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Coastally trapped disturbances in the lower atmosphere Reason, Christopher James Charles


Coastally trapped disturbances that propagate in the marine layers of western North America, Southern Africa and southeastern Australia are examined. These areas of the world are considered to be most favourable for the propagation of the disturbances because they all possess pronounced subsidence inversions and barrierlike coastal mountain ranges. Trapping of the disturbance energy within a coastal zone then occurs through this inversion being situated below the mountain crests and through Coriolis effects on the propagating disturbances themselves. Coriolis effects are also responsible for the propagation occurring with the coast on the right (left) in the Northern (Southern) Hemisphere. As this propagation occurs, marked changes in the inversion height and local weather conditions below the inversion are observed. These changes are similar in all three regions with the exception that the inversion is raised in the North American and Australian cases but lowered for the South African disturbances. This difference is shown to arise because the forcing flow is on- or alongshore for the former but offshore in Southern Africa. It is argued that the fundamental dynamics of these disturbances are identical (hydrostatic and semigeostrophic) in each area but that regional differences in the forcing and boundary conditions are responsible for the various manifestations of the disturbances. Based on the observed commonality between the three theory of coastally trapped disturbances is developed from the shallow water equations for a rotating, stratified and flat-bottomed fluid. It is shown that the theory will admit two types of solution, a Kelvin wave and a coastal gravity current, which if higher order effects are included, are found to be related. Comparisons of the different forcings and boundary conditions are made to show the potential importance of nonlinearities. It is concluded that the Southern African case is best described as a continuously forced, linear Kelvin wave, while the North American and Australian disturbances exhibit both gravity current and nonlinear Kelvin wave characteristics. In each case, the theoretical predictions of the evolution time scale, propagation characteristics and speed are shown to be consistent with the available observations.

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