Multidecadal decline in sea ice meltwater volume and Pacific Winter Water salinity in the Bering Sea revealed by ocean observations

Large amounts of freshwater and nutrients pass through the Bering Strait to the Arctic Ocean, making the Bering Sea a crucial marginal sea of the North Pacific Ocean. The hydrography and biological production of the Bering Sea are strongly influenced by the amount of sea ice produced and melted. The sea ice extent and production exhibited large interannual variability but no visible trend until 2016 when a strong decrease began. However, records of sea ice before 1979 and the beginning of satellite-based estimates do not exist. In this paper, we devised a methodology using historical temperature and salinity data, supplemented by historical oxygen isotope (delta 18O) data, to estimate sea ice melt and its temporal variability in the Bering Sea from 1950 onward. Our results, consistent with estimates of sea ice thickness, indicate that the sea ice melt volume has declined significantly -following lower sea ice extent and production- with a decrease between 35 and 50 km3 (from 442 km3) between pre-1980 and post-1980 climatologies. In particular, our meltwater time series reveals a decline of 160 km3 between 2012 and 2018, which also reflects the strong decrease in sea ice volume between 2016 and 2018 that numerous previous studies have highlighted. We also evaluated the change in the salinity of the Pacific Winter Water (PWW), whose formation is also related to sea ice production. The time series of PWW salinity exhibits a strong decreasing trend, with a freshening of about 0.3 between the mid-1950s and the mid-2010s, that we attribute to a combination of a reduced sea ice production and the freshening of the Alaskan Coastal Current water. The decline in meltwater volume and PWW salinity that we observed strongly influences the stratification over the Bering shelf, with a significant weakening of the stratification in coastal polynya regions, and a stronger and increasingly temperature-controlled stratification in the rest of the shelf. These changes could have adverse consequences on the biological productivity of the northern Bering Sea.