The Changing Nature Of Rainfall Annual Cycle And The Propagation Characteristics Of The Intraseasonal Oscillations In Flood And Drought Years Of The Indian Monsoon

Abstract

Using a 50-year (1951-2000) gridded (1-degree) daily rainfall data set over the Indian land region, we study two main aspects of the Indian monsoon. The first aspect deals with the changing nature of the rainfall annual cycle. This, to our knowledge, is the first attempt at studying the changing behaviour of the Indian monsoon rainfall annual cycle in a systematic way. The annual cycle is defined as a combination of the first few Fourier harmonics of daily rainfall. We then identify five attributes of the annual cycle for each year and location (grid): (a) the day of maximum intensity (peak day); (b) maximum intensity (peak value); (c) beginning; (d) end; and (e) duration of the annual cycle. An extensive statistical analysis of these five attributes over the central Indian region (16.5 – 26.5N; 74.5 – 86.5E) shows that the probability distributions of all attributes, barring the peak value, show a significant change in the last 25 years (1976-2000) compared to the first 25 years (1951-1975). The second issue addressed in this thesis deals with the behaviour of the intraseasonal oscillations in flood and drought years. Previous studies on this issue have been limited to only specific flood or drought years. Our analysis confirms earlier findings such as the northwestward propagation of the 10-20 day ISO. However, we also find, for the first time, based on 9 flood and 9 drought years, that the 20-60 day has an eastward propagation during drought years and remains stationary in flood years. The analysis is primarily statistical in nature, and providing a physical explanation for some of our findings is beyond the scope of our work. Finally, it is worth noting here that without the long-term gridded data, it would have been difficult to assess coherent changes over a large region and long time-period.