Late season tropical cyclogenesis in the northeastern Atlantic Ocean: 1975-2005

In the past thirty years, twenty named tropical cyclones formed in the northeastern Atlantic Ocean in the months of October, November, and December. By the accepted definition of a favorable environment for tropical cyclogenesis, most of them should not have developed. The past seven seasons produced ten of the twenty, with 2001 and 2005 spawning three and four systems, respectively.

The study period begins in 1975, the year in which the Hebert-Poteat technique for satellite identification of subtropical systems was published. The northeastern Atlantic is defined as the portion of the Atlantic north of 20°N and east of 60°W. Purely subtropical storms are excluded from the study to focus on the conditions for tropical transformation.

The climatology of the twenty late-season tropical cyclones (LSTCs) is discussed including the peak development periods, location, maximum strength, and type of non-tropical origin. LSTCs in this region arise from four unique origins, providing a convenient way to classify them. Type I originate as pre-existing, non-frontal and non-tropical cyclones. Type II systems develop along dissipating frontal systems. Type III develop directly from an occluded frontal cyclone, while Type IV have tropical origins. Thirteen of the twenty systems are Types I and II, and the remaining systems are split relatively evenly between Types III and IV.

The environment of each LSTC is examined for the twenty-four hours prior to attainment of tropical storm status. Wind fields and temperature profiles are calculated on a 13x13 Lagrangian grid. Wind shear is calculated for the 850-200 hPa, 850-300hPa, and 850-500 hPa layers. The shear values are averaged on 3x3 and 1x1 grids for quantitative comparison of systems. The synoptic environments are also visualized on a 13x13 Lagrangian grid; of particular interest are the locations of upper-level troughs, ridges, and upper-level cold lows. Lastly, thermodynamic profiles are studied for the emergence of a warm core and the development of conditions favorable for deep convection. With these data the conditions favorable for the formation of LSTCs are analyzed.