FY 2020

Antarctic Bottom Water Warming in the Brazil Basin: 1990s through 2020, from WOCE to Deep Argo

Johnson, G.C., C. Cadot, J.M. Lyman, K.E. McTaggart, and E.L. Steffen

Geophys. Res. Lett., 47(18), e2020GL089191, doi: 10.1029/2020GL089191, View online (2020)

A warming trend of 2.1 (±0.4) m °C yr⁻¹ in bottom waters (4,500 to 5,900 dbar) spreading north from Antarctica through the Brazil Basin is quantified by comparing 2019–2020 data from a new Deep Argo regional pilot array to 1989–1995 data from full‐depth zonal and meridional hydrographic sections occupied across the basin before and during the World Ocean Circulation Experiment (WOCE). Additionally, float temperatures are about 0.046°C warmer than those from a long‐term climatology in those same bottom waters. Lower North Atlantic Deep Water in the basin shows no detectable warming, but Upper North Atlantic Deep Water exhibits evidence of warming in both analyses. The bottom‐intensified warming results in a reduction in vertical density stratification between the bottom and deep waters of about 1% per decade. This change is in contrast with the effects of surface‐intensified warming, which tends to increase vertical density stratification.

Plain Language Summary. Surface waters around Antarctica become very cold, salty, and dense through atmosphere and ice exchanges. After cascading down the continental shelf to the abyssal ocean, they spread northward as “Antarctic Bottom Waters.” Widespread warming of these waters over the past few decades has been detected by analyses of data from decadal occupations of a set of worldwide, transoceanic research cruises. The better to monitor and understand these and other deep ocean changes, international oceanographers are working together to deploy arrays of oceanic robots (Deep Argo floats) that collect oceanographic data from the sea surface to its floor and report them in real time. We analyze data from one of these arrays, recently deployed in the Brazil Basin. By comparing the Deep Argo data collected from 2019 to 2020 with data collected from 1989–1995 during research cruises in the region, we quantify the warming trend of Antarctic Bottom Water as 0.002°C yr⁻¹ over that time period, which is about one seventh the global sea surface temperature warming trend from 1989–2019. Furthermore, since the bottom waters are warming, but the deep waters above them are not, the density difference between them is lessening over the decades, potentially influencing ocean mixing and circulation.