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

The Antarctic Slope Current (ASC) circumnavigates the Antarctic continent following the continental slope and separating the waters on the continental shelf from the deeper offshore Southern Ocean. Water mass exchanges across the continental slope are critical for the global climate as they impact the global overturning circulation and the mass balance of the Antarctic ice sheet via basal melting. Despite the ASC’s global importance, little is known about its spatial and subannual variability, as direct measurements of the velocity field are sparse. Here, we describe the ASC in a global eddying ocean-sea ice model and reveal its large-scale spatial variability by characterising the continental slope using three regimes: the surface-intensified ASC, the bottom-intensified ASC and the reversed ASC. Each ASC regime corresponds to a distinct classification of the density field as previously introduced in the literature, suggesting that the velocity and density fields are governed by the same leading-order dynamics around the Antarctic continental slope. Only the surface-intensified ASC regime has a strong seasonality. However, large temporal variability at a range of other timescales occurs across all regimes, including frequent reversals of the current. We anticipate our description of the ASC’s spatial and subannual variability to be helpful to guide future studies of the ASC aiming to advance our understanding of the region’s response to a changing climate.

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

The 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.

Pathways and timescales of connectivity around the Antarctic continental shelf

<p><span xml:lang="EN-US" data-contrast="auto"><span>The Antarctic margin is surrounded by two westward flowing currents: the Antarctic Slope Current and the Antarctic Coastal Current. The former influences key processes near the Antarctic margin by regulating the flow of heat and nutrients onto and off the continental shelf, while together they </span></span><span xml:lang="EN-US" data-contrast="auto"><span>advect</span></span><span xml:lang="EN-US" data-contrast="auto"><span> nutrients, biological organisms, and temperature and salinity anomalies around the coastline, providing a connective link between different shelf regions. However, the extent to which these currents transport water from one sector of the continental shelf to another, and the timescales over which this occurs, remain poorly understood. Concern that crucial water formation sites around the Antarctic coastline could respond to non-local freshwater forcing </span></span><span><span xml:lang="EN-US" data-contrast="auto"><span>from ice shel</span></span></span><span><span xml:lang="EN-US" data-contrast="auto"><span>f meltwater</span></span></span> <span xml:lang="EN-US" data-contrast="auto"><span>motivates a more thorough understanding of zonal connectivity around Antarctica. In this study, we use daily velocity fields from a global high-resolution ocean-sea ice model, combined with the <span>Lagrangian</span> tracking software Parcels, to investigate the pathways and timescales connecting different regions of the Antarctic continental shelf<span> with a view to understanding</span><span> the timescales of meltwater transport around the continent</span>. Virtual particles are released over the continental shelf, poleward of the 1000 <span>metre</span> isobath, and are tracked for 20 years. Our results show a strong seasonal cycle connecting different sectors of the Antarctic continent, with more particles arriving further downstream during winter than during summer months. Strong advective links exist between West Antarctica and the Ross Sea while shelf geometry in some other regions acts as barriers to transport. We also highlight the varying importance of the Antarctic Slope Current and Antarctic Coastal Current in connecting different sectors of the coastline. Our results help to improve our understanding of circum-Antarctic connectivity <span>and the timescales </span><span>of meltwater transport from source regions to downstream </span><span>shelf locations. </span><span>Further</span><span>more, t</span><span>he timescales and pathways we </span><span>present </span><span>p</span>rovide a baseline from which to assess long-term changes in Antarctic coastal circulation due to local and remote forcing.<br></span></span></p>

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

<p> <span><span>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 sit</span><span>e</span><span>s 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.</span></span></p>

Antarctic Slope Current controls warm ocean intrusions towards Totten Glacier

<p><strong>The Totten Glacier in East Antarctica has received increasing attention in recent years for its ice loss and warm oceanographic conditions observed at the ice shelf front. Here, we developed satellite estimates of temporally varying Totten Ice Shelf (TIS) melt rates and a high-resolution ocean model. </strong><strong>We show that the Antarctic Slope Current (ASC) impedes ocean heat intrusions, and on-shelf intrusions enhance when the ASC weakens</strong><strong>. </strong><strong>The interannually varying strength of the ASC is primarily controlled by lateral ocean boundary conditions (and thus atmosphere and ocean circulations outside of the model domain) but also likely influenced by local wind stress curl and upstream decent of shelf water. We further </strong><strong>show that heat intrusions towards the TIS are enhanced with coastal freshening, suggesting that freshening from ice loss in West Antarcticacould start a chain reaction, leading to increased melt in East Antarctica, and further coastal freshening. </strong></p>

Cross-Slope Observations in the Bellingshausen Sea, Southern Ocean

<p>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.</p><p>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.</p><p>We compare the cross-slope sections from the observations with sections from the NEMO 1/12 ° 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.</p>