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All 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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Mesoscale, Tidal, and Seasonal/Interannual Drivers of the Weddell Sea Overturning Circulation
The Weddell Sea supplies 40–50% of the Antarctic BottomWaters that fill the global ocean abyss, and therefore exerts significant influence over global circulation and climate. Previous studies have identified a range of different processes that may contribute to dense shelf water (DSW) formation and export on the southern Weddell Sea continental shelf. However, the relative importance of these processes has not been quantified, which hampers prioritization of observational deployments and development of model parameterizations in this region. In this study a high-resolution (1/12°) regional model of the southern Weddell Sea is used to quantify the overturning circulation and decompose it into contributions due to multi-annual mean flows, seasonal/interannual variability, tides, and other sub-monthly variability. It is shown that tides primarily influence the overturning by changing the melt rate of the Filchner-Ronne Ice Shelf (FRIS). The resulting ~0.2 Sv decrease in DSW transport is comparable to the magnitude of the overturning in the FRIS cavity, but small compared to DSW export across the continental shelf break. Seasonal/interannual fluctuations exert a modest influence on the overturning circulation due to the relatively short (8-year) analysis period. Analysis of the transient energy budget indicates that the non-tidal, sub-monthly variability is primarily baroclinically-generated eddies associated with dense overflows. These eddies play a comparable role to the mean flow in exporting dense shelf waters across the continental shelf break, and account for 100% of the transfer of heat onto the continental shelf. The eddy component of the overturning is sensitive to model resolution, decreasing by a factor of ~2 as the horizontal grid spacing is refined from 1/3° to 1/12°.
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- Award ID(s):
- 1751386
- PAR ID:
- 10328142
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 51
- Issue:
- 12
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 3695-3722
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The Antarctic Slope Current (ASC) is a coherent circulation feature that rings the Antarctic continental shelf and regulates the flow of water toward the Antarctic coastline. The structure and variability of the ASC influences key processes near the Antarctic coastline that have global implications, such as the melting of Antarctic ice shelves and water mass formation that determines the strength of the global overturning circulation. Recent theoretical, modeling, and observational advances have revealed new dynamical properties of the ASC, making it timely to review. Earlier reviews of the ASC focused largely on local classifications of water properties of the ASC's primary front. Here we instead provide a classification of the current's frontal structure based on the dynamical mechanisms that govern both the along‐slope and cross‐slope circulation; these two modes of circulation are strongly coupled, similar to the Antarctic Circumpolar Current. Highly variable motions, such as dense overflows, tides, and eddies are shown to be critical components of cross‐slope and cross‐shelf exchange, but understanding of how the distribution and intensity of these processes will evolve in a changing climate remains poor due to observational and modeling limitations. Results linking the ASC to larger modes of climate variability, such as El Niño, show that the ASC is an integral part of global climate. An improved dynamical understanding of the ASC is still needed to accurately model and predict future Antarctic sea ice extent, the stability of the Antarctic ice sheets, and the Southern Ocean's contribution to the global carbon cycle.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JPO-D-20-0320.1
