Interannual variability of the South Atlantic subtropical mode water: Investigating the early aughts shift and its relationship to ENSO modulations

Abstract

The South Atlantic subtropical mode water (SASTMW) is a conspicuous volume of water formed during winter and early spring that occupies the mid-latitudes of the South Atlantic. Here, we aim to investigate the interannual variability of the SASTMW's volume in response to remote processes influenced by atmospheric teleconnection. We utilized the Simple Ocean Data Assimilation (SODA) ocean reanalysis to identify this particular mode water. The SASTMW's volume time series underwent a shift during the early aughts, with the frequency spectrum changing from broad and sparse interannual peaks during 1980–2001 to narrow and frequent peaks during 2002–2018, following the interdecadal shift of El Niño–Southern Oscillation (ENSO) that occurred in 1999. Hence, the influence of ENSO on SASTMW was evaluated for each of the two periods. From 1980 to 2001, in the western side of the basin, SASTMW's volume reached its peak 21 months after the occurrence of an El Niño, and after 43 months it became thinner. The opposite pattern occurred after a La Niña event, with stronger events having a greater impact on the SASTMW. In the 2002–2018 period, the largest correlations were observed in the eastern part of the basin, with significantly shorter lags of 6–20 months.