  It has recently been shown that an interesting inverse relationship exists
between the strength of the overturning in the regional boreal Hadley
circulation and tropical cyclone genesis and lifetime maximum intensity
latitudes in the eastern North Pacific. One of the particularly curious aspects
of this association is its uniqueness to this ocean basin. We show here that
this result can be understood as the outcome of an equatorward shift in the
local Intertropical Convergence Zone (ITCZ) and a moderate reduction of
tropical vertical shear. These two factors are not necessarily concomitant. The
magnitude of the vertical shear change is low enough to suggest that the
primary physical mechanism behind this inverse relationship is the equatorward
shift in the ITCZ. Since a significant proportion of tropical cyclone genesis
results from aggregated convective cells forming into a coherent convective
vortex and being shed from the ITCZ, this shift is apparently sufficient to
explain the observed association. The equatorward shift in the ITCZ is
potentially the result of a relative warming of the Southern Hemisphere in the
boreal tropical cyclone season. This more equatorward ITCZ, located over warmer
surface waters, presumably explains the more intense Hadley circulation,
despite a reduction of the large-scale boreal meridional temperature gradient.
Although, these links require further research.

Atmospheric and Oceanic Physics