A subset of the global ocean temperature climatology is used to characterize the observed seasonal variability in the near-surface thermal structure of the tropical Indian Ocean. In the near-surface isothermal layer the seasonal variability is least in the warm pool region and increases poleward. Over much of the tropical Indian Ocean, local surface heat fluxes overwhelm horizontal advection in the seasonal evolution of the mixed layer temperature. Entrainment cooling is weaker by an order of magnitude than the other two processes. Several dynamical processes produce the most prominent signals in the thermocline in different geographic regions, such as coastal upwelling/downwelling with associated reversals in the flow off Arabia, off southwest India, and off the east coast of India, Ekman-driven thermocline deepening in the central Arabian Sea, convergence of waters caused by Wyrtki Equatorial Jets in the eastern equatorial Indian Ocean, Ekman divergence of a dome of cold waters just south of the equator in the west, propagating Kelvin waves in the coastal regions of the Bay of Bengal, and propagating Rossby waves from off southwest India to Somalia and in the Indo-Pacific throughflow region. For the spatio-temporal resolutions considered in this study the penetration of the seasonal cycle of temperature appears to be limited to 150 m depth or so. Numerical model solutions have revealed the causative mechanisms for some of the prominent signals seen in the observations, in particular, those driven by long-period propagating waves in regions of large stratification, whose structure was not revealed by earlier climatologies to date, as revealed by this new climatology. ¿ 2000 American Geophysical Union