Limited Width of Tropical Cyclone–Induced Baroclinic Geostrophic Response

Abstract

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 has 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. Significance Statement Despite the large radius of gale-force wind of a tropical cyclone, the effects of a tropical cyclone on mesoscale and large-scale ocean are confined in a limited cross-track width; this strange but important phenomenon is interpreted here. In essence, the effects are exerted by the wind stress curl rather than by the wind stress. However, constrained by atmospheric dynamics, a tropical cyclone has most of the positive wind stress curl in the inner core and nearly zero wind stress curl far away from the inner core. Consequently, albeit violent, the winds outside the inner core cannot make an appreciable contribution to the physical processes below the mixed layer.