Tropical Intraseasonal Oscillations in a Simple Nonlinear Model

This study examines two possible mechanisms responsible for the selection of the preferred period of the Madden and Julian (40–50 day) oscillation. A global two-level nonlinear model with a positive-only CISK-type cumulus heating parameterization is used to simulate the oscillation, which appears when the SST exceeds a critical value for instability of CISK type. Longitudinal variations of tropical SST are imposed so that a stable and an unstable region coexist. When the cold SST sector is sufficiently stable, the CISK wave propagates efficiently through the stable region in the form of a damped moisture-modified Kelvin wave, and reemerges in the unstable region where its amplitude grows. When the SST in the stable sector is set closer to the instability threshold, the moist Kelvin wave slows down and decays before reentering the unstable region, but the CISK perturbation periodically regenerates over the warm waters in response to a local buildup of instability. This last experiment implies a new mechanism for setting the time scale of the oscillation, alternative to that of simple zonal propagation around the globe. A “discharge-recharge” theory is proposed whereby the 40-day recurrence period in the model is set by the growth and duration times of the convective episode together with the recharge time for the instability. It is shown that the midlatitude baroclinic eddies provide the quasi-stochastic forcing necessary to excite each new intraseasonal episode by organizing a region of subtropical convection, which then grows and expands equatorwards due to the effect of the latent heating. The dynamical picture that emerges from the above results is consistent with observations that suggest a local thermodynamically based time scale for the oscillation, and with case studies indicating that extratropical processes might be responsible for the onset of 40–50-day events.