Abstract
AbstractThe response of the subpolar Southern Ocean (sSO) to wind forcing is assessed using satellite radar altimetry. sSO sea level exhibits a phased, zonally coherent, bimodal adjustment to circumpolar wind changes, involving comparable seasonal and interannual variations. The adjustment is effected via a quasi‐instantaneous exchange of mass between the Antarctic continental shelf and the sSO to the north, and a 2‐month‐delayed transfer of mass between the wider Southern Ocean and the subtropics. Both adjustment modes are consistent with an Ekman‐mediated response to variations in surface stress. Only the fast mode projects significantly onto the surface geostrophic flow of the sSO; thus, the regional circulation varies in phase with the leading edge of sSO sea level variability. The surface forcing of changes in the sSO system is partly associated with variations of surface winds linked to the Southern Annular Mode and is modulated by sea ice cover near Antarctica.
Highlights
The subpolar Southern Ocean, extending from Antarctica to the southern boundary of the Antarctic Circumpolar Current (Figure 1), is a region of vigorous and complex interactions between the atmosphere, cryosphere, and ocean, with an influence on Earth's climate that is disproportionate to its area
The adjustment is effected via a quasi‐instantaneous exchange of mass between the Antarctic continental shelf and the subpolar Southern Ocean (sSO) to the north, and a 2‐month‐delayed transfer of mass between the wider Southern Ocean and the subtropics
Our work extends that of Armitage et al (2018) in that (i) it dissects the adjustment of the sSO to forcing in a way that illuminates the key drivers and dynamics, as well as their divergent impacts on the two major oceanic features of the sSO outlined above, and (ii) it considers the role of sea ice in modulating sSO variability and demonstrates its significance
Summary
The subpolar Southern Ocean (sSO), extending from Antarctica to the southern boundary of the Antarctic Circumpolar Current (Figure 1), is a region of vigorous and complex interactions between the atmosphere, cryosphere, and ocean, with an influence on Earth's climate that is disproportionate to its area. Evidence is mounting that the sSO is undergoing a significant transition in its climatic state and global influence, entailing an intensification of the atmospheric polar vortex (Thompson et al, 2011), accelerated melting of the Antarctic Ice Sheet (Rye et al, 2014; Shepherd et al, 2018), a redistribution of sea ice (Holland, 2014), and extensive warming, freshening, and poleward contraction of Antarctic Bottom Water (Purkey & Johnson, 2012) Understanding these changes and their mechanistic connections is confounded by the historical scarcity of oceanic measurements in the region, which presents a challenging environment to in situ observing platforms and, through its pervasive sea ice cover, satellite measurements.
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