Phenomenological Paradigm for Midtropospheric Cyclogenesis in the Indian Summer Monsoon

Mid-tropospheric cyclones (MTCs) are a distinct class of synoptic disturbances, characterized by quasi-stationary cyclonic circulation in mid-tropospheric levels, which often produce heavy rainfall and floods over western India during the summer monsoon. This study presents a composite and diagnostic process study of long-lived (>5 days) mid-tropospheric cyclonic circulation events identified by the India Meteorological Department (IMD). Reanalysis data confirm earlier studies in revealing that the MTC composite has its strongest circulation in mid-troposphere. Lagged composites show that these events co-occur with broader-scale monsoon evolution, including larger synoptic-scale low-pressure systems over the Bay of Bengal (BoB) and east coast, and the active phase of regional-scale poleward-propagating intraseasonal rain belts, with associated drying ahead (north) of the convectively active area. Diabatic heating composites, in particular the TRMM latent heating and MERRA2-derived radiative cooling in the dry inland areas of Southwest Asia north of the rain belt, are used to drive a nonlinear multi-layer dynamical model in a forced-damped reconstruction of the global circulation. Results show that the midlevel circulation is largely attributable to top-heavy latent heating, indicative of the prevalence of stratiform-type precipitation in mesoscale convective systems in these moist, active larger-scale settings. Both the west coast and BoB latent heating are important, while the radiative cooling over Southwest Asia plays a modest role in sharpening some of the simulated features. A conceptual model encapsulates the paradigm based on this composite and diagnostic modeling, a diabatic update of early theoretical studies that emphasized hydrodynamic flow instabilities.