Downstream and In Situ: Two Perspectives on the Initiation of Monsoon Low-Pressure Systems Over the Bay of Bengal

Abstract

Monsoon low-pressure systems (LPSs) contribute to more than half of the total summer monsoon rainfall over central India. As their genesis mechanism is not well understood, the LPS-related precipitation contribution is ill represented in climate models, which has contributed to the underestimation of rainfall over central India in climate model simulations. Two hundred fifty-six cases of LPS initiations over the Bay of Bengal during 1979 to 2017summer seasons were analyzed, and it was found that 68% of the systems were initiated in situ, while the remaining 32% were initiated by downstream amplification of wave disturbances from the western Pacific. Detailed analysis reveals that the LPS generated by the two mechanisms have similar dynamic and thermodynamic features. A declining trend is also observed in the number of downstream generated cases indicating that it would become increasingly difficult to predict the initiation of LPS in the future.