Interannual Variability and Multiyear Trends of Sea Surface Salinity in the Amazon-Orinoco Plume Region From Satellite Observations and an Ocean Reanalysis

Abstract

The Amazon and Orinoco Rivers form the largest river system on Earth with wide-reaching impacts on the biophysical climate of the region. Based on 11 years of satellite observations, here we show strong interannual variability of sea surface salinity (SSS) in the plume region that is about three times larger than in the surrounding region. This variability has been driven mainly by changes in horizontal advection of the plume with smaller contributions from precipitation, river discharge, and vertical mixing. Interannual variability of horizontal advection occurs mainly near the eastern boundary of the plume region and especially in the latitude range of the North Equatorial Countercurrent (3°–10°N). The North Atlantic Oscillation (NAO) and El Niño Southern Oscillation (ENSO) strongly modulate SSS in the plume region during the boreal spring-summer (flood) and fall-winter (dry) seasons, respectively, through their impacts on the tropical North Atlantic Ocean circulation. Overall, the NAO exerts a stronger influence on plume SSS than ENSO. The plume SSS also shows a significant upward trend during 2010–2016, corresponding to transitions of the NAO and ENSO from negative to positive phases. This was followed by a downward trend during 2017–2020 when both the NAO and ENSO decreased from their 2015–2016 peaks. These results show that year-to-year changes in the spatial mean SSS of the Amazon-Orinoco plume are driven mainly by large-scale climate forcings and their associated imprints on tropical Atlantic Ocean circulation and very little by changes in river outflow and associated rainfall over land.