Antarctic Bottom Water Formation Research Articles

Controlling high-latitude Southern Ocean convection in climate models

Earth System Models (ESMs) generally suffer from a poor simulation of the High-Latitude Southern Ocean (HLSO). Here we aim at a better understanding of the shortcomings by investigating the sensitivity of the HLSO to the external freshwater flux and the horizontal resolution in forced and coupled simulations with the Max-Planck-Institute Ocean Model (MPIOM). Forced experiments reveal an immediate reduction of open-ocean convection with additional freshwater input. The latter leads to a remarkably realistic simulation of the distinct water-mass structure in the central Weddell Sea featuring a temperature maximum of +0.5°C at 250m depth. Similar, but more modest improvements occur over a time span of 40years after switching from a forced to a coupled simulation with an eddy-resolving version of MPIOM. The switch is accompanied with pronounced changes of the external freshwater flux and the wind field, as well as a more realistic heat flux due to coupling. Similar to the forced freshwater-flux experiments, a heat reservoir develops at depth, which in turn decreases the vertically integrated density of the HLSO and reduces the Antarctic Circumpolar Current to rather realistic values. Coupling with a higher resolution version of the atmosphere model (ECHAM6) yields distinct improvements of the HLSO water-mass structure and sea-ice cover. While the coupled simulations reveal a realistic amount of Antarctic runoff, its distribution appears too concentrated along the coast. Spreading the runoff over a wider region, as suggested in earlier studies to mimic the effect of freshwater transport through icebergs, also leads to noticeable improvements of the HLSO water-mass properties, predominantly along the coast. This suggests that the spread of the runoff improves the representation of Antarctic Bottom Water formation through enhanced near-boundary convection rather than weakened open-ocean convection.

Species diversity variations in Neogene deep-sea benthic foraminifera at ODP Hole 730A, western Arabian Sea

Deep-sea benthic foraminifera are an important and widely used marine proxy to understand paleoceanographic and paleoclimatic changes on regional and global scales, owing to their sensitivity to oceanic and climatic turnovers. Some species of benthic foraminifera are sensitive to changes in water mass properties whereas others are sensitive to organic fluxes and deep-sea oxygenation. Benthic faunal diversity has been found closely linked to food web, bottom water oxygen levels, and substrate and water mass stability. The present study is aimed at analyzing species diversity trends in benthic foraminifera and their linkages with Indian monsoon variability during the Neogene. Species diversity of benthic foraminifera is examined in terms of number of species (S), information function (H), equitability (E) and Sanders’ rarefied values, which were combined with relative abundances of high and low productivity benthic foraminifera at Ocean Drilling Program Hole 730A, Oman margin, western Arabian Sea. The Oman margin offers the best opportunity to understand monsoon-driven changes in benthic diversity since summer monsoon winds have greater impact on the study area. The species diversity was higher during the early Miocene Climatic Optimum (∼17.2–16.4 Ma) followed by a decrease during 16.4–13 Ma coinciding with a major increase in Antarctic ice volume and increased formation of Antarctic Bottom Water. All the diversity parameters show an increase during 13–11.6 Ma, a gradual decrease during 11.6–9 Ma and then an increase with a maximum at 7 Ma. Thereafter the values show little change until 1.2 Ma when all the parameters abruptly decrease. The benthic foraminiferal populations and diversity at Hole 730A were mainly driven by the Indian monsoon, and polar waters might have played a minor or no role since early Neogene period as the Arabian Sea is an enclosed basin.

Antarctic glaciation caused ocean circulation changes at the Eocene-Oligocene transition.

Two main hypotheses compete to explain global cooling and the abrupt growth of the Antarctic ice sheet across the Eocene-Oligocene transition about 34 million years ago: thermal isolation of Antarctica due to southern ocean gateway opening, and declining atmospheric CO2 (refs 5, 6). Increases in ocean thermal stratification and circulation in proxies across the Eocene-Oligocene transition have been interpreted as a unique signature of gateway opening, but at present both mechanisms remain possible. Here, using a coupled ocean-atmosphere model, we show that the rise of Antarctic glaciation, rather than altered palaeogeography, is best able to explain the observed oceanographic changes. We find that growth of the Antarctic ice sheet caused enhanced northward transport of Antarctic intermediate water and invigorated the formation of Antarctic bottom water, fundamentally reorganizing ocean circulation. Conversely, gateway openings had much less impact on ocean thermal stratification and circulation. Our results support available evidence that CO2 drawdown--not gateway opening--caused Antarctic ice sheet growth, and further show that these feedbacks in turn altered ocean circulation. The precise timing and rate of glaciation, and thus its impacts on ocean circulation, reflect the balance between potentially positive feedbacks (increases in sea ice extent and enhanced primary productivity) and negative feedbacks (stronger southward heat transport and localized high-latitude warming). The Antarctic ice sheet had a complex, dynamic role in ocean circulation and heat fluxes during its initiation, and these processes are likely to operate in the future.

Externally Forced and Internally Generated Decadal Climate Variability Associated with the Interdecadal Pacific Oscillation

AbstractGlobally averaged surface air temperatures in some decades show rapid increases (accelerated warming decades), and in other decades there is no warming trend (hiatus decades). A previous study showed that the net energy imbalance at the top of the atmosphere of about 1 W m−2 is associated with greater increases of deep ocean heat content below 750 m during the hiatus decades, while there is little globally averaged surface temperature increase or warming in the upper ocean layers. Here the authors examine processes involved with accelerated warming decades and address the relative roles of external forcing from increasing greenhouse gases and internally generated decadal climate variability associated with interdecadal Pacific oscillation (IPO). Model results from the Community Climate System Model, version 4 (CCSM4), show that accelerated warming decades are characterized by rapid warming of globally averaged surface air temperature, greater increases of heat content in the upper ocean layers, and less heat content increase in the deep ocean, opposite to the hiatus decades. In addition to contributions from processes potentially linked to Antarctic Bottom Water (AABW) formation and the Atlantic meridional overturning circulation (AMOC), the positive phase of the IPO, adding to the response to external forcing, is usually associated with accelerated warming decades. Conversely, hiatus decades typically occur with the negative phase of the IPO, when warming from the external forcing is overwhelmed by internally generated cooling in the tropical Pacific. Internally generated hiatus periods of up to 15 years with zero global warming trend are present in the future climate simulations. This suggests that there is a chance that the current observed hiatus could extend for several more years.

Insolation-induced mid-Brunhes transition in Southern Ocean ventilation and deep-ocean temperature

Glacial-interglacial cycles characterized by long cold periods interrupted by short periods of warmth are the dominant feature of Pleistocene climate, with the relative intensity and duration of past and future interglacials being of particular interest for civilization. The interglacials after 430,000 years ago were characterized by warmer climates and higher atmospheric concentrations of carbon dioxide than the interglacials before, but the cause of this climatic transition (the so-called mid-Brunhes event (MBE)) is unknown. Here I show, on the basis of model simulations, that in response to insolation changes only, feedbacks between sea ice, temperature, evaporation and salinity caused vigorous pre-MBE Antarctic bottom water formation and Southern Ocean ventilation. My results also show that strong westerlies increased the pre-MBE overturning in the Southern Ocean via an increased latitudinal insolation gradient created by changes in eccentricity during austral winter and by changes in obliquity during austral summer. The stronger bottom water formation led to a cooler deep ocean during the older interglacials. These insolation-induced differences in the deep-sea temperature and in the Southern Ocean ventilation between the more recent interglacials and the older ones were not expected, because there is no straightforward systematic difference in the astronomical parameters between the interglacials before and after 430,000 years ago. Rather than being a real 'event', the apparent MBE seems to have resulted from a series of individual interglacial responses--including notable exceptions to the general pattern--to various combinations of insolation conditions. Consequently, assuming no anthropogenic interference, future interglacials may have pre- or post-MBE characteristics without there being a systematic change in forcings. These findings are a first step towards understanding the magnitude change of the interglacial carbon dioxide concentration around 430,000 years ago.

Effect of opening the Drake Passage on the oceanic general circulation: A box model study

The impacts of opening the Drake Passage (DP) on the oceanic general circulation are examined. When the DP is open, wind stress at mid- and high latitudes gives rise to a wind-driven gyre, which induces a meridional heat exchange between mid- and high latitudes in the Southern Ocean. After the opening of the DP, the Antarctic Circumpolar Current (ACC) forms and its associated strong temperature front blocks the heat transport from mid-latitudes to high latitudes. A simple box model is formulated, in which the effects of the wind stress (for the case of DP closed) and the thermal front (for the case of DP open) on the variability of Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) are explored. The sensitivity experiments demonstrate that: (1) When the DP is closed, the enhancement of the wind-driven gyre leads to the decline of AABW formation in the Southern Ocean and the increase of NADW formation in the North Atlantic. As a result, water in high latitudes of the Southern Ocean becomes warmer, so does the bottom water of global ocean. (2) When the DP is open, there is no formation of AABW until the intensity of thermal front along ACC exceeds a threshold value (it is 4.03°C in our model). Before the formation of AABW, temperature in most of the oceans is higher than that after the formation of AABW, which usually leads to the cooling of high latitudes of the Southern Hemisphere and the bottom water in global ocean. When the strength of the thermal front is lower than the critical value, there is no AABW formation, and temperature in most of the oceans is slightly higher. These results demonstrate that during the opening of the DP, changes in wind stress and the formation of the thermal front in the Southern Ocean can substantially affect the formation of AABW and NADW, thus changing the state of meridional overturning circulation in the global ocean.

Tracing Southwest Pacific Bottom Water Using Potential Vorticity and Helium-3

Abstract This study uses potential vorticity and other tracers to identify the pathways of the densest form of Circumpolar Deep Water in the South Pacific, termed “Southwest Pacific Bottom Water” (SPBW), along the 28.2 kg m−3 surface. This study focuses on the potential vorticity signals associated with three major dynamical processes occurring in the vicinity of the Pacific–Antarctic Ridge: 1) the strong flow of the Antarctic Circumpolar Current (ACC), 2) lateral eddy stirring, and 3) heat and stratification changes in bottom waters induced by hydrothermal vents. These processes result in southward and downstream advection of low potential vorticity along rising isopycnal surfaces. Using δ3He released from the hydrothermal vents, the influence of volcanic activity on the SPBW may be traced across the South Pacific along the path of the ACC to Drake Passage. SPBW also flows within the southern limb of the Ross Gyre, reaching the Antarctic Slope in places and contributes via entrainment to the formation of Antarctic Bottom Water. Finally, it is shown that the magnitude and location of the potential vorticity signals associated with SPBW have endured over at least the last two decades, and that they are unique to the South Pacific sector.

On the relationship between Nd isotopic composition and ocean overturning circulation in idealized freshwater discharge events

Using a cost‐efficient climate model, the effect of changes in overturning circulation on neodymium isotopic composition,ϵNd, is systematically examined for the first time. Idealized sequences of abrupt climate changes are induced by the application of periodic freshwater fluxes to the North Atlantic (NA) and the Southern Ocean (SO), thus mainly affecting either the formation of North Atlantic Deep Water (NADW) or Antarctic Bottom Water (AABW). Variations in ϵNd reflect weakening and strengthening of the formation of NADW and AABW, changes in ϵNdof end‐members are relatively small. Relationships betweenϵNd and the strength of NADW or AABW are more pronounced for AABW than for NADW. Atlantic patterns of variations in ϵNd systematically differ between NA and SO experiments. Additionally, the signature of changes in ϵNd in the Atlantic and the Pacific is alike in NA but opposite in SO experiments. Discrimination between NA and SO experiments is therefore possible based on the Atlantic pattern of variations in ϵNd and the contrariwise behavior of ϵNd in the Atlantic and the Pacific. In further experiments we examined the effect of variations in magnitudes of particle export fluxes. Within the examined range, and although settling particles represent the only sink of Nd, their effects on ϵNd are relatively small. Our results confirm the large potential of ϵNd as a paleocirculation tracer but also indicate its limitations of quantitative reconstructions of changes in the Atlantic Meridional Ocean Circulation.

Eddy-Resolving Model Estimate of the Cabbeling Effect on the Water Mass Transformation in the Southern Ocean

Abstract Cabbeling effect on the water mass transformation in the Southern Ocean is investigated with the use of an eddy-resolving Southern Ocean model. A significant amount of water is densified by cabbeling: water mass transformation rates are about 4 Sv (1 Sv ≡ 106 m3 s−1) for transformation from surface/thermocline water to Subantarctic Mode Water (SAMW), about 7 Sv for transformation from SAMW to Antarctic Intermediate Water (AAIW), and about 5 Sv for transformation from AAIW to Upper Circumpolar Deep Water. These diapycnal volume transports occur around the Antarctic Circumpolar Current (ACC), where mesoscale eddies are active. The water mass transformation by cabbeling in this study is also characterized by a large amount of densification of Lower Circumpolar Deep Water (LCDW) into Antarctic Bottom Water (AABW) (about 9 Sv). Large diapycnal velocity is found not only along the ACC but also along the coast of Antarctica at the boundary between LCDW and AABW. It is found that about 3 Sv of LCDW is densified into AABW by cabbeling on the continental slopes of Antarctica in this study. This densification is not small compared with observational and numerical estimates on the AABW formation rate, which ranges from 10 to 20 Sv.

North Atlantic ventilation of “southern‐sourced” deep water in the glacial ocean

One potential mechanism for lowering atmospheric CO2 during glacial times is an increase in the fraction of the global ocean ventilated by the North Atlantic, which produces deep water with a low concentration of unused nutrients and thus drives the ocean's biological pump to a high efficiency. However, the data indicate that during glacial times, a water mass low in 13C/12C and 14C/C occupied the deep Atlantic, apparently at the expense of North Atlantic Deep Water (NADW). This water is commonly referred to as “southern‐sourced” because of its apparent entry into the Atlantic basin from the South, prompting the inference that it was ventilated at the Southern Ocean surface. Here, we propose that this deep Atlantic water mass actually included a large fraction of North Atlantic‐ventilated water, the chemical characteristics of which were altered by recirculation in the deep Southern and Indo‐Pacific oceans. In an ocean model sensitivity experiment that reduces Antarctic Bottom Water formation and weakens its overturning circulation, we find that a much greater fraction of NADW is transported into the Southern Ocean without contacting the surface and is entrained and mixed into the southern‐sourced deep water that spreads into the global abyssal ocean. Thus, North Atlantic ventilation takes over more of the ocean interior, lowering atmospheric CO2, and yet the abyssal Atlantic is filled from the South with old water low in 13C/12C and 14C/C, consistent with glacial data.

The CSIRO Mk3L climate system model version 1.0 – Part 2: Response to external forcings

Abstract. The CSIRO Mk3L climate system model is a coupled general circulation model, designed primarily for millennial-scale climate simulation and palaeoclimate research. Mk3L includes components which describe the atmosphere, ocean, sea ice and land surface, and combines computational efficiency with a stable and realistic control climatology. It is freely available to the research community. This paper evaluates the response of the model to external forcings which correspond to past and future changes in the climate system. A simulation of the mid-Holocene climate is performed, in which changes in the seasonal and meridional distribution of incoming solar radiation are imposed. Mk3L correctly simulates increased summer temperatures at northern mid-latitudes and cooling in the tropics. However, it is unable to capture some of the regional-scale features of the mid-Holocene climate, with the precipitation over Northern Africa being deficient. The model simulates a reduction of between 7 and 15% in the amplitude of El Niño-Southern Oscillation, a smaller decrease than that implied by the palaeoclimate record. However, the realism of the simulated ENSO is limited by the model's relatively coarse spatial resolution. Transient simulations of the late Holocene climate are then performed. The evolving distribution of insolation is imposed, and an acceleration technique is applied and assessed. The model successfully captures the temperature changes in each hemisphere and the upward trend in ENSO variability. However, the lack of a dynamic vegetation scheme does not allow it to simulate an abrupt desertification of the Sahara. To assess the response of Mk3L to other forcings, transient simulations of the last millennium are performed. Changes in solar irradiance, atmospheric greenhouse gas concentrations and volcanic emissions are applied to the model. The model is again broadly successful at simulating larger-scale changes in the climate system. Both the magnitude and the spatial pattern of the simulated 20th century warming are consistent with observations. However, the model underestimates the magnitude of the relative warmth associated with the Mediaeval Climate Anomaly. Finally, three transient simulations are performed, in which the atmospheric CO2 concentration is stabilised at two, three and four times the pre-industrial value. All three simulations exhibit ongoing surface warming, reduced sea ice cover, and a reduction in the rate of North Atlantic Deep Water formation followed by its gradual recovery. Antarctic Bottom Water formation ceases, with the shutdown being permanent for a trebling and quadrupling of the CO2 concentration. The transient and equilibrium climate sensitivities of the model are determined. The short-term transient response to a doubling of the CO2 concentration at 1% per year is a warming of 1.59 ± 0.08 K, while the long-term equilibrium response is a warming of at least 3.85 ± 0.02 K.

Impact of tide‐topography interactions on basal melting of Larsen C Ice Shelf, Antarctica

Basal melting of ice shelves around Antarctica contributes to formation of Antarctic Bottom Water and can affect global sea level by altering the offshore flow of grounded ice streams and glaciers. Tides influence ice shelf basal melt rate (wb) by contributing to ocean mixing and mean circulation as well as thermohaline exchanges with the ice shelf. We use a three‐dimensional ocean model, thermodynamically coupled to a nonevolving ice shelf, to investigate the relationship between topography, tides, andwb for Larsen C Ice Shelf (LCIS) in the northwestern Weddell Sea, Antarctica. Using our best estimates of ice shelf thickness and seabed topography, we find that the largest modeled LCIS melt rates occur in the northeast, where our model predicts strong diurnal tidal currents (∼0.4 m s−1). This distribution is significantly different from models with no tidal forcing, which predict largest melt rates along the deep grounding lines. We compare several model runs to explore melt rate sensitivity to geometry, initial ocean potential temperature (θ0), thermodynamic parameterizations of heat and freshwater ice‐ocean exchange, and tidal forcing. The resulting range of LCIS‐averagedwb is ∼0.11–0.44 m a−1. The spatial distribution of wb is very sensitive to model geometry and thermodynamic parameterization while the overall magnitude of wb is influenced by θ0. These sensitivities in wbpredictions reinforce a need for high‐resolution maps of ice draft and sub‐ice‐shelf seabed topography together with ocean temperature measurements at the ice shelf front to improve representation of ice shelves in coupled climate system models.

Quantifying the ocean's role in glacial CO<sub>2</sub> reductions

Abstract. A series of Last Glacial Maximum (LGM) marine carbon cycle sensitivity experiments is conducted to test the effect of different physical processes, as simulated by two atmosphere-ocean general circulation model (AOGCM) experiments, on atmospheric pCO2. One AOGCM solution exhibits an increase in North Atlantic Deep Water (NADW) formation under glacial conditions, whereas the other mimics an increase in Antarctic Bottom Water (AABW) associated with a weaker NADW. None of these sensitivity experiments reproduces the observed magnitude of glacial/interglacial pCO2 changes. However, to explain the reconstructed vertical gradient of dissolved inorganic carbon (DIC) of 40 mmol m−3 a marked enhancement in AABW formation is required. Furthermore, for the enhanced AABW sensitivity experiment the simulated stable carbon isotope ratio (δ13C) decreases by 0.4‰ at intermediate depths in the South Atlantic in accordance with sedimentary evidence. The shift of deep and bottom water formation sites from the North Atlantic to the Southern Ocean increases the total preformed nutrient inventory, so that the lowered efficiency of Southern Ocean nutrient utilization in turn increases atmospheric pCO2. This change eventually offsets the effect of an increased abyssal carbon pool due to stronger AABW formation. The effects of interhemispheric glacial sea-ice changes on atmospheric pCO2 oppose each other. Whereas, extended sea-ice coverage in the Southern Hemisphere reduces the air-sea gas exchange of CO2 in agreement with previous theoretical considerations, glacial advances of sea-ice in the Northern Hemisphere lead to a weakening of the oceanic carbon uptake through the physical pump. Due to enhanced gas solubility associated with lower sea surface temperature, both glacial experiments generate a reduction of atmospheric pCO2 by about 20–23 ppmv. The sensitivity experiments presented here demonstrate the presence of compensating effects of different physical processes in the ocean on glacial CO2 and the difficulty of finding a simple explanation of the glacial CO2 problem by invoking ocean dynamical changes.

Spatial distribution of Ice Shelf Water in front of the Amery Ice Shelf, Antarctica in summer

As a unique low-temperature water mass in Antarctic coastal region, the Ice Shelf Water (ISW) is an important component for the formation of the Antarctic Bottom Water (AABW). In this paper, we present a criterion for ISW identification based on freezing point at the sea surface, and we study spatial distribution of ISW in front of the Amery Ice Shelf (AIS) and its flow path in Prydz Bay by analyzing hydrographic data from Australian cruises in 2001 and 2002, as well as Chinese cruises in 2003, 2005, 2006, and 2008, all being made in the austral summer. The relatively cold and fresh ISW occurred as several discrete water blocks with cold cores in front of the AIS, within the depth range of 100–600 m, under the seasonal thermocline. ISW had obvious temporal and spatial variations and the spatial distribution pattern changed greatly after 2005. Most of ISW was concentrated west of 73°E during 2001 to 2003 and 2006, but it was widespread to east in 2005 and 2008. In all observation years, a small amount of cold ISW always occurs at the west end of the AIS front section, where the coldest ISW in the whole section also occurred in 2001, 2003 and 2006. Considering general cyclonic circulation pattern under the AIS, the ISW flowing out from west end of the AIS front might have experienced the longest cooling period under ice shelf, so it would have the lowest temperature. Analysis of data from meridian sections in Prydz Bay in 2003 implied that ISW in the west could spread north to the continental break along the east flank of the Fram Bank near 70.5°E, mix with the upwelling Circumpolar Deep Water and possibly contribute to the formation of AABW.

Research Spotlight: Climate response to melting of the West Antarctic Ice Sheet

The West Antarctic Ice Sheet (WAIS) could collapse in the future as the rising sea level and warming climate destabilize the sheet. Previous studies have shown that melting of WAIS could contribute to several meters of global sea level rise over the next few centuries. Some studies have pointed to evidence that WAIS has collapsed in the past during previous interglacial periods, which could give clues to what might happen in the future. To gain additional insight into how the Earth's climate would respond to a collapse of WAIS, Menviel et al. simulated the effects on global climate and the carbon cycle of a massive meltwater discharge from the collapse of the ice sheet. They found that a large meltwater discharge into the Southern Ocean would lead to a substantial cooling of the Southern Ocean, causing a northward expansion of the sea ice margin. Southern Hemisphere westerly winds would intensify. Furthermore, the formation of cold Antarctic Bottom Water would be suppressed, and subsurface warming would take place in areas where Antarctic Bottom Water is formed under present‐day conditions. This subsurface warming would lead to positive feedback that would accelerate the melting of the ice sheet. (Paleoceanography, doi:10.1029/2009PA001892, 2010)

A multi-variable box model approach to the soft tissue carbon pump

Abstract. The canonical question of which physical, chemical or biological mechanisms were responsible for oceanic uptake of atmospheric CO2 during the last glacial is yet unanswered. Insight from paleo-proxies has led to a multitude of hypotheses but none so far have been convincingly supported in three dimensional numerical modelling experiments. The processes that influence the CO2 uptake and export production are inter-related and too complex to solve conceptually while complex numerical models are time consuming and expensive to run which severely limits the combinations of mechanisms that can be explored. Instead, an intermediate inverse box model approach of the soft tissue pump is used here in which the whole parameter space is explored. The glacial circulation and biological production states are derived from these using proxies of glacial export production and the need to draw down CO2 into the ocean. We find that circulation patterns which explain glacial observations include reduced Antarctic Bottom Water formation and high latitude upwelling and mixing of deep water and to a lesser extent reduced equatorial upwelling. The proposed mechanism of CO2 uptake by an increase of eddies in the Southern Ocean, leading to a reduced residual circulation, is not supported. Regarding biological mechanisms, an increase in the nutrient utilization in either the equatorial regions or the northern polar latitudes can reduce atmospheric CO2 and satisfy proxies of glacial export production. Consistent with previous studies, CO2 is drawn down more easily through increased productivity in the Antarctic region than the sub-Antarctic, but that violates observations of lower export production there. The glacial states are more sensitive to changes in the circulation and less sensitive to changes in nutrient utilization rates than the interglacial states.

Climate and biogeochemical response to a rapid melting of the West Antarctic Ice Sheet during interglacials and implications for future climate

interglacials onto the global climate‐carbon cycle system using the Earth system model of intermediate complexity LOVECLIM. Prescribing a meltwater pulse in the Southern Ocean that mimics a rapid disintegration of the WAIS, a substantial cooling of the Southern Ocean is simulated that is accompanied by an equatorward expansion of the sea ice margin and an intensification of the Southern Hemispheric Westerlies. The strong halocline around Antarctica leads to suppression of Antarctic Bottom Water (AABW) formation and to subsurface warming in areas where under present‐day conditions AABW is formed. This subsurface warming at depths between 500 and 1500 m leads to a thermal weathering of the WAIS grounding line and provides a positive feedback that accelerates the meltdown of the WAIS. Our model results further demonstrate that in response to the massive expansion of sea ice, marine productivity in the Southern Ocean reduces significantly. A retreat of the WAIS, however, does not lead to any significant changes in atmospheric CO2. The climate signature of a WAIS collapse is structurally consistent with available paleoproxy signals of the last interglacial MIS5e. Citation: Menviel, L., A. Timmermann, O. E. Timm, and A. Mouchet (2010), Climate and biogeochemical response to a rapid melting of the West Antarctic Ice Sheet during interglacials and implications for future climate, Paleoceanography, 25, PA4231, doi:10.1029/2009PA001892.

Modeling sea ice production and dense shelf water formation in coastal polynyas around East Antarctica

Using an ice‐ocean coupled model with fine horizontal resolution around East Antarctica, sea ice production and dense shelf water (DSW) formation in coastal polynyas are investigated. The model reproduces well the locations of coastal polynyas and the high sea ice production there. DSW is formed over the continental shelves under a number of coastal polynyas. A threshold density, beyond which net production of DSW takes place, is largely different among coastal polynyas. The densest and most vigorous DSW formation occurs in the Cape Darnley and Mertz‐Ninnis Glacier polynyas followed by somewhat less but still significant DSW formation in the Prydz‐Barrier and Vincennes polynyas. Assuming mixing of the DSW outflowing across the shelf break with typical Modified Circumpolar Deep Water over the continental slope, the maximum possible formation rate of Antarctic Bottom Water (AABW) is estimated to be 7.58 Sv around East Antarctica between 60°E and 150°E, with the Cape Darnley and Mertz‐Ninnis Glacier polynyas exhibiting the most active formation rates of 2.13 and 1.97 Sv, respectively. From a series of numerical experiments, it is found that the treatment of coastline and grounded icebergs has a large impact on both sea ice production and formation of DSW and AABW.

Antarctic Bottom Water from the Adélie and George V Land coast, East Antarctica (140–149°E)

We report on observations of dense shelf water overflows and Antarctic Bottom Water (AABW) formation along the continental margin of the Adélie and George V Land coast between 140°E and 149°E. Vertical sections and bottom layer water mass properties sampled during two RVIB Nathaniel B Palmer hydrographic surveys (NBP00–08, December 2000/January 2001 and NBP04–08, October 2004) describe the spreading of cold, dense shelf water on the continental slope and rise from two independent source regions. The primary source region is the Adélie Depression, exporting high‐salinity dense shelf water through the Adélie Sill at 143°E. An additional eastern source region of lower‐salinity dense shelf water from the Mertz Depression is identified for the first time from bottom layer properties northwest of the Mertz Sill and Mertz Bank (146°E–148°E) that extend as far as the Buffon Channel (144.75°E) in summer. Regional analysis of satellite‐derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40% of the total ice production for the region occurs over the Mertz Depression and therefore this area is likely to make a significant contribution to the total dense shelf water export. Concurrent time series from bottom‐mounted Microcats and ADCP instruments from the Mertz Polynya Experiment (April 1998 to May 1999) near the Adélie Sill and on the upper continental slope (1150 m) and lower continental rise (3250 m) to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses. The critical density for shelf water to produce AABW is examined and found to be 27.85 kg m−3 from the Adélie Depression and as low as 27.80 kg m−3 from the Mertz Depression. This study suggests previous dense shelf water export estimates based on the flow through the Adélie Sill alone are conservative and that other regions around East Antarctica with similar ice production to the Mertz Depression could be contributing to the total AABW in the Australian‐Antarctic Basin.

Modelling the impact of ocean warming on melting and water masses of ice shelves in the Eastern Weddell Sea

The Eastern Weddell Ice Shelves (EWIS) are believed to modify the water masses of the coastal current and thus preconditions the water mass formation in the southern and western Weddell Sea. We apply various ocean warming scenarios to investigate the impact on the temperature–salinity distribution and the sub-ice shelf melting in the Eastern Weddell Sea. In our numerical experiments, the warming is imposed homogeneously along the open inflow boundaries of the model domain, leading to a warming of the warm deep water (WDW) further downstream. Our modelling results indicate a weak quadratic dependence of the melt rate at the ice shelf base on the imposed amount of warming, which is consistent with earlier studies. The total melt rate has a strong dependence on the applied ocean warming depth. If the warming is restricted to the upper ocean (above 1,000 m), the water column (aside from the mixed surface layer) in the vicinity of the ice shelves stabilises. Hence, reduced vertical mixing will reduce the potential of Antarctic Bottom Water formation further downstream with consequences on the global thermohaline circulation. If the warming extends to the abyss, the WDW core moves significantly closer to the continental shelf break. This sharpens the Antarctic Slope Front and leads to a reduced density stratification. In contrast to the narrow shelf bathymetry in the EWIS region, a wider continental shelf (like in the southern Weddell Sea) partly protects ice shelves from remote ocean warming. Hence, the freshwater production rate of, e.g., the Filchner–Ronne Ice Shelf increases much less compared with the EWIS for identical warming scenarios. Our study therefore indicates that the ice-ocean interaction has a significant impact on the temperature-salinity distribution and the water column stability in the vicinity of ice shelves located along a narrow continental shelf. The effects of ocean warming and the impact of increased freshwater fluxes on the circulation are of the same order of magnitude and superimposed. Therefore, a consideration of this interaction in large-scale climate studies is essential.