Mechanisms of locking of the El Nino and La Nina mature phases to boreal winter

The peaks of El Nino in the Cane-Zebiak (CZ) model tend to appear most freq uently around November when the ocean Rossby waves, which were amplified du ring the previous unstable season (February-May), turn back to the eastern Pacific and when the local instability in the eastern Pacific is very weak. The peaks of La Nina in the CZ model occur most frequently in boreal summe r, in contrast to the observed counterpart that usually occurs in boreal wi nter. Sensitivity experiments indicate that the phase locking of the La Nin a to boreal summer is primarily caused by seasonal variations of the tropic al convergence zone, which regulate convective heating through atmospheric convergence feedback. The observed thermocline and the wind anomalies in th e western Pacific exhibit considerable seasonal variations. These were miss ed in the original CZ model. In a modified CZ model that includes the seaso nal variations of the western Pacific wind anomalies and the basic-state th ermocline depth, the peaks of La Nina preferably occur in boreal winter, su ggesting that the seasonal variation of the western Pacific surface wind an omalies and the mean thermocline depth are critical factors for the phase l ocking of the mature La Nina to boreal winter. The mechanisms by which thes e factors affect ENSO phase locking are also discussed.