Northern Hemisphere Tropical Cyclogenesis Environments

Recent modeling studies have collectively suggested that environmental vertically sheared flows are not necessarily unfavorable for tropical cyclone (TC) formation, and have even gone as far as suggesting that modest westerly shear, coupled with counter-aligned (easterly) surface winds may be intrinsically more favorable for genesis than easterly shear. The present study investigates this hypothesis by considering statistics and composites of developing and non-developing disturbances in the Northern Hemisphere tropics and subtropics. Candidate disturbances are identified in the ERA-Interim dataset by a vortex-tracking algorithm. Disturbance tracks that coincide with named TCs in the Best Track (IBTrACS) database are classified as developing TCs and all other systems as nondeveloping disturbances. After controlling for thermodynamic conditions, we find that counter-aligned shear favors genesis over the West Pacific in easterly shear. However, this same environment is decidedly unfavorable over the eastern Atlantic. Aligned and counter-aligned easterly shear are comparably favorable over the eastern Pacific. Westerly shear is generally not favorable in the tropics (except in the Caribbean Sea), but counter-aligned westerly shear in the subtropics of the Atlantic and West Pacific is the most efficient environment anywhere.

Spatial composites show that favorability for genesis for a given environment is governed by surface latent heat fluxes on the left side (relative to the shear vector) of the vortex. We hypothesize that these fluxes, occurring well inside the pouch, favor convection upshear from the circulation center, thereby providing a moisture source and a vorticity tendency that offsets the shear. Counter-aligned shears in the tropical west Pacific and subtropical west Pacific and Atlantic are characterized by enhanced surface fluxes left-of-shear, which contributes to increased favorability for genesis compared to aligned shear flows in these regions. Both aligned and counter-aligned shears over the eastern Pacific are characterized by enhanced surface fluxes left-of-shear due to well-defined cross-equatorial low-level flow from the Southern Hemisphere associated with the Hadley circulation, which may explain (in part) why both environments are favorable for genesis in that region.