A Unified Moisture Mode Theory for the MJO and the BSISO
Abstract
The Madden Julian Oscillation (MJO) and Boreal Summer Intraseasonal Oscillation (BSISO) are fundamental modes of variability in the tropical atmosphere on the intraseasonal time scale. A linear model, using a moist shallow water equation set on an equatorial beta plane, is developed to provide a unified treatment of the two modes and to understand their growth and propagation over the Indian Ocean. Moisture is assumed to increase linearly with longitude and to decrease quadratically with latitude. Solutions are obtained through linear stability analysis, considering the gravest (n=1) meridional mode with nonzero meridional velocity. Zonal moisture advection and surface fluxes are both proportional to zonal wind but of opposite sign. With observation-based estimates for both effects, the zonal advection dominates, and drives the planetary-scale instability. With a sufficiently small meridional moisture gradient, the horizontal structure exhibits oscillations with latitude and a northwest-southeast horizontal tilt in the northern hemisphere, qualitatively resembling the observed BSISO. As the meridional moisture gradient increases, the spatial pattern transforms toward the "swallowtail" structure associated with the MJO, with cyclonic gyres in both hemispheres straddling the equatorial precipitation maximum. These results suggest that the magnitude of the meridional moisture gradient shapes the horizontal structures, leading to the transformation from the BSISO-like structure to the MJO-like neutral wave pattern as the gradient changes with the seasons. It is suggested that the existence and behavior of these intraseasonal modes can be understood as a consequence of phase speed matching between the equatorial v=0 mode and a local off-equatorial component by considering the behavior of an otherwise similar system on an f plane.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.A51G..08S