Observational exploration of the subtropical gyre of the South Atlantic

Past modeling and observational studies have indicated that the South Atlantic subtropical gyre is intensifying and shifting poleward, as well as experiencing an increase in its mean sea surface height. In this study, we further investigate these changes using: (1) absolute dynamic topography derived from AVISO for 1993-2017, (2) the Roemmich-Gilson Argo Climatology for 2005-2017, and (3) NOAA’s Optimum Interpolation Sea Surface Temperature fields from 1993-2017. We estimate basin-wide trends, as well as the seasonal and inter-annual variability, of the sea surface height field in this subtropical gyre. We find that the observed basin-wide trend in sea surface height can be attributed to the increase in basin-wide temperatures rather than to dynamic changes. Furthermore, an EOF analysis of the annual sea surface height field reveals the dominant mode of variability (62%) to be a basin-scale rise in sea surface height. EOF analysis of the annual detrended sea surface height reveals the South Atlantic Ocean Dipole to be the primary mode, explaining 23% of the variability. On the gyre-scale, observed seasonal variability in sea surface height is primarily explained by the seasonal heating and cooling cycle. We find no significant trend in the gyre strength, nor any poleward shift of the gyre.