Intense cooling of the sea surface at intraseasonal time scales takes place in the southern tropical Indian Ocean during austral summer. Mechanisms that cause intraseasonal cooling are investigated using in situ observations by Argo floats, remote sensing data sets and simulation using a high resolution ocean general circulation model of the Indian Ocean. Temperature profiles from Argo floats within the cooling region show evidences for the entrainment of cool thermocline water into the mixed layer. The cooling events are accompanied by an increase in sea surface chlorophyll concentration which provides additional evidence for entrainment. The Indian Ocean model reproduces intraseasonal cooling events and demonstrates that mixed layer cooling originate from vertical processes at the base of the mixed layer for certain events. The air-sea flux tends to be the dominant cooling mechanism when the thermocline is deep and mixed layer is thick whereas entrainment dominates when the thermocline is shallow.