Rising Temperatures, Falling Isopycnals and Decreasing AOU: The Story of Abyssal Property Change at 24°S in the Atlantic (2009-2018)

Decadal-scale changes in temperature, salinity, density, and oxygen at 24°S in the Atlantic are investigated using observations from the 2009 and 2018 repeat occupations of the U.K. CLIVAR/GO-SHIP A095 line. Antarctic Bottom Water (AABW) saw significant warming across the entire basin. Warming was particularly striking in the Cape Basin and Brazil Basin. The latter saw significant warming that exceeded a 2005-2014 25°W estimate.However, only the warming on the eastern side was significant. Previous investigation suggested that the warming signal in AABW originated with the 1970’s polynya, which moved northward from the Scotia Sea and through the Argentine and Brazil Basins. Here, we find that the progression of the warming signal has continued to spread within the Brazil Basin through 2018. The lack of significant change in the west is partially due to strong variability in the boundary current. However, as the western boundary represents the most direct route northward for AABW, the lack of a strong signal may also be due to the warming having already passed to the north of 24°S in the west. The strong warming in the east indicates either the slower passage on this side of the basin of the signal that entered through the Vema Channel and/or the effect of the longer and somewhat more indirect journey for waters entering through the Hunter Channel. Substantial increases in dissolved oxygen are observed basin-wide, but again are most striking in the Brazil Basin. The falling of density surfaces bringing higher oxygen concentrations from above to abyssal waters contributes to the increasing oxygen content but cannot account for all of it.