Reversal of freshening trend of Ross Sea shelf water links to an abrupt transition from high to low state of Antarctic sea ice since the mid-2010s

Despite the increase in global mean temperature and massive sea ice loss in the Arctic, the Antarctic sea ice extent has not changed significantly throughout reliable satellite records starting in 1979. Long-term trends rather show an increase in the Antarctic sea ice area, resulting in record high anomalies in 2014 and 2015. However, after the moderate expansion in the sea ice extent, a sharp decline occurred in 2016 and has remained low since then. The record Antarctic sea ice loss in recent years may be a sign the region has entered a new regime of low sea ice coverage in a warming world. Meanwhile, Antarctic Bottom Water (AABW), driving the lower limb of the global meridional overturning circulation and ventilating the abyssal ocean interior has warmed and freshened in recent decades, leading to a decrease in AABW formation. Ross Sea shelf water which is responsible for 20–40% of the total AABW production, has experienced the largest freshening. However, repeat hydrographic data have shown that since the mid-2010s the salinity of Ross Sea shelf water has sharply rebounded from the multidecadal freshening trend. Here, it is interesting that the abrupt transition from a high to low state of Antarctic sea ice since the mid-2010s coincides with the onset of the salinity rebound of dense shelf water on the Antarctic continental shelf.As the planet warms global sea ice has continued to get a lot of attention due to the substantial implications for planetary albedo, ice sheet and ice shelf stability, atmosphere-ocean interactions, cryosphere ecosystems, biogeochemical cycle, and the Southern Ocean freshwater cycle. Particularly, sea ice’s growth and melting play an important role in water mass transformations. Here, we investigate how the rapid decline in the Antarctic sea ice in recent years has contributed to the rebound of shelf water salinity in the Ross Sea, using satellite observations of sea ice, as well as oceanic and atmospheric reanalysis data. Our result shows that despite the rapid decrease in the Antarctic sea ice in recent years, the sea ice formation rate in the Ross Sea continental shelf has increased. During the salinification period since the mid-2010s, local anomalous winds and surface heat flux associated with the remote and large-scale forcing that drive the recent change in the Antarctic sea ice, induced the reduced sea ice cover and larger polynya area on the Ross Sea continental shelf, increasing sea ice formation rate. Furthermore, data-based sea ice budget analysis indicates that due to the anomalous wind forcing, the sea ice has moved to the outer shelf through dynamic processes such as advection and divergence, creating a sustained favorable environment for sea ice formation and brine rejection.