REVISITING TROUGH INTERACTIONS AND TROPICAL CYCLONE INTENSITY CHANGE

Upper-tropospheric troughs may affect tropical cyclone (TC) intensity in both positive and negative ways. Favorably, the increase of angular momentum near the tropopause by eddy flux convergence (EFC) causes the central pressure of the TC to decrease and the surface winds to increase. Unfavorably, high wind shear tilts the TC vortex from an upright configuration and ventilates the TC with cooler, drier air. Despite increased values of vertical wind shear associated with TC–trough interactions, as many as 78% of these interactions have been found to be favorable for TC intensification (Hanley et al. 2001).

Revisiting the problem of TC–trough interactions with a more sophisticated (ERA-Interim) and larger (1979–2014) data set, our preliminary research has focused on the roles of EFC and vertical wind shear in TC intensity change. Results show that the magnitude of the 850–200-hPa vertical wind shear is significantly different between favorable and unfavorable interactions, and shear dominates the effect of EFC on intensification; only 34% of TCs in our sample intensify in the 24 hours following a trough interaction.

To ascertain why some troughs were more detrimental to TC intensity, composites of TC–trough interactions were made. These composites suggest that unfavorable trough interactions tend to be zonally broader and vertically deeper relative to favorable trough interactions. Broader and deeper troughs induce larger environmental wind shear over the TC and lead to a more hostile environment. Additionally, the unfavorable trough is positively titled and more progressive, while the favorable trough is slower and neutral or negatively tilted. The progressive nature of the unfavorable trough allows less time for the TC convection to erode the trough as it approaches, resulting in higher wind shear.