In a horizontally sheared environmental current, a barotropic, but otherwise hurricane--like, vortex exhibits a wavenumber 2 asymmetry forced directly by mutual advection of the shear flow and the axisymmetric vortex swirling flow. The asymmetries are Rossby waves that propagate upon the radial gradient of axisymmetric potential vorticity. The waves move against the swirling flow so that their phase lines remain stationary. In a cyclonically sheared environment, eddy transports of angular momentum and wave energy are inward and outward, respectively. Although they arise in a barotropic model, these asymmetries have exactly the properties expected of the upper tropospheric eddies hypothesized to modulate tropical cyclone intensity changes. In the above sketch, alternating cyclonic (red) and anticyclonic (blue) bands transport cyclonic angular momentum inward through the black circle, nominally 2000 km in radius. This transport tends to spin up the vortex primary circulation (magenta). In a real hurricane, upper-level transports of this sort would force a quasi balanced outflow that would, in turn, destabilize the thermodynamic sounding and invigorate the convection.


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