Seasonal variation of the Antarctic Slope Front occurence and position estimated from an interpolated hydrographic climatology

AbstractThe Antarctic Slope Front (ASF) is a fundamental feature of the subpolar Southern Ocean that is still poorly observed. In this study we build a statistical climatology of the temperature and salinity fields of the upper 380 m of the Antarctic margin. We use a comprehensive compilation of observational datasets including the profiles gathered by instrumented marine mammals. The mapping method consists first of a decomposition in vertical modes of the combined temperature and salinity profiles. Then the resulting principal components are optimally interpolated on a regular grid and the monthly climatological profiles are reconstructed, providing a physically plausible representation of the ocean. The ASF is located with a contour method and a gradient method applied on the temperature field, two complementary approaches that provide a complete view of the ASF structure. The front extends from the Amundsen Sea to the eastern Weddell Sea and closely tracks the continental shelf break. It is associated with a sharp temperature gradient that is stronger in Winter and weaker in Summer. The emergence of the front in the Amundsen and Bellingshausen sector appears seasonally variable (slightly more westward in Winter than in Summer). The investigation of the density gradients across the shelf break indicates a Winter slowdown of the baroclinic component of the Antarctic Slope Current at the near-surface, in contrast with the seasonal variability of the temperature gradient.

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Variability of the Antarctic Slope Current System in the Northwestern Weddell Sea

Journal of Physical Oceanography ◽

10.1175/jpo-d-17-0030.1 ◽

2017 ◽

Vol 47 (12) ◽

pp. 2977-2997 ◽

Cited By ~ 11

Author(s):

Marina Azaneu ◽

Karen J. Heywood ◽

Bastien Y. Queste ◽

Andrew F. Thompson

Keyword(s):

Current System ◽

Water Layer ◽

Bottom Layer ◽

Weddell Sea ◽

The South ◽

Orkney Islands ◽

Dense Water ◽

South Orkney Islands ◽

Antarctic Slope Current ◽

The Antarctic

AbstractThe dense water outflow from the Antarctic continental shelf is closely associated with the strength and position of the Antarctic Slope Front. This study explores the short-term and spatial variability of the Antarctic Slope Front system and the mechanisms that regulate cross-slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012. The 22 sections along the eastern Antarctic Peninsula and west of the South Orkney Islands are grouped regionally and composited by isobaths. There is consistency in the front position around the Powell Basin, varying mostly between the 500- and 800-m isobaths. In most of the study area the flow is bottom intensified. The along-slope transport of the Antarctic Slope Current (upper 1000 m) varies between 0.2 and 5.9 Sv (1 Sv ≡ 106 m3 s−1) and does not exhibit a regional pattern. The magnitude of the velocity field shows substantial variability, up to twice its mean value. Higher eddy kinetic energy (0.003 m2 s−2) is observed in sections with dense water, possibly because of baroclinic instabilities in the bottom layer. Distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer. Glider sections located west of the South Orkney Islands indicate a northward direction of the flow associated with the Weddell Front, which differs from previous estimates of the mean circulation. This study provides some of the first observational confirmation of the high-frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region.

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Acceleration and Overturning of the Antarctic Slope Current by Winds, Eddies, and Tides

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0221.1 ◽

2019 ◽

Vol 49 (8) ◽

pp. 2043-2074 ◽

Cited By ~ 10

Author(s):

Andrew L. Stewart ◽

Andreas Klocker ◽

Dimitris Menemenlis

Keyword(s):

Sea Ice ◽

Continental Shelf ◽

Continental Slope ◽

Mesoscale Eddy ◽

Shelf Break ◽

Ekman Transport ◽

Global Ocean ◽

Antarctic Slope Current ◽

The Antarctic ◽

Continental Shelf Break

AbstractAll exchanges between the open ocean and the Antarctic continental shelf must cross the Antarctic Slope Current (ASC). Previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability, yet the mechanisms responsible for setting the ASC’s transport and structure have received relatively little attention. In this study the roles of winds, eddies, and tides in accelerating the ASC are investigated using a global ocean–sea ice simulation with very high resolution (1/48° grid spacing). It is found that the circulation along the continental slope is accelerated both by surface stresses, ultimately sourced from the easterly winds, and by mesoscale eddy vorticity fluxes. At the continental shelf break, the ASC exhibits a narrow (~30–50 km), swift (>0.2 m s−1) jet, consistent with in situ observations. In this jet the surface stress is substantially reduced, and may even vanish or be directed eastward, because the ocean surface speed matches or exceeds that of the sea ice. The shelfbreak jet is shown to be accelerated by tidal momentum advection, consistent with the phenomenon of tidal rectification. Consequently, the shoreward Ekman transport vanishes and thus the mean overturning circulation that steepens the Antarctic Slope Front (ASF) is primarily due to tidal acceleration. These findings imply that the circulation and mean overturning of the ASC are not only determined by near-Antarctic winds, but also depend crucially on sea ice cover, regionally-dependent mesoscale eddy activity over the continental slope, and the amplitude of tidal flows across the continental shelf break.

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Abundance and Biodiversity of Top Predators - Seabirds and Marine Mammals - in Antarctic Seas

Journal of Marine Science Research and Oceanography ◽

10.33140/jmsro.03.03.07 ◽

2020 ◽

Vol 3 (3) ◽

Keyword(s):

Sea Level ◽

Marine Mammals ◽

Weddell Sea ◽

Amundsen Sea ◽

Top Predators ◽

Very High

This article concerns the comparison of data collected in different Antarctic seas by the same team, same platform (mainly from the bridge of icebreaking RV Polarstern, 18 m above sea level), and thus the same methodology. Drastic differences were noted, from very high numbers in the Weddell Sea to very low ones in the Amundsen Sea. Biodiversity was low, as reflected by low numbers of species, a few of them representing the vast majority in numbers of individuals: between 85% and 95% of the total.

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Spatial and temporal variability of the Antarctic Slope Current in an eddying ocean-sea ice model

10.5194/egusphere-egu21-13947 ◽

2021 ◽

Author(s):

Wilma Huneke ◽

Adele Morrison ◽

Andy Hogg

Keyword(s):

Continental Shelf ◽

Temporal Variability ◽

Shelf Break ◽

Spatial And Temporal Variability ◽

Shelf Water ◽

Antarctic Continental Shelf ◽

Antarctic Slope Current ◽

The Antarctic ◽

Continental Shelf Break ◽

Ice Model

The basal melt rate of Antarctica's ice shelves is largely controlled by heat delivered from the Southern Ocean to the Antarctic continental shelf. The Antarctic Slope Current (ASC) is an almost circumpolar feature that encircles Antarctica along the continental shelf break in an anti-clockwise direction. Because the circulation is to first order oriented along the topographic slope, it inhibits exchange of water masses between the Southern Ocean and the Antarctic continental shelf and thereby impacts cross-slope heat supply. Direct observations of the ASC system are sparse, but indicate a highly variable flow field both in time and space. Given the importance of the circulation near the shelf break for cross-shelf exchange of heat, it is timely to further improve our knowledge of the ASC system. This study makes use of the global ocean-sea ice model ACCESS-OM2-01 with a 1/10 degree horizontal resolution and describes the spatial and temporal variability of the velocity field. We categorise the modelled ASC into three different regimes, similar to previous works for the associated Antarctic Slope Front: (i) A surface-intensified current found predominantly in East Antarctica, (ii) a bottom-intensified current found downstream of the dense shelf water formation sites in the Ross, Weddell, and Prydz Bay Seas, and (iii) a reversed current found in West Antarctica where the eastward flowing Antarctic Circumpolar Current impinges onto the continental shelf break. We find that the temporal variability of the Antarctic Slope Current varies between the regimes. In the bottom-intensified regions, the variability is set by the timing of the dense shelf water overflows, whereas the surface-intensified flow responds to the sub-monthly variability in the wind field.

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The Impact of Open Oceanic Processes on the Antarctic Bottom Water Outflows

Journal of Physical Oceanography ◽

10.1175/2011jpo4571.1 ◽

2011 ◽

Vol 41 (10) ◽

pp. 1941-1957 ◽

Cited By ~ 15

Author(s):

Shinichiro Kida

Keyword(s):

Bottom Water ◽

Water Mass ◽

Shelf Break ◽

Weddell Sea ◽

Antarctic Bottom Water ◽

Antarctic Continental Shelf ◽

Deep Channel ◽

Major Branch ◽

The Antarctic ◽

The Impact

Abstract The impact of open oceanic processes on the Antarctic Bottom Water (AABW) outflows is investigated using a numerical model with a focus on outflows that occur through deep channels. A major branch of the AABW outflow is known to occur as an overflow from the Filchner Depression to the Weddell Sea through a deep channel a few hundred kilometers wide and a sill roughly 500 m deep. When this overflow enters the Weddell Sea, it encounters the Antarctic Slope Front (ASF) at the shelf break, a density front commonly found along the Antarctic continental shelf break. The presence of an AABW outflow and the ASF create a v-shaped isopycnal structure across the shelf break, indicating an interaction between the overflow and oceanic processes. Model experiments show the overflow transport to increase significantly when an oceanic wind stress increases the depth of the ASF. This enhancement of overflow transport occurs because the channel walls allow a pressure gradient in the along-slope direction to exist and the overflow transport is geostrophically controlled with its ambient oceanic water at the shelf break. Because the ASF is associated with a lighter water mass that reaches the depth close to that of the channel, an increase in its depth increases the density gradient across the shelf break and therefore the geostrophic overflow transport. The enhancement of overflow transport is also likely to result in a lighter overflow water mass, although such an adjustment of density likely occurs on a much longer time scale than the adjustment of transport.

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Cross-Slope Observations in the Bellingshausen Sea, Southern Ocean

10.5194/egusphere-egu2020-463 ◽

2020 ◽

Author(s):

Ria Oelerich ◽

Karen J. Heywood ◽

Gillian M. Damerell ◽

Andrew F. Thompson

Keyword(s):

Current System ◽

Amundsen Sea ◽

Ice Shelves ◽

West Antarctic ◽

Bellingshausen Sea ◽

Average Current ◽

Adcp Data ◽

Basal Melting ◽

Antarctic Slope Current ◽

The Antarctic

The Bellingshausen Sea, located between the West Antarctic Peninsula and the Amundsen Sea, is poorly observed, compared with its neighbours. The Antarctic Slope Front (ASF), that rings the continental slope of Antarctica, supports a westward current (the Antarctic Slope Current). The structure and variability of this current affect exchange processes close to Antarctica such as the transport of warm Circumpolar Deep Water onto the Antarctic continental shelf. This water mass is responsible for the transport of heat across the shelf and therefore the basal melting of ice shelves. Due to the lack of observations, it is still unclear if the ASF even exists in the Bellingshausen Sea or if there are other processes moderating the transport of warm water onto the shelf.We present ship-based and glider-based CTD data collected in 2007 and 2019, which in total provide 7 cross-slope sections in the Bellingshausen Sea. Geostrophic velocities are referenced to lowered ADCP data, shipboard ADCP data and the Dive Average Current. Cumulative transports show remarkable differences between the years 2007 and 2019. The sections of 2007 provide cumulative transports of up to 3.5 Sv eastward. In contrast, the sections in 2019 have cumulative transports up to 2 Sv westward. The sections from 2007 and 2019 are in very similar locations, indicating a temporal change rather than a spatial change.We compare the cross-slope sections from the observations with sections from the NEMO 1/12 &#176; model output. A time series of cumulative transports from the model, covering the years from 2000 to 2010, allows us to identify seasonality and interannual variability in this current system.

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Changes in Antarctic temperature, wind and precipitation in response to the Antarctic Oscillation

Annals of Glaciology ◽

10.3189/172756404781814654 ◽

2004 ◽

Vol 39 ◽

pp. 119-126 ◽

Cited By ~ 81

Author(s):

Michiel R. van den Broeke ◽

Nicole P. M. van Lipzig

Keyword(s):

Climate Model ◽

Weddell Sea ◽

East Antarctica ◽

Antarctic Oscillation ◽

West Antarctica ◽

Ice Shelf ◽

Near Surface ◽

Ross Ice Shelf ◽

Temperature And Precipitation ◽

The Antarctic

AbstractOutput of a 14 year integration with a high-resolution (55 km ×55 km) regional atmospheric climate model is used to study the response of Antarctic near-surface climate to the Antarctic Oscillation (AAO), the periodical strengthening and weakening of the circumpolar vortex in the Southern Hemisphere. In spite of the relatively short record, wind, temperature and precipitation show widespread and significant AAO-related signals. When the vortex is strong (high AAO index), northwesterly flow anomalies cause warming over the Antarctic Peninsula (AP) and adjacent regions in West Antarctica and the Weddell Sea. In contrast, cooling occurs in East Antarctica, the eastern Ross Ice Shelf and parts of Marie Byrd Land. Most of the annual temperature signal stems from the months March–August. The spatial distribution of the precipitation response to changes in the AAO does not mirror temperature changes but is in first order determined by the direction of flow anomalies with respect to the Antarctic topography. When the vortex is strong (high AAO index), the western AP becomes wetter, while the Ross Ice Shelf, parts of West Antarctica and the Lambert Glacier basin, East Antarctica, become drier.

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Significant additional Antarctic warming in atmospheric bias-corrected ARPEGE projections with respect to control run

The Cryosphere ◽

10.5194/tc-15-3615-2021 ◽

2021 ◽

Vol 15 (8) ◽

pp. 3615-3635

Author(s):

Julien Beaumet ◽

Michel Déqué ◽

Gerhard Krinner ◽

Cécile Agosta ◽

Antoinette Alias ◽

...

Keyword(s):

Atmospheric Circulation ◽

Bias Correction ◽

Ice Sheet ◽

Atmospheric Model ◽

Bias Reduction ◽

Climate Projections ◽

Amundsen Sea ◽

Near Surface ◽

Precipitation Increase ◽

The Antarctic

Abstract. In this study, we use run-time bias correction to correct for the Action de Recherche Petite Echelle Grande Echelle (ARPEGE) atmospheric model systematic errors on large-scale atmospheric circulation. The bias-correction terms are built using the climatological mean of the adjustment terms on tendency errors in an ARPEGE simulation relaxed towards ERA-Interim reanalyses. The bias reduction with respect to the Atmospheric Model Intercomparison Project (AMIP)-style uncorrected control run for the general atmospheric circulation in the Southern Hemisphere is significant for mean state and daily variability. Comparisons for the Antarctic Ice Sheet with the polar-oriented regional atmospheric models MAR and RACMO2 and in situ observations also suggest substantial bias reduction for near-surface temperature and precipitation in coastal areas. Applying the method to climate projections for the late 21st century (2071–2100) leads to large differences in the projected changes of the atmospheric circulation in the southern high latitudes and of the Antarctic surface climate. The projected poleward shift and strengthening of the southern westerly winds are greatly reduced. These changes result in a significant 0.7 to 0.9 K additional warming and a 6 % to 9 % additional increase in precipitation over the grounded ice sheet. The sensitivity of precipitation increase to temperature increase (+7.7 % K−1 and +9 % K−1) found is also higher than previous estimates. The highest additional warming rates are found over East Antarctica in summer. In winter, there is a dipole of weaker warming and weaker precipitation increase over West Antarctica, contrasted by a stronger warming and a concomitant stronger precipitation increase from Victoria to Adélie Land, associated with a weaker intensification of the Amundsen Sea Low.

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Verification of the regional atmospheric model CCLM v5.0 with conventional data and Lidar measurements in Antarctica

10.5194/gmd-2019-141 ◽

2019 ◽

Author(s):

Rolf Zentek ◽

Günther Heinemann

Keyword(s):

Wind Speed ◽

Sea Ice ◽

Climate Model ◽

Regional Climate ◽

Weddell Sea ◽

Good Representation ◽

Near Surface ◽

Turbulence Parametrization ◽

Lidar Measurements ◽

The Antarctic

Abstract. The non-hydrostatic regional climate model CCLM was used for a long-term hindcast run (2002–2016) for the Weddell Sea region with resolutions of 15 and 5 km and two different turbulence parametrizations. CCLM was nested in ERA-Interim data. We prescribed sea-ice concentration from satellite data, and used a thermodynamic sea-ice model. The performance of the model was evaluated in terms of temperature and wind using data from Antarctic stations, AWS over land and sea ice, operational forecast model and reanalyses data, and lidar wind profiles. For the reference run we found a warm bias for the near-surface temperature over the Antarctic plateau. This bias was removed in the second run by adjusting the turbulence parametrization, which results in a more realistic representation of the surface inversion over the plateau. Differences in other regions were small. A comparison with measurements over the sea ice of the Weddell Sea by three AWS buoys for one year showed small biases for temperature around 1 K and for wind speed of 1 m s−1. Comparisons of radio soundings showed a model bias around zero and a RMSE of 1–2 K for temperature and of 3–4 m s−1 for wind speed. The comparison of CCLM simulations at resolutions down to 1 km with wind data from Doppler Lidar measurements during December 2015 and January 2016 yielded almost no bias in wind speed and RMSE of ca. 2 m s−1. Overall CCLM shows a good representation of temperature and wind for the Weddell Sea region. These results encourage for further studies using CCLM data for the regional climate in the Antarctic at high resolutions and the study of atmosphere-ice-ocean interactions processes.

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Sea Ice–Ocean Feedbacks in the Antarctic Shelf Seas

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0229.1 ◽

2019 ◽

Vol 49 (9) ◽

pp. 2423-2446 ◽

Cited By ~ 1

Author(s):

R. C. Frew ◽

D. L. Feltham ◽

P. R. Holland ◽

A. A. Petty

Keyword(s):

Sea Ice ◽

Air Temperature ◽

Mixed Layer ◽

Surface Air Temperature ◽

Weddell Sea ◽

Surface Energy Budget ◽

Atmospheric Conditions ◽

Ocean Mixed Layer ◽

Amundsen Sea ◽

The Antarctic

AbstractObserved changes in Antarctic sea ice are poorly understood, in part due to the complexity of its interactions with the atmosphere and ocean. A highly simplified, coupled sea ice–ocean mixed layer model has been developed to investigate the importance of sea ice–ocean feedbacks on the evolution of sea ice and the ocean mixed layer in two contrasting regions of the Antarctic continental shelf ocean: the Amundsen Sea, which has warm shelf waters, and the Weddell Sea, which has cold and saline shelf waters. Modeling studies where we deny the feedback response to surface air temperature perturbations show the importance of feedbacks on the mixed layer and ice cover in the Weddell Sea to be smaller than the sensitivity to surface atmospheric conditions. In the Amundsen Sea the effect of surface air temperature perturbations on the sea ice are opposed by changes in the entrainment of warm deep waters into the mixed layer. The net impact depends on the relative balance between changes in sea ice growth driven by surface perturbations and basal-driven melting. The changes in the entrainment of warm water in the Amundsen Sea were found to have a much larger impact on the ice volume than perturbations in the surface energy budget. This creates a net negative ice albedo feedback in the Amundsen Sea, reversing the sign of this typically positive feedback mechanism.

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