Performance of all-India southwest monsoon seasonal rainfall when monthly rainfall reported as deficit/excess

Abstract

Performance of all-India summer monsoon rainfall (June to September) has been studied when June or July or June + July rainfall is reported as being in excess/deficit, using monthly and seasonal rainfall data for the period 1871–2010. The study shows that seasonal rainfall was above/below the long term mean in almost all the years when rainfall of June or July or June + July was in excess/deficit. The probabilities of occurrence of seasonal rainfall below the long term mean are 77, 83, and 92% respectively when June, July, and June + July rainfalls are in deficit. The years in which the rainfalls of June and July were deficit, were mostly El Nino years and their corresponding seasonal rainfalls were below the mean. The deficiencies in June/July or June + July rainfall during El Nino years thus give an indication of seasonal rainfall being below the mean, and vice versa for La Nina years. The study has brought out an important result that rainfall during June gives a first signal of the performance of seasonal rainfall. In order to explain the relationship between June excess/deficit rainfall and seasonal above/below mean rainfall, an hypothesis is proposed which states that the circulation anomalies developed over the Indian region during an excess/deficit June persist throughout the season. The hypothesis has been proved by examining the composites of excess/deficit June wind anomalies at 850 and 200 hPa and observing their subsequent persistence from July to September. Copyright © 2013 Royal Meteorological Society