Relationship Between Intraseasonal Variability of Upper Ocean Heat Content in the Bay of Bengal and Monsoon Intraseasonal Oscillations

Abstract The northward propagation of the Monsoon Intraseasonal Oscillations (MISOs) in the Bay of Bengal (BoB) is an intrinsic characteristic of the Indian summer monsoon (ISM). Previous studies have demonstrated the critical role of air‐sea interactions in modulating MISO propagation. This study elucidates the intraseasonal variability of ocean heat content (OHC) in the upper 200 m of the BoB and its dynamic relationship with MISO. Similar to sea surface temperature (SST), the positive OHC anomalies lead MISO's northward propagation, showing two prominent maxima located east of Sri Lanka and in the northwestern BoB. The OHC anomalies are stronger east of Sri Lanka, penetrating the thick barrier layer during MISO events, whereas temperature anomalies in the northwestern BoB remain confined to the mixed layer. Diagnostic analyses reveal that the intraseasonal OHC variability, unlike that of SST, stems from intensified downward vertical advection driven by intraseasonal vertical velocity. In contrast to the wind‐dominated intraseasonal vertical velocity in NBOX, the pronounced intraseasonal OHC variability east of Sri Lanka stems from sea level anomaly generated by both westward‐propagating Rossby waves and MISO‐related winds. Subsequently, with the arrival of MISO, upward vertical advection and thick barrier layer prolong warm SST anomalies east of Sri Lanka, providing additional heat and moisture to enhance MISO convection and rainfall intensity. These results highlight the essential role of the memory effect of upper ocean heat exchange and redistribution processes in sustaining MISO propagation.