The effects of the phase and amplitude of seasonal cycle and external monsoon wind forcing over the western Pacific on the evolution of El Nino are investigated using the intermediate atmosphere-ocean coupled model. The results indicate that northern spring and summer mean conditions in the tropical atmosphere-ocean provide a favorable basic state for interannual variations of sea surface temperature(SST) in the eastern Pacific. A strong seasonal cycle shortens the periodicity of El Nino but intensifies its amplitude.
In order to investigate the relation between interannual variation of East Asian monsoon and El Nino, simple anomalous monsoon flows over the western Pacific region are introduced to model as external forcing. The monsoon wind forcing is parameterized as a function of SST anomalies in the eastern Pacific with time lag. The external wind forcing superimposed over western Pacific region reduced the wind stress curl generated during the mature phase of El Nino. Therefore the evolution of El Nino is accelerated and the period of El Nino is shortened. When the external forcing is increased, the evolution of El Nino shows QBO like feature.
Effect on the change of the mean thermocline depth due to the climate change is also studied. As a first step, the variation of cold tongue is only considered. Timeseries of the simulated El Nino index is very chaotic and most sensitive to the zonal extend of cold tongue and/or the mean depth of equatorial eastern Pacific. When the cold tongue is extended to the central Pacific and/or the mean depth of equatorial eastern Pacific ocean is shallow, El Nino index shows very distinctive quasi-periodic features. On the other hand, model El Nino is damped out when the cold tongue is shrinked