Thermal changes in the western tropical Pacific in relation to the wind field

Vertical displacement of the thermocline may be due either to a convergence or a divergence of Ekman transport in the surface layer. Data from ships of opportunity between New Caledonia an¿ Japan are used to compute the wind stress curl, from 1973 to 1980, neglecting the zonal gradients. The pattern is compared to the changes of the 25OC isotherm depth between 5ON\ and 20°S from all data available between 160°E and 180°. Alternate long periods of lifting and deepening of the thermocline appear in the western Pacific that may be roughly related to El Niño. Preceding El Niño the thermocline is deep in both the equatorial and the tropical western Pacific. The feature is due to the baroclinic response to the trade winds forcing on the equator associated with an Ekman convergence in the tropical zone. After El Niño, the thermocline is liîted up. This is due to the relaxation of the trade winds on the equator coupled with an Ekman divergence in the tropical zone induced by the presence of the intertropical convergence zone of the winds on the equator. High values of vertical velocities (more than 20cmday-') are, however, confined to the 10°N to IOOS band.