Seasonal dynamics of the East Pacific Fresh Pool and Sea Surface Salinity front off Panama

The freshest waters in the tropical Pacific are found off Panama at the south-east border of the East Pacific Warm Pool. In this region, the meeting of the Intertropical Convergence Zone (ITCZ) and the American Cordillera gives birth to monsoon-like wind and rain patterns. Using more than 30 years of thermosalinograph data from ships of opportunity, we describe the seasonal cycle in Sea Surface Salinity (SSS) which results from this ocean-atmosphere-land interaction. Its main feature is the presence of a Fresh Pool (SSS<32 pss) with a strong salinity front at its western boundary whose zonal migration is around 1000 km. We explore the seasonal dynamics of this Fresh Pool with the help of satellite wind, rain and surface current data, and Argo hydrographic profiles. The Fresh Pool appears along the Panama-Colombia coast following the strong summer rains, when the ITCZ is in its northern-most position. Advection of saltier waters by the North Equatorial Counter Current limits its westward extension and contributes to the creation of a salinity front. In winter, as the ITCZ moves south, the Panama wind jet creates an Ekman pumping dipole and associated geostrophic eddies. As a result, upwelling in the Panama bight brings to the surface saltier waters which erode the Fresh Pool on its eastern side, while the South Equatorial Current moves it westward. Also, low SSS is generally associated with high Sea Surface Temperature (SST) in this region and Argo profiles show that barrier layers are a frequent feature in the Fresh Pool. It may be that barrier layers created by heavy rains locally enhance SST and atmospheric convection, providing a positive feedback on rain, as suggested by the location of the rainfall maximum over the ocean.