Modulation of dense shelf water salinity variability in the western Ross Sea associated with the Amundsen Sea Low

Abstract

Dense shelf water (DSW) produced in the western Ross Sea (RS) is one of the major sources of Antarctic bottom water (AABW). Thus the understanding of long-term variability of DSW salinity and its controlling factors in the western RS is critical to assess the variability of globally distributed AABW. Here we analyze a long time record of hydrographic data (1984–2020) collected in the western RS, as well as sea ice drift vectors, surface wind speed, sea level pressure and Amundsen Sea low (ASL) indices. We confirm recent findings that there is a rapid increase of DSW salinity in the western RS after a minimum in 2013, although the DSW has experienced substantial freshening in the past few decades, indicating a significant multidecadal variability of DSW salinity in the western RS. Over the past four decades, multidecadal variability in the DSW salinity has been strongly coupled with westward zonal flow changes along the coastal current, and the post-2013 rapid enhancement of DSW salinity is accompanied by reduced freshwater input due to weakening of the westward zonal flow from the upstream Amundsen Sea (AS) into the RS. Large-scale circulation determining the strength of the zonal flow is closely linked to the ASL variability. The accelerated deepening of the ASL and the resulting southwestward extension of low pressure induce an eastward coastal current anomaly. This reduces the freshwater input from the AS to the RS and is responsible for the subsequent enhancement of DSW salinity in recent years in the western RS. These dynamical processes demonstrated here explain how the ASL changes modulate the DSW salinity in the western RS, and will help to understand the implication of climate changes in the Southern Ocean on AABW formation.

Keywords

• dense shelf water
• freshwater input
• large-scale circulation
• Ross Sea
• Amundsen Sea low