Limited width of tropical cyclone-induced baroclinic geostrophic response

Abstract

Despite the large radius (R17) of gale-force wind of a tropical cyclone (TC), the observed TC-induced effects on mesoscale and large-scale ocean via the baroclinic geostrophic response are found to have a limited cross-track width; this strange but important phenomenon is interpreted here. Driven by the wind stress curl (WSC), the TC-induced geostrophic response is in fact regulated by along-track integration of the WSC (AIWSC). Constrained by atmospheric TC dynamics, the violent winds outside the radius (Rmax) of maximum wind of any TC must have nearly zero WSC. Consequently, the AIWSC function can be fit as a boxcar function with an extraordinarily large positive value between ±Rmax about the track. Based on this boxcar function, the theoretical estimate of the cross-track length scale of the baroclinic geostrophic response, Ld + Rmax, is presented, where Ld is the first-mode baroclinic Rossby deformation radius. Further, this scale is validated by numerical experiments to well explain the width of the altimetry-observed geostrophic response induced by any TC. Evidently, Ld + Rmax is far smaller than R17 and thus the baroclinic geostrophic response generally have a limited width. This study implies that, although for a TC the violent winds outside Rmax are generally ∼90% of all winds, in an open ocean these winds may be useless to perturb the ocean interior due to the nearly zero WSC.