Impact of Dry Spells over Central India in Modulating Subsequent Northward Propagation of Intraseasonal Convective Bands in the Indian Summer Monsoon

Indian summer monsoon exhibits a strong northward propagation (roughly 100km/day) on the intraseasonal timescale (known as intraseasonal oscillations) roughly every 30 to 50 days from the equatorial Indian Ocean towards the Indian region. The passage of these waves is manifested as active-break spells over central India. Satellite derived rainfall estimates and reanalysis products are used to unravel different characteristics of intraseasonal oscillations in terms of strength, extent and speed for different lobes of northward propagation. Northward propagation from near the equatorial region is observed to be much stronger and slower when central India experiences stronger dry anomaly at the time of initiation of northward march of convection near the equator. Weaker dry anomalies over central India show opposite features. A first northward propagating dry lobe seems to impact the behavior of the next arriving wet lobe. If a dry lobe arrives with large amplitude over central India, it was noted that it impacted the next wet lobe, whose arrival over central India ended up with large rains. This implies that when a dry lobe arrives at central India it influences the behavior of the next wet lobe arriving from the equator.

Distinctive features in Rossby waves emanated from the eastern equatorial Indian Ocean and atmospheric conditions over central India prevail in the two different cases, modulating the nature of intraseasonal oscillations. Strong dry conditions over central India co-exists with strong easterly vertical wind shear, which eventually helps destabilizing the atmosphere. These conditions are favorable for generation of strong boundary layer convergence to the north of the convective band, thus strong northward propagation. These results indicate a dual role for the convection near the equator and dryness over central India in modulating the northward march of rainfall over India. The results also have large implications towards short-to-medium range forecasting of Indian rainfall.