During the last few decades, the ability to predict tropical cyclone and hurricane tracks has increased substantially principally because of improvements in numerical prediction models and the ready availability of satellite data. Similar advances have not occurred in the prediction of the formation of tropical storms. It has been known for a considerable time that storms form in regions where the sea-surface temperature (SST) is greater than 26.5°C and then only in regions of weak vertical shear. Yet, no explanation has been given as to why so few easterly waves intensify to tropical storm or hurricane intensity or why, occasionally, a train of tropical cyclones develop one after the other.

We pose a third necessary condition for tropical cyclone formation which depends on the sign of the zonal stretching deformation (i.e., ). A climatololgy of tropical storms shows that they form almost universally in regions where the stretching deformation is negative (i.e., ). There is good reason why easterly waves entering such a regime tend to spawn tropical cyclones. Under constraints of conservation of wave action flux, a wave must decrease its wavelength as it propagates westward through a region of where its propagation speed also slows. The impact is two-fold: wave energy accumulates in the region of negative stretch and local vorticity (and hence Ekman pumping out of the boundary layer) increases substantially. Thus, a benign easterly wave may develop into a more concentrated entity during its propagation. In essence, the impact of the negative stretch is to provide a downscale cascade of energy and the organization of a more intense and smaller scale circulation.

When this process occurs over a warm ocean, an incipient cyclone may form at low levels. This environment favors the formation of organized mesoscale convective systems (MCSs). Each MCS develops a mesoscale stratiform region and thus a “top heavy” profile of latent heating. Such a heating profile promotes positive potential vorticity development on the scale of the MCS and a mesoscale vortex develops in midlevels in the stratiform region of the MCS. When several MCSs form in this environment they pinwheel around the center of the incipient cyclone, and each MCS feeds vorticity upscale into an annular region surrounding the center of the developing storm. Thus, mesoscale processes favored by the concentrated cyclonic circulation of the easterly wave feeds vorticity upscale into the developing tropical cyclone. The full fledged tropical cyclone forms as the midlevel vorticity intensified by MCSs joins with the lower level cyclone initially spawned by the easterly wave moving into a zone of negative stretching deformation.