On the dynamics of easterly waves, monsoon depressions, and tropical depression type disturbances

Abstract

The authors argue that certain aspects of the rotational, synoptic-scale disturbances of the wind field that are observed in ITCZ or monsoon trough regions can be understood by considering the linear response of a dry, initially resting atmosphere to a pulse of heating whose amplitude and spatial and temporal scales are characteristic of a large mesoscale convective system. The key points are that short Rossby waves have small intrinsic group and phase velocities, and that a heating pulse projects much more energy on the Rossby modes if it is located slightly off rather than on the equator. It follows that synoptic-scale Rossby waves, with characteristics broadly similar to those of observed disturbances, should be present in off-equatorial regions of persistent deep convection, since large mesoscale convective systems tend to develop in such regions.