A coupled atmosphere-ocean GCM study of the ENSO cycle

This study examines interannual variability produced by a recent version of the University of California, Los Angeles, coupled atmosphere-ocean genera l circulation model (CGCM). The CGCM is shown to produce ENSO-like climate variability with reasonable frequency and amplitude. A multichannel singula r spectrum analysis identifies the simulated ENSO cycle and permits examina tion of the associated evolution of atmospheric and oceanic states. During the cycle, the evolution of upper-ocean heat content in the tropical Pacifi c is characterized by a zonal oscillation between the western and eastern e quatorial Pacific and a meridional oscillation between the equator and 10 d egreesN. The zonal oscillation is related to the amplification of the cycle , and the meridional oscillation is related to the transition between phase s of the cycle. It is found that the north-south ocean heat content differe nce always reaches a threshold near the onset of a warm/cold event. The three-dimensional evolution of ocean temperature anomalies in the tropi cal Pacific during the simulated ENSO cycle is characterized by four major features: 1) a build up in the subsurface of the western equatorial sector during the pre-onset stage, 2) a fast spread from the western subsurface to the eastern surface along the equator during the onset stage, 3) a zonal e xtension and amplification at the surface during the growth stage, and 4) a northward and downward spread during the decay stage. Ocean temperature budget analyses show that the buildup of subsurface tempe rature anomalies is dominated by the vertical advection process in the west ern sector and the meridional advection process in the central sector. The former process is associated with vertical displacements of the thermocline , which is an important element of the delayed oscillator theory. The latte r process is associated with a Sverdrup imbalance between trade wind and th ermocline anomalies and is emphasized as the primary charge-discharge proce ss by the recharge oscillator theory. It is argued that both processes play key roles in producing subsurface ocean memory for the phase transitions o f the ENSO cycle.