A composite study of Extratropical Transformation in the western North Atlantic Ocean, 1963-1996

The transformations of 45 tropical cyclones into extratropical cyclones over the western North Atlantic Ocean between 1963 and 1996 are studied. The cases are chosen from the National Hurricane Center's 'best track' archive that passed through a six-degree latitude-longitude region that is centered at 40 degrees N and 69 degrees W. National Centers for Environmental Prediction (NCEP) global reanalyses of geopotential height, wind and temperature are used to construct composite dynamic and thermodynamic fields during the systems' passage through the region. Thermodynamic and wind soundings, derived from coastal and maritime regions, are also used to composite these transformations. We find that a) a statistically-significant 1000-500 hPa warm anomaly (with respect to the 1963-96 climatology) persists over central and eastern North America for the two-week period prior to the passage of the tropical systems into the region of study; 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 warm core and convectively-unstable tropical air mass are maintained for at least 18 hours after transition; 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.