Forced Waves on a Zonally Aligned Jet Stream

Abstract

The potential vorticity (PV) pattern in the vicinity of the jet stream takes the form of a narrow tube of enhanced PV gradient on the in situ isentropic surfaces. It is asserted that this distinctive structure can serve as a waveguide and a seat for trapped Rossby waves and that a neighboring vortexlike anomaly can trigger such waves and/or interact strongly with the jet. These conjectures are examined theoretically in an idealized setting comprising a finite-scale vortex forcing of a zonally aligned PV discontinuity. The quintessential dynamics of the vortex's influence upon the PV interface are first elucidated in the linear barotropic ?-plane limit, and thereafter other aspects of the jet?vortex interaction are examined in a hemispheric primitive equation setting using a nonlinear numerical model. It is shown that for the selected setting the interface can sustain trapped waves, a strong response is favored by larger-scale forcing, and a quasi-resonant response can prevail for some ambient flow settings, provided the vortex advects zonally at approximately the Doppler-shifted velocity of a trapped Rossby wave. It is also deduced that (i) a mesoscale perturbing vortex can retain its coherency despite the deforming effect of the ambient flow; (ii) the enhanced PV gradient can indeed serve as an effective waveguide; and (iii) the backreaction of the interface perturbations upon a weak mesoscale vortex need not be appreciable, and conversely for a stronger synoptic-scale vortex the interaction can lead to significant deformation of both vortex and interface with a tendency for a pairing of the vortex with an oppositely signed anomaly on the distorted interface. Comments are made on the relationship of the results to observed phenomena.