DIRECTION OF HURRICANE BETA-DRIFT IN HORIZONTALLY SHEARED FLOWS

The impacts of linear environmental shears on beta drift direction are assessed through numerical experiments with a single-layer, primitive equation model. It is found that cyclonic (anticyclonic) shears turn the beta drift more westward (northward) in the Northern Hemisphere, I n addition, the longitudinal shear of meridional flows (delta V/delta x) is much more effective than the meridional shear of zonal flows (de lta U/delta y) in deflection of the beta drift. A theoretical model, t he beta gyre dynamic system, describing evolution of the beta gyre amp litude and phase angle is advanced to interpret the numerical model re sults. In this model, the nonlinear energy transfer from the beta gyre s to the primary vortex and higher asymmetric modes was partially para meterized by linear damping. The semi-empirical theory predicts that 1 ) beta drift direction is independent of the planetary vorticity gradi ent; 2) in a quiescent environment, the drift angle is primarily deter mined by the outer azimuthal flows of the vortex; and 3) in a sheared environmental flow, the deflection of beta drift induced by environmen tal shears depends mainly on the longitudinal shear of meridional flow s. The authors show that the environmental shear changes beta drift an gle by advection of beta gyre vorticity and planetary vorticity, which affects beta gyre orientation.