PHASE-TRANSITION OF THE EL-NINO SOUTHERN OSCILLATION - A STATIONARY SST MODE

A stationary SST mode is proposed to understand the physical mechanism s responsible for the phase transition of the El Nino-Southern Oscilla tion. This stationary SST mode differs from the original delayed oscil lator mode and the slow SST mode in the sense that it considers both b alanced and unbalanced thermocline depth variations and does nor take into account the zonal propagation of SST. Within this mode, the Walke r circulation acts as a positive feedback mechanism to amplify and mai ntain an existing interannual SST anomaly, whereas the Hadley circulat ion acts as a negative feedback mechanism that dismisses the original anomaly and causes the phase shift from a warm (cold) to a cold (warm) episode. The key to the cause of interannual oscillations in the stat ionary SST mode lies in the zonal-mean thermocline depth variation tha t is not in equilibrium with the winds. Because of the nonequilibrium, this part of the thermocline depth anomaly tends to have a phase lag with the wind (or SST) anomaly and therefore holds a key for the inter annual oscillation. The zonally asymmetric part of the thermocline dep th anomaly, on the other hand, is always in Sverdrup balance with the winds. Such a phase relationship agrees well with observations and wit h GCM simulations. The stationary SST mode strongly depends on the bas in width, on the air-sea coupling strength, and on the seasonal-cycle basic state. For a reasonable parameter regime, it depicts an interann ual oscillation with a period of 2-7 years. This stationary SST mode i s also season dependent: it has a maximum growth rate during the later part of the year and a negative growth rare during the northern sprin g, which may explain the occurrence of the mature phases of the El Nin o in the northern winter and a rapid drop of the lagged correlation of the Southern Oscillation index in the boreal spring.