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

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. Plain Language Summary: About 200,000 m3 of fresh water per second is discharged into the tropical North Atlantic by the Amazon and Orinoco Rivers, creating a large low‐salinity plume that sometimes exceeds a million km2. The plume is often more turbid and stable than surrounding waters and can trap more heat from solar radiation, triggering atmospheric convection that can increase rainfall over the Caribbean Sea and Central America and strengthen tropical cyclones. Here, we find strong interannual variability of surface salinity in the Amazon‐Orinoco plume region over the 2010–2020 satellite period. Additional data from an ocean reanalysis reveal that interannual variability of the plume is mainly driven by large‐scale ocean circulation, which in turn is driven by large‐scale climate phenomena: El Niño Southern Oscillation (ENSO) in the dry season and the North Atlantic Oscillation (NAO) in the flood season. The plume surface salinity also shows a significant upward trend (i.e., weakening of the plume) during 2010–2016, corresponding to transitions of the NAO and ENSO from negative to positive phases. Key Points: Horizontal advection influenced by the North Atlantic Oscillation (NAO) is the main driving force for plume interannual variabilityThe NAO and El Niño Southern Oscillation (ENSO) are the ultimate drivers of plume salinity in the flood and dry season, respectivelyPlume salinity shows a significant positive trend during 2010–2016 when NAO and ENSO transitioned from a negative to a positive phase [ABSTRACT FROM AUTHOR]