Patterns of variability of Indian monsoon Rainfall

Abstract

The study of variability of southwest monsoon (SWM) rainfall is of considerable scientific importance. The complexities in understanding the behaviour of SWM rainfall are due to the strong space–time variation involved in the phenomenon. Thus, a comprehensive knowledge of spatial and temporal variability is essential for a meaningful study. This requires identification of spatially homogeneous regions and a subsequent study of their temporal variability. Both these aspects are studied in the present thesis.

Chapter I presents a brief review of the literature. Past investigations related to interannual variability of SWM rainfall in India are reviewed in some detail. Also, a few methods for identifying homogeneous regions are discussed. Limitations of the existing approaches are brought out to focus on the motivation for the present work.

This is followed by Chapter II, wherein a systematic statistical study of SWM rainfall at 200 stations of India for 80 years (1901–1980) is presented. Principal Component Analysis (PCA) is used for the analysis. Based on a correlation criterion, twelve principal regions connected to the first six principal components are identified.

Chapter III develops a hierarchical method of refining the homogeneous regions found previously. In this method, coherent regions are obtained in a sequential manner. Only the first principal component is used at every level for this purpose. An application of the method leads to 10 sequentially homogeneous regions. Automatically, 9 more complementary regions emerge.

In Chapter IV, a statistical study of the area-weighted rainfall and the significant principal components of the homogeneous regions is presented. Possible existence of signals is investigated. A significant interannual variability signal is found to exist in a particular principal region. The isotropy of the identified regions with respect to the spatial correlations has also been studied.

The thesis concludes in Chapter V with a summary and a set of conclusions.