Variability of El Nino-Southern Oscillation-related noise in the equatorial Pacific Ocean

This study documents the variability of noise in the equatorial Pacific associated with the El Nino and La Nina events based on the ensemble retrospective forecasts of the Climate Forecast System. It is found that the noise (measured by the ensemble spread) in the western equatorial Pacific zonal wind stress is enhanced around and before the peak of El Nino events and weakened around 2-3 months after the peak of El Nino events. The change in the wind stress noise is communicated to the thermocline depth in the eastern equatorial Pacific with about 1 month time lag. The eastern equatorial Pacific SST noise, however, decreases during the decay stages of El Nino events while the corresponding noise in the surface zonal wind and heat flux increases. This decrease in the SST noise is related to the weakening of the SST front and the suppression of the tropical instability waves along the flanks of the equatorial Pacific cold tongue that is associated with the SST warming due to El Nino events. Similar relations are seen during La Nina events except that the changes are in opposite sense. As such, the signal-to-noise ratio and, thus, the predictability for the eastern equatorial Pacific SST is relatively high during warm events compared to cold events.