ABSTRACT A stationary SST mode is proposed to understand the physical mechanisms responsible for the phase transition of the El Niño-Southern Oscillation. This stationary SST mode differs from the original delayed oscillator mode and the slow SST mode in the sense that it considers both balanced and unbalanced thermocline depth variations and does not take into account the zonal propagation of SST. Within this mode, the Walker circulation acts as a positive feedback mechanism to amplify and maintain an existing interannual SST anomaly, whereas the Hadley circulation 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 stationary SST mode lies in the zonal-mean thermocline depth variation that 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 interannual oscillation. The zonally asymmetric part of the thermocline depth anomaly, on the other hand, is always in Sverdrup balance with the winds. Such a phase relationship agrees well with observations and with GCM simulations. The stationary SST mode strongly depends on the basin width, on the air-sea coupling strength, and on the seasonal-cycle basic state. For a reasonable parameter regime, it depicts an interannual oscillation with a period of 2-7 years. This stationary SST mode is also season dependent: it has a maximum growth rate during the later part of the year and a negative growth rate during the northern spring, which may explain the occurrence of the mature phases of the El Niño in the northern winter and a rapid drop of the lagged correlation of the Southern Oscillation index in the boreal spring
