Climatological data derived from the Comprehensive Ocean-Atmosphere Data Set (COADS) combined with the climatological temperature fields from XBT observations in the tropical Pacific Ocean are used to assess the importance of various oceanic and atmospheric processes in the seasonal variation of the mixed layer temperature. The analyses indicate that in most areas of the extra-equatorial oceans, where the mixed layer is deep and oceanic mixing is weak, the seasonal change in SSTs can be attributed to the net local heating received by the mixed layer. However, in other parts of the equatorial oceans, oceanic processes are found to play an important role in determining the seasonal fluctuations of the SST. In particular, in the eastern equatorial Pacific Ocean where the mixed layer is shallow, the thermal-inertia of the upper ocean is reduced significantly by the intense vertical mixing so that the response of the SST is nearly in phase with the surface heating. Using the Kraus-Turner type of entrainment parameterization, the analyses further demonstrate that the vertical heat flux induced by the mean entrainment velocity contributes significantly to the mixed layer heat budget. ¿ American Geophysical Union 1993