Parameterization of vertical mixing in the Weddell Sea

A series of vertical mixing schemes implemented in a circumpolar coupled ice–ocean model of the BRIOS family is validated against observations of hydrography and sea ice coverage in the Weddell Sea. Assessed parameterizations include the Richardson number-dependent Pacanowski–Philander scheme, the Mellor–Yamada turbulent closure scheme, the K-profile parameterization, a bulk mixed layer model and the ocean penetrative plume scheme (OPPS). Combinations of the Pacanowski–Philander parameterization or the OPPS with a simple diagnostic model depending on the Monin–Obukhov length yield particularly good results. In contrast, experiments using a constant diffusivity and the traditional convective adjustment cannot reproduce the observations. An underestimation of wind-driven mixing in summer leads to an accumulation of salt in the winter water layer, inducing deep convection in the central Weddell Sea and a homogenization of the water column. Large upward heat fluxes in these simulations lead to the formation of unrealistic, large polynyas in the central Weddell Sea after only a few years of integration. Furthermore, spurious open-ocean convection affects the basin-scale circulation and leads to a significant overestimation of meridional overturning rates. We conclude that an adequate parameterization of both wind-induced mixing and buoyancy-driven convection is crucial for realistic simulations of processes in seasonally ice-covered seas.     

Some thoughts on the freezing and melting of sea ice and their effects on the ocean

The high-latitude freezing and melting cycle can variously result in haline convection, freshwater capping or freshwater injection into the interior ocean. An example of the latter process is a secondary salinity minimum near 800 m-depth within the Arctic Ocean that results from the transformation on the Barents Sea shelf of Atlantic water from the Norwegian Sea and its subsequent intrusion into the Arctic Ocean. About one-third of the freshening on the shelf of that initially saline water appears to result from ice melt, although the actual sea ice flux is small, only about 0.005 Sv. A curious feature of this process is that water distilled at the surface of the Arctic Ocean by freezing ends up at mid-depth in the same ocean. This is a consequence of the ice being exported southward onto the shelf, melted, and then entrained into the northward Barents Sea throughflow that subsequently sinks into the Arctic Ocean. Prolonged reduction in sea ice in the region and in the concomitant freshwater injection would likely result in a warmer and more saline interior Arctic Ocean below 800 m.     

海冰消融背景下北极增温的季节差异及其原因探讨

运用哈德莱中心第一套海冰覆盖率(HadISST1)、欧洲中心(ERA_Interim)的温度以及NCEP第一套地表感热通量、潜热通量等资料,研究了1979—2011年33a来北极海冰消融的季节特点和空间特征,并从反照率——温度正反馈与地表感热通量、潜热通量等方面分析了海冰减少对北极增温影响的季节差异。结果表明,北极海冰在秋季和夏季的减少范围明显大于冬季和春季,而北极地表升温却在秋季和冬季最显著,夏季最为微弱,且夏季的增温趋势廓线也与秋冬季显著不同。这主要是因为夏季是融冰季,海冰融化将吸收潜热。且此时北极低空大气温度高于海表温度,海水相当于大气的冷源。随着海冰的消融,更多的热量由大气传入海洋用于融冰和加热上层海水,这使得夏季的低空大气不能显著升温。而在秋冬季,海冰凝结释放潜热,且此时低空大气温度远低于海水温度,海冰的减少使得海水将更多热量释放到大气中导致低空大气显著增暖。海水对大气的这种延迟放热机制是北极低空在夏季增温不显著而在秋冬季增温显著的主要原因。此外,秋冬季的海冰减少与北极近地面升温具有非常一致的空间分布,北冰洋东南边缘和巴伦支海北部分别是秋季和冬季海气相互作用的关键区域。     

Deep convection seesaw controlled by freshwater transport through the Denmark Strait

Observations of deep ocean temperature and salinity in the Labrador and Greenland Seas indicate that there is negative correlation between the activities of deep convection in these two sites. A previous study suggests that this negative correlation is controlled by the North Atlantic Oscillation (NAO). In this study, we discuss this deep convection seesaw by using a coupled atmosphere and ocean general circulation model. In this simulation, the deep convection is realistically simulated in both the Labrador and Greenland Seas and their negative correlation is also recognized. Regression of sea level pressure to wintertime mixed layer depth in the Labrador Sea reveals strong correlation between the convection and the NAO as previous studies suggest, but a significant portion of their variability is not correlated. On the other hand, the convection in the Greenland Sea is not directly related to the NAO, and its variability is in phase with changes in the freshwater budget in the GIN Seas. The deep convection seesaw found in the model is controlled by freshwater transport through the Denmark Strait. When this transport is larger, more freshwater flows to the Labrador Sea and less to the Greenland Sea. This leads to lower upper-ocean surface salinity in the Labrador Sea and higher salinity in the Greenland Sea, which produces negative correlation between these two deep convective activities. The deep convection seesaw observed in the recent decades could be interpreted as induced by the changes in the freshwater transport through the Denmark Strait, whose role has not been discussed so far.     

南极海冰快速下降历史事件的时空特征分析

卫星记录以来,南极海冰范围发生5次快速下降事件,研究这5次事件的时空特征,对进一步认识海冰快速下降事件的物理机制具有重要意义。基于海冰范围和海冰密集度的卫星数据,从时间和空间两个维度总结5次南极海冰快速下降事件的特征,再结合大气和海洋各项环境因素的再分析数据,探讨海冰快速下降的影响因素及其驱动过程。结果显示:南极海冰快速下降的空间分布存在季节性差异, 2021年8～12月以及2016年8～12月的春季南极海冰快速下降由别林斯高晋海、威德尔海、印度洋和西太平洋区域的海冰减少所主导; 2010年12月至2011年4月以及1985年12月至1986年4月的夏季南极海冰快速下降由威德尔海、罗斯海沿岸和西太平洋区域的海冰减少所主导;2008年4～8月的冬季南极海冰快速下降则由别林斯高晋海和西太平洋的部分区域的海冰减少所主导。探究影响海冰的环境因素发现,海表面温度和海表面净热通量对海冰减少的热力效应影响具有区域性差异。此外,南极海冰快速下降受阿蒙森低压的影响,相应的海表面风异常既通过经向热输运的热力效应导致海冰减少,也通过风的动力效应驱动海冰漂移使得海冰密集度降低。     

The Effect of Seasonal Variability of Atlantic Water on the Arctic Sea Ice Cover

Under the influence of global warming, the sea ice in the Arctic Ocean (AO) is expected to reduce with a transition toward a seasonal ice cover by the end of this century. A comparison of climate-model predictions with measurements shows that the actual rate of ice cover decay in the AO is higher than the predicted one. This paper argues that the rapid shrinking of the Arctic summer ice cover is due to its increased seasonality, while seasonal oscillations of the Atlantic origin water temperature create favorable conditions for the formation of negative anomalies in the ice-cover area in winter. The basis for this hypothesis is the fundamental possibility of the activation of positive feedback provided by a specific feature of the seasonal cycle of the inflowing Atlantic origin water and the peaking of temperature in the Nansen Basin in midwinter. The recently accelerated reduction in the summer ice cover in the AO leads to an increased accumulation of heat in the upper ocean layer during the summer season. The extra heat content of the upper ocean layer favors prerequisite conditions for winter thermohaline convection and the transfer of heat from the Atlantic water (AW) layer to the ice cover. This, in turn, contributes to further ice thinning and a decrease in ice concentration, accelerated melting in summer, and a greater accumulation of heat in the ocean by the end of the following summer. An important role is played by the seasonal variability of the temperature of AW, which forms on the border between the North European and Arctic basins. The phase of seasonal oscillation changes while the AW is moving through the Nansen Basin. As a result, the timing of temperature peak shifts from summer to winter, additionally contributing to enhanced ice melting in winter. The formulated theoretical concept is substantiated by a simplified mathematical model and comparison with observations.     

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.     

The connection between Labrador Sea buoyancy loss, deep western boundary current strength, and Gulf Stream path in an ocean circulation model

The sensitivity of the North Atlantic gyre circulation to high latitude buoyancy forcing is explored in a global, non-eddy resolving ocean general circulation model. Increased buoyancy forcing strengthens the deep western boundary current, the northern recirculation gyre, and the North Atlantic Current, which leads to a more realistic Gulf Stream path. High latitude density fluxes and surface water mass transformation are strongly dependent on the choice of sea ice and salinity restoring boundary conditions. Coupling the ocean model to a prognostic sea ice model results in much greater buoyancy loss in the Labrador Sea compared to simulations in which the ocean is forced by prescribed sea ice boundary conditions. A comparison of bulk flux forced hindcast simulations which differ only in their sea ice and salinity restoring forcings reveals the effects of a mixed thermohaline boundary condition transport feedback whereby small, positive temperature and salinity anomalies in subpolar regions are amplified when the gyre spins up as a result of increased buoyancy loss and convection. The primary buoyancy flux effects of the sea ice which cause the simulations to diverge are ice melt, which is less physical in the diagnostic sea ice model, and insulation of the ocean, which is less physical with the prognostic sea ice model. Increased salinity restoring ensures a more realistic net winter buoyancy loss in the Labrador Sea, but it is found that improvements in the Gulf Stream simulation can only be achieved with the excessive buoyancy loss associated with weak salinity restoring.     

Winter convection in the Irminger Sea in 2004–2014

Winter convection in the Irminger Sea leading to the formation of Labrador Sea Water (LSW) is analyzed using CTD data collected along the 59.5° N transatlantic section in 2004–2014, winter Argo data from 2012–2014, and daily North American regional reanalysis (NARR). The interannual variability of LSW in the Irminger Sea is investigated. The dissolved oxygen saturation rate of 93% is used to indicate maximal local convection depth. It is shown that the deepest convection (up to 1000 m) resulting in the largest LSW volume that formed in the Irminger Sea in 2008 and 2012. These years were characterized by numerous storms with anomalously strong turbulent heat loss from the ocean to the atmosphere and negative air temperature to the east of the southern tip of Greenland in January–March. LSW became warmer by 0.42°C, saltier by more than 0.03 PSU, and more oxygenated by 8 µmol/kg between 2004 and 2014. A strong LSW decay in the Iceland Basin is also noted.     

Sea-ice retreat in the Sea of Okhotsk and the ice–ocean albedo feedback effect on it

Sea-ice retreat processes are examined in the Sea of Okhotsk. A heat budget analysis in the sea-ice zone shows that net heat flux from the atmosphere at the water surface is about 77 W m⁻² on average in the active ice melt season (April) due to large solar heating, while that at the ice surface is about 12 W m⁻² because of the difference in surface albedo. The temporal variation of the heat input into the upper ocean through the open water fraction corresponds well to that of the latent heat required for ice retreat. These results suggest that heat input into the ice–upper ocean system from the atmosphere mainly occurs at the open water fraction, and this heat input into the upper ocean is an important heat source for ice melting. The decrease in ice area in the active melt season (April) and the geostrophic wind just before the melt season (March) show a correlation: the decrease is large when the offshoreward wind is strong. This relationship can be explained by the following process. Once ice concentration is decreased (increased) by the offshoreward (onshoreward) wind just before the melt season, solar heating of the upper ocean through the increased (decreased) open water fraction is enhanced (reduced), leading to (suppressing) a further decrease in ice concentration. This positive feedback is regarded as the ice–ocean albedo feedback, and explains in part the large interannual variability of the ice cover in the ice melt season.     

Transient sea-ice polynya forced by oceanic flow variability

In the Weddell Sea during the winters of 1974–1976 a significant opening in the sea-ice cover occurred in the vicinity of a large bathymetric feature — the Maud Rise seamount. The event is commonly referred to as the Weddell Polynya. Aside from such a large-scale, relatively persistent polynya in the Weddell Sea, transient, small-scale polynya can also appear in the sea-ice cover at various times throughout the winter and at various locations with respect to the Maud Rise. The underlying causes for the occurrence of such transient polynya have not been unambiguously identified. We hypothesize that variations in the mean ocean currents are one major contributor to such variability in the sea-ice cover. Analysis of the sea-ice equations with certain idealized patterns of ocean currents serving as forcing is shown to lead to Ekman transports of sea ice favorable to the initiation of transient polynya. Aside from the actual spatial pattern of the idealized ocean currents, many other factors need also be taken into account when looking at such transient polynya. Two other such factors discussed are variations in the sea-ice thickness field and the treatment of the sea-ice rheology. Simulations of a sea-ice model coupled to a dynamical ocean model show that the interaction of (dynamical) oceanic currents with large-scale topographic features, such as the Maud Rise, does lead to the formation of transient polynya, again through Ekman transport effects. This occurs because the seamount has a dynamic impact on the three-dimensional oceanic flow field all the way up through the water column, and hence on the near surface ocean currents that are in physical contact with the sea ice. Further simulations of a sea-ice model coupled to a dynamic ocean model and forced with atmospheric buoyancy fluxes show that transient polynya can be enhanced when atmospheric cooling provides a positive feedback mechanism allowing preferential open-ocean convection to occur. The convection, which takes hold at sites where transient polynya have been initiated by sea-ice–ocean stress interaction, has an enhancing effect arising from the convective access to warmer, deeper waters. To investigate all of these effects in a hierarchical manner we use a primitive equation coupled sea-ice–ocean numerical model configured in a periodic channel domain with specified atmospheric conditions. We show that oceanic flow variability can account for temporal variability in small-scale, transient polynya and thus point to a plausible mechanism for the initiation of large-scale, sustained polynya such as the Weddell Polynya event of the mid 1970s.     

A coupled ice-ocean model for the Bohai Sea Ⅱ. Case study

The coupled ice-ocean model for the Bohai Sea is used for simulating the freezing, melting, and variation of ice cover and the heat balance at the sea-ice, air-ice, and air-sea interfaces of the Bohai Sea during the entire winter in 1998～1999 and 2000～2001. The coupled model is forced by real time numerical weather prediction fields. The results show that the thermodynamic effects of atmosphere and ocean are very important for the evolvement of ice in the Bohai Sea, especially in the period of ice freezing and melting. Ocean heat flux plays a key role in the thermodynamic coupling. The simulation also presents the different thermodynamic features in the ice covered region and the marginal ice zone. Ice thickness, heat budget at the interface, and surface sea temperature, etc. between the two representative points are discussed.     

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.     

渤海冰-海洋耦合模式Ⅱ.个例试验

以渤海1998~1999和2000~2001年度冬季的海冰发展过程为例,采用实时气象数值预报场作为大气强迫,利用渤海冰海洋耦合模式模拟渤海整个冬季的海冰生消和演变以及渤海冰季冰气、冰水和气水界面的热收支.模拟结果显示大气和海洋的热力效应对渤海的海冰发展非常重要,特别在海冰的冻结和融化阶段,海洋热通量在热力耦合中起着重要作用.模拟还显示了冰覆盖内部区域和冰外缘线附近不同的热力特征,分析讨论了冰区内和冰边缘两个特征点冰厚分布、界面热量收支和海表水温等.     

A coupled ice-ocean model for the Bohai Sea Ⅱ. Case study

The coupled ice-ocean model for the Bohai Sea is used for simulating the freezing, melting, and variation of ice cover and the heat balance at the sea-ice, air-ice, and air-sea interfaces of the Bohai Sea during the entire winter in 1998-1999 and 2000-2001. The coupled model is forced by real time numerical weather prediction fields. The results show that the thermodynamic effects of atmosphere and ocean are very important for the evolvement of ice in the Bohai Sea, especially in the period of ice freezing and melting. Ocean heat flux plays a key role in the thermodynamic coupling. The simulation also presents the different thermodynamic features in the ice covered region and the marginal ice zone. Ice thickness, heat budget at the interface, and surface sea temperature, etc. between the two representative points are discussed.     

The Sea of Okhotsk: A Window on the Ice Age Ocean

The modern Sea of Okhotsk and the high-latitude glacial ocean share similar radiolarian faunas suggesting they also share environmental similarities. This sea favors deep- (>200 m) over shallow-living species as evidenced by collections of sediment traps set at 258 and 1061 m in the central part of the Sea. Of the twelve dominant polycystine radiolarian species, four live above and eight below 258 m. The shallow-living species’ productivity maxima coincide with spring and fall phytoplankton blooms while deep-living species’ annual production, nearly twice that of the shallow-living species, is concentrated in fall. Previous workers have shown that summer plankton tows collect higher concentrations of polycystine Radiolaria below than above 200 m and that Radiolaria, fish and zooplankton have unusual concentration maxima between 200 and 500 m. The paucity of Radiolaria and other consumers above 200 m coincides with an upper (0–150 m) cold (−1.5°C to 1.5°C), low salinity layer while higher concentrations below 200 m occur within warmer saltier water. This unusual biological structure must produce a lower ratio of shallow (<200 m) to deep carbon remineralization than elsewhere in the world ocean.Deep-living radiolarian species, similar to those of the modern Sea of Okhotsk, dominate glacial high-latitude deep-sea sediments. If the hydrographic and biological structures that produced these glacial faunas were like those of the modern Sea of Okhotsk, then glacial high-latitude oceans would have differed from today's in at least two respects. Surface waters were less saline and more stable enhancing the spread of winter sea ice. This stability, combined with a deepening of nutrient regeneration, reduced surface water nutrients contributing to a reduction of atmospheric carbon dioxide.     

Thickness sensitivities in the CICE sea ice model

Passive microwave satellite observations of ice extent and concentration form the foundation of sea ice model evaluations, due to their wide spatial coverage and decades-long availability. Observations related to other model quantities are somewhat more limited but increasing as interest in high-latitude processes intensifies. Sea ice thickness, long judged a critical quantity in the physical system, is now being scrutinized more closely in sea ice model simulations as more expansive measurements become available. While albedo is often the first parameter chosen by modelers to adjust simulated ice thickness, this paper explores a set of less prominent parameters to which thickness is also quite sensitive. These include parameters associated with sea ice conductivity, mechanical redistribution, oceanic heat flux, and ice–ocean dynamic stress, in addition to shortwave radiation. Multiple combinations of parameter values can produce the same mean ice thickness using the Los Alamos Sea Ice Model, CICE. One of these “tuned” simulations is compared with a variety of observational data sets in both hemispheres. While deformed ice area compares well with the limited observations available for ridged ice, thickness measurements differ such that the model cannot agree with all of them simultaneously. Albedo and ice–ocean dynamic parameters that affect the turning of the ice relative to the ocean currents have the largest effect on ice thickness, of the parameters tested here. That is, sea ice thickness is highly sensitive to changes in external forcing by the atmosphere or ocean, and therefore serves as a sensitive diagnostic for high-latitude change.     

Ice–ocean interactions during shallow convection under conditions of steady winds: three-dimensional numerical studies

A three-dimensional hydrodynamic ocean model coupled to a thermohydrodynamic model for young sea ice is applied to study shallow haline convection in the central Greenland Sea, with an emphasis on sub-mesoscale ice–ocean interactions. Two types of young sea ice are distinguished; i.e., frazil and pancake ice, both acting different on surface heat, salt, and momentum fluxes. Two scenarios are considered: (a) continued frazil-ice production during steady winds, and (b) the same scenario but with the intermittent formation of pancake ice during a short intervening period of low winds. Brine release due to new-ice production creates shallow convection in both cases. Under conditions of continued frazil-ice production, ice streaks develop at the sea surface, finally becoming oriented roughly parallel to the wind. These streaks are the result of convective plumes that induce organized patterns of convergent and divergent surface currents. Frazil-ice is herded into convergence zones where it becomes as thick as 6 m within 24 h. The studies suggest a strong relationship between streak spacing and the penetration depth of convection, given by an aspect ratio in the range of 2–3. After pancake ice has been formed, however, the organized ice streaks vanish, developing into complex patterns of pancake ice. This finding is in agreement with recent field observations in the Greenland Sea Is-Odden ice tongue. With the existence of pancake ice, moreover, the surface-averaged buoyancy flux decreases and is determined from the integral of local sub-mesoscale ice–ocean interactions.     

2010年马卡罗夫海盆冰站观测期间垂向热通量时间变化研究

Based on hydrographic data obtained at an ice camp deployed in the Makarov Basin by the 4th Chinese Arctic Research Expedition in August of 2010, temporal variability of vertical heat flux in the upper ocean of the Makarov Basin is investigated together with its impacts on sea ice melt and evolution of heat content in the remnant of winter mixed layer(r WML). The upper ocean of the Makarov Basin under sea ice is vertically stratified. Oceanic heat flux from mixed layer(ML) to ice evolves in three stages as a response to air temperature changes, fluctuating from 12.4 W/m2 to the maximum 43.6 W/m2. The heat transferred upward from ML can support(0.7±0.3) cm/d ice melt rate on average, and daily variability of melt rate agrees well with the observed results. Downward heat flux from ML across the base of ML is much less, only 0.87 W/m2, due to enhanced stratification in the seasonal halocline under ML caused by sea ice melt, indicating that increasing solar heat entering summer ML is mainly used to melt sea ice, with a small proportion transferred downward and stored in the r WML. Heat flux from ML into r WML changes in two phases caused by abrupt air cooling with a day lag. Meanwhile, upward heat flux from Atlantic water(AW) across the base of r WML, even though obstructed by the cold halocline layer(CHL), reaches0.18 W/m2 on average with no obvious changing pattern and is also trapped by the r WML. Upward heat flux from deep AW is higher than generally supposed value near 0, as the existence of r WML enlarges the temperature gradient between surface water and CHL. Acting as a reservoir of heat transferred from both ML and AW, the increasing heat content of r WML can delay the onset of sea ice freezing.     

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.