Can geostrophic adjustment of baroclinic disturbances in the tropical atmosphere explain MJO events?

Abstract

AbstractUsing the two‐layer moist‐convective rotating shallow‐water model, we study the process of relaxation (adjustment) of localized large‐scale pressure anomalies in the lower equatorial troposphere, and show that it engenders coherent structures strongly resembling Madden–Julian Oscillation (MJO) events, as seen in vorticity, pressure, and moisture fields. We demonstrate that baroclinicity and moist convection substantially change the scenario of the quasi‐barotropic “dry” adjustment, which was established in the framework of the one‐layer shallow‐water model and consists, in the long‐wave sector, of the emission of equatorial Rossby waves, with dipolar meridional structure, to the west, and of equatorial Kelvin waves to the east. If moist convection is strong enough, a dipolar cyclonic structure, which appears in the process of adjustment as a Rossby‐wave response to the perturbation, transforms into a coherent modon‐like structure in the lower layer, which couples with a baroclinic Kelvin wave through a zone of enhanced convection and produces, at initial stages of the process, a self‐sustained slowly eastward‐propagating zonally dissymmetrical quadrupolar vorticity pattern. At the same time, a weaker quadrupolar structure of opposite sign arises in the upper layer, the whole picture similar to the active phase of MJO events. The baroclinic Kelvin wave then detaches from the dipole, which keeps slow eastward motion, and circumnavigates the Equator, catching up and interacting with the dipole.