Diurnal Cycle Induced Amplification of Sea Surface Temperature Intraseasonal Oscillations Over the Bay of Bengal in Summer Monsoon Season

In spite of strong mean summer monsoon winds, the magnitudes of diurnal and intraseasonal oscillations (ISO) of the sea surface temperature (SST) in the Bay of Bengal (BoB) are as strong as the respective magnitudes in the western Pacific. Using continuous observations during the peak summer monsoon of 1998 at BoB buoy (DS4) located at (89° E, 19° N), we show that the strong near-surface diurnal variation in the BoB during warming phases of the ISO leads to almost double the magnitude of the diurnal SST over the BoB as compared to that during the cooling phases. The simulation experiments with and without the diurnal cycle of surface fluxes indicate that more than one-third of the observed SST ISO amplitude could arise from the rectification of the diurnal cycle through the influence of late night and early daytime upper-ocean mixing processes during the warming phases. The rapid shoaling of the upper-ocean mixed layer occurs during afternoon while it deepens slowly during late night and early daytime which tends to retain the warm SSTs at the end of the nighttime cooling. The insight derived from these experiments on the influence of the diurnal cycle on ISOs of the SST underlines the need for a proper simulation of the diurnal cycle of the SST in climate models.