ON THE TIMING OF WARM AND COLD EL-NINO SOUTHERN-OSCILLATION EVENTS

Using wind and atmospheric pressure observations, the authors find that the ENSO signal has a previously unnoticed structure fundamental to ENSO dynamics and prediction. Specifically, the time at from the maximum of a warm ENSO event io the minimum of the next cold ENSO event increases linearly with the size of the warm ENSO event. A similar result holds, but with marginal correlation, in going from a cold to a subsequent warm event. These results are consistent with a version of delayed oscillator physics. A larger warm event implies that the westerly zonal equatorial wind anomaly is farther to the east. Consequently, the oceanic Rossby waves that the zonal wind anomaly generates take longer to propagate to the western boundary, reflect, and return as an equatorial Kelvin wave to the region of the wind anomaly. According to delayed oscillatory theory, the time at taken to replace the westerly wind anomaly with an easterly one is a multiple of the wave transit time, so at should increase when the size of the warm event increases. The effect is marginal in going from a cold event to a warm one because a larger cold event does not imply a greater eastward displacement of the wind anomaly.