A Climatology of the Extratropical Transition of Atlantic Tropical Cyclones

• Published in: Journal of climate Vol. 14; no. 4; pp. 546 - 564
• Main Authors: Hart, Robert E., Evans, Jenni L.
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 15.02.2001
• Subjects: Atlantic Ocean; Climate; Climatology; Cyclones; Datasets; Earth, ocean, space; Exact sciences and technology; External geophysics; Hurricanes; Marine; Meteorology; Oceans; Rain; Seasons; Storms; Storms, hurricanes, tornadoes, thunderstorms; Temperate regions; Tropical climates; Tropical cyclones; Tropical regions; Canada; United States; Europe; Atlantic Ocean; America; North America; Western Europe; United States > US; USA; A, Atlantic; Extratropical zone; Climatology; Life cycle; Extratropical cyclone; Tropical cyclone; Seasonal variation; Intensity; Intensification; Trajectory; Thunderstorm; Hurricane
• ISSN: 0894-8755; 1520-0442
• DOI: 10.1175/1520-0442(2001)014<0546:acotet>2.0.co;2

A comprehensive climatology of extratropically transitioning tropical cyclones in the Atlantic basin is presented. Storm tracks and intensities over a period from 1899 to 1996 are examined. More detailed statistics are presented only for the most reliable period of record, beginning in 1950. Since 1950, 46% of Atlantic tropical cyclones transitioned to the extratropical phase. The coastal Atlantic areas most likely to be impacted by a transitioning tropical cyclone are the northeast United States and the Canadian Maritimes (1–2 storms per year), and western Europe (once every 1–2 yr). Extratropically 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, with October having the highest probability (50%) of transition. Atlantic transition occurs from 24° to 55°N, with a much higher frequency between the latitudes of 35° and 45°N. Transition occurs at lower latitudes at the beginning and end of the season, and at higher latitudes during the season peak (August–September). This seasonal cycle 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 is pushed northward. Conversely, the climatologically favored region for baroclinic development expands southward late in the season, pinching off the oceanic surface area over which tropical development can occur. The relative positions of these two areas define the typical life cycle of a transitioning tropical cyclone: tropical intensification, tropical decay, extratropical 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 changes 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 intensification. Over 60% of cyclones that underwent post-transition intensification originated south of 20°N. In contrast, 90% of tropical cyclones that underwent post-transition decay originated north of 20°N. This suggests that strong baroclinic characteristics during formation are not necessary for strong post-transition development; in fact, they appear to hinder post-transition intensification and, therefore, the post-transition life span of the cyclone itself.