Rossby Wave–Coastal Kelvin Wave Interaction in the Extratropics. Part II: Formation of Island Circulation

The formation of an island circulation is investigated both theoretically and numerically in light of the dynamics of coastal Kelvin waves and Rossby waves. An island circulation is formed in three stages. First, the direction of the circulation is initiated by the coastal Kelvin wave; second, the transport of the circulation is established by the short Rossby wave dissipated against the eastern coast of the island; and finally, the basinwide circulation pattern is completed by the long Rossby wave radiated from the western coast of the island. An island circulation can be forced by either a local alongshore wind or a remote vorticity forcing to the east of the island; the initial Kelvin wave is directly forced by the alongshore wind in the former case, but indirectly forced by a planetary wave incident on the eastern coast of the island in the latter case. A comparison is also made between the spinup of an island circulation and a basin circulation. In addition, this spinup study also provides an alternative derivation of the island rule in light of the dynamics of Kelvin and Rossby waves. The implication for understanding the temporal response of an island circulation to a variable forcing is also discussed.