Coastally trapped disturbances in the lower atmosphere: dynamic commonalities and geographic diversity

Abstract

The dynamics of coastally trapped disturbances in the lower atmosphere are considered. These disturbances are trapped by the existence of a subsidence inversion at a height below the crests of barrier-like coastal mountains and propagate alongshore with the coast on the right (left) in the Northern (Southern) Hemisphere. Favourable areas on the Earth for the existence of the disturbances are California, southeastern Australia and southern Africa. It is shown that the fundamental dynamics of the disturbances is in each case governed by the hydrostatic and semigeostrophic approximations. The latter implies that a geostrophic balance exists between the alongshore velocity of the motion and the across-shore pressure gradient. Although the fundamental dynamics are identical, it is shown that regional differences in forcing and boundary conditions lead to the observed diversity in the manifestation of the disturbances in each area. These differences include variations in the width of the coastal mountains, the strength and height of the inversion, in the scale of the forcing and whether this forcing leads to on- or offshore flow in the generation region of the disturbance.