Persistent Ross Sea Freshening From Imbalance West Antarctic Ice Shelf Melting

A 63-year observational record in the southwest Ross Sea shows a continuing, near-linear salinity decrease of 0.170 and slight warming of 0.013 degrees C through 2020. That freshening exceeded any increase in sea ice production and brine release from stronger southerly winds, while melting and freezing at the Ross Ice Shelf base contributed little to the salinity change. The parallel seawater density decline appears not to have enhanced warm deep water intrusions onto the continental shelf (CS). Confirming prior inferences, the salinity change has been mainly caused by a growing imbalance in the meltwater available from thinning ice shelves and increased iceberg calving in the upstream Amundsen and Bellingshausen Seas. Shorter-term salinity variability has tracked winds near the Amundsen Sea CS break, in turn coherent with a broader Pacific climate variability index, and with salinity reversals on and seaward of the Ross CS. The melt driven freshening is positively correlated with global atmospheric CO2 and temperature increases, and adds to the rise in sea level from increased glacier flow into weakened ice shelves. Continued erosion of those ice shelves could end the production of high salinity shelf and bottom waters, as defined in the Ross Sea, by the 2050s. Plain Language Summary Prior reports of salinity change in the Ross Sea have relied on selected observations over a limited area, and assumptions about a probable cause. In this study we documented freshening over 63 years, accompanied by slight warming, from measurements obtained during 43 summers in a wider region on the southwest Ross continental shelf. Minor net melting under the adjacent Ross Ice Shelf may have helped to offset additional brine release by sea ice formation under stronger surface winds. However, most of the freshening has resulted from an increasing volume of West Antarctic ice shelf and iceberg meltwater transported westward by coastal and slope currents. Seawater density is dominated by salinity at cold temperatures, parameters that began to reveal change similar to 50 years ago as a shifting atmospheric circulation increased "warm" deep water access to vulnerable ice shelves. Downstream freshening of the Ross Sea and its Antarctic Bottom Water then resulted from meltwater sources that have increased by more than 10 billion tons each year, a remote impact of anthropogenic climate change with regional to global effects on ocean properties and sea level.