Abstract
Midtropospheric cyclones (MTCs) are moist synoptic systems with distinct midtropospheric vorticity maxima and weak signatures in the lower troposphere. Composites and statistics of tropical MTCs are constructed and compared with monsoon lows and depressions [together, lower-troposphere cyclones (LTCs)]. We begin with South Asia, where tracking reveals that MTCs change character during their life, i.e., their track comprises MTC and LTC phases. The highest MTC-phase density and least motion are 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-core above a 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. Globally, systems with MTC-like morphology are observed over west and central Africa and the east and west Pacific in boreal summer. In boreal winter, regions that support MTCs include northern Australia, the southern Indian Ocean, and southern Africa. MTC fraction is higher equatorward where there is a cross-equatorial low-level jet that advects oppositely signed vorticity, whereas LTCs are more prevalent farther poleward. Finally, a histogram of differential vorticity (the difference between middle and lower levels) versus the height of peak vorticity for cyclonic centers is shown to be bimodal. One peak, around 600 hPa, corresponds to MTCs, while the second, at approximately 900 hPa, comes from LTCs. Thus, moist cyclonic systems in the tropics have a natural tendency to reside in either the MTC or LTC category.
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