Warming Trend in Antarctic Bottom Water in the Vema Channel in the South Atlantic

Abstract

The excess heat absorbed from the atmosphere has increased the temperature in the upper layers of the ocean (<2,000 m). In the abyss, infrequently repeated ship sections, deep Argo float measurements, and sparse moored observations have found signs of warming in the Southwest Atlantic, possibly linked to changes in the Weddell Sea. We present a new moored temperature time series sampled near the bottom in the Vema Channel, from February 2019 to August 2020. Together with historical data, the combined record confirms the warming of the abyssal waters, with an increase of 0.059°C in potential temperature between January 1991 and August 2020, embedded within intense high‐frequency variability. Moreover, the data suggest the possibility of an accelerated warming, with a change in the temperature trend from 0.0016°C yr−1, between the early 1990s and 2005, to 0.0026°C yr−1 afterwards.

Energetic overturning flows, dynamic interocean exchanges, and ocean warming observed in the South Atlantic

“…The observed AABW warming is strongest in the Southwest Atlantic where the younger variety of AABW has to cross through the Vema Channel near 31°S, 39°W. Steady and significant AABW warming has been observed from moored observations and hydrographic data in the Vema Channel of about 0.02 °C decade −1 since the 1980s, which has accelerated to approximately 0.026 °C decade −1 since 2005 76,162 . Abyssal warming of 0.02 ± 0.01 °C decade −1 was also observed over the period 2009-2019 from continuous hourly measurements from two moored instruments located near the 4500 m and 4800 m isobaths in the northwestern Argentine Basin along the SAMBA line at 34.5°S 75 (Fig.…”

“…As a result of the excess heat absorbed from the atmosphere associated with ongoing planetary warming, AMOC variability, interocean exchanges from the Pacific and Indian basins, and other factors elucidated in this review, the South Atlantic Ocean has warmed from the surface to the deep and abyssal ocean 12,75,76,147,148,151,163 (Fig. 3a, b, e).…”

“…At the deepest layers (purple pathway in Fig. 1), newly formed abyssal waters of Antarctic origin flow northward into the Argentine and the Brazil Basins [72][73][74][75][76] , cross the equator, and ultimately mix with other deep and abyssal waters.…”

“…The lower limb of the upper AMOC cell involves the export of NADW to the Southern Ocean where it joins the Antarctic Circumpolar Current (ACC). NADW export is compensated by a northward flow of surface, central, and intermediate waters through the upper layers in the South Atlantic and across the equator, and by the northward flow of Antarctic Bottom Water (AABW) carried by the abyssal cell 58,69,74,76,114,[117][118][119] . Heat and freshwater transported by these interocean exchanges substantially impact the Atlantic Ocean as a whole 58,65,66,69,120 .…”

“…Pan-Atlantic observational efforts are crucial for improving climate models and tracking the projected decline of AMOC in the next century in response to anthropogenic forcing 19,41 . Sustained and continuous monitoring of AMOC changes in the South Atlantic is needed to assess their impacts on climate patterns, such as shifts in regional surface temperature and precipitation patterns 32 and extreme events (e.g., marine heat waves, hurricanes), the variability of the key boundary current systems 85,113,122,177 , coastal sea level rise, warming/cooling or salinification/freshening of the ocean 75,76 , deoxygenation and acidification processes. All of these phenomena have large societally-relevant planetary impacts on agriculture, fisheries, infrastructure, and health around the globe 103,245 .…”