Baroclinically Induced Tropical Cyclogenesis

The authors investigate the transition of numerous subtropical cyclones into late season tropical storms and hurricanes during the 2000 and 2001 Atlantic tropical cyclone seasons. In all transitioning cases (10), the 900-200-hPa wind shear was initially near or in excess of the upper limit of vertical shear deemed suitable for tropical cyclogenesis. In many of these cases, the vertical shear decreased markedly near or prior to the time of tropical cyclone formation. In cases that did not become named tropical cyclones, either the tropospheric vertical shear remained in excess of 15-20 m s 1 or the underlying sea surface temperature (SST) dropped below about 26C prior to or during the weakening of the shear. Cases in which the shear remained large featured multiple, shortwave upper-tropospheric troughs interacting with the developing lower-tropospheric disturbance such that classical occlusion did not occur.