For over half a century it has been accepted that the majority of tropical cyclones in the North Atlantic basin form from African easterly waves. Yet little emphasis is given to the unique nature of tropical cyclogenesis in the Atlantic proper, where alternative forms of origin are possible. We note that, other than the classical form of development from tropical seedlings, tropical cyclones can result from diffuse frontal troughs/low-level baroclinic zones, upper tropospheric cold lows (initially without surface reflections), decaying occluded (extratropical) cyclones, and on occasion mesoscale convective complexes (MCCs) and their associated mesoscale convective vortices (MCVs); in fact, nearly a quarter of all hurricane activity in the North Atlantic since 1950 has resulted from these sources and approximately a third of all named storms; these have been termed
baroclinically-initiated (BI) tropical cyclones.
Here we illustrate various cases of BI tropical cyclones and contrast
them with easterly wave development at low latitudes. First, the
canonical origin and development of a BI hurricane from a frontal troughis shown (Hurricane Josephine of 1984). Next, tropical cyclogenesis resulting from a summertime MCC originating over the central Plains receives some attention (Claudette 1997). The genesis of an upper tropospheric cold low into a tropical cyclone (Beryl 1994) and finally the infrequent birth of a tropical cyclone from an occluded (decaying extratropical) cyclone are shown
(Unnamed Hurricane 1991)