Extremes in June rainfall during the Indian summer monsoons of 2013 and 2014: observational analysis and extended-range prediction

The onset/progression phase of theIndian summer monsoon (ISM) is very crucial for the agricultural sector of the country as it has strong bearing on the sowing of kharif crops, which in turn affects overall food grain production and hence food security. The recent ISMs of 2013 and 2014 exhibited quite distinct progression phases. While 2013 had one of the fastest advancement in the last 70 years, 2014 witnessed a comparatively lethargic progression phase. The major difference was felt in the early monsoon month of June, with 2013 (2014) monthly rainfall being +34% (-43%) of its long period average. Observational investigations reveal that, during June 2013, the monsoon trough was very active in its normal position favouring low-level positive vorticity generation and moisture convergence, whereas the absence of monsoon trough during June 2014 facilitated the prevalence of a strong low-level anticyclonic circulation over central India hampering the northward progression of the ISM. It is found that June 2013 (2014) was associated with (i) stronger (weaker) north-south tropospheric temperature (TT) gradient with positive (negative) TT anomalies over Eurasia and north of 60 degrees N; (ii) negative (positive) SST anomalies over the equatorial Indian Ocean, northwestern Arabian Sea and equatorial eastern Pacific; (iii) stronger (weaker) monsoonal Hadley circulation; and (iv) stronger (weaker) Walker circulation in response to the negative (positive) SST anomalies over the equatorial Pacific. The study also examines the skill of an Ensemble Prediction System (EPS) in predicting the observed contrasting behaviour during June 2013/2014 on extended range (approximate to 15-20 days in advance) in real time. The EPS not only forecasted the observed discrepancy, but also predicted the influential role of the large-scale meteorological conditions prevalent during June 2013 (2014), thus demonstrating the remarkable skill of the EPS in predicting June extremes.