Freshwater plume seen by SMOS (top) and Aquarius (bottom) in January–March 2012. This freshwater plume west of Panama is known as the Far Eastern Pacific Fresh Pool. It is an interesting area for research because it exhibits very low salinity, reaching a minimum of 33 psu (practical salinity units) compared with the average of 35 psu. However, there are also large variations in this value depending on season.
The fresh pool results because of the heavy summer rains over Central America. During the second half of the year, the eastward-flowing North Equatorial Counter-Current keeps it trapped near the coast and strengthens the salinity front on its western edge. During winter, the north-easterly Panama gap wind creates a south-westerly jet-like current in its path. As a result, upwelling in the Panama Bight brings cold salty waters to the surface which erode the fresh pool on its eastern side while both the jet current and the enhanced South Equatorial Current stretch the fresh pool westward until it virtually disappears in May.
Data from SMOS and Aquarius are in good agreement, depicting similar large scale features. Local difference are still subject to scientific research.