ENSO Recharge Oscillator Theory Integrating the Southward Wind Shift

The El Niño-Southern Oscillation (ENSO) is among the most well understood climate phenomena. The Recharge Oscillator (RO) theory is widely used to conceptualize the key ENSO physics in observations and state-of-the-art models. It is well known that the ENSO-associated equatorial zonal wind anomalies shift southward in boreal winter, contributing to ENSO termination. Thus far, this effect has not been explicitly incorporated into the RO framework. Here we derive a new form of the RO, which incorporates the seasonal meridional migration of the zonal wind anomalies under the low-frequency limit. In our theory, wind stress centered off the equator forces equatorial waves and acts with a delayed effect on SST. Meanwhile, ENSO is stabilized as the central latitude of the zonal wind anomalies shift southward, owing to exponentially weakened thermocline feedback. A stochastic RO simulation with a prescribed observed southward wind shift reproduces ENSO seasonal synchronization and combination tones.