The interannual variability in the tropical Indian Ocean is examined using 41-year (1958--1998) seasonal anomalies of the upper-ocean heat content (HCA), sea surface temperature (SSTA), and surface wind stress. Precipitation anomalies from a shorter period (1979--1998) have also been analyzed. This analysis demonstrates that a coupled ocean--atmosphere interannual oscillation with a period ranging from 2 to 5 years is the major variability in the tropical Indian Ocean. At the peak phase, anomalous equatorial zonal winds over the central and the eastern ocean and anomalous trade winds to the south induce zonal SSTA and HCA gradients near the equator and an east--west shift of the convection. This interannual oscillation is the dominant signal from the boreal autumn to the next spring. The westward propagating HCA causes a phase delay between the peaks of the surface cooling near the eastern coast and the warming near the western coast near the equator. During its propagation, the southern HCA branch is strengthened by the anomalous wind curl of the equatorial and southeast trade wind anomalies over the southern ocean. As a result, the southern HCA is maintained near the western coast for a much longer period. This Indian Ocean oscillation is significantly correlated with the El Ni¿o/Southern Oscillation (ENSO) variability in the Pacific Ocean.