INTERANNUAL SEA-LEVEL VARIATIONS IN THE TROPICAL INDIAN-OCEAN FROM GEOSAT AND SHALLOW-WATER SIMULATIONS

Sea level variations of the Indian Ocean north of 20-degrees-S are ana lyzed from Geosat satellite altimeter data over April 1985-September 1 989. These variations are compared and interpreted with numerical simu lations derived from a reduced gravity model forced by FSU observed wi nds over the same period. After decomposition into complex empirical o rthogonal functions, the low-frequency anomalies are described by the first two modes for observations as well as for simulations. The sums of the two modes contain 34% and 40% of the observed and simulated var iances, respectively. Averaged over the basin, the observed and simula ted sea level changes are correlated by 0.92 over 1985-1988. The stron gest change happens during the El Nino 1986-1987: between winter 1986 and summer 1987 the basin-averaged sea level rises by approximately 1 cm. These low-frequency variations can partly be explained by changes in the Sverdrup circulation. The southern tropical Indian Ocean betwee n 10-degrees and 20-degrees-S is the domain where those changes are st rongest: the averaged sea level rises by approximately 4.5 cm between winter 1986 and winter 1987. There, the signal propagates southwestwar d across the basin at a speed similar to free Rossby waves. Sensitivit y of observed anomalies is examined over 1987-1988, with different orb it ephemeris, tropospheric corrections, and error reduction processes. The uncertainty of the basin-averaged sea level estimates is mostly d ue to the way the orbit error is reduced and reaches approximately 1 c m. Nonetheless, spatial correlation is good between the various observ ations and better than between observations and simulations. Sensitivi ty of simulated anomalies to the wind uncertainty, examined with FSU a nd ECMWF forcings over 1985-1988, shows that the variance of the simul ations driven by ECMWF is 52% smaller, as FSU winds are stronger than ECMWF. Results show that the wind strength also affects the dynamic re sponse of the ocean: anomalies propagate westward across the basin mor e than twice as fast with FSU than with ECMWF. It is found that the di screpancy is larger between ECMWF and FSU simulations than between obs ervations and FSU simulations.