The Dynamics of Coastally Trapped Mesoscale Ridges in the Lower Atmosphere

The dynamics of coastally trapped ridges that propagate in the marine layers of western North America and southeastern Australia is examined. A nonlinear semigeostrophic theory shows that the coastal ridges develop initially as an alongshore intrusion of denser marine air that is driven by the synoptic-scale pressure gradient. Nonlinear Kelvin waves evolve with the intruding flow on a slower time scale governed by the dynamic parameters. If dispersive effects balance the nonlinearities, then these waves evolve into solitary form. Otherwise, the nonlinear waves steepen so that the leading edge of the ridge eventually propagates as a shock.The theory is applied to two ridging events in California and one in southeastern Australia. In each case, good agreement is found between theory and observations of the evolution times and propagation speeds of the coastal ridges. The theory also explains the observed behavior of the events at prominent convex bends and gaps in the coastal topography.