Abstract
The origin of an air-sea coupled disturbance in the Anderson-McCreary model (Anderson and McCreary, 1985) with external land heating west of the Pacific is investigated in detail. It is demonstrated that nonlinearities in the Anderson-McCreary model dramatically change the state predicted by the linear theory. The repetitious generation of the coupled disturbance has nothing to do with the linear U1 mode of Hirst (1988) as well as off-equatorial Rossby waves. It is perfectly determined by the amplitude of the external heating. The mass budget analysis demonstrates that the change of zonal wind direction in the western Pacific, which is due to relative importance between the external land heating and the heating associated with the previous coupled disturbance, modulates the oceanic heat content relevant to the origin of the following coupled disturbance. This mechanism gives the oscillation between two stable equilibria (La Nina and El Nino), which is very different from those described in Battisti (1988), Schopf and Suarez (1987) and Zebiak and Cane (1987). Despite lots of model limitations the present results seem to be compatible with the recent analyses of the air-sea-land system in the western Pacific.
