Seminar Series on Interannual to Decadal Variations and Prediction of Monsoon
We are organizing a seminar series on the variabilities of the boreal summer monsoon. We focus on the interannual to decadal variations of different characteristics of the summer monsoon over Asia in general and India in particular, intending to improve understanding and modeling.

The details can be found here.

If last winter was La-Nina, this summer monsoon over India would reduce a bit
Normally, remote forcing (teleconnection) to Indian summer monsoon are quantified simultaneously. This includes connecting summer monsoon rainfall to summer El Nino and Southern Oscillation (ENSO) condition. Can what happened in the past affect the present because of its signature in present climate through slow evolution of the climate system?

Using past data, we showed that if previous winter was a La Nina, the rainfall in current summer over India would reduce a bit. In other words, preceding winter La Nina increases the probability of a decrease in summer monsoon rainfall intensity. How much reduction? We showed that even if the present summer is ENSO neutral, the mean rainfall over India is on an average 4% below its long-term normal when it was preceded by La Nina. And if the present summer is El Nino (that normally causes decrease in rainfall), last winter's La Nina increases probability of drought. This was the first time we showed the asymmetry between La Nina and El Nino of the past season on Indian summer monsoon rainfall. Such delay in impact happens because of slow propagation of surface pressure anomalies in the subtropics as ENSO changes its state with season.

References: Chakraborty (2018).

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), and Chakraborty (2016).

Connecting Extremes and Intraseasonal Oscillations of Seasonal Mean Monsoon Rainfall

The frequency of extreme rainfall events of Indian summer monsoon has increased significantly during past seven decades. At the same time, the variance of low frequency intraseasonal oscillation (LF-ISO) and mean of the Indian summer monsoon rainfall decreased steadily. Are those contrasting changes physically connected?

In a recent research, we found that increase in extreme events during break phase of Indian summer monsoon can decrease its LF-ISO variance and seasonal mean. This is due to thermodynamic stabilization of the atmosphere and disorganization of large-scale circulation after an extreme rainfall event.

References: Karmakar et al. (2015), Karmakar et al. (2016), Karmakar et al. (2017).