Abstract
A simple moist model useful to understand some basic properties relevant to the origin of the intraseasonal disturbances in the tropics is constructed by use of the linear shallow-water equations on a sphere. The dynamical equations are supplemented by the moisture equation as proposed by Gill (1982). The mutual interaction of equatorial dynamics and convective activity is demonstrated by releasing localized, initial anomalies for either temperature or velocity. In particular, it is found that there are two intrinsic ways of large-scale moist adjustment processes in the tropics.
Firstly, it is shown that the active heating region associated with the convergence of low-level winds propagates eastward spontaneously with expanding the zonal scale of the velocity field. The phase speed of this organized pattern is much smaller than that of the corresponding free Kelvin wave because the buoyancy effects are much reduced by the convergence-dependent heating. These results are consistent with the recent observational studies of the 30-50 day oscillation in the tropics.
Secondly, the present model also demonstrates that the burst of westerly winds may excite a westward propagating cross-equatorial cyclone pair more easily than the burst of easterly winds. This vortex pair is mainly composed of the gravest Rossby waves with moist processes and is quite similar to the long-lived synoptic cyclone pair often observed over the warm SST region in the tropics.
