Dr. M. R. Ramesh Kumar

The Indian economy is primarily dependent upon agriculture and hence spatial distribution of rainfall over the Indian subcontinent is a matter of grave concern. Most of the meteorological sub divisions of the subcontinent receive over 75–90% of the mean annual rainfall during the summer monsoon season (June–September). The south east peninsular India receives a reasonable amount of rainfall during the post monsoon season (October-December).In addition to the large inter-annual variability in the amount of monsoon rainfall, there is large intra-seasonal variability, which determines the capacity of the monsoon to provide a water resource useful for agriculture. A positive rainfall anomaly of 11.2% translates into 8% food grain production for India (WMO, 1997). The summer monsoon exhibits large variability in three important aspects. They are: • Onset of Monsoon over the Kerala coast (MOK); • Different phases of monsoon activity within the life cycle of the monsoon (active, weak and break in monsoon conditions), and, • The amount of rainfall received during the monsoon season. In my present talk, I would like to bring out the role of the various Ocean Atmospheric Processes on the all the above three important aspects of the Indian Summer Monsoon, using a suite of data sets such as in situ, satellite and Reanalysis products. Further, we also look into the role of El Nino, La Nina, IOD etc on the monsoon activity over the Indian Subcontinent. The association between the intensity of El Nino/La Nina (weak, moderate, strong and very strong) and Indian summer monsoon rainfall activity over the Indian subcontinent has been studied for the period of 1957-2017 (61 years). The intensity of El Nino/La Nina is determined based on the Oceanic Nino Index sea surface temperature. The characteristics of the convective systems such as frequency, duration and direction of the movement of them over the Bay of Bengal and the Western Pacific region have been studied using the data from the UNISYS website (http://weather.unisys.com) and their association with the monsoon activity over the Indian subcontinent. During the study period, the earliest MOK date was on the 14th May 1960 (neutral phase) and most delayed was on 18th June, 1972 (Strong El Nino). The delayed onsets were in general associated with strong and very strong El Nino years; whilst La Nina years were associated with early MOKs. Further, La Nina years, in general had equal number of active and break spells. El Nino years on the other hand had, very less number of active spells. In order to bring out the differences in geographic location, duration and tracks of the convective systems over Bay of Bengal and Western Pacific, composites for weak, moderate, strong and very strong El Nino, La Nina and Neutral conditions were plotted. A study of tracks of the convective systems formed over the Western Pacific region showed a significant change in the tracks during the El Nino years as compared to Neutral and La Nina years, indicating their predominant role on the monsoon.