. Institute, UK),
A.J. Willmott (Keele Univ., Dept. of Mathematics, UK)
DYNAMICS OF SEA AND OCEAN IN POLAR SEAS
P13/W/19-B2 0930
SIMULATIONS OF ANTARCTIC SEA ICE IN GLOBAL CLIMATE MODELS
Siobhan P. O'FARRELL (CSIRO Atmospheric Research Aspendale, 3195, Australia.
email : Siobhan.O'Farrell@dar.csiro.au)
The CSIRO coupled ocean-ice-atmosphere climate model includes a full dynamic thermodynamic ice model with cavitating fluid rheology. A number of simulations have been undertaken with the model for different greenhouse forcings and for different formulations of ocean eddy mixing (the latter alters the density structure in the Southern Ocean). This talk will examine the response of the ice in the Southern Ocean to the greenhouse signal. A number of simulations will be presented: a) where the seasonal cycle has been improved over the original simulation and b) simulations where different values for the ocean-ice heat flux have been employed.
One of the shortcomings of the CSIRO coupled model is the coarse resolution (3.2 degrees in latitude by 5.6 degrees in longitude). While this enables i more climate simulations to be undertaken, the ice is poorly resolved around the Antarctic continent which leads to problems with the seasonal cycle of ice retreat in some locations and poor representation of the thicker ice of the Weddell sea. These shortcomings do have significant implications for the freshwater coupling between the ice and ocean components of the coupled model. Currently the Antarctic bottom water generated in the coupled model is driven more by salinity flux adjustment terms than by levels of brine release onto the continental shelf in areas of high ice production.
The experience gained in running the atmosphere- ice component of the coupled model at higher resolution has been used in the upgraded CSIRO climate model which is at finer resolution (1.875 degrees in latitude and longitude). Results from this new atmosphere-ice model will be shown. The improved rates of ice freezing around Antarctica will reduce the dependence on flux adjustment terms in the coupling process. It is noted that two of the recent generation coupled models which have been run without flux adjustment terms (NCAR CSM, HADCM3) still encounter some of their greatest problems in the Antarctic region where the models have difficulty getting the ice to drive realistic amounts of bottom water. There has been some discussion recently over the likelihood of the rates of formation of Antarctic bottom water decreasing under a warmer climate. To be able to predict such changes with any confidence, it is crucial that the model has realistic ice production rates which may balance any increases in freshwater from precipitation and glacial melt.
P13/E/09-B2 0950
SEA-ICE WITH FREE-DRIFT DYNAMICS IN THE HADLEY CENTRE GCM
Doug CRESSWELL and Jonathan Gregory (Hadley Centre for Climate Prediction and Research, The UK Met. Office, London Road, Bracknell, Berkshire, RG12 2SY, UK, email: dcresswell@meto.gov.uk, jmgregory@meto.gov.uk)
Results are presented from the current Hadley Centre coupled model, HadCM3, which has completed a 1000 year simulation of pre-industrial climate. The model comprises a 2.5 x 3.75 degree latitude longitude atmospheric GCM coupled to a 1.25x1.25 degree ocean model and a sea-ice model based on Semtner zero layer thermodynamics in which the ice drifts with the ocean current. The coupled model runs stably without flux adjustment. The drift in global-average surface air temperature is only 0.1K in the first 1000 years. The sea-ice climatology of the model will be described, and compared with observations. The model ice has a realistic seasonal cycle, slightly over-estimating the extent of the ice in winter. There is good agreement with satelite data on the geographical distribution of the ice.
A revised version of the model has been run with free-drift sea-ice dynamics, in which the ice has an independent velocity calculated to give a balance of forces, including the effect of windstress. The rheology of the ice is represented in both schemes by a restriction on the convergence of thick ice. The free-drift scheme generally increases ice velocity and causes an increase of ice volume in winter. An improvement was seen in the simulation of the areas of ice production off the Siberian and Antarctic coasts. Results of these experiments will be presented.
P13/E/06-B2 1010
ICE CLASSES BASED MODELLING OF ICE THICKNESS REDISTRIBUTION
Jari HAAPALA (Department of Geophysics, P.O. Box 4, FIN-00014 University of Helsinki, Finland, Email: Jari.J.Haapala@helsinki.fi)
Pack ice is a mixture of several ice types and open water. Each ice type has its own characteristic thickness, temperature, roughness etc., and has an specific effect on the ice dynamics, and heat and momentum exchange between the atmosphere and the ocean. Present ice models resolve an amount of the open water, the mean ice thickness and distinguish ridged ice from level ice. That approach has been extended and a new ice thickness redistribution model has been formulated where the pack ice is decomposed to open water, two different type of undeformed ice, and rafted, rubble and ridged ice. The ice thickness distribution model has been included to a coupled ice-ocean model and numerical experiments has been made for the simulation of the Baltic Sea ice season. The benefits of the extended ice classification are a separation of the thermally and mechanically produced ice and a better description of the minimum ice strength. Also different thermodynamic growth/melting rates of the ice types can be introduced to the model, hence giving more detailed seasonal evolution of the pack ice. In addition, the six level ice thickness distribution model gives more information about the surface properties (surface temperature, albedo, roughness) of pack ice.
P13/W/02-B2 1050
OPEN-OCEAN DEEP CONVECTION DUE TO THERMOBARICITY
Kazunori AKITOMO (Department of Geophysics, Kyoto University, Kyoto 606-8502, JAPAN, Email: akitomo@kugi.kyoto-u.ac.jp)
Properties of open-ocean deep convection due to thermobaricity at high latitudes have been investigated. Scaling argument shows that two types of open-ocean deep convection are possible. The first type appears in a homogeneous ocean, or in a deepening mixed layer. The increased apparent buoyancy flux due to thermobaricity makes the scales of convective properties larger than those without thermobaricity. The ratio of thermobaric to nonthermobaric scales is determined only by the ratio of the nonthermobaric convective size to the characteristic depth for thermobaricity to be effective. In a nonrotating frame, then, thermobaricity becomes effective when the ocean depth is comparable with the characteristic depth, as previous studies showed. In a rotating frame, thermobaricity is most effective with slow rotation and low temperatures. In the actual situation, thus, thermobaricity is not so effective even in polar oceans because the earth rotation confines the convective size to a smaller level. The second type of deep convection is driven by pure thermobaric instability in a two-layer ocean where a cold, fresh mixed layer overlies a warm, saline deep layer, as often observed in polar oceans. It causes an abrupt overturning of the water column. Scales of convective properties are dependent on the difference of water temperature between the two layers, the initial size of plume and the Coriolis parameter (or time), but not on the surface cooling rate directly. With actual parameters, convective properties and the associated buoyancy flux are much larger than those of the first type. Observed vertical profiles of water temperature and salinity suggest that the first type of deep convection could occur in the Greenland Basin while the second type in the Weddell Sea. Numerical experiments with a three-dimensional nonhydrostatic model confirms the above results based on the scaling argument except that the convective size is determined by the product of characteristic scales of velocity and time whether the ocean is shallow or deep, rotating or nonrotating.
P13/W/12-B2 1110
REGIONAL FOCUS OF A GLOBAL TIDAL MODEL: THE POLAR REGIONS
F. LEFEVRE (email: Fabien.Lefevre@cnes.fr), F. Lyard (email: Florent.Lyard@cnes.fr), C. Le Provost (email: Christian.Le-Provost@cnes.fr) (all at LEGOS/GRGS, UMR5566 CNES-CNRS-UPS, 14 Avenue E. Belin, 31400 Toulouse, France)
The CEFMO hydrodynamic model and the associated data assimilation code is the only one global model to include the ice-shelf covered seas like the Weddell Sea, the Ross Sea, or the Amery ice shelf basin. A new version of our hydrodynamic finite element tide model combined with a revised data assimilation procedure is now available, referred as the FES98 solutions. These global solutions, including the polar regions which are not included in the altimetric empirical models, show now an overall accuracy comparable with the one of the latter. The modelling of the tides in the polar regions is made more difficult due to the lack of accurate data, especially on the ocean bottom topography. The situation is even more complicated below the permanent ice shelves. Nevertheless, a significant effort was made to prescribe the correct water column height and grounding line in these regions. Its finite element discretization allows a realistic modelling in the coastal areas (with horizontal resolution ranging from 10 to 1 kilometer). The model output are the tidal elevation and currents. The tides can interact with many processes, such as, among others, the ice melting below the ice shelves (through a background turbulence) or transport, grounding line variations, the water mixing etc... Actually, the regions with intense tidal dissipation are known to show strong water mixing. In addition, the tides can be responsible of a mean transport, which is somehow connected to the tidal energy flux. A collection of model zooms on regions of interest are presented, with a particular focus on ice covered seas, shelf edges and energetically active regions.
P13/P/01-B2 1130
ICE AND SALT TRANSPORTS IN THE WEDDELL SEA
Sabine HARMS, Eberhard Fahrbach, Volker H. Strass (all at Alfred Wegener Institute for Polar and Marine Research, Postfach 12 01 61, 27515 Bremerhaven, Germany, email: sharms@awi-bremerhaven.de).
Time series of ice thickness in the Weddell Sea are evaluated together with hydrographic observations and ice drift for estimation of the freshwater fluxes into and out of the Weddell Sea. Ice draft is measured with moored Upward Looking Sonars (ULS) since 1989 along two transacts across the Weddell Sea. One transect, extending from the tip of the Antarctic Peninsula to Kapp Norvegia, covers the flow into and out of the southern Weddell Sea. The other transect, extending from the Antarctic continent northward along the Greenwich Meridian, covers the exchange of water masses between the eastern and the western Weddell Sea. On average wind and current force a westward ice transport in the east into the Weddell Sea and a northward ice transport in the west out of the Weddell Sea. Near the coast, in the southern Weddell Sea and along the tip of the Antarctic Peninsula, offshore winds push ice away from the coast and new ice is continuously formed in these regions, indicated by low ice draft modes (< 0.3 m) and high surface salinities (> 34.25). When offshore winds weaken or reverse to onshore ice is deformed. In the presence of opposing currents and the coast the ice thickness increases. On its way along the southern part of the Weddell Gyre the ice grows continuously. The mean ice draft increases from 1.6m in the eastern inflow at the Greenwich Meridian to 2.2m in near Kapp Norvegia and 2.7m in the western outflow. The mean ice export out of the Weddell Sea is (57±21) x I03m3 s- 1, corresponding to a net freshwater export due to drifting sea ice of (0.06±0.02) Sv. This freshwater export exceeds the average amount of freshwater gained by precipitation (Bromwich and Cullather,1997) bv a factor of three. The net amount of salt gained by sea ice formation is (15±7) x 105 kg s-1.
P13/L/01-B2 1150
A LINK BETWEEN SEA ICE EXPORT FROM THE ARCTIC AND THE NAO
Bruno TREMBLAY (Lamont-Doherty Earth Observatory of Columbia University, USA)
In the past decade, an increasing trend in the North Atlantic Oscillation (NAO) index has been observed. A high NAO index will influence the strength of the westerlies in the mid-latitude of the North Atlantic and could also result in different atmospheric circulation pattern in the Arctic. Since regional and large scale anomalous wind patterns in this region can produce fluctuations in ice export from the Arctic through Fram Strait, a link between the state of the NAO and the export of ice out of the Arctic is possible. A dynamic-thermodynamic sea-ice model is used to study the interannual variability of the sea ice export from the Arctic through Fram Strait during the 1958-98 period. The model is forced with monthly varying wind stresses and air temperatures from the NCEP reanalysis data. The simulation results show a significant correlation between the ice export and the NAO index time series. When the NAO index is positive, the Icelandic Low is extending further north into the eastern Arctic and the Arctic High is located closer to the Beaufort Sea. During those periods, the Transpolar Drift Stream (TDS) is well developed, leading to a large ice export from the Arctic. During a negative phase of the NAO, the Arctic High moves closer to the Siberian coast and the TDS tends to feed into the Beaufort Gyre leading to a smaller ice export. During these periods, the atmospheric circulation is such that more ice is found in the East Siberian Sea (at the source of the TDS); these ice thickness anomalies are advected out of the Arctic during subsequent periods of high exports, tending to reinforced the ice export anomalies.
Tuesday 27 July PM
Presiding Chairs: P. Wadhams (Scott Polar Res. Institute, Univ. of Cambridge, UK),
A.J. Willmott (Dept. of Mathematics, Keele Univ., UK)
P13/E/02-B2 Invited 1400
SATELLITE OBSERVED SEASONAL AND INTERANNUAL VARIABILITY OF THE ODDEN FROM 1973 THROUGH 1997 AND ENVIRONMENTAL EFFECTS
Josefino C. COMISO (Laboratory for Hydrospheric Processes, Code 971, NASA/GSFC, Greenbelt, MD, USA, 20771, email: comiso@joey.gsfc.nasa.gov), Peter Wadhams (Scotts Polar Research Institute, University of Cambridge, Cambridge, England, email: pw11@cam.ac.uk), and Lef Toudal Pedersen, Technical University of Denmark, Dept. of Electromagnetic Systems, Bygning 348, DK-2800 Lyngby, Danmark, email: ltp@emi.dtu.dk)
The Odden is a regular ice feature phenomenon in the Greenland Sea that is believed to have a profound influence on the convection and circulation characteristics of the ocean in the region. Although it had been observed and reported by early explorers, it was not until the satellite era that it's size, shape, and characteristics became apparent. It usually occurs early in the winter and forms as an ice tongue, a bulge, or an island depending on atmospheric and oceanographic forcing. The most frequently observed development of the Odden is as an area of locally-grown frazil and pancake ice along the cold polar Jan Mayen Current which diverts eastward from the East Greenland Current. Such composition has been confirmed during field programs carried out in this region. However, on some occasions, the ice cover in the Odden has been observed late in the season to be made up of multiyear or old ice advected from the Arctic Ocean through the Fram Strait and the Greenland Sea. Our study makes use of satellite passive microwave historical data to investigate the seasonal and interannual variability of the size, extent, and type of the Odden since 1973. High resolution satellite SAR and AVHRR data was used to improve our interpretation of the passive microwave data, the resolution of which is relatively poor. Our analysis reveals large interannual variability in time of occurrence, persistence, and type. Also, the Odden appeared every year except in 1984, 1994, and 1995. We observed that the size and extent varies with wind and surface temperature but the correlation of the distribution of the ice cover with the bathymetry is weak although the feature is usually along the Jan Mayen fracture zone. Analysis of the influence of environmental effects that sometimes cause transistions from a bulge feature to an ice tongue, and from an ice tongue to an island will be presented. Also, the potential impact of rapid ice growth to convection and of the presence of multiyear and old ice in late spring to the carbon cycle in the region will be discussed.
P13/E/16-B2 1440
SATELLITE-DERIVED DYNAMICS OF SOUTHERN OCEAN SEA ICE
Mark R. DRINKWATER and Xiang Liu (Jet Propulsion Laboratory MS: 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109, USA, email: mrd@pacific.jpl.nasa.gov)
Antarctic ERS-2, RADARSAT Synthetic Aperture Radar and ERS-1/2 Scatterometer images were analyzed with SMMI radiometer image time-series data to investigate seasonal variability in satellite-tracked sea-ice dynamics in the Southern Ocean during 1992. Supporting field data were acquired during 'in-situ' experiments including the winter 1992 Ice Station Weddell and Winter Weddell Gyre studies. A variety of surface measurements were made during these experiments including Argos-buoy deployment and GPS drift measurements. These are used in conjunction with International Program for Antarctic Buoys drift trajectories for ice-motion tracking validation. Comparisons between gridded SSMI ice-motion vectors and ECMWF/NCEP analyses indicate that large-scale drift is forced predominantly by the long-term mean, large-scale synoptic pressure field. Only sub-daily SAR sea-ice tracking can capture high-frequency fluctuations, driven by polar lows or tidal forcing. In these cases, sea-ice drift can respond rapidly to changes in forcing on semi-diurnal time scales depending on the location with respect to the coastline. Seasonality of ice drift, particularly in the Weddell and Ross Seas, is linked to ice extent and compactness, and internal ice stresses transmitted through the pack ice from the coast. Three-monthly seasonal climatologies are presented of austral winter of ice drift in the Southern Ocean. The large Weddell and Ross Sea gyres are clearly resolved along with key seasonal and spatial attributes of their cyclonic circulation. Regional time series of ice dynamics parameters are used to illustrate correlations with meteorological forcing. Persistent divergence such as that occurring in the Ronne-Filchner polynya system results in large fractions of new ice. Similarly, convergence zones produce large fractions of deformed ice and characterize the dynamics of regions where perennial ice is observed. High shear strains also help delineate the axis of the Antarctic divergence in many places in the ice cover. In these regions, the separation between the coastal 'east wind drift' and ACC-dominated drift regimes is characterised by zonally extended regions of intense shear.
P13/E/15-B2 1500
SEA ICE THICKNESS DERIVED FROM ERS-1 AND 2 ALTIMETER OBSERVATIONS
Neil PEACOCK, Seymour Laxon (University College London 17-19 Gordon Street, London, WC1H 0AH, UK)
A new technique to directly measure sea ice freeboard from ERS-1 and 2 spaceborne altimetry has recently been developed. This offers the potential to provide up to 250,000 estimates of ice thickness per month over a large fraction of the Arctic Ocean. There is a critical need to calibrate and validate these thickness estimates prior to their widespread application to climate-related problems.
In this paper, we present the results of comparisons of these measurements, obtained between 1993 and 1998, with in situ Upward-Looking Sonar (ULS) measurements from moored buoys in Fram Strait, and ULS measurements made from US Navy submarines. The statistical properties of the thickness data sets acquired using these methods will also be investigated, and estimates of the corresponding errors in our thickness measurements will be derived.
Potential applications of this unique data set will then be discussed, which include the monitoring of ice thickness in the Arctic Ocean and Fram Strait, and the ability to estimate ice flux from the Arctic Ocean into the Greenland Sea. Future spaceborne altimeter missions, such as ENVISAT, will allow observations of this important climatological parameter well into the next century.
P13/W/14-B2 1520
POLAR SEA ICE MOTIONS CHARACTERIZED FROM 20 YEARS OF PASSIVE MICROWAVE SATELLITE IMAGERY
W.J. EMERY, C. Fowler, J.A. Maslanik (CCAR Box 431, University of Colorado, Boulder, CO 80309, USA)
An approximately 20 year time series of daily passive microwave maps of polar sea ice have been constructed using the Special Sensor Microwave/ Imager (SSM/I) for the years 1987-1997, and the Scanning Multichannel Microwave Radiometer (SMMR) for the years 1979-1987. While the sensor channels and the specific ice concentration algorithms are different for these two sensors the basic imaging of the ice surface as used for the computation of sea ice motion, is quite similar between the two instruments. Arctic sea ice motions are analyzed in conjunction with ice trajectories from the Arctic ice buoy program while the Antarctic ice motions are based on the passive microwave imagery alone. Means are calculated for the entire time period, for both decades and then for individual years to characterize the interannual variability of the ice motion fields. Standard deviations for these same periods reveal where the ice movements are most variable. Empirical orthogonal functions (EOFS) are computed from the ice motion maps to indicate the basic patterns of ice motion. These patter are found to correlate well with structures of the atmospheric pressure and hence geostrophic wind patterns. The EOFS for the Arctic and the Antarctic are quite different reflecting the very different forcing functions operating in the Antarctic.
P13/E/13-B2 Poster 1600-01
THE ESTIMATIONS OF SEA ICE EXCHANGE BETWEEN THE ARCTIC SHELF SEAS AND THE ARCTIC BASIN
Alexander MAKSHTAS and Sergey Shoutilin (both at Arctic and Antarctic Research Institute, 38 Bering Street, St.Petersburg, 199397, Russia, email: maksh@aari.nw.ru)
Sea ice exchange between the Barents, Kara and Laptev seas and the Arctic Basin was investigated using a large-scale dynamic-thermodynamic model ( for period 1958 - 1997 years ) and semi-empirical method, based on empirical relation between atmospheric surface pressure gradient and drift velocity (for period 1936 - 1996 years ). Estimates of sea ice exchange of the Laptev sea, calculated using empirical relation between ice flux and air pressure difference at polar stations Kotelnyy and Cape Cheluskin, showed strong seasonal and interannual variability with an absolute minimum of sea ice export to the Arctic Basin in 1957 year and did not reveal significant trends for the entire period.In opposite estimates for the Kara sea, obtained with the same method and atmospheric surface pressure data from meteorological stations Cape Arcticheskiy and Ostrov Rudolf, showed strong positive trend of ice export to the Arctic Basin, especially during last three decades, when sea ice export from the Kara sea almost reached values from the Laptev sea. The dynamic-thermodynamic model, validated with empirical method for sea ice area exchange allowed to estimate sea ice mass exchange and to made some numerical experiments on sensitivity of the Arctic Basin sea ice cover to the hypothetical Arctic climate changes. This experiments showed strong dependence of sea ice mass exchange to increasing of atmospheric surface layer temperature and incoming longwave radiation.
P13/E/08-B2 Poster 1600-02
INTERANNUAL AND DECADAL VARIABILITY OF THE GIN/BARENTS SEAS SYSTEM
S. PIACSEK, A.Warn-Varnas, R.Allard (Ocean Science Division, Naval Research Laboratory, Stennis Space Center, MS, 39529, USA; email:piacsek@nrlssc.navy.mil); A.Mehra, D.Dietrich (CAST, Mississippi State University, Stennis Space Center, MS, 39529)
The variability of the Greenland-Iceland-Norwegian (GIN)/ Barents Seas system has been investigated via analysis of hydrographic data and numerical models. Data has been extracted mainly from the NODC's WOA94 database and from 9 successive cruises by the SACLANT Research Center in the 1986-1993 period. A watermass census was performed for regions where the data density warranted it.
Model results were generated with two models, one spherical including the North Atlantic and one polar stereographic including the whole Arctic Basin. Forcing was derived from the NCEP reanalysis files (2.5 deg) and the US Navy's NOGAPS GCM output (1 deg).
The model results revealed an increased inflow of Atlantic water (AW) into the Arctic Basin during the 90's, with the dominant circulation patterns and transport magnitudes shifting from the West Spitsbergen Current to the Barents Sea branch flowing north along the Novaya Zemlya coast. A recirculation of AW southward along the east coast of Svalbard, very prominent in the '80s, almost disappeared except for 1994. The outflow of deep water (DW) from the Arctic through the Fram Strait consisted of two branches, one fed by waters coming from the Eurasian Basin and one from the Canadian Basin. The hydrographic data analysis revealed a gradual decrease of AW until 1989, and then a gradual increase in the 90's.
The DW exhibited an opposite behavior, peaking in 1989 and then decreasing. Furthermore, the DW constitution changed from Norwegian Sea Deep Water (NSDW) to a class warmer than -.5 deg C.
P13/E/03-B2 Poster 1600-03
SPATIAL AND TEMPORAL VARIABILITY OF WATER MASSES IN FRANZ-VICTORIA TROUGH
Ivanov V.V., Korablev A.A. (Arctic and Antarctic Research Institute, 38 Bering st., 199397, St.Peterburg, Russia, vivan@ aari.nw.ru); PISAREV S.V. ( P.P.Shirshov Institute of Oceanology, RAS, 36 Nachimovsky ave.,117218, Moscow, Russia, sergey@pisarev.msk.ru)
The WODB-98, MOODS, local Russian and Norwegian databases, recent measurements of RV "Polarstern" in 1993 and RV "Lance" in 1993,94,95 were used to describe the spatial and temporal variability of water masses in Franz-Victoria Trough (between the Victoria Isl. and Franz Joseph Land). The data obtained during the multidisciplinary research cruise carried out in October 1998 from the board of Russian RV "Ak. Fedorov" were used also. About 750 stations were collected for 1923-1998. The water masses of the Franz-Victoria Trough were classified based mainly on the T,S characteristics. It was noted, that the main features of spatial distribution of the water masses of the region were determined by bottom relief. The conclusion concerning general circulation within the Trough was made. The interannual variability of atlantic water temperature, heat content, and spreading was also determined.
P13/W/06-B2 Poster 1600-04
DEVELOPMENT OF AN IMPROVED DYNAMIC-THERMODYNAMIC SEA ICE THICKNESS DISTRIBUTION MODEL FOR CLIMATE APPLICATIONS
Todd ARBETTER (McGill University, Montreal, Quebec, CANADA); Judy Curry, Julie Schramm (University of Colorado,Boulder, Colorado, USA)
One of the major challenges in climate modeling is development and implementation into general circulation models of a sea ice model that accurately predicts sea ice mass balance, ice extent, interfacial fluxes, and the associated feedbacks with the atmosphere and ocean. Initially, general circulation models contained simple parameterizations of sea ice thermodynamic and dynamic processes. The development of sophisticated stand-alone ice dynamic models facilitated improvement in the treatment of ice dynamics in GCMs. However, while there has been substantial progress in the development of single-column thermodynamic sea ice ice models, little work has been done to unify sophisticated thermodynamics and dynamics into a single model.
Towards this end, a new sea ice model is described which incorporates the sophisticated thermodynamics of a single-column sea ice model developed at the University of Colorado into an existing basin-scale dynamic-thermodynamic model. Using a viscous-plastic dynamic model and an ice strength parameterization which accounts for a distribution of sea ice thicknesses, the model resolves a domain covering the Arctic Ocean and much of its surround seas. Beneath the ice at each grid cell is an interactive ocean mixed layer. A preliminary comparison of baseline characheristics of the model with observations indicates that the new model performs reasonably well in terms of its reproduction of ice surface albedo, ice surface temperature, and ice thickness distribution.
P13/W/09-B2 Poster 1600-05
CHANGES IN THE NORTH SLOPE CLIMATE AND THE SEA ICE CONCENTRATION IN THE ADJACENT BEAUFORT SEA
Gerd WENDLER and Blake Moore (both at Geophysical Institute, University of Alaska, Fairbanks. 903 Koyukuk Dr., Fairbanks, AK, 99775-7320 USA, email: gerd@gi.alaska.edu)
Significant climate change has been observed in recent decades in the Western Arctic. Two climate variables in particular show this quite clearly; temperature and sea ice concentration. Western Arctic temperatures have increased; the observed temperature rise, however, varies significantly from one season to another (it is most pronounced in the winter and spring) and over multi-year time scales. This makes it difficult to explain the trend as a direct consequence of increasing greenhouse gases in the atmosphere. At the same time, the sea ice concentration has decreased in the southern Beaufort Sea. Temperature variations were found to correlate with changes incloudiness associated with specific synoptic patterns. Increased cloudiness was correlated with a warming of the temperature in winter, but with a cooling in summer, indicating that the temperature regime in winter is strongly influenced by the long wave radiation.We were able to analyze systematically the area in the southern Beaufort Sea using a portion of the international Sea Ice Grid (SIGRID) produced by National Ice Center. Weekly sea ice concentration consisting of 0.25 degree latitude by 0.50 degree longitude grids derived from satellites for the period 1972-1994. We limited our preliminary study to the coastal Beaufort Sea south of 72° North, and between 142° and 152° West. There are two main processes which influence the sea ice conditions, namely thermodynamics and dynamics. The thermodynamics determines the formation and decay of the sea ice and is directly related to the surface energy budget, while the dynamics determines the transport of sea ice, which is strongly related to atmospheric dynamics. Other factors such as ocean currents also play a major role.
The annual average ice concentrations shows a decreasing ice concentration. Forcing the best linear fit through the data points, a decrease in the yearly average sea ice concentration from over 88% to 81% can be observed for the 24 year time period. A temperature increase of about 1°C was also observed during this time. If the annual temperature values are plotted against the annual values of the sea ice concentration, a good statistical variance (r2 = 0.48) was found. For shorter time periods, especially during times when the ice concentration can vary considerably (e.g. fall), the relationships become even better. This presentation will concentrate on the underlying causes of these changes which are not well understood. The primary forcing mechanisms that underlie these changes will be discussed. The observed trend of decreasing sea ice concentration is continuing. This fall (1998) the lowest ice concentration since the start of the record has been observed.
P13/W/18-B2 Poster 1600-06
THE ROLE OF TRANSIENT EDDIES IN ACC DYNAMICS IN CROZET PLATEAU REGION OF THE SOUTHERN OCEAN
Vladimir IVCHENKO (Jet Propulsion Laboratory/NASA, M/S 300-323, 4800 Oak Grove Drive, Pasadena, CA 91109, USA, Email:voi@sundog.jpl.nasa.gov)
A vigorous eddy field is observed in TOPEX altimeter data around the Crozet Plateau, especially around the northern branch and downstream of the Plateau. The important question that is addressed in this study is how transient eddies affect the mean flow? This problem has been studied using the output of two fine resolution numerical models: the Fine Resolution Antarctic Model (FRAM) and the Parallel Ocean Programm (POP). The level of eddy kinetic energy (EKE) throughout the region is much greater in POP than FRAM, due to better horizontal resolution.
Although maximum values in FRAM are just as high as in POP, these values are restricted around the northern flank and downstream of the Plateau. The most important source of the EKE is the eddy buoyancy term. In both models instability analysis showed that baroclinic instability is likely to be the main mechanism responsible for generating EKE. Transient eddies are found to be responsible in shaping the flow structure and transferring momentum in both the Horizontal and vertical directions. The eddy flux of the potential vorticity has been split into rotating and divergent parts by solving a Poisson-type equation, using iteration schemes with appropriate boundary conditions. The rotational part is found to be quite large in the western and north-western part of the Crozet storm track.
P13/W/17-B2 Poster 1600-07
BEAUFORT AND CHUKCHI SEA SEASONAL VARIABILITY
Tatiana Proshutinsky and Andrey PROSHUTINSKY (Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK 99775-7220, USA e-mail: prosh@ims.alaska.edu) James Maslanik (Colorado Center for Astrodynamics Research, University of Colorado, Boulder, CO 80309 USA, e-mail: jimm@northwind.colorado.edu)
Arctic navigation, oil and gas exploration, and arctic pollutant transport depend on arctic environmental conditions. Existing atlases, manuals, and reference books contain multi-year mean environmental variables and their multi-year mean seasonal variability; however, uncertainties sometimes result from the existing atlases because they do not take into account climate change and climate variability. Our work is motivated by the recent finding of two regimes (or two climate states) of arctic atmosphere--ice--ocean circulation described by Proshutinsky and Johnson [1997]. Seasonal variations in the ice concentration, ice thickness, and ice drift, ocean currents, ocean temperature, and salinity, atmospheric pressure, wind speed, cloudiness, and precipitation, river discharge, and permafrost temperature are different for cyclonic and anticyclonic arctic climate states. In this poster we present the atmospheric, ice, oceanic, and terrestrial signals showing seasonal variability of environmental parameters during cyclonic and anticyclonic climate states in the Beaufort and Chukchi seas using observational data and results of numerical modeling.
P13/W/20-B2 Poster 1600-08
THE COUPLED ATMOSPHERIC BOUNDARY LAYER- SEA ICE - UPPER LAYER - DEEP OCEAN MODEL FOR INVESTIGATION OF SEASONAL, INTERANNUAL AND INTERDECADE VARIABILITY OF WATER CIRCULATION AND TERMOHALINE STRUCTURE IN THE ARCTIC AND ATLANTIC OCEANS
G.A. SEMYONOV (Arctic and Antarctic Research Institute, 38 Bering St., St-Petersburg, 199397, Russia)
For simulation and investigation of climate large-scale circulation evaluation in the Atlantic Ocean (AO) and the Arctic basin (AB) the three-dimensional efficient hydro-thermodynamical model, based on the primitive equations with used of a hydrostatics and Boussinesque approximation with free surface, was built. The calculating area includes the regions from the equator to the Bering strait. To improve the finite-difference approximation in the Denmark, Frame and Faeroe-Shetland straits the spherical coordinate system with pole placed in point 75 N & 40 W (in Greenland) was designed. The ocean model has a variable resolution from 28 km (Greenland Sea) to 312 km (near equator). The model uses a total of 47 levels in the vertical. The method of solving equations this model is based on the split method. The atmospheric boundary layer is parametrized with the help of integrated model. The model of n ice cover is based on the primitive equations of dynamics both thermodynamics of ice and snow. The upper mixed - layer of ocean is parametrized by using the K - theory, that allows to calculate coefficients of vertical turbulent exchange of momentum, heat and salt by solving the equation of balance of kinetic turbulent energy. Application of the effective non-explicit numerical schemes allows to execute calculation using time steps equal 24 hour for period of 100 years and more. The model reproduces three-dimensional circulation of waters, evolution of a sea surface elevation, termohalin of structure, annual course of variability of an ice cover, distribution of a firm suspension and various pollutions both on a surface of the sea and in thickly waters at the task of temperature of air and height of geopotential of 850 mb of a surface above the AB and AO.
P13/W/04-B2 Poster 1600-09
NEW OBSERVATIONS FROM BOTTOM PRESSURE RECORDERS IN THE WEDDELL SEA
M.J. SMITHSON (CCMS-Proudman Oceanographic Laboratory, Birkenhead, Merseyside, L43 7RA, U.K., e-mail msm@ccms.ac.uk)
Recent studies of the southern Weddell Sea using a hydrodynamic numerical tide model have shown that it is not possible to model simultaneously the tides of the open ocean and the tidal motion of the ice shelves (Ronne and Filchner). Specifically, in order to reproduce the ice shelf movement near the grounding line unrealistic model parameters in the region of permanent ice cover, in particular the bottom friction coefficient, must be used. The effect of this is to destroy any agreement between model and observations in the open ocean, especially near to the permanent ice front. Existing observational data indicate the presence of an amphidrome in the principal semidiurnal tides close to the Ronne Ice Shelf but its position is uncertain. Its existence is also borne out by the model results and its position is very sensitive to model parameters.
A number of bottom pressure recorders were deployed in the region at the beginning of 1998, from H.M.S. Endurance. These are due to be recovered in January/February 1999. The data from them should provide valuable information to help fix the location of the amphidrome and hence reduce significantly the uncertainties in the model parameters.
P13/W/15-B2 Poster 1600-10
A STRONG FRONT OVER THE NORTHERN ARCTIC MID-OCEAN RIDGE
Robin D. MUENCH, John T. Gunn (Earth & Space Research, 1910 Fairview East, Suite 102, Seattle, WA 98102-3620, USA, email rmuench@esr.org); Tim Boyd (College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, 97330, USA, email tboyd@oce.orst.edu)
Oceanographic temperature (T), salinity (S) and dissolved oxygen concentration (O2) were measured continuously along a closely spaced array of transects that were made across and roughly normal to the northern portion of the Arctic Mid-Ocean Ridge system from the U.S. Navy submarine Hawkbill. Additional T and S data were collected as vertical profiles at selected sites using expendable probes. The transects were run at a sensor depth of 212m, and the vertical profiles extended from about 20m down to 1000m depths, providing detailed documentation of the Atlantic Water layer. A strong T and S front was seen to coincide approximately with the axis of the Ridge. Maximum Atlantic Water T increased from about 1.1°C to nearly 2.3°C across the front from north to south, and S increased by about 0.06 psu. The frontal distributions of T and S showed complex wavelike undulations and discrete features. Some were consistent with the presence of mesoscale eddies, while others appeared to be associated with the bottom topography. An eastward flowing current filament about 20 km wide coincided over part of the system with the southern slope of the central rift valley. Current speeds in this filament were estimated based on offsets in the cruise track to exceed 25 cm/s. Neither the high T nor the eastward current is consistent with the concept of a cyclonic gyre occupying the Nansen Basin. The eastward current, whose presence is consistent with topographic control by the steep axial rift valley, may be an extension of a continuous northward flow through Fram Strait. The high T suggests, also, a more direct route from Fram Strait than would be consistent with flow within a large cyclonic gyre.
P13/W/13-B2 Poster 1600-11
MODELLING OF THE SEASONAL DYNAMICS OF THE WATER MASSES, ICE AND RADIONUCLIDE TRANSPORT IN THE KARA SEA
Liudmila Koziy, Vladimir Maderich, Nugzar MARGVELASHVILI, Mark Zheleznyak (all at Institute of Mathematical Machine and System Problems NASU, Glushkova pr. 42, Kiev 252187, Ukraine.)
The numerical THREETOX code was used to simulate 3-D hydrodynamic fields, ice transport, suspended sediment transport and the dispersion of the radionuclide in the Kara Sea and adjacent areas of the Barents Sea and Arctic Ocean. The code includes the hydrodynamics sub-model, ice sub-model, suspended sediment transport and radionuclide transport sub-models. The hydrodynamics is simulated using the, time-dependent, free surface, primitive equation model. The modified Hibler type dynamic-thermodynamic model describes the momentum balance, nonlinear ice rheology, mass balance and the ice strength. The ice thickness distribution is a two-level representation (compactness and ice thickness). An important role of seasonally varying ice cover in the seasonal dynamics of the Kara Sea was shown. At summer, the western and northern flows of the Ob and Enisey plume are transformed in the slope current that flows to the Severnaya Zemlya. Another current flows along the north coast of the Novaya Zemlya. The residual tidal currents are important in narrows (e.g. Karskie Vorota Strait and Ob and Enisey estuary mouths). The water exchange through the Karskie vorota is bi-directional. At winter the ice sheet covers almost all area of the Kara Sea. The strong current flows from the Karskie Vorota and merges with Ob-Enisey flow in the northern part of the sea. The numerical results agree quite well with the observations. These peculiarities of the Kara Sea circulation strongly affected on the radionuclide transport. Weak and seasonally varied currents near Novaya Zemlya coast result in the long residence time for radionuclides dumped in the Novaya Zemlya fjords and the Novaya Zemlya trough.
P13/W/21-B2 Poster 1600-12
ON PHYSICAL NATURE OF LARGE-SCALE ANTICYCLONIC GYRE IN THE NORWEGIAN SEA WATER DEPTH
Anatoly PERESKOKOV (Russian Research Institute of Hydrometeorological Information - World Data Center, 6 Korolyov St, Obninsk 249020, Russia, email: peres@meteo.ru)
Anticyclonic vorticity in the thick layer of deep water over Lofoten hollow in the Norwegian Sea is a paradox, especially in view of the cyclonic winds of the Icelandic lows, and the fact that neither the density field nor the available measurements suggest an anticyclonic circulation of the surface waters. No doubt, the topography largely determines the characteristic features of the stationary circulation of the Norwegian Sea waters. However, the circumstantial evidence suggests that an important role may be attributed to a redistribution of the Atlantic waters heat from upper layers to the lower ones through salt finger convection. Our calculations show close agreement between the area with the highest likelyhood for salt fingers, given through the density ratio, and the region of the relative temperature maximum at every horizontal surface of 800-1500 m water thickness. The centre of the temperature maximum is quasi-stationary in this deep layer near 69,5 N, 3,5 E. A quite natural response to the generation of this quasi-stationary horizontal inhomogeneity of physical and topographical origin (with less density) is the initiation of an anticyclonic circulation. Since variations in the gire intensity can not but influence the transfer of the Atlantic waters heat to the higher latitudes, the monitoring of the parameters of dynamic and thermal regime at some certain depths could possibly be the positive contribution to the ice prediction.
P13/W/23-B2 Poster 1600-13
THERMAL INSTABILITY OF WATER SATURATED SEA ICE SHEET BOUNDED BY MELT POND
Pjotr BOGORODSKY (Arctic & Antarctic Research Institute, 38 Bering str., 199397 St.Petersburg, Russia, email vivanv@aari.nw.ru)
Fresh melting water accumulates during summer period in the melt ponds at the ice surface and percolate it forming so-called "underice melt ponds". Heating from above by radiation flux can induce convection resulting intensifying processes of air-sea energy and mass exchange. However, the permeability of intermediate ice sheet provides interaction of fresh liquid layers by the way that all they form unit combine sysrem. At the present work the analysis of its convective stability is performed. The external boundaries of the system are considered as free or hard surfaces with constant heat fluxes. The critical values of Rayleigh numbers depending of determining parameters (coefficients of heat diffusivity of ice and fresh water, thickness and permeability of ice) are found out from the analysis of linearized spectrum problem. At dedaction of equations for eigenvalues the method of series expansion is used. The study is restricted by the longwave instability of monotonic perturbations. The simplicity of such an approach allows to easy transform to extreme cases cases of approximation of dimensionless ice thickness to zero (unit). In these cases the expression obtained for critical Rayleigh number transforms to the known value for single liquid (porous) layer with heat insulated boundaries. When summerizing results it may be concluded that 1) the dynamical boundary conditions on the system outer surfaces do not principally affect the instability character; 2) the ice sheet permeability changes at its fixed thickness do not influence on the value of critical Rayleigh numbers; 3) the increasing ice sheet thickness lead to significant stabilization on the longwave mode of instability.