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 (Ty...