On the mechanisms in a tropical ocean-global atmosphere coupled general circulation model. Part II: interannual variability and its relation to the seasonal cycle

The thirty year simulation of the coupled global atmosphere-tropical Pacifi c Ocean general circulation model of the Laboratoire de Meterologie Dynamiq ue and the Laboratoire d'Oceanographie Dynamique et de Climatologie present ed in Part I is further investigated in order to understand the mechanisms of interannual variability. The model does simulate interannual events with ENSO characteristics; the dominant periodicity is quasi-biennial, though s trong events are separated by four year intervals. The mechanism that is re sponsible for seasonal oscillations, identified in Part I, is also active i n interannual variability with the difference that now the Western Pacific is dynamically involved. A warm interannual phase is associated with an equ atorward shift of the ITCZ in the Western and Central Pacific. The coupling between the ITCZ and the ocean circulation is then responsible for the coo ling of the equatorial subsurface by the draining mechanism. Cold subsurfac e temperature anomalies then propagate eastward along the mean equatorial t hermocline. Upon reaching the Eastern Pacific where the mean thermocline is shallow, cold subsurface anomalies affect surface temperatures and reverse the phase of the oscillation. The preferred season for efficient eastward propagation of thermocline depth temperature anomalies is boreal autumn, wh en draining of equatorial waters towards higher latitudes is weaker than in spring by a factor of six. In that way, the annual cycle acts as a dam tha t synchronizes lower frequency oscillations.