Early and Late Onset of Indian Summer Monsoon Linked to West Asian Surface Pressure
The date of onset of Indian summer monsoon shows substantial interannual variations. Monsoon onset isochrones gradually advance from southern tip of India to north and central regions, and then to northwest regions of the country. What determines the interannual variations of onset date over central India?
We show that
in May, at least a month before the onset over central India, surface pressure anomalies over west Asia is lower than normal for an early onset year. A lower than normal surface pressure increases the strength of the cross equatorial Somaji Jet than brings more moisture over Indian land. Increase in moisture decreases stability of the atmosphere, and helps early onset over both south and central India. Similar results are found in numerical experiments
using a state of the art general circulation model.
Such surface pressure anomalies propagate from west to east along 20-30N latitude band with speed equivalent to mid-latitude Rossby wave. This advances the potential predictability of date of onset of monsoon.
References: Chakraborty and Agrawal (2017), Agrawal and Chakraborty (2016).