New modeling evidence of the MJO influence on ENSO

This presentation summarizes recent numerical modeling results addressing issues related to the influence of the Madden-Julian Oscillation (MJO) on the El Nino-Southern Oscillation (ENSO). Controversy regarding this issue concerns the frequency gap between the two phenomena and the physical mechanisms for the influence. Using a group of atmosphere-ocean coupled models with various complexities assisted by diagnoses of global model reanalyses and observations, we demonstrate that, when the atmosphere-ocean climate system is stable or neutral, the MJO as a source of stochastic forcing affects ENSO through repeatedly exciting oceanic downwelling Kelvin waves. Their accumulative effect of deepening the thermocline in the eastern Pacific, when amplified by air-sea coupling, effectively induces interannual variability in SST resembling the observed ENSO evolution. The process is linear and therefore the low-frequency component of the MJO, due to its irregularity and interannual variability, is vital to its influence on ENSO. The large spatial extent of the stress anomalies associated with the MJO differentiates this kind of forcing from other sources of stochastic variability. The low-frequency component of the total stochastic forcing (linearly independent of ENSO SST) not associated with the MJO can induce interannual variability in SST with an amplitude smaller than that induced by the MJO. Hindcast experiments demonstrates that ENSO predictability can be substantially extended (from 4 to 12 months) by adding stochastic forcing that includes the MJO.