Variability and Trends of the South Atlantic Subtropical Gyre

Previous studies have suggested that the South Atlantic subtropical gyre has been expanding, shifting poleward, and intensifying. We investigate these possibilities from an observational standpoint by analyzing the seasonal and interannual variability, and the long-term trends, in sea surface height, steric height, and ocean mass. For our analysis, we employ satellite altimetry from AVISO (1993-2018), steric height fields reconstructed from the Roemmich-Gilson Argo Climatology (2004-2016), and ocean mass fields from GRACE (2002-2019). Seasonal variabilities in the gyre position and size are influenced by the positions of the northern and eastern boundaries, as well as the Brazil-Malvinas Confluence. The gyre strength follows the seasonal heating and cooling of the upper ocean and is largely influenced by variability in the sea surface height gyre maximum. Variability in the ocean mass is driven by the wind stress curl and out of phase with the steric signal. Interannual changes to the southern boundary and strength of the gyre are related to the wind stress curl. We find a significant southward migration of the southern gyre boundary but no trends in the gyre size or gyre strength from 1993 to 2018.