Rossby waves in May and the Indian summer monsoon rainfall

Abstract

Large amplitude stationary Rossby wave trains with wavelength in the range 50° to 60° longitudehave been identified in the upper troposphere during May, through the analysis of 200 hPawind anomalies. The spatial phase of these waves has been shown to differ by about 20° oflongitude between the dry and wet Indian monsoon years. It has been shown empirically thatthe Rossby waves are induced by the heat sources in the ITCZ. These heat sources appear inthe Bay of Bengal and adjoining regions in May just prior to the onset of the Indian summermonsoon. The inter-annual spatial phase shift of the Rossby waves has been shown to be relatedto the shift in the deep convection in the zonal direction.