El Niño–Southern Oscillation Simulated in an MRI Atmosphere–Ocean Coupled General Circulation Model

Abstract

A coupled atmosphere-ocean general circulation model (GCM) was time integrated for 30 years to study interannual variability in the tropics. The atmospheric component of the coupled model is a global GCM with 5 levels in the vertical and 4° latitude ?5° longitude grids in the horizontal. This atmospheric GCM includes standard physical processes (e.g., interactive clouds). The oceanic component of the coupled model is a GCM for the Pacific with 19 levels in the vertical and 1°?2.5° grids in the horizontal. The coupled model includes seasonal varying solar radiation as forcing. The model succeeded in reproducing interannual variations that resemble the El Niño?Southern Oscillation (ENSO) with realistic seasonal variations in the atmospheric and oceanic fields. The model ENSO cycle has a time scale of approximately 5 years and the model El Niño (warm) events are locked roughly in phase to the seasonal cycle. The cold events, however, are less evident in comparison with the El Niño events. The evolution of the model ENSO can be summarized as follows. Easterly wind anomalies in the central-eastern Pacific excite westward-propagating ocean waves that deepen the thermocline off the equator. The deepening of the thermocline is intensified with easterly anomalies induced in the atmospheric-ocean coupling in the western Pacific. Through reflection of the waves in the western boundary and weakening of the easterly anomalies in accordance with seasonal march, eastward-propagating ocean waves are generated and travel along the equator. These waves reach the eastern Pacific and deepen the thermocline depth there, resulting in warming of sea surface temperature (SST) in the eastern Pacific. This warming of SST (the El Niño event) produces westerly anomalies in the central-eastern Pacific. Then, the same processes opposite in sign proceed. When SST becomes cool in the eastern Pacific (the cold event), the conditions favoring easterly anomalies are prepared. The time scale of the model ENSO cycle is determined primarily by propagation time of signals from the central-eastern Pacific to the western Pacific and back to the eastern Pacific. However, some of the aforementioned processes are phase locked in the seasonal cycle. Especially, the wind anomalies in the central-eastern Pacific are phenomena occurring in summer and the atmosphere-ocean coupling in the western Pacific operates efficiently in the first half of the year. Therefore, the seasonal timing is also important in the ENSO time scale.