The Oceanic Response to Winds in the Antarctic Sea Ice Loss at the end of the 1970s

F Feba , Hugues Goosse, Pierre-Yves Barriat, François Massonnet

The sea ice extent (SIE) in the Southern Ocean experienced a substantial decline in the late 1970s, although less pronounced than the one observed in 2016. Though several studies explain the decline since 2016, the 1970s drop is critical in understanding the long-term variability of SIE. To investigate the underlying mechanisms for this decline, we conducted wind stress-forced multi- ensemble experiments using the general circulation model EC-Earth3 with constant pre-industrial CO2. Using this anomaly-nudging technique, we reproduce the drop during the late 1970s, despite the control (no-nudging) run showing no such trend. The loss in SIE expresses spatial heterogeneity across different sectors of the Southern Ocean, with the maximum absolute loss occurring in the King Hakon sector, followed by the Weddell Sea and Indian Ocean sectors. The loss of SIE co-occurred with an increase in temperature and salinity at the surface. Our analysis shows that this loss of SIE can be attributed to wind-driven oceanic twofold responses. 1) During the late 1970s, the wind stress curl was anomalously negative, driving a stronger Ekman upwelling, bringing salt and warmer water from the subsurface to the surface. 2) The winds shifted to a stronger phase during the same period, influencing the vertical mixing, inducing a deeper mixed layer and the entrainment of warmer and saltier water at depth in the mixed layer. This study highlights the importance of winds in driving ocean processes and thereby influencing the interannual variability of SIE in the Southern Ocean. Our results finally indicate that the late 1970s rapid sea ice loss event shares several characteristics with the recent post-2016 retreat, including a significant surface salinification despite substantial sea ice loss, suggesting that common mechanisms might be at play between the two periods and thus providing an insightful analogy to understand the current decline.