View the simulated ENSO cycle here.

Simulation

The performance of the UCLA coupled GCM (CGCM) has been greatly improved in recent years. The model produces ENSO-type variability with large amplitudes. The simulated cycle is characterized by predominantly standing oscillations of SST in the eastern Pacific, almost simultaneous zonal wind stress anomalies to the west of the SST anomalies, and preceding thermocline anomalies near the western edge of the basin. The above figure shows the three-dimensional evolution of ocean temperature anomalies in the tropical Pacific during the ENSO cycle. The evolution is characterized by four major movements: (1) the build up of temperature anomalies in the subsurface of the western equatorial Pacific during the pre-onset phase of ENSO; (2) the fast spread of the anomalies from the western subsurface to the eastern surface of the equatorial Pacific during the onset phase, (3) the zonal extension and amplification of surface anomalies during the growth phase, and (4) the northward and downward spread of temperature anomalies during the decay phase.

Dynamics

The fundamental physical processes that give rise to El Nino-Southern Oscillation (ENSO) are believed to be within the tropical Pacific. We use the simulated ENSO cycle obtained by Yu and Mechoso (2001) from a 53-year long CGCM simulation to examine two conceptual models of ENSO: the recharge-oscillator and the delayed-oscillator model. During the simulated ENSO cycle, ocean heat content anomalies in the tropical Pacific are characterized by a meridional oscillation between the equator and 10N and a zonal oscillation between the western and eastern equatorial Pacific. The meridional mode appears to be related to the phase-transition of the cycle, and the zonal mode is related to the growth of the cycle. We also perform ocean temperature budget analysis to determine the relative importance of various ocean processes in producing subsurface ocean memory for the ENSO cycle. Our analyses indicate that the simulated ENSO cycle is consistent with several major features described by two leading simple models of ENSO: the recharge-oscillator and the delayed-oscillator.

Prediction

Encouraging results are produced from the experimental long-range prediction made with the UCLA CGCM for the '97-'98 ENSO event. The SST anomalies predicted by the forecast initialized in June 1997 showed that the '97-'98 ENSO had a rapid growth from the June initial condition to the September-October maximum. The model also predicted that, after October 1997, this ENSO event would enter a slowly decaying phase and will be terminated after the spring of 1998. This experimental prediction shows that the '97-'98 ENSO is characterized by a rapid growing phase followed a slow decay. Its forecast of the NINO3 SST anomalies appears to be close to those forecasts made with several models that have been routinely used for long-range prediction. It is shown that the UCLA CGCM has the potential of making long-term prediction with reasonable success.

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