A gridded daily rainfall data set at 3700 stations is used to analyze the intraseasonal and interannual variability of the summer monsoon rainfall over India. It is found that the major drought years are characterized by large scale negative rainfall anomalies covering nearly all of India and persisting for the entire monsoon season. The intraseasonal variability of rainfall during a monsoon season is characterized by the occurence of active and break phases. During the active phase, the rainfall is above-normal over central India and below-normal over Northern India (foothills of the Himalayas) and southern India. This pattern is reversed during the break phase. It is found that the nature of the intraseasonal variability is not different during the years of major droughts or major floods. This suggests that a simple conceptual model to explain the interannual variability of the Indian monsoon rainfall should consist of a linear combination of a large scale persistent seasonal mean component and a statistical average of intraseasonal variations. The large scale persistent component can be part of low frequency components of the coupled ocean-land-atmosphere system including influences of sea surface temperature, snow, etc. The mechanisms responsible for the intraseasonal variations are not well understood. This simple conceptual framework suggests that our ability to predict the seasonal mean rainfall over India will depend on the relative contributions of the externally forced component and the intraseasonal component. To the extent that the intraseasonal component is intrinsically unpredictable, success is long range forecasting will largely depend on accurate quantitative estimates of the externally forced component.
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Center for Ocean-Land-Atmosphere Studies
last update: 10 Sebtember 1999
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