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.