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

The authors argue that certain aspects of the rotational, synoptic-scale di sturbances of the wind field that are observed in ITCZ or monsoon trough re gions can be understood by considering the linear response of a dry, initia lly resting atmosphere to a pulse of heating whose amplitude and spatial an d 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 th e Rossby modes if it is located slightly off rather than on the equator. It follows that synoptic-scale Rossby waves, with characteristics broadly sim ilar to those of observed disturbances, should be present in off-equatorial regions of persistent deep convection, since large mesoscale convective sy stems tend to develop in such regions.