Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

S Aoki,K Yamazaki,K Katsumata,K Shimada,D Hirano,H Sasaki,H Murase,Y Kitade

doi:10.1038/s41598-020-71290-6

Abstract

The Antarctic continental margin supplies the densest bottom water to the global abyss. From the late twentieth century, an acceleration in the long-term freshening of Antarctic Bottom Waters (AABW) has been detected in the Australian-Antarctic Basin. Our latest hydrographic observations reveal that, in the late 2010s, the freshening trend has reversed broadly over the continental slope. Near-bottom salinities in 2018–2019 were higher than during 2011–2015. Along 170° E, the salinity increase between 2011 and 2018 was greater than that observed in the west. The layer thickness of the densest AABW increased during the 2010s, suggesting that the Ross Sea Bottom Water intensification was a major source of the salinity increase. Freshwater content on the continental slope decreased at a rate of 58 ± 37 Gt/a in the near-bottom layer. The decadal change is very likely due to changes in Ross Sea shelf water attributable to a decrease in meltwater from West Antarctic ice shelves for the corresponding period.

Highlights

The Antarctic continental margin supplies the densest bottom water to the global abyss
Off the Adélie/George V Land (AGVL) Coast, Antarctic Bottom Waters (AABW) is formed through the export of dense shelf water, originating from intense sea-ice production in the George V Land polynya system, which eventually joins the bottom water originating in the Ross Sea[7]
Significant freshening occurred in benthic water masses over a wide area in the Antarctic Basin (AAB) until the early 2010s, but the latest observations reveal that this tendency has reversed (Fig. 2)

Summary

Introduction

The Antarctic continental margin supplies the densest bottom water to the global abyss. From the late twentieth century, an acceleration in the long-term freshening of Antarctic Bottom Waters (AABW) has been detected in the Australian-Antarctic Basin. The layer thickness of the densest AABW increased during the 2010s, suggesting that the Ross Sea Bottom Water intensification was a major source of the salinity increase. Cold and dense water on the continental shelf around Antarctica feeds the abyssal waters of the global oceans to produce Antarctic Bottom Water (AABW)[1], which plays a crucial role in global mass, heat and freshwater transport[2]. AABW flows equator-ward as the deep western boundary current[8], partly extending into the Princess Elizabeth Trough (PET) From the latter half of the twentieth century to the early 2010s, changes have been observed in the water mass properties of AABW in the AAB. In the high-latitude North Atlantic Ocean, which is another source region of deep water, decadal and multi-decadal variability are vigorous in comparison, and advection plays a role in carrying the v ariability[3]

Methods

Results

Conclusion