Antarctic Bottom Water Warming and Circulation Slowdown in the Argentine Basin From Analyses of Deep Argo and Historical Shipboard Temperature Data

A decadal warming trend of 2.1 (+/- 0.2) m degrees C yr(-1) in Antarctic Bottom Water within the western Argentine Basin is found by comparing Deep Argo temperature profiles from 2021 to 2022 to nearby historical shipboard data from 1972 to 1998. This trend is similar in magnitude, but about 10 times more certain, than a previously published trend in the eastern Argentine Basin estimated using repeat hydrographic section data (World Ocean Circulation Experiment Section A16S) from 1989, 1995, and 2014. The present analysis also detects a warming rate in the coldest water entering the basin about double that in the interior. The observed reduction in deep meridional temperature gradient indicates a reduction in geostrophic shear, consistent with a reduced flow rate, and transport, of the coldest, deepest water entering the basin. The falling isotherms in the basin are consistent with an 0.6 x 10(6) m(-3) s(-1) reduction in inflow of the coldest waters to the basin. Plain Language Summary The coldest, densest waters that sink to the ocean floor around Antarctica and spread northward have been warming in recent decades, as observed in much of the global ocean by shipboard oceanographic surveys since the 1990s. However, large portions of the ocean are unsampled by these surveys, which are only repeated at decadal intervals. Quantifying the warming of the deep ocean, the sea level rise that this warming drives, and associated changes in ocean currents are important for validating projections of future warming and understanding climate change impacts. As a result, regional pilot arrays of freely drifting robotic profiling floats capable of measuring temperature and salinity from the sea surface to the seafloor, called Deep Argo floats, are being deployed to demonstrate their capabilities and build toward a global array. We compare data from Deep Argo floats recently deployed in the Argentine Basin of the western South Atlantic Ocean to historical temperature profiles measured from ships prior to the year 2000. By doing so, we quantify the warming rate of these bottom waters over the bottom 1.5 km of that basin with about 10 times more certainty than by previously published studies using shipboard data alone.

Automated summary

A study finds a decadal warming trend in Antarctic bottom water within the western Argentine Basin, showing a similar magnitude but greater certainty compared to previous studies. The analysis also reveals a faster warming rate in the coldest water entering the basin than in the interior. The reduction in deep meridional temperature gradient suggests a decrease in geostrophic shear and the flow rate of the coldest water.