In the tropical oceans, where the temperature is roughly homogenous in the upper layer, vertical variations of salinity may be responsible for significant density stratifications. In this study on the western Tropical Atlantic Ocean, it is shown that fresh surface waters of the Amazon River discharge may induce a strong halocline in the 3--30 m depth range. This halocline induces a pycnocline that acts as a barrier for mixing between the surface and the subsurface waters. The fresh surface waters are generally associated with positive anomalies of the sea surface temperature. These positive anomalies cover a large part of the basin in boreal summer-fall, following the maximum Amazon River discharge, which may have a significant importance for the ocean-atmosphere exchanges. ¿ 1999 American Geophysical Union