Sensitivity of El Niño – Southern Oscillation to Madden Julian Oscillation

Several time-series of idealized canonical Madden Julian Oscillation (MJO) were constructed. In each series, individual MJO events are defined solely by their essential parameters – propagation speed, interval, amplitude, duration and seasonality. The time-series differ in terms of magnitude and stochasticity of MJO parameters. The series are systematically used to force a coupled ocean-atmosphere model of intermediate complexity. The El Niño – Southern Oscillation (ENSO) simulated by the model was analyzed in order to study the dependence of ENSO characteristics on MJO parameters.

Two sets of experiments were conducted, using additive and multiplicative MJO forcing, respectively. In the additive case, all parameters for all MJO events were prescribed. In the multiplicative case, the values of some MJO parameters are made a function of the underlying SST. It was found that reasonable interannual variability in the model arises only if (a) the MJO parameters (duration and frequency) exceed some threshold values, and (b) MJO activity varies from year-to-year. If the prescribed sensitivity of MJO duration and amplitude to SST is higher, it tends to cause more periodic ENSO. However, the regularity of simulated ENSO decreases as the sensitivity of MJO propagation speed to SST is increased.