Abstract
The space-borne measurements of the SMOS mission reveal for the first time the complete features of the sea surface salinity (SSS) signature at the full scale of the Pacific basin. The SSS field in the equatorial cold tongue is typically found to be larger than 35.1 within a narrow 2° band of latitude that is positioned slightly south of the equator and that stretches across the eastern Pacific basin up to the Galapagos Islands. On the northern edge of the eastern equatorial Pacific this signature results in a very strong horizontal gradient (larger than 2 units over 100 km) with the fresh waters of the Panama warm pool. By considering a water density criterion, it can be shown that the cold tongue is characterized by a strong seasonal cycle with a 3°C amplitude in SST where the warm season of February-March contrasts with the cold period extending from September to November. If the present ocean reanalyzes are able to capture these features, then the assimilation of the SMOS data becomes a worthwhile objective in order to depict more accurately the salinity signature of the cold tongue of the tropical Pacific.
Highlights
The classic paradigm of interannual climate variability in the Tropics involves positive feedback of ocean? atmosphere interaction, the Pacific cold tongue and the El Ni?o/Southern Oscillation (ENSO) phenomenon [4, 11]
The cold tongue is characterized by surface waters with the greatest density in the equatorial zone, typically more than 22.5 kg/m3, that marks the eastern limit of the western Pacific warm pool [15, 22, 23]
Between 4N? and 10?N, low salinity tropical surface water extends across the Pacific and coincides with a region of the InterTropical Convergence Zone (ITCZ) where, in the mean, precipitation exceeds evaporation
Summary
The classic paradigm of interannual climate variability in the Tropics involves positive feedback of ocean? atmosphere interaction, the Pacific cold tongue and the El Ni?o/Southern Oscillation (ENSO) phenomenon [4, 11]. The equatorial cold tongue results from the divergence of surface Ekman currents in response to the large-scale southeast trade winds, which brings cool waters through upwelling into the surface layers As it represents a substantial part of the equatorial band where the ocean gains a large contribution of heat from the atmosphere, nuch attention has been given to various mechanisms involved in the maintenance of the cold tongue. As opposed to the attention given to the western equatorial Pacific where salinity anomalies can significantly influence SST and ENSO (i.e., [22, 33]), the salinity variability has been largely ignored in the eastern Pacific cold tongue In his pioneering study of the Pacific equatorial upwelling by means of a simple box model Wyrtki [32] underlined the difficulty of closing the salt budget due to the presence of a strong north? The recent satellite observations by the ESA Soil Moisture and Ocean Salinity (SMOS) and the
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