Energetic processes underlying the interannual variability of South Asian summer monsoon

The South Asian summer monsoon (SASM) interannual variability significantly impacts regional climates, with its first mode featuring a lower-level anomalous anticyclone over the northern Bay of Bengal (BOB) and the second mode displaying an anomalous anticyclone over central-northern India. Here, we diagnose the energy budget of these two SASM modes. Barotropic energy conversion supplies eddy kinetic energy (EKE) to the first mode from lower-level climatological confluent westerlies downstream of the Somali Jet and over the BOB–western North Pacific. In contrast, the second mode derives EKE from the subtropical westerly confluence induced by Tibetan Plateau topography. Baroclinic energy conversion, extracting eddy available potential energy (EAPE) from the mean thermal structure, sustains the second mode while dampening the first. Convective heating generates EAPE for both modes, acting as feedback. Further analyses suggest these modes likely stem from internal dynamics. Our findings highlight the importance of internal energetic processes in SASM modulation.