The All-India area-weighted mean summer monsoon rainfall, based on a homogeneous rainfall data set of 306 raingauges in India, developed by the Indian Institute of Tropical Meteorology, is widely considered as a reliable index of summer monsoon activity over the Indian region. Long time series of this index since 1871 have revealed several interesting aspects of the interannual and decadal-scale variations in the monsoon as well as its regional and global teleconnections.
This figure shows the time series evolution of AISMR anomalies, expressed as percent departures from its long-term mean, over more than a century in the past. Prediction of the future evolution of the monsoon activity, at least a season in advance, remains a difficult challenge.
FLOOD YEARS: During the period 1871-2009, there were 19 major flood years, defined as years with AISMR in excess of one standard deviation above the mean (i.e., anomaly exceeding +10%; blue bars above):
1874, 1878, 1892, 1893, 1894, 1910, 1916, 1917, 1933, 1942, 1947, 1956, 1959, 1961, 1970, 1975, 1983, 1988, 1994.
DROUGHT YEARS: During the period 1871-2009, there were 24 major drought years, defined as years with AISMR less than one standard deviation below the mean (i.e., anomaly below -10%; red bars above):
1873, 1877, 1899, 1901, 1904, 1905, 1911, 1918, 1920, 1941, 1951, 1965, 1966, 1968, 1972, 1974, 1979, 1982, 1985, 1986, 1987, 2002, 2004, 2009.
It is interesting to note that there
have been alternating periods extending to 3-4 decades with less and
more frequent weak monsoons over India. For example, the 44-year period
1921-64 witnessed just three drought years; during such epochs, the
monsoon was found to be less correlated with the ENSO. During the other
periods like that of 1965-87 which had as many as 10 drought years out
of 23, the monsoon was found to be strongly linked to the ENSO
(Parthasarathy et al., 1991).
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Dr. A.A. Munot Dr. D.R. Kothawale
Monthly data on Indian summer monsoon rainfall during 1871-2008 are available on-line from HERE .