Is summer monsoon rainfall decreasing over India in the global warming era?

Abstract

Using long-term (1871-2005) summer monsoon rainfall data over 30 meteorological subdivisions of India, overall tendencies of the rainfall have been studied. Further, the subseasonal (monthly) trends have been evaluated. For this purpose, simple linear regression technique is applied. To examine the trends in different segments for summer monsoon rainfall, 11-year running averages are calculated. Most of the subdivisions reveal systematic increasing and decreasing trends in different segments of the time series. India experienced wet monsoon conditions during the late 1950s and dry monsoon conditions around the early 1900s. In the global warming era (1970-2005), 19 out of 30 meteorological subdivisions showed negative tendencies in summer monsoon rainfall. This decrease in summer monsoon rainfall is associated with a weakening of the Southern Oscillation and relaxation of the meridional temperature gradient over the Indian Ocean. The gamma distribution is used to study the distribution of the rainfall by calculating scale and shape parameters. In general, larger values of scale parameter are found over west coast during summer monsoon and individual months. The shape parameter is high over northeast India. © 2009 by the American Geophysical Union.