The warm-water “Halo” around Maud Rise: Properties,dynamics and Impact

Regional hydrographic and current observations from the 2005 MaudNESS winter field campaign in the Maud Rise seamount region of the eastern Weddell Sea show that an annular Halo consisting largely of Warm Deep Water (WDW) encircled the Rise at depths just below the mixed layer. The Halo was associated with elevated isopycnals and, on the northern flank of the Rise, strong subsurface velocities up to 20 cm s⁻¹. Intercomparison of these observations with winter 1986 and 1994 conditions confirms the presence of the Halo and suggests that it, and associated warm pools west of the Rise, are at least semipermanent features of the region. These observational results compare well with the output from an isopycnic ocean model for a variety of parameters including shape of the seamount, inflow conditions and vertical stratification. The model captures processes associated with a steady westward flow impinging on the isolated seamount and shows (1) that the dynamics of the warm-water Halo with a shallow mixed layer are related to the formation of a jet surrounding the Rise and the overlying Taylor column and (2) that eddies of alternating sign (cyclones and anticyclones) are formed from instability of the jet-like flow structure, and are subsequently shed from the western flanks of the Rise. The eddies closest to the rise are dominated by cyclones which tend to adhere to the flanks more strongly than anticyclones. The formation and passage of approximately 3–5 eddies per year is seen in the sea-surface-height anomalies over a 12-year period. Despite apparent spatial and temporal variability in the dynamics of the Halo and shedding of eddies, the time-mean picture is such that significantly elevated isopycnals with WDW below the mixed layer are always present on the flanks of Maud Rise. This mechanism likely contributes annually to earlier seasonal ice loss in the eastern Weddell Sea than farther west. For unusually strong inflow conditions, possibly due to large-scale interannual variability, the Halo becomes more intense and overlies a much larger part of Maud Rise, potentially preconditioning the area for deep ocean ventilation and a subsequent polynya event such as observed in the 1970s.     

Formation mechanism of the Weddell Sea Polynya and the impact on the global abyssal ocean

An experiment using a global ocean–ice model with an interannual forcing data set was conducted to understand the variability in the Southern Ocean. A winter-persisting polynya in the Weddell Sea (the Weddell Polynya, WP) was simulated. The process of WP breaking out after no-WP years was explored using the successive WPs found in the late 1950s. The results suggested that the anomalously warm deep water, saline surface layer, and a cyclonic wind stress over the Maud polynya region in early winter are essential for the surface layer to be dense enough to trigger deep convections which maintain a winter-persisting polynya; also, the reanalyzed surface air temperature (SAT) over the observed polynya region is too high for an ocean–ice model’s bulk formula to yield sufficient upward heat fluxes to induce WP formation. Therefore the Weddell Polynya, a series of WPs observed from satellite in the mid-1970s, is reproduced by replacing the SAT with a climatological one. Subsequent to the successive WP events, density anomalies excited in the Weddell Sea propagate northward in the Atlantic deep basins. The Antarctic Circumpolar Current (ACC) is enhanced through the increased meridional density gradient. The enhanced ACC and its meandering over the abyssal ridges excite buoyancy anomalies near the bottom at the southwestern end of the South Pacific basin. The buoyancy signals propagate northward and eventually arrive in the northern North Pacific.     

On the parameterisation of oceanic sensible heat loss to the atmosphere and to ice in an ice-covered mixed layer in winter

In high-latitude oceans with seasonal ice cover, the ice and the low-salinity mixed layer form an interacting barrier for the heat flux from the ocean to the atmosphere. The presence of a less dense surface layer allows ice to form, and the ice cover reduces the heat loss to the atmosphere. The ice formation weakens the stability at the base of the mixed layer, leading to stronger entrainment and larger heat flux from below. This heat transport retards, and perhaps stops, the growth of the ice cover. As much heat is then entrained from below as is lost to the atmosphere. This heat loss further reduces the stability, and unless a net ice melt occurs, the mixed layer convects. Two possibilities exist: (1) A net ice melt, sufficient to retain the stability, will always occur and convection will not take place until all ice is removed. The deep convection will then be thermal, deepening the mixed layer. (2) The ice remains until the stability at the base of the mixed layer disappears. The mixed layer then convects, through haline convection, into the deep ocean. Warm water rises towards the surface and the ice starts to melt, and a new mixed layer is reformed. The present work discusses the interactions between ice cover and entrainment during winter, when heat loss to the atmosphere is present. One crucial hypothesis is introduced: “When ice is present and the ocean loses sensible heat to the atmosphere and to ice melt, the buoyancy input at the sea surface due to ice melt is at a minimum”. Using a one-dimensional energy-balance model, applied to the artificial situation, where ice melts directly on warmer water, it is found that this corresponds to a constant fraction of the heat loss going to ice melt. It is postulated that this partitioning holds for the ice cover and the mixed layer in the high-latitude ocean. When a constant fraction of heat goes to ice melt, at least one deep convection event occurs, before the ice cover can be removed by heat entrained from below. After one or several convection events the ice normally disappears and a deep-reaching thermal convection is established. Conditions appropriate for the Weddell Sea and the Greenland Sea are examined and compared with field observations. With realistic initial conditions no convection occurs in the warm regime of the Weddell Sea. A balance between entrained heat and atmospheric heat loss is established and the ice cover remains throughout the winter. At Maud Rise convection may occur, but late in winter and normally no polynya can form before the summer ice melt. In the central Greenland Sea the mixed layer generally convects early in winter and the ice is removed by melting from below as early as February or March. This is in agreement with existing observations.     

南极威德尔海冰间湖形成机制的研究进展

针对南极威德尔海冰间湖的特点及影响，本文介绍了有关威德尔海冰间湖形成研究的进展情况，并对存在的问题以及未来的发展进行了分析和阐述。结果说明：动力因素、热力因素和热动力因素都只是威德尔海冰间湖形成和维持的某一方面的控制和影响因子。对威德尔海冰间湖更深入的研究，应该充分考虑较大尺度的海洋-海冰-大气相互作用。     

High-resolution sea ice in long-term global ocean GCM integrations

The resolution of the sea-ice component of a coarse-resolution global ocean general circulation model (GCM) has been enhanced to about 22 km in the Southern Ocean. The ocean GCM is designed for long-term integrations suitable for investigations of the deep-ocean equilibrium response to changes in southern hemisphere high-latitude processes. The space and time scales of the high-resolution sea-ice component are commensurate with those of the resolution of satellite passive-microwave sea-ice data. This provides the opportunity for a rigorous evaluation of simulated sea-ice characteristics. It is found that the satellite-derived continuous high ice concentration of the interior winter ice pack can only be captured when vertical oceanic mixing is modified in a way that less local, intermittent convection occurs. Furthermore, the width and the variability of the coastal polynyas around the Antarctic continent and its ice shelves are best captured when some form of ice-shelf melting is accounted for. The width of the wintertime ice edge is reasonably reproduced, while its variability remains underestimated, closely following the coarse-grid pattern of the ocean model due to its high dependence on ocean temperature. Additional variability besides daily winds, e.g. in form of idealized tidal currents, improves the temporal and spatial ice-edge variability, while leads in the interior ice pack become more abundant, more in line with the fine-scale satellite-derived texture. The coast- or ice-shelf line is described on the fine grid based on satellite passive-microwave data. This method requires parts of a coarse coastal ocean grid cell to be covered by an inert layer of “fast ice” or “ice shelf”. Reasonable long-term global deep-ocean properties can only be achieved when these areas are not inert, i.e. are exposed to heat flux and ice growth, or when the vertical mixing parameterization allows for excessive open-ocean convection. The model area exposed to cold high-latitude atmospheric conditions thus being most decisive for a realistic representation of the long-term deep-ocean properties, suggests that high-latitude coastlines are definitely in need of being represented at high resolution, including ice sheets and their effects on the heat and freshwater flux for the ocean.     

Spring–summer cycling of DOC, DON and inorganic N in a highly seasonal system encompassing the Northeast Water Polynya, 1993

Dissolved organic nitrogen (DON), dissolved organic carbon (DOC) and inorganic nutrient concentrations were determined in samples from an area encompassing the Northeast Water Polynya from June to August 1993. In June, still ice-covered polynya area surface waters (PySW) had significantly higher (p<0.05) DOC concentrations (110 μM, n=68) than surface water outside the polynya area (96 μM, n=6). Melting ice and ice algae are suggested as DOC sources. DOC concentrations found in this study are consistent with other studies showing higher DOC concentrations in the Arctic than in other ocean areas. As the productive season progressed, DOC concentrations in Polynya surface water (PySW) decreased (p<0.05) from 110 to 105 μM, while DON concentrations increased (p<0.05) from 5.6 to 6.1 μM, causing a significant decrease (p<0.05) in the C : N ratios of DOM from spring (C : N ratio 20) to summer (C : N ratio 17). We found a significant (p<0.05) decrease in the DOM C : N ratio in all water masses within the polynya area as the productive season progressed. DON was the largest fraction of total dissolved nitrogen (TDN) in PySW and surface waters outside the polynya area. TDN was calculated as the sum of DON, nitrate, nitrite and ammonium concentrations. DON increased (p<0.05) from 62% to 73% of TDN in PySW from spring to summer, a result of increasing DON concentrations and decreasing inorganic nitrogen concentrations over the productive season. The seasonal accumulation of DON and the corresponding decrease in nitrate concentrations in waters with primary production indicate that it is important to take the DON pool into account when estimating export production from nitrate concentration decreases in surface waters. PySW TDN concentrations decreased (p<0.05) from 9.1 (n=61) to 8.6 μM (n=60) from spring (May 25 through June 19) to summer (July 1 through July 27). The seasonal decrease in surface water TDN concentrations corresponded to increases in TDN concentrations in deeper water masses within the Polynya. Most of the TDN increase in deep water was in the form of DON. A possible explanation is that PON was dissolved (partially remineralized) in the water column at mid depths, causing increases in the DON concentration. Transfer of N from PySW (with a short residence time in the polynya area) to Polynya Intermediate Water and deep waters of the Norske and Westwind Trough with multi-year residence times keeps N from leaving the polynya area. In spring, nutrients from degradation of OM in PyIW could support primary production. The role of PyIW as an OM trap could be important in supporting primary production in the polynya area.     

On the transport of water masses by long-period waves in seas and oceans

Parallel with the traditional hypotheses on the predominantly wind (Ekman), gradient, and thermohaline nature of large-scale currents in seas and oceans, there exist hypotheses on the participation of long-period waves in the formation of these currents. As the physical mechanisms of generation of currents by waves, one can mention nonlinear wave transport and the phenomenon of negative viscosity in waves. From the viewpoint of existence of these mechanisms, numerous scientists explain high velocities of jet currents and their stable character. The predominant role of winds and thermohaline processes in the formation of large-scale currents is also accepted. On the basis of the experimental data, we demonstrate additional possibilities for the formation of large-scale currents given by the transport of water masses by long-period waves.Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 24–34, September–October, 2004.This revised version was published online in May 2005 with corrections to cover date.     

Ocean circulation and exchanges through the northern Bering Sea—1979–2001 model results

We have developed and run a model with sufficiently high resolution (9 km and 45 levels) and a large enough spatial domain to allow for realistic representation of flow through the narrow and shallow straits in the northern Bering Sea. This is potentially important for quantification of long-term mean and time-dependent ocean circulation, and water mass and property exchanges between the Pacific and Arctic Oceans. Over a 23 year interval (1979–2001), mean transport through Bering Strait is estimated to be 0.65 Sv. Comparison of our model results with published observations indicates that ocean circulation is not only variable at seasonal to interdecadal scales but it is also responsive to short-term atmospheric forcing. One of such events occurred during the winter of 2000–2001 with reversed oceanic flow in some areas and much reduced sea-ice cover. Analyses of eddy kinetic energy fields identify some high biological productivity regions of the Chirikov Basin coincident with persistent high energy (up to 2700 cm² s⁻² in the surface layer and up to 2600 cm² s⁻² at mid-depth) throughout the annual cycle. Model output in the Bering Strait region is validated against several time series of moored observations of water mass properties. Comparison with shipboard observations of near-bottom salinity from late winter through autumn indicates that the model reasonably represents the major water-mass properties in the region. The modeled vertical water-column structure in the northern Bering Sea allows increased understanding of the mechanisms of water transformation and transport northward through Bering Strait into the Chukchi and Beaufort Seas. We conclude that the long-term model results for the northern Bering Sea provide important insights into the ocean circulation and fluxes and they are a useful frame of reference for limited observations that are short-term and/or cover only a small geographic region.     

Open‐ocean convection and polynya formation in a large‐scale ice‐ocean model

The physical processes responsible for the formation in a large‐scale ice–ocean model of an offshore polynya near the Greenwich meridian in the Southern Ocean are analysed. In this area, the brine release during ice formation in autumn is sufficient to destabilise the water column and trigger convection. This incorporates relatively warm water into the surface layer which, in a first step, slows down ice formation. In a second step, it gives rise to ice melting until the total disappearance of the ice at the end of September. Two elements are crucial for the polynya opening. The first one is a strong ice‐transport divergence in fall induced by south‐easterly winds, which enhances the amount of local ice formation and thus of brine release. The second is an inflow of relatively warm water at depth originating from the Antarctic Circumpolar Current, that sustains the intense vertical heat flux in the ocean during convection. The simulated polynya occurs in a region where such features have been frequently observed. Nevertheless, the model polynya is too wide and persistent. In addition, it develops each year, contrary to observations. The use of a climatological forcing with no interannual variability is the major cause of these deficiencies, the simulated too low density in the deep Southern Ocean and the coarse resolution of the model playing also a role. A passive tracer released in the polynya area indicates that the water mass produced there contributes significantly to the renewal of deep water in the Weddell Gyre and that it is a major component of the Antarctic Bottom Water (AABW) inflow into the model Atlantic.     

Density Current Descending along Continental Slope and the Associated Deep Water Formation: Two-Dimensional Numerical Experiments with a Nonhydrostatic Model

Numerical experiments with a two-dimensional nonhydrostatic ocean model have been carried out to investigate the dynamical process of descending density current on a continental slope. The associated deep water formation has been also examined by tracking labeled particles. The descending flow along the continental slope occurs in the bottom Ekman layer. The net pressure gradient determining the volume transport consists of not only the pressure gradient due to density deviation but also the surface pressure gradient due to the depth-mean alongshore flow. Since these constituents have the opposite signs and strengthen each other, the oscillation with an alternation of intense up- and downslope flows appears around the shelf break. This temporal variation of the flow field causes the effective mixing on the slope between descending shelf and interior waters and forms the deep water as a mixture of them at a ratio of about 1:3. The present result is applied to the slope current around Antarctica, using velocity and density fields calculated by an ocean general circulation model. The Ekman volume transport is estimated at 0.97 Sv (1 Sv = 10⁶ m³s^–1) in the Weddell Sea, 0.35 Sv in the Ross Sea, and 1.8 Sv in total. About 70% of them is attributed to the depth-mean alongshore flow, such as the East Wind Drift and the Weddell Gyre driven by the wind. This suggests that the pressure gradient due to other factors than density deviation may play an important role in the deep and bottom water formation in the actual oceans.     

The relation of surface forcing of the Bering Sea to large-scale climate patterns

Climate fluctuations, or modes, are largely manifested in terms of coherent, large-scale (3000 km) patterns of anomalous sea-level pressure or geopotential height at various altitudes. It is worthwhile to investigate how these modes relate to the specific processes associated with atmospheric forcing of the ocean, in this case for the southeast Bering Sea. This approach has been termed “downscaling.” Climate-scale patterns in this study are derived from covariance-based empirical orthogonal functions (EOFs) of low-pass filtered (10-day cut-off) 700-mb geopotential height fields for 1958–1999. By design, this EOF analysis elicits sets of patterns for characterizing the variability in the large-scale atmospheric circulation centered on the Bering Sea. Four modes are considered for each of three periods, January–March, April–May, and June–July. These modes are compared with atmospheric circulation patterns formed by compositing 700-mb height anomalies based on the individual elements constituting the local forcing, i.e. the surface heat and momentum fluxes.In general, different aspects of local forcing are associated with different climate modes. In winter, the modes dominating the forcing of sea-ice include considerable interannual variability, but no discernible long-term trends. A prominent shift did occur around 1977 in the sign of a winter mode resembling the Pacific North American pattern; this mode is most significantly related to the local wind-stress curl. In spring, forcing of currents and stratification are related to the two leading climate modes, one resembling the North Pacific (NP) pattern and one reflecting the strength of the Aleutian low; both exhibit long-term trends with implications for the Bering Sea. In summer, an NP-like mode and a mode featuring a center over the Bering Sea include long-term trends with impacts on surface heating and wind mixing, respectively. Rare events, such as a persistent period of strong high pressure or a major storm, also can dominate the summer Bering Sea forcing in particular years.     

简单海-气边界层模式中的埃克曼流动Ⅰ——无限深海的情形

近十余年来,海洋和大气相互作用这一课题日益受到人们的关注。从海-气边界层结构以及通过海面的物理量和化学量输送机制的研究到大尺度海洋和大气相互作用的研究都取得了可喜的成果。此外,还从海洋和大气相互作用的观点探索了风暴潮预报的新的可能途径[2]。然而,由于问题的复杂性,通过海面以及海-气边界层的物理量的小尺度输送机制迄未得到澄清,企图从本质上改善大尺度海洋和大气相互作用的理论是不现实的。     

Local oceanic response to atmospheric forcing in the Gulf Stream region

The dominance of shifts in the location of the Gulf Stream (GS) in the local heat balance was observed in an hourly 15-month record of unprecedented surface mooring measurements at a site in the western North Atlantic occupied from November 2005 to January 2007. Instrumentation on the buoy provided a high quality record of air-sea exchanges of momentum, heat, and freshwater flux; and oceanographic sensors recorded ocean variability in the upper 640 m. The mooring was at times in the GS and at other times north of the GS. Our intent was to isolate the local oceanic response to the atmosphere from the influence of the GS shifts. A one-dimensional heat budget analysis indicated that the advective contribution from the GS shifts dwarfed the heat contribution by atmospheric forcing and therefore played the dominant role for upper oceanic thermal variability during the whole time record. A GS case study (i.e., when the surface mooring was in the GS), isolated the upper oceanic response to the atmospheric forcing in the GS and supported the critical role of GS shifts in total oceanic heat content. Through both an Empirical Orthogonal Function (EOF) analysis and by referencing temperatures to that observed at 200 m, the impact of GS shifts and atmospheric forcing were decomposed, allowing the local oceanic thermal response to be isolated. This local oceanic response was particularly prominent during the period of sustained heating during summer. A case study of summer conditions revealed a near surface flow consistent with Ekman dynamics within a shallow, warm ocean mixed layer.     

Interdecadal climate regime dynamics in the North Pacific Ocean: theories, observations and ecosystem impacts

Basin-scale variations in oceanic physical variables are thought to organize patterns of biological response across the Pacific Ocean over decadal time scales. Different physical mechanisms can be responsible for the diverse basin-scale patterns of sea-surface temperature (SST), mixed-layer depth, thermocline depth, and horizontal currents, although they are linked in various ways. In light of various theories and observations, we interpret observed basinwide patterns of decadal-scale variations in upper-ocean temperatures. Evidence so far indicates that large-scale perturbations of the Aleutian Low generate temperature anomalies in the central and eastern North Pacific through the combined action of net surface heat flux, turbulent mixing and Ekman advection. The surface-forced temperature anomalies in the central North Pacific subduct and propagate southwestwards in the ocean thermocline to the subtropics but apparently do not reach the equator. The large-scale Ekman pumping resulting from changes of the Aleutian Low forces western-intensified thermocline depth anomalies that are approximately consistent with Sverdrup theory. These thermocline changes are associated with SST anomalies in the Kuroshio/Oyashio Extension that are of the same sign as those in the central North Pacific, but lagged by several years. The physics of the possible feedback from the SST anomalies to the Aleutian Low, which might close a coupled ocean–atmosphere mode of decadal variability, is poorly understood and is an area of active research. The possible responses of North Pacific Ocean ecosystems to these complicated physical patterns is summarized.     

Decadal-Scale Climate and Ecosystem Interactions in the North Pacific Ocean

Decadal-scale climate variations in the Pacific Ocean wield a strong influence on the oceanic ecosystem. Two dominant patterns of large-scale SST variability and one dominant pattern of large-scale thermocline variability can be explained as a forced oceanic response to large-scale changes in the Aleutian Low. The physical mechanisms that generate this decadal variability are still unclear, but stochastic atmospheric forcing of the ocean combined with atmospheric teleconnections from the tropics to the midlatitudes and some weak ocean-atmosphere feedbacks processes are the most plausible explanation. These observed physical variations organize the oceanic ecosystem response through large-scale basin-wide forcings that exert distinct local influences through many different processes. The regional ecosystem impacts of these local processes are discussed for the Tropical Pacific, the Central North Pacific, the Kuroshio-Oyashio Extension, the Bering Sea, the Gulf of Alaska, and the California Current System regions in the context of the observed decadal climate variability. The physical ocean-atmosphere system and the oceanic ecosystem interact through many different processes. These include physical forcing of the ecosystem by changes in solar fluxes, ocean temperature, horizontal current advection, vertical mixing and upwelling, freshwater fluxes, and sea ice. These also include oceanic ecosystem forcing of the climate by attenuation of solar energy by phytoplankton absorption and atmospheric aerosol production by phytoplankton DMS fluxes. A more complete understanding of the complicated feedback processes controlling decadal variability, ocean ecosystems, and biogeochemical cycling requires a concerted and organized long-term observational and modeling effort. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stochastic study of the temperature response of the upper ocean to uncertainties in the atmospheric forcing in an Atlantic OGCM

The impact of errors in atmospheric forcing on the behaviour of ocean models is a fundamental issue for ocean modellers and data assimilation and one that has yet to be fully addressed. In this study, we use a stochastic modelling approach with 50 7-months (September–March) primitive equation eddy permitting (1/4°) integrations. We investigate the response of the oceanic circulation to atmospheric uncertainties, focusing principally on their impact on the upper oceanic temperature field. The ensemble is generated by perturbing the wind, atmospheric temperature and incoming solar radiation of the ERA40 reanalysis. Each perturbation consists of a random combination of the 20 dominant EOFs of the difference between the ERA40 and NCEP/CORE reanalysis datasets. The ensemble standard deviation of various interfacial and oceanic quantities is then examined in the upper 200 m of three distinct regions of the North Atlantic: in the Gulf Stream, in the Northern Tropical band and in the North East Atlantic. These show that even a very small perturbation of the atmospheric variables can lead to significant changes in the ocean properties and that regions of oceanic mesoscale activity are the most sensitive. The ocean response is driven by vertical diffusivity and eddy activity. The role of subsurface currents is also crucial in carrying the eddy signal away from the regions of mesoscale activity. Finally, the decorrelation time scale of the mesoscale activity is critical in determining the amplitude of the oceanic response.     

Magnetic susceptibility and ice-rafted debris in surface sediments of the Atlantic sector of the Southern Ocean

Spatial distribution of magnetic susceptibility and the gravel fraction in surface sediments in the Atlantic sector of the Southern Ocean were investigated to reconstruct source areas and recent transport pathways of magnetominerals and ice-rafted debris. Maxima of magnetic susceptibility were observed offshore from areas where mafic source rocks occur, e.g. Queen Maud Land and the northern Antarctic Peninsula. The glacigenic input of debris and subsequent redeposition of fine material by bottom and turbidity currents on the continental margins result in regional variations of the gravel and susceptibility values. In the deep sea, however, the mixing of ice-rafted debris and turbidites from distal source areas causes a homogenous distribution of the susceptibility signal. On submarine elevations such as Maud Rise and Astrid Ridge, dust input may be an additional source for magnetominerals.     

基于卫星遥感数据的全球表层流场反演重构

利用2000—2008年的卫星高度计资料和QuikSCAT风场资料,反演了全球的海表的地转流和Ekman流,将两者合成后生成了0.5°×0.5°的逐周全球表层流产品。在计算Ekman流的时候,引入了权重函数,改进了Lagerloef方法中Ekman流在25°S和25°N上的不连续问题。分析表明:卫星资料反演的流产品能够反映出海表流场的特征,将其分别与TAO观测和SCUD流产品进行定量化的比较显示,所得流产品具有较高的反演精度和可信度,说明改进的方法是有效的。     

Autumn atmospheric preconditioning for interannual variability of wintertime sea-ice in the Okhotsk Sea

Relations in year-to-year variability between wintertime Sea-Ice Concentrations (SICs) in the Okhotsk Sea and atmospheric anomalies consisting of zonal and meridional 1000-hPa wind speeds and 850-hPa air temperatures are studied using a singular value decomposition analysis. It is revealed that the late autumn (October–November) atmospheric conditions strongly influence sea-ice variability from the same season (late autumn) through late winter (February—March), in which sea-ice extent is at its maximum. The autumn atmospheric conditions for the positive sea-ice anomalies exhibit cold air temperature anomalies over the Okhotsk Sea and wind anomalies blowing into the Okhotsk Sea from Siberia. These atmospheric conditions yield anomalous ocean-to-atmosphere heat fluxes and cold sea surface temperature anomalies in the Okhotsk Sea. Hence, these results suggest that the atmospheric conditions affect the sea-ice through heat anomalies stored in sea-ice and oceanic fields. The late autumn atmosphere conditions are related to large 700-hPa geopotential height anomalies over the Bering Sea and northern Eurasia, which are related to a stationary Rossby wave propagation over the North Pacific and that from the North Atlantic to Eurasia, respectively. In addition, the late autumn atmospheric preconditioning also plays an important role in the decreasing trend in the Okhotsk sea-ice extent observed from 1980 to the mid-1990s. Based on the lagged sea-ice response to the late autumn atmosphere, a simple seasonal prediction scheme is proposed for the February–March sea-ice extent using four-month leading atmospheric conditions. This scheme explains 45% of the variance of the Okhotsk sea-ice extent.     

Improved sea-ice radiative processes in a global coupled climate model

1Introduction Seaiceplaysanimportantroleinmoderating heatandmoistureexchangesbetweentheatmosphere andtheoceanathighlatitudes.Seaicealsointeracts withthebroaderclimatesystembythepositiveice albedofeedback(Curryetal.,1995),whichamplifies projectedclimatewarmingatthehighlatitudes,andby theoceanicfeedbackinvolvingicegrowthandmelt, whichinfluencesglobalthermohalinecirculation(i.e., theNorthAtlanticDeepWaterandtheAntarcticBot- tomWater)(Walsh,1983;Barryetal.,1993). Recently,theimplementationofas…