Space–Time Scales of Northward Propagation of Convection during Boreal Summer

Abstract

This study uses precipitation estimates from the Tropical Rainfall Measuring Missionto quantify the spatial and temporal scales of northward propagation of convection over the Indian monsoon region during boreal summer. Propagating modes of convective systems inthe intraseasonal time scales such as the Madden-Julian oscillation can interact with theintertropical convergence zone and bring active and break spells of the Indian summer monsoon. Wavelet analysis was used to quantify the spatial extent (scale) and center of these propagating convective bands, as well as the time period associated with different spatial scales. Results presented here suggest that during a good monsoon year the spatial scale of this oscillation is about 308 centered around 108N. During weak monsoon years, the scale of propagation decreases and the center shifts farther south closer to the equator.A strong linear relationship is obtained between the center/scale of convective wave bands and intensity of monsoon precipitation over Indian land on the interannual time scale. Moreover, the spatial scale and its center during the break monsoon were found to be similar to an overall weak monsoon year. Based on this analysis, a new index is proposed to quantify the spatial scales associated with propagating convective bands. This automated wavelet-based technique developed here can be used to study meridional propagation of convection in a large volume of datasets from observations and model simulationsThe information so obtained can be related to the interannual and intraseasonal variation of Indian monsoon precipitation.