A hybrid coupled tropical atmosphere ocean model: sensitivities and hindcast skill

The behaviour of a hybrid coupled model consisting of a simple atmospheric model and a tropical Pacific ocean general circulation model is examined. The atmosphere is a Gill type dynamical model forced by latent heating determined from a moisture equation. The model is shown to oscillate for suitable choices of the coupling strength and this oscillatory behaviour is analysed with a view to determining mechanisms. Evidence is presented that the phase reversal from warm to cold involves the formation of a "dipole pattern" of opposite signed SST between the eastern and central Pacific. The cold eastern water is then advected westward causing the termination of the "warm event". Central to the formation of the dipole pattern in the coupled oscillation, is the strong local oceanic response to easterly atmospheric anomalies in the eastern Pacific and weak remote response there to strong central Pacific westerlies. The model is examined for forecast skill using a large ensemble of two year hindcasts. The relationship between this skill and the mechanisms elucidated in the oscillatory case is discussed