Models of coastally trapped disturbances

During the spring and summer, the climatological northerly flow along the U.S. west coast is occasionally interrupted by transitions to southerly flow that have a limited offshore scale and appear to be manifestations of marine-layer flow that is rotationally trapped by the coastal mountains. Existing climatological and observational studies suggest that a synoptic-scale offshore flow initiates these coastally trapped disturbances (CTDs). Using idealized simulations produced with a 3D nonhydrostatic model, the authors find that an imposed offshore flow will produce CTDs in idealized coastal environments. The imposed flow first weakens the prevailing northerly flow in the marine layer and lowers the pressure at the coast. The marine-layer flow around this low pressure evolves toward geostrophic balance, but is retarded as it encounters the coastal mountains to the south of the low and subsequently deepens the marine layer in this region. The elevated marine layer then begins progressing northward as a Kelvin wave and later may steepen into a bore or gravity current, this progression being the CTD. Many observed features accompanying CTDs are found in the numerical simulations, including the formation of a mesoscale pressure trough offshore and deep southerlies in the CTD at the coast. Stability in the atmosphere above the marine layer can give rise to topographically trapped Rossby waves and stronger CTD winds. In these stable conditions, propagation of wave energy away from the disturbance does not preclude strong, quasi-steady, propagating CTDs.