Influence of the Amazon-Orinoco Discharge Interannual Variability on the Western Tropical Atlantic Salinity and Temperature

Over the last three decades, extreme floods have become increasingly frequent in the Amazon basin, affecting strongly the population and ecosystems of the area. However, the impact of these extreme events on the tropical Atlantic Ocean is still poorly known. In this study, we use a 1/4 degrees coupled ocean-atmosphere model to assess the impact of the runoff interannual variability on the sea surface salinity and sea surface temperature of the area. Twin sensitivity experiments are performed, forced alternatively with interannually-varying and climatological river runoff. Composite fields for the highest floods and lowest floods are also compared. This combination of sensitivity tests and composite extremes allows to separate the effect of runoff interannual variability from the rest of the variability (typically driven by mixing, advection and precipitation). We show that the runoff interannual variability modulates the sea surface salinity of the Amazon plume with the same order of magnitude as the salinity variability driven by ocean dynamics and atmospheric forcing. However, due to vertical mixing, this oceanic imprint of the extreme floods is limited to a few months and a few hundred kilometers from the mouth. Years of extreme floods generally coincide with anomalous phases of the Atlantic Meridional Mode, which are associated with large-scale sea surface temperature anomalies over the tropical Atlantic Ocean. Our results did not reveal any significant modulation of these temperature anomalies by the runoff interannual variability, at any time of the year, questioning the relevance of a hydrological feedback on the tropical Atlantic sea surface temperature.

Automated summary

In the past three decades, extreme floods in the Amazon basin have had a significant impact on the population and ecosystems of the area. However, their impact on the tropical Atlantic Ocean is still poorly understood. This study uses a coupled ocean-atmosphere model to investigate the effect of interannual variability in river runoff on sea surface salinity and temperature. The results show that while extreme floods can modulate the sea surface salinity, their impact on sea surface temperature is limited.