What Controls Seasonal Variations of the Diurnal Cycle of Sea Surface Temperature in the Eastern Tropical Indian Ocean?*

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Abstract Recent in situ buoy observations revealed interesting seasonal features of the diurnal sea surface temperature cycle (DSST) in the eastern tropical Indian Ocean. Composite analysis shows that areas away from the equator exhibit stronger seasonal variations of DSST, while weaker seasonal variations appear near the equator. The most interesting characteristic is the distinctive contrast of the seasonal variations of DSST between the Bay of Bengal (BOB) and the region south of the equator (particularly around 12°S). While the range of DSST is weakest in the BOB during boreal summer, it has its largest range around 12°S in austral summer. Furthermore, BOB DSST exhibits two peaks that occur during the monsoon transitions (March–April and October), whereas DSST south of the equator shows only a single peak in its annual cycle. Using a one-dimensional, oceanic, mixed layer model, the authors examined the cause of the distinctive annual cycles of DSST north and south of the equator. Two parallel experiments were conducted at buoy sites 12°N, 90°E and 12°S, 80.5°E driven by surface forcing from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) product. The results demonstrated that, in the BOB, both surface shortwave radiation and wind stress contribute to the March maximum, whereas the wind stress alone drives the October maximum. In contrast, the seasonal variation of DSST south of the equator is primarily caused by the annual cycle of the wind stress, which is extremely weak in austral summer near the intertropical convergence zone (ITCZ). How the monsoon and ITCZ modulate the distinctive annual cycles of DSST is discussed.