Observed seasonal variability of heat content in the upper layers of the tropical Indian Ocean from a new global ocean temperature climatology

A subset of the recently published temperature climatology for the global oceans is utilized to characterize the observed seasonal variability in the heat content (HC) of the upper layers, estimated from surface to fixed depths (50, 100, 200 and 300 m), and with respect to fixed isotherm (27, 26, 25 and 20°C) depths, for the entire tropical Indian Ocean (TIO). The most prominent features of the observed annual cycle of the HC are described in terms of amplitude and phase of the annual and semi-annual frequencies employing Fourier analysis. The relationship between the SST and the upper layer HC over an annual cycle is explored through correlation analysis. stributions of the HC of the uppermost 50 m and with respect to 27°C depth show similar seasonal variability with peak values during May, registering a dramatic drop during the summer monsoon in the Arabian Sea and the Bay of Bengal. The distributions of HC show the most prominent annual mode in the uppermost 50 m in the southern TIO and in the 50–300 m off Arabia and southwest and southeast India. The annual mode of HC is weak in the equatorial band. On the other hand, both the western and eastern equatorial regions show the most pronounced semi-annual mode in the 50–300 m layer. This mode is also strong in the uppermost 100 m over the northwestern Arabian Sea. The dominant annual mode in the HC with respect to isotherms is most prominent in the southeastern Arabian Sea. The semi-annual mode of HC with respect to 20°C is most prominent in the northwestern Arabian Sea and the western and the eastern equatorial regions. The correlation between the SST and the HC of 50 m is high over the entire basin. In the southern and western TIO, the regions of strong surface wind field with deep and diffuse thermocline, the correlation between the SST and the HC is high and penetrates deeper. The correlation degrades with depth, in particular in the warm pool region where the surface wind field is weak and the thermocline is shallow and sharp. The correlation between the SST and the HC with respect to isotherm depths also shows a similar degradation with depth in the warm pool region with the exception of a pocket in the southeastern TIO, where the thermocline topography is determined by propagating Rossby waves.