Orographically Trapped Coastal Wind Events and Their Oceanic Response: a Study in Nonlinear Adjustment.

Three ageostrophic, orographically trapped wind events in the coastal zone of the Pacific Northwest are examined. Atmospheric and oceanic time series are utilized to establish the general features of these events and to characterize the oceanic response. Infrared (AVHRR) satellite imagery is used to evaluate the broad-scale response to sea surface temperature (SST) to each wind event. Observed oceanic responses include a drop in SST of 1-2 ^circC offshore to approximately 150 km (~ the atmospheric Rossby radius) and a rise in SST of similar magnitude within ~15 km of the shore. These oceanic results are interpreted as a combination of mixed layer deepening and relaxation of upwelling. A set of reduced-gravity, fully-nonlinear single layer models is utilized to investigate the dynamics of the atmospheric features and their oceanic response, and a possible air-sea interaction. The models reproduce several of the first-order features of the air and sea phenomena, such as the speed of progression of the atmospheric front, and the wake of near-inertial waves in the ocean. The inviscid dynamics of small vs. large amplitude, coastally trapped disturbances are explored through an idealized adjustment problem. Nonlinear advective adjustment of a discontinuity in free surface height under gravity and rotation in a channel is considered, using the method of contour dynamics. After linear wave-adjustment has set up an interior jet and boundary currents in a wide (gg one Rossby radius) channel, fluid surges down-channel on both walls, rather than only that wall supporting a down-channel Kelvin wave. For narrower channels, a coherent boundary-trapped structure of low potential vorticity fluid is ejected at one wall, and shoots ahead of its parent fluid. Comparisons of the contour dynamics solutions, valid for small amplitude, with the nonlinear shallow water model results for large amplitude, show great similarity. Boundary friction strongly modifies the results. The relevance of these findings to the observed atmospheric phenomena is discussed.