New investigations of monsoon depressions using observational data, theory and numerical experiments

Abstract

Monsoon depressions (MDs) are synoptic-scale storms that exist embedded within the planetary-scale summer monsoon circulation. Their structure and propagation is distinct to other tropical and sub-tropical systems. The last five years have seen particular advances in the understanding of the atmospheric dynamics and growth mechanisms of MDs. This thesis builds upon this recent literature, and then covers the development and results of new analyses of atmospheric reanalysis data and a series of idealised modelling experiments that have been conducted to gain new understanding and insight into MDs. This work is introduced with a discussion of the global monsoon focusing on the particular background conditions during the South Asian Summer Monsoon that support a high-frequency of MDs. A tracking algorithm for low-pressure events is developed and used to construct composite South Asian MDs in high-resolution ERA5 data. A sensitivity of faster westward MD propagation to the mid-tropospheric meridional temperature gradient is identified. Such insights then motivate the development of a new idealised moist-thermal quasi-geostrophic (QG) model as a dynamical toolbox to understand the fundamental dynamics of MDs. Linear-modes of this QG model demonstrate a sensitivity to background temperature and precipitation gradients, but fail to organise precipitation and dynamics in a manner similar to that of observed MDs. A variety of experiments, with large-amplitude MD-analogues, are shown to capture MD-like propagation and sensitivity to background temperature gradients by diabatic-barotropic processes and thermally-modulated beta-drift. Finally, a set of numerical experiments are conducted in an idealised general circulation model using a series of idealised surface configurations. Results of these experiments demonstrate an idealised configuration that captures South Asian monsoon and MD-like variability, and provide insights into the relationship between MDs and meridional temperature gradients.