Modulation of Western South Atlantic Marine Heatwaves by Meridional Ocean Heat Transport

Marine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short-lived nature and dependence on air-sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3-5 years. The pattern has a significant in-phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 82% of the daily extreme warm (90th percentile) and cold (10th percentile) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward-propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r > 0.6) with the strength of the meridional overturning circulation at 25 degrees S and 30 degrees S with a lag of approximately 3-9 months. The lagged response provides a potential source of predictability of extreme events in the western South Atlantic.