Changes in Antarctic bottom water off the Wilkes Land coast in the Australian-Antarctic Basin

Abstract

Long-term changes in Antarctic Bottom Water (AABW) off the Wilkes Land coast were examined using historical hydrographic surveys and data from Deep Argo floats. Since the 1980s at the latest, AABW has contracted by approximately 11–14 m yr−1 and undergone freshening by approximately −0.6 × 10−3 yr−1. The contraction and freshening of AABW has accelerated slightly over the past few decades, with the former being led by the latter. The contraction of AABW was mainly derived from thinning of the denser layers of AABW, which was compensated for by thickening of the upper AABW. This contrast between thinning and compensative thickening has strengthened gradually over time, suggesting that the meridional overturning circulation driven by AABW formation has weakened in the Australian-Antarctic Basin. Dissolved oxygen on isotherms in AABW appears to have remained constant since 1970, except for the possibility of some oxygenation after 2000, which was not statistically significant. After 2010, changes in AABW deviated from the long-term trends, with contraction and freshening of AABW accelerating significantly from 2011 to 2015 and then slowing down from 2015 to 2019. Meanwhile, the bottommost AABW, which is defined as the bottommost 300 m of the water column, increased in salinity significantly in the latter 2010s, mainly in response to contraction of AABW. These processes have resulted in the recent salinification of the bottommost AABW in the eastern Australian-Antarctic Basin, even in areas where the saltier Ross Sea Bottom Water has not arrived yet. In 2018, marked freshening and oxygenation of AABW was observed in the 110°E section, probably due to newly formed bottom water in the Vincennes Bay Polynya.