The seasonal cycle of the depth of several isotherms representative of the main thermocline near the western coast of South America shows important semiannual and annual components. Annual thermocline oscillations are mainly related to wind stress curl changes and, secondarily, to alongshore wind and the annual equatorial Kelvin wave. In contrast, the important semiannual component observed in hydrographic data off Peru and Chile is mainly of equatorial origin. Based on satellite sea level anomalies and Rossby wave theory, we show that the coastal seasonal signal --annual and semiannual-- propagates seaward near 21¿S in the form of long Rossby waves, which may modulate the thermocline depth several hundreds of kilometers offshore.