A Tropical Survey of Mid-Tropospheric Cyclones, their Classification and Genesis over the Arabian Sea

Abstract

Middle Tropospheric Cyclones (MTCs) are moist synoptic tropical systems with vorticity maxima in the middle troposphere and weak signature in the lower troposphere. We begin with a tropical survey of MTCs; in South Asia, manual tracking reveals that MTCs change character during their life, i.e., their track is composed of MTC and LTC (lower troposphere cyclone) phases. The highest MTC-phase density and least motion is over the Arabian Sea, followed by the Bay of Bengal and the South China Sea. An MTC-phase composite shows an east-west tilted warm above deep cold-core temperature anomaly with maximum vorticity at 600 hPa. In contrast, the LTC phase shows a shallow cold-core below 800 hPa and a warm upright temperature anomaly with a lower tropospheric vorticity maximum. Further, the systems with MTC-like morphology are observed over the west and central Africa and east and west Pacific in boreal summer. In boreal winter, regions that support MTCs include northern Australia, the southern Indian Ocean, and South Africa. The MTC’s kinematic and thermal structure exhibit remarkable similarity among different basins, suggesting a common underlying maintenance mechanism. Given that the Arabian Sea is a hot spot of devastating MTCs, their classification and genesis mechanisms in this region are explored. Both k-means and cyclone tracking approaches reveal four dominant weather patterns that lead to the genesis of these systems; specifically, re-intensification of westward-moving synoptic systems from Bay of Bengal (Type 1, 51%), in-situ formation with a coexisting cyclonic system over the Bay of Bengal that precedes (Type 2a, 31%) or follows (Type 2b, 10%) genesis in the Arabian Sea, and finally in-situ genesis within a northwestward propagating cyclonic anomaly from the South Bay of Bengal (Type 2c, 8%). Thus, a significant fraction of rainy middle tropospheric synoptic systems in this region form in association with cyclonic activity in the Bay of Bengal (BOB). While in-situ formation with a BOB cyclonic anomaly (Type 2a and 2b) primarily occurs in June, downstream development is more likely in the core of the monsoon season. Type 2a is associated with the highest rain rate and points towards the dynamical interaction between a low-pressure system over the BOB and the development of MTCs over western India and the northeast Arabian Sea. The frequent coexistence BOB lows during the Type 2a formation of MTCs is not merely a coincidence. Rather the BOB system induces an off-equatorial Gill type response which deepens the middle tropospheric trough and zonal shear over the Arabian Sea. In turn, this enhances the cyclonic vorticity and intensifies the middle troposphere anomalous easterlies north of 20 ◦ N. This results in reduction of dry and warm desert air advection, depletion of low-level inversion and destabilization of the lower troposphere. Following which, the eddy-induced moisture flux convergence and advection of climatological moisture increases the saturation fraction. These favorable conditions within the middle troposphere trough region triggers the genesis of MTCs over the Arabian Sea. The proposed role of BOB lows in MTC formation is validated by numerical experiments using the state-of-the-art Weather Research & Forecast (WRF) model. Twenty one ensemble members were generated through addition of balanced vortices to the climatological flow in the BOB. Consistent with observations, in simulations, the BOB low deepens the monsoon trough over western India, enhances the cyclonic shear, reduces the inversion, and increases the middle troposphere relative humidity; supporting the genesis of an MTC over the Arabian Sea within 2.5 − 4 days of model integration. During the first 24 − 36 hours of intensification, advection of absolute vorticity and tilting account for the entire vorticity tendency, while during the rapid intensification phase, vortex stretching is the dominant source of vorticity enhancement. Mechanism Denial Runs with cooling and drying of BOB were then performed to show that this hinders the intensification of the low over the Bay and consequently, the MTC did not form over the Arabian Sea. This global survey, classification, identification of precursors, connection with cyclonic activity over the Bay of Bengal, and dependence on the large-scale environment provide an avenue for better understanding and predicting rain-bearing MTCs.