Identifying a Barotropic Growth Mechanism in East Pacific Tropical Cyclogenesis Using Adjoint-Derived Sensitivity Gradients

Abstract

he eastern Pacific tropical cyclone basin is typified by a low-level westerly jet with the main development region residing on its northern, cyclonic-shear side. The persistent meridional shear of the zonal flow associated with the jet allows for the possibility of barotropic conversion of energy from the mean state into the kinetic energy of vortices?possibly contributing to tropical cyclogenesis, but this is difficult to quantify by perturbing the model based on intuition since there is no guarantee that perturbations will favorably interact with the jet to facilitate cyclogenesis.Here, sensitivity gradients of vortex intensity through cyclogenesis are calculated for a set of cases spanning from 2004 to 2010 and are interpreted dynamically to determine which cases have sensitivities describing structures that can grow barotropically from the low-level jet. The adjoint model is run with adiabatic physics linearized about a basic state that contains moist convection. Optimal perturbations derived from these sensitivities are inserted into the model to observe the impact. Roughly 34% of observed cases exhibited structures in sensitivity to zonal flow that strongly imply barotropic growth, while about 21% exhibited no such structures. The remainder (roughly 45%) exhibit some reliance on barotropic growth. Cases with sensitivities exhibiting strong barotropic growth structures are typified by low-level westerly jets with larger meridional shear. In these cases, optimal perturbations require less initial energy to increase vortex intensity by a specified amount, the energy is more strongly focused at jet level, and the localized energy growth rate of perturbations is most efficient.