Variability of summer monsoon depressions over the Bay of Bengal with special emphasis on El Niño cycle

Interannual variability of summer monsoon depressions (MDs) over the Bay of Bengal (BoB) is investigated with special emphasis on developing and decaying El Niño phases during the period 1979–2020. Previous studies have overlooked the relationship between MDs frequency and El Niño phenomena, despite of their stronger association with Indian Summer Monsoon (ISM) rainfall. Our analysis revealed that the number of MDs has reduced remarkably by a factor of 2 during El Niño decay summers (more than half) as compared to climatology and in fact, no such decline is depicted in developing El Niño years. The strong easterlies extending from the southern flank of the Western North Pacific (WNP) anticyclone to the southern BoB cause negative low-level vorticity anomalies and reduce the Barotropic instability during El Niño decay summers. These conditions are unfavourable for the formation of MDs in the BoB. It is also noted that the Genesis Potential Parameter (GPP) for the MDs over the BoB and WNP region is weaker in the El Niño decay summers compared to developing El Niño and normal years. Moreover, during the El Niño decay summers less number of Tropical Cyclones (TCs) over WNP due to weak GPP, reduced the chances of westward propagating remnants of TCs from the Philippine Sea to the BoB, which is not favourable for the formation of MDs. Essentially, the reduction in the frequency of MDs would influence the seasonal rainfall distribution over the Indian subcontinent and largely impacts the regional agriculture activities and water management sector. Further, the Indo-western Pacific capacitor (IPOC) effect, which is strongly tied to decaying El Niño phase plays a critical role in altering the number of MDs by maintaining WNP anticyclone. Thus, predicting IPOC pattern during the decay phase of El Niño would provide useful insights on a number of MDs and ISM rainfall prediction.