Rebound of shelf water salinity in the Ross Sea

Pasquale Castagno,Giannetta Fusco,Giorgio Budillon,Stephen R Rintoul,Vincenzo Capozzi,Giacomo R Ditullio,Giancarlo Spezie,Pierpaolo Falco

doi:10.1038/s41467-019-13083-8

Abstract

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean. In recent decades, AABW has warmed, freshened and reduced in volume. Ross Sea Bottom Water (RSBW), the second largest source of AABW, has experienced the largest freshening. Here we use 23 years of summer measurements to document temporal variability in the salinity of the Ross Sea High Salinity Shelf Water (HSSW), a precursor to RSBW. HSSW salinity decreased between 1995 and 2014, consistent with freshening observed between 1958 and 2008. However, HSSW salinity rebounded sharply after 2014, with values in 2018 similar to those observed in the mid-late 1990s. Near-synchronous interannual fluctuations in salinity observed at five locations on the continental shelf suggest that upstream preconditioning and large-scale forcing influence HSSW salinity. The rate, magnitude and duration of the recent salinity increase are unusual in the context of the (sparse) observational record.

Highlights

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean
A decrease in High Salinity Shelf Water (HSSW) salinity of 0.05 dec−1 was observed in Terra Nova Bay (TNB) between 1995 and 200613 and near the continental shelf break in the western (1995–2006) and central Ross Sea (1998–2006)[10]
We focus on five areas, two where HSSW is formed in persistent polynyas (TNB and Ross Island (RI)) and three troughs through which HSSW is exported to the deep ocean (Drygalski Trough (DT), Joides Trough (JT) and Glomar Challenger Trough (GCT)) (Fig. 1)

Summary

Introduction

Antarctic Bottom Water (AABW) supplies the lower limb of the global overturning circulation and ventilates the abyssal ocean. In recent decades AABW has warmed[4], freshened[1,4] and decreased in volume and density[5], contributing to the increase in global ocean heat content and sea level rise[4,5]. High salinity shelf water (HSSW) produced on the continental shelf of the Ross Sea is a precursor for RSBW. Freshening of RSBW has been linked to a decrease in salinity of HSSW in the Ross Sea, which freshened by 0.03 dec−1 between 1958 and 2008 on the inner continental shelf[6,12]. A decrease in HSSW salinity of 0.05 dec−1 was observed in Terra Nova Bay (TNB) between 1995 and 200613 and near the continental shelf break in the western (1995–2006) and central Ross Sea (1998–2006)[10]. We find the multi-decadal freshening identified in earlier studies persisted until 2014 and was followed by a rapid increase in HSSW salinity to values previously observed in the 1990s

Methods

Results

Conclusion