Normal mode projections of GCM response to tropical forcing

The transient response of a general circulation model (GCM) is examined for the first 10 days of integration after a tropical heating source is switched on. Model output from 10 control and 10 experiment GCM runs are projected onto the normal modes of an adiabatic primitive equation model linearized about a basic state at rest to quantify the contributions from different modes to the 200 mb wind response. The response in the vicinity of the source is similar to that predicted by models linearized about uniform basic states, with important contributions from the internal Rossby and Kelvin modes in the tropics. The heating also excites internal gravity modes with contributions to the 200 mb winds about 40% of those of the internal Rossby modes through the 10-day integration time. The response in the external Rossby modes in the extratropics becomes statistically significant after 5 days. The internal mode contributions exhibit less spread from the ensemble average than those of external modes, a result due to the deterministic nature of the imposed heating.