Observations from the TOPEX/POSEIDON (T/P) altimeter reveal that the sea level in the Indian Ocean varies strongly with season. Surface undulations propagate eastward along the equator and westward outside the equatorial waveguide. The forcing mechanisms responsible for such variations are investigated in this paper by analyzing T/P data and surface wind-stress forcing, and by using a nonlinear, 2.5-layer, reduced gravity model. Both the data and the model show that Kelvin and Rossby waves forced by the Indian Ocean monsoon are primarily responsible for the seasonal sea-level change in the tropical and the subtropical Indian Ocean. ¿ 1998 American Geophysical Union