Acceleration of the Hurricane Beta Drift by Shear Strain Rate of an Environmental Flow

Abstract

An energetics analysis is presented to reveal the mechanisms by which the environmental flows affect hurricane beta-gyre intensity and beta-drift speed. The two-dimensional environmental flow examined in this study varies in both zonal and meridional directions with a constant shear. It is found that a positive (negative) shear strain rate of the environmental flow accelerates (decelerates) beta drift. The horizontal shear of the environmental flow contains an axially symmetric component that is associated with vertical vorticity and an azimuthal wavenumber two component that is associated with shear strain rate. It is the latter that interacts with the beta gyres, determining the energy conversion between the environmental flow and beta gyres. A positive shear strain rate is required for transfering kinetic energy from the environmental flow to the beta gyres. As a result, the positive shear strain rate enhances the beta gyres and associated steering flow that, in turn, accelerates the beta drift.