Synoptic response to mountain gravity waves encountering turning critical levels

The synoptic scale response of a stratified rotating shear flow to small scale mountain gravity waves (GWs) encountering turning critical levels is analysed, quantified and compared to the response to a large scale mountain. The large scale synoptic dynamics are described by a semi-geostrophic version of the Eady model, and forced by two independent processes, both resulting from the same large scale complex mountain that consists of a finite size ensemble of small scale ridges embedded within a large scale envelope. The first mountain forcing is due to the momentum deposited by the mountain GWs generated by the small-scale ridges, which interact with the large scale flow at turning critical levels. They produce a dipolar potential vorticity (PV) anomaly advected and steered by the shear in the midtroposphere. The second forcing is due to the large scale envelope, which produces a vertical velocity at the ground but no inflow PV. With no upper boundary, we show that, under a geometrical configuration such that the majority of the mountain GWs encounter critical levels, the potential vorticity they produce can force steady boundary Eady waves as much substantial as those produced by the corresponding large scale mean orography. Furthermore, we find that the warm front configurations tend to be cyclogenetic: the GWs can reinforce (i) the anticyclonic circulation and (ii) the downslope low which are produced by the mean orography. On the contrary, the cold front configurations are cyclolytic. In the presence of a rigid lid, baroclinic instabilities can develop but the above results remain valid at least within the first 36 hours. In the long term, the PV advected in the far field can sustain very efficiently the development of unstable baroclinic Eady modes. These calculations, essentially analytical, take profit of a well known model to analyse the mechanism and evaluate the significance of breaking mountain GWs in turning critical levels action on the synoptic circulation. They may help to appreciate the needs for the parametrization of turning critical levels in GCMs.