A climatology of the extratropical transition of Atlantic tropical cyclones

A comprehensive climatology of extratropically transitioning tropical cyclo nes in the Atlantic basin is presented. Storm tracks and intensities over a period from 1899 to 1996 are examined. More detailed statistics are presen ted only for the most reliable period of record, beginning in 1950. Since 1950, 46% of Atlantic tropical cyclones transitioned to the extratrop ical phase. The coastal Atlantic areas most likely to be impacted by a tran sitioning tropical cyclone are the northeast United States and the Canadian Maritimes (1-2 storms per year), and western Europe (once every 1-2 yr). E xtratropically transitioning tropical cyclones represent 50% of landfalling tropical cyclones on the east coasts of the United States and Canada, and the west coast of Europe, combined. The likelihood that a tropical cyclone will transition increases toward the second half of the tropical season, wi th October having the highest probability (50%) of transition. Atlantic transition occurs from 24 degrees to 55 degreesN, with a much high er frequency between the latitudes of 35 degrees and 45 degreesN. Transitio n occurs at lower latitudes at the beginning and end of the season, and at higher latitudes during the season peak (August-September). This seasonal c ycle of transition location is the result of competing factors. The delayed warming of the Atlantic Ocean forces the location of transition northward late in the season, since the critical threshold for tropical development i s pushed northward. Conversely, the climatologically favored region for bar oclinic development expands southward late in the season, pinching off the oceanic surface area over which tropical development can occur. The relativ e positions of these two areas define the typical life cycle of a transitio ning tropical cyclone: tropical intensification, tropical decay, extratropi cal transition and intensification, occlusion. Using a synthesis of National Hurricane Center Best-Track data and European Centre for Medium-Range Weather Forecasts reanalyses data, the intensity c hanges during and after transition are evaluated. It is extremely rare for a transitioning tropical cyclone to regain (in the extratropical phase) its peak (tropical phase) intensity. However, of the 61 transitioning tropical storms during the period 1979-93, 51% underwent post-transition intensific ation. Over 60% of cyclones that underwent post-transition intensification originated south of 20 degreesN. In contrast, 90% of tropical cyclones that underwent post-transition decay originated north of 20 degreesN. This sugg ests that strong baroclinic characteristics during formation are not necess ary for strong post-transition development; in fact, they appear to hinder post-transition intensification and, therefore, the post-transition life sp an of the cyclone itself.