Seasonal and interannual variation of the upper southern tropical Indian Ocean (STIO) is described by harmonic and empirical orthogonal function (EOF) analysis of the depth of the 20 ¿C isotherm (D20) derived from expendable bathythermograph (XBT) data and from an ocean general circulation model (OGCM). The harmonic analysis shows a band of large annual amplitude between 8¿ and 20 ¿S extending across the STIO with a steady westward propagation in both the XBT and model data. The generation of the annual wave is discussed in terms of Ekman pumping and the westward propagation of long, nondispersive, baroclinic Rossby waves. The Ekman pumping on a large scale over the open ocean strongly modifies waves radiating from the eastern boundary and generates much of the structure in the amplitude and the phase of the annual signal in the STIO. The EOF analysis shows strong interannual variation of the anomaly of D20 on two XBT lines covering the western and eastern sides of the basin. Large interannual anomalies are observed in a band between 6¿ and 14 ¿S on the western side, with deep D20 in 1988, 1991, and 1994 and shallow D20 in 1990 and 1992. On the eastern side, large interannual anomalies are observed close to the coast of Java, and sign is opposite to that on the western side, suggesting an oscillation in the zonal tilt of the thermocline across the STIO. Those interannual variations of D20 are also well simulated in the model. The generation of the interannual anomalies in the model are also due mainly to the wind stress curl integrated along the Rossby wave trajectories in the STIO. ¿ 1998 American Geophysical Union