SEA-LEVEL ANOMALIES IN THE TROPICAL ATLANTIC FROM GEOSAT DATA ASSIMILATED IN A LINEAR-MODEL, 1986-1988

Along-track sea level anomalies derived from Geosat altimeter data fro m November 1986 to November 1988 are assimilated by Kalman filtering i nto a wind-forced second-baroclinic vertical mode linear model of the tropical Atlantic Ocean. To save computer time, the filter is degraded , mostly by fixing the error covariance matrix of the estimate once th e filter has reached its asymptotic behavior. Geosat altimeter data ha ve been processed using improved corrections. The sea surface height v ariability signal is extracted using the classical along-track techniq ue, relative to a complete reference cycle, and using only tracks long er than 2200 km. This processing has preserved oceanic signals both on large scales (above 1000 km) and on the mesoscale (around 200 km). Se a level anomalies predicted at Principe Island are close to in situ ti de gage data, though some differences can be partly related to tidal o r orbit error corrections. Oceanographic signals are analyzed from two different sets of fields: one issued from anisotropic space-time obje ctive analysis of Geosat data and the other from the model assimilatio n. The latter appears as an interesting method to extract low-frequenc y and propagating signals. Along the equator, eastward propagating fea tures are consistent with Kelvin waves correlated with zonal wind stre ss anomalies. Upwelling in the Gulf of Guinea is 1 month earlier in 19 87 than in 1988. After elimination of the annual and semiannual signal s by harmonic analysis, the residual signal over the whole tropical ba sin, decomposed into complex empirical orthogonal functions, is found dominated by variations between the 2 years, equatorial and tropical s ignals being anticorrelated.