Tropical Intraseasonal Oscillations in a Simple Nonlinear Model

Publisher Website
Google Scholar
Abstract
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 ne... Abstract 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 ne...