TROPICAL ATLANTIC SEA-LEVEL VARIABILITY FROM GEOSAT (1985-1989)

Geosat altimeter data between April 1985 and September 1989 are analyzed in the tropical Atlantic Ocean. First, improvements due to the use of new corrections and orbit computations are found to be effective, especially in the Gulf of Guinea, where part of the previously missing signal is recovered. Then, the variability of the ocean is examined using empirical orthogonal functions (EOFs). Only three EOFs are needed to describe 80% of the seasonal variance. The first one describes the meridional tilting of the tropical Atlantic along the mean location of the Intertropical Convergence Zone, with an annual period. The second describes a mass redistribution due to the equatorial upwelling peaking in June-July. The third function presents a clear semiannual signal. Looking at interannual variability, the first EOF reveals a mass redistribution between the equatorial region (10-degrees-N to 10-degrees-S) and the northern and southern ones (10-degrees-N to 30-degrees-N, 10-degrees-S to 30-degrees-S). In the equatorial region the upper layer volume increases from about - 1.5 to 1.5 x 10(14) m3 between 1987 and 1989. Occurring 1 year after the Pacific El Nino, this phenomenon recalls the 1984 anomaly observed during the Programme Francais Ocean et Climat en Atlantique Tropical/Seasonal Equatorial Atlantic experiments.