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A study of extratropical transition in the Western North Atlantic Ocean, 1963–1996

Abstract A synoptic‐ dynamic climatology of the forty‐five extratropical‐transitioning tropical cyclones, (as compiled by the United States National Hurricane Center) travelling through offshore regions of the North American east coast during the 1963–96 hurricane seasons, is presented. The National Centers for Environmental Prediction global reanalyses of geopotential height are used to construct large‐scale circulation features associated with the passage of these systems. Soundings of six near‐track stations are used to study the stratification of temperature and moisture during the cyclones ‘ passage. Sea‐surface temperature (SST) structures are also studied with respect to these events. Primary results of the study include: a) a statistically‐significant 1000–500 hPa warm anomaly (with respect to a 1963–96 climatology) persists over central and eastern North America during the two‐day period prior to the passage of the tropical systems into the Canadian Maritime provinces; b) a northwestward extension of the surface subtropical anticyclone exists over the Canadian Atlantic Provinces during the two‐day period prior to the arrival of the cyclone; c) the tropical cyclone's convectively‐unstable tropical air mass is maintained for at least eighteen hours after transition; and d) the presence of quasi‐geostrophic forcing for ascent, typically seen in extratropical cyclones, is observed during periods in which the systems are still classified as tropical cyclones. This forcing for ascent continues during the extratropical transformation, and typically occurs ahead and to the left of the storm track. Additionally, these transforming systems travel through a region of SST gradients that are anomalously strong compared with the climatology for that region. Storms are also classified according to whether they were deepening or filling. These classifications were composited to distinguish circulation structures unique to each classification. Considering that several of the systems passing through the maritime‐coastal regions are filling and several are intensifying, we distinguish the circulation structures between the deepening and filling systems. The primary distinction of the deepening extratropical transition is the statistically‐significant colder tropospheric air mass to the north of the storm track. Such a finding is similar to that found in other studies for cold‐season deepening extratropical cyclones.