Forced Waves on a Zonally Aligned Jet Stream

Cornelia Schwierz, Sébastien Dirren, Huw C. Davies
2004 Journal of the Atmospheric Sciences  
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
more » ... cing 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.
doi:10.1175/1520-0469(2004)061<0073:fwoaza>;2 fatcat:m2qplacptnhjtjzivee34tudri