Characteristics of Stochastic Variability Associated with ENSO and the Role of the MJO

Abstract

Abstract To shed light onto the possible role of stochastic forcing of the El Niño–Southern Oscillation (ENSO), the characteristics of observed tropical atmospheric variability that is statistically uncoupled from slowly evolving sea surface temperature (SST) are diagnosed. The Madden–Julian oscillation (MJO) is shown to be the dominant mode of variability within these uncoupled or “stochastic” components. The dominance of the MJO is important because the MJO generates oceanic Kelvin waves and perturbs SST in the equatorial Pacific that may feed back onto the El Niño–Southern Oscillation. The seasonality present in the uncoupled zonal stress (maximum in austral summer), which reflects the seasonality of MJO activity, is also transmitted to the eastern Pacific thermocline variability by these Kelvin waves. Hence, the MJO component of the uncoupled stress could plausibly contribute to the observed phase locking of ENSO to the seasonal cycle. During an El Niño event, maximum uncoupled zonal stress variance shifts eastward from the western Pacific along with the coupled surface westerly wind and warm SST anomalies. The MJO accounts for less than half of this low-frequency behavior of the uncoupled stress but accounts for nearly two-thirds of the resultant thermocline variability. The uncoupled zonal stress also exhibits weak, westerly anomalies in the western Pacific some 8–10 months prior to El Niño, which is mostly accounted for by the low-frequency (period ≫ 50 days) behavior of the MJO. This low-frequency behavior possibly explains why observed El Niño variability is recovered when weakly damped models are forced with similar estimates of observed stochastic zonal stress.