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Ocean dynamics played key role in Antarctic sea ice changes during past decades

AOML Oceanographer Sang-Ki Lee and his colleagues used an ocean and sea-ice coupled model to show that ocean dynamics played key role in Antarctica's sea ice changes during past decades. This research* was selected as an Editor's highlight for the Journal of Geophysical Research.

Figure 1 : Linear trends of Antarctic sea-ice concentration during (a) the warm (December–May) and (b) cold (June–November) seasons, obtained from the Hadley Center sea-ice and sea surface temperature data sets over the period of 1979–2014. The units are % in 35 years.

Since late 1978, Antarctic sea-ice extent in the East Pacific has retreated persistently over the Amundsen and Bellingshausen Seas in warm seasons, but expanded over the Ross and Amundsen Seas in cold seasons, while an almost opposite trend has occurred in the Atlantic over the Weddell Sea (Figure 1). By using a surface-forced ocean and sea-ice coupled model that reasonably reproduces the observed sea-ice trends around West Antarctica, here we show that regional wind-driven ocean dynamics played a key role in driving these trends. In the East Pacific, the strengthening Southern Hemisphere (SH) westerlies in the region enhanced the Ekman upwelling of warm upper Circumpolar Deep Water and increased the northward Ekman transport of cold Antarctic surface water. The associated surface ocean warming south of 68°S and the cooling north of 68°S directly contributed to the retreat of sea ice in warm seasons and the expansion in cold seasons, respectively (Figure 2a). In the Atlantic, the poleward shifting SH westerlies in the region strengthened the northern branch of the Weddell Gyre, which in turn increased the meridional thermal gradient across it as constrained by the thermal wind balance (Figure 2b). Ocean heat budget analysis further suggests that the strengthened northern branch of the Weddell Gyre acted as a barrier against the poleward ocean heat transport, and thus produced anomalous heat divergence within the Weddell Gyre and anomalous heat convergence north of the gyre (Figure 2b). The associated cooling within the Weddell Gyre and the warming north of the gyre contributed to the expansion of sea ice in warm seasons and the retreat in cold seasons, respectively (Figure 2b). The leading EOFs of the detrended Antarctic sea-ice variability (i.e., interannual variability) also show spatially and seasonally contrasting sea-ice variations around West Antarctica

Figure 2: Sketch of the physical mechanisms that link the wind-driven ocean dynamics and the Antarctic sea-ice trends in (a) the East Pacific sector and (b) Atlantic sector.

*Lee, S.-K., D. Volkov, H. Lopez, W. G. Cheon, A. L. Gordon, Y. Liu, and R. Wanninkhof, 2017: Wind-driven ocean dynamics impact on the contrasting sea-ice trends around West Antarctica. J. Geophys. Res., 122, 4413-4430, doi:10.1002/2016JC012416.