Abstract
Floating ice shelves buttress the Antarctic Ice Sheet, which is losing mass rapidly mainly due to ocean-driven melting and the associated disruption to glacial dynamics. The local ocean circulation near ice shelves is therefore important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf and the resultant melting. However, the dynamics controlling the near-coastal circulation are not fully understood. A cyclonic (i.e. clockwise) gyre circulation (27 km radius) in front of the Pine Island Ice Shelf has previously been identified in both numerical models and velocity observations. Here we present ship-based observations from 2019 to the west of Thwaites Ice Shelf, revealing another gyre (13 km radius) for the first time in this habitually ice-covered region, rotating in the opposite (anticyclonic, anticlockwise) direction to the gyre near Pine Island Ice Shelf, despite similar wind forcing. We use an idealised configuration of MITgcm, with idealised forcing based on ERA-5 climatological wind fields and simplified sea ice conditions from MODIS satellite images, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf. The model driven solely by wind forcing in the presence of ice can reproduce the horizontal structure and direction of both gyres. We show that the modelled gyre direction depends upon the spatial difference in the ocean surface stress, which can be affected by the applied wind stress curl filed, the percentage of wind stress transferred through the ice, and the angle between the wind direction and the sea ice edge. The presence of ice, either it is fast ice/ice shelves blocking the effect of wind, or the mobile sea ice enhancing the effect of wind, has the potential to reverse the gyre direction relative to ice-free conditions.
Highlights
The local ocean circulation near ice shelves is important for the prediction of future ice mass loss and related sea-level rise as it determines the water mass exchange, heat transport under the ice shelf and the resultant melting
A cyclonic gyre circulation (27 km radius) in front of the Pine Island Ice Shelf has previously been identified in both numerical models and velocity observations
We use an idealised configuration of MIT general circulation model (MITgcm), with idealised forcing based on ERA-5 climatological wind fields and simplified sea ice conditions from MODIS satellite images, to reproduce key features of the observed gyres near Pine Island Ice Shelf and Thwaites Ice Shelf
Summary
Antarctic ice shelves are thinning rapidly due primarily to basal melting, allowing the ice sheets to accelerate and lose mass (e.g. Pritchard et al, 2012) to significantly contribute to the future sea-level rise (e.g. Bamber et al, 2019; Golledge et al, 2019; DeConto et al, 2021). Understanding the local circulation that determines the flow of mCDW and its associated heat transport toward the ice shelves is crucial for a better prediction of ice shelf melting, future sea level and climate. Schodlok et al (2012) use a high-resolution model to infer that the strength of the PIB gyre is the main determinant of heat transport toward the ice shelf. We note the different sea ice coverage over the Thwaites and PIB gyres, and hypothesise that sea ice can influence the wind stress field felt by the ocean (i.e. ocean surface stress, hereafter OSS) to alter the gyre direction. 3, we introduce an idealised model designed to explore the roles of wind and sea ice in determining the gyre features. We discuss the limitations and applications of the results and summarise the results in Sect
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