Zonal Circulations

The response of the tropical atmosphere to steady forcing for all four seasons is computed using a simple two-layer linear primitive-equation model allowing an arbitrary basic flow with two-dimensional shear. The character of each seasonal response is studied, and two distinct forms of behavior are found. Near the Equator, the forcing excites a slowly varying Kelvin wave of the time scale of months while, in midlatitudes, the atmosphere responds via the Rossby mode. Both regimes are separated by the critical latitudes existing on the poleward limits of the easterly channel of the basic flow. The longwave scale response of the equatorial motions noted by Krishnamurti are shown to be the results of the natural filtering of the slowly varying Kelvin wave. The temporal variation of this zonal circulation is discussed. The analysis is extended to infer the behavior and location of transient disturbances in the upper tropical tropo-