Antarctic Bottom Water seasonality observed by Deep Argo in the Australian Antarctic Basin

Abstract: Antarctic Bottom Water (AABW) supplies 40% of the ocean volume and maintains the deep limb of the global overturning circulation. AABW is produced when Dense Shelf Water overflows from the continental shelf into the deep ocean and mixes with surrounding water. Despite strong seasonality on the shelf, the seasonality of AABW is largely unknown. We use 7.5~years of full-depth profiles collected every ten days by Deep Argo floats to quantify the seasonal variability of AABW in the Australian Antarctic Basin. Monthly mean anomalies, relative to the local multi-year average, show that while seasonality is weak at basin-scale and within the Antarctic margin, it becomes significant downstream of the Adelie Land and Ross Sea sources of AABW. Adelie-sourced AABW at 140$^\circ$E is coldest (-0.023$^\circ$C) and densest (+0.003~kg~m$^{-3}$) in November, while Ross-sourced AABW at 150$^\circ$E is coldest, most saline, densest, and thickest in June. The arrival time of the densest AABW at each longitude, along with that previously observed by moorings near the sources, is used to estimate advective speeds. AABW from the Adélie Land source to 140$^\circ$E (0.04 – 0.06~m~s$^{-1}$) is about twice the average speed of AABW from the Ross Sea source to 150$^\circ$E (0.025~m~s$^{-1}$). Deep Argo resolves the seasonality of AABW in the Australian Antarctic Basin, with implications for the deep circulation and controls on Dense Shelf Water export.