资源环境科技发展态势分析平台

Indian summer monsoon exhibits strong northward propagation in intraseasonal timescale [intraseasonal oscillations (ISO)] from the equatorial Indian Ocean (EIO) to the foothills of the Himalayas. Initiation of this northward march is often marked by a strong convective activity over the EIO, which could be associated with Madden-Julian Oscillation. Satellite derived rainfall and reanalysis products are used to unravel different characteristics of ISO in terms of strength, extent and speed at different times. Eastern EIO convective events are grouped into two categories based on the intensities of the simultaneous rainfall anomalies over central India (CI). Northward propagation from EIO is observed to be much stronger and slower when CI experiences strong dry anomaly at the time of initiation of northward march of convection near the equator. Essentially, a first northward propagating dry lobe seems to impact the behavior of the next arriving wet lobe. Distinctive features in Rossby waves emanated from the eastern EIO and different surface and atmospheric conditions over the region in the two different cases are observed. Strong dry conditions over CI favors strong easterly vertical wind-shear, which eventually helps destabilizing the atmosphere. Upper tropospheric meridional temperature gradient over the region modulates the variation in vertical wind-shear via thermal wind balance. These conditions are favorable for generation of strong boundary layer convergence to the north of the convective band, producing strong northward propagation. These results indicate the dual role of convection near the equator and dryness over CI in modulating the northward march of rainfall over India.