Local Wave Activity and the Onset of Blocking along a Potential Vorticity Front

Interaction between a train of transient waves and a diffluent westerly jet is examined using a regional, quasigeostrophic equivalent barotropic model with a (nearly) binary potential vorticity (PV) distribution. Unlike most previous studies but consistent with observation, cross-stream variation in the layer thickness is allowed to contribute to the discontinuity in PV. In all cases examined, short (i.e. barotropic) edge waves are continuously forced in the upstream, then they migrate downstream and eventually exit the domain. A quasilinear 1D theory based on the conservation of local wave activity predicts that no steady wavetrain can be maintained where the westerly zonal flow is decelerated below one half of the initial value, at which point the wave envelope develops a migratory shock analogous to the Lighthill-Whitham-Richards traffic flow problem. Fully nonlinear, high-resolution 2D calculations show that the wavetrain indeed undergoes a significant transformation once the zonal flow along the jet axis is decelerated below the threshold. The subsequent flow evolution depends on the nature of the incipient discontinuity in PV. When the discontinuity is entirely due to vorticity profile, waves are compressed and partially deflected sideways but no complete blocking occurs. When the discontinuity in PV is augmented by the layer thickness variation, the incident wavetrain is blocked and split into two tracks at the stagnation point, eventually leading to a formation of a modon-like vortex pair, reminiscent of a 'Rex-type' atmospheric blocking. Implications for low-frequency variability of the atmosphere are discussed.