Seasonal variability of the surface dynamic topography in the tropical Atlantic Ocean

Abstract

The seasonal changes of the surface dynamic topography are studied using a merged data set including bathymetric thermograph, expendable bathymetric thermograph and Nansen observations, collected in the 20S-30N tropical Atlantic until 1978. Two regions of maximum seasonal variability appear. One lies north of the equator extending zonally approximately under the mean position of the Intertropical Convergence Zone (ITCZ). The 2nd is in the eastern equatorial region (Gulf of Guinea). The 2 regions vary out of phase and are separated by a pivot point around 25W in the equatorial plane. Another .apprx. 180.degree. phase change is observed at the northern edge of the region of maximum seasonal variability north of the equator. These amplitude and phase changes in the surface dynamic topography are related to a double seasonal tilt of the thermocline recently simulated by several 2-layer and multilayer models. The seasonal variability of the geostrophic part of the North Equatorial Countercurrent (NECC) and the South Equatorial Current (SEC) is related to these near-equatorial areas of large seasonal dynamic height variability. Their seasonal variations are almost in phase and are maximum in the northern summer season (July-Oct.) and near zero or reversed in the northern winter-spring season (Feb.-May). The seasonal variability of the surface dynamic topography of the tropical Atlantic appears to be related to 2 phenomena: a northern equatorial oceanic response due to the local effect of the seasonal migration of the ITCZ associated with wind stress change and an equatorial eastern oceanic response due to the remote effect of this wind stress change and the associated local thermodynamic effects.