Presented on November 13, 2024, by

Professor Tim Li
Department of Atmospheric Sciences
University of Hawaii

ABSTRACT

Energetic synoptic-scale disturbances often develop over the South Asian Monsoon region, but mechanisms responsible for the observed phenomenon remains open. Given a strong easterly vertical shear of the background mean flow, a possible candidate is baroclinic instability (BI), but this mechanism was denied by Cohen and Boos (2016) based on the observed vertical structure of the monsoon depressions. Another candidate is the moisture-vortex instability (MVI) proposed by Adames (2021) in which perturbation moisture caused by anomalous meridional and zonal advection holds a key for eddy available potential energy generation.

In this work we re-examine the BI and MVI, by examining the characteristics of general synoptic disturbances and conducting a theoretical study with a 2.5-layer model that extends the classic 2-level quasi-geostrophic model to including a prognostic lower-tropospheric moisture tendency equation and an interactive planetary boundary layer. It is found that the BI does work in the South Asian monsoon, and that the most unstable mode prefers a zonal wavelength of 4000 km, a westward phase speed of 6 m s -1 , an eastward tilt vertical structure and a westward shift of maximum moisture and precipitation centers relative to the lower-tropospheric vorticity center, all of which agree with the observations. Sensitivity experiments show that low-level westerly is not a necessary condition for the MVI to occur and the growth rate is reversely proportional to convective adjustment time, both of which contradict to Adames (2021). Thus, the current work sheds light on understanding the fundamental dynamics of the South Asian monsoon.