HOMOGENEOUS INDIAN MONSOON RAINFALL - VARIABILITY AND PREDICTION

The Indian summer monsoon rainfall is known to have considerable spati al variability, which imposes some limitations on the all-India mean w idely used at present. To prepare a spatially coherent monsoon rainfal l series for the largest possible area, fourteen subdivisions covering the northwestern and central parts of India (about 55% of the total a rea of the country), having similar rainfall characteristics and assoc iations with regional/global circulation parameters are merged and the ir area-weighted means computed, to form monthly and seasonal Homogene ous Indian Monsoon (HIM) rainfall series for the period 1871-1990. Thi s paper includes a listing of monthly and seasonal rainfall of HIM reg ion. HIM rainfall series has been statistically analysed to understand its characteristics, variability and teleconnections for long-range p rediction. HIM rainfall series is found to be homogeneous, Gaussian di stributed and free from persistence. The mean (RBAR) rainfall is 757 m m (87% of annual) and standard deviation (S) 119 mm, with a Coefficien t of Variation (CV) of 16%. There were 21 dry (R(i) less-than-or-equal -to RBAR - S) and 19 wet (R(i) greater-than-or-equal-to R + S) years d uring 1871-1990. There were clusters of frequent negative departures d uring 1899-1920 and 1965-1987 and positive departures during 1942-1961 . The recent three decades show very high rainfall variability with 10 dry and 6 wet years. The decadal averages were alternatively positive and negative for three consecutive decades, viz., 1871-1900 (positive ), 1901-1930 (negative); 1931-1960 (positive) and 1961-1990 (negative) respectively. Significant QBO and autocorrelation at 14th lag have be en found in HIM rainfall series. To delineate the changes in the clima tic regime of the Indian summer monsoon, sliding correlation coefficie nts (CCs) between HIM rainfall series and (i) Bombay msl pressure, (ii ) Darwin msl pressure and (iii) Northern Hemisphere surface air temper ature over the period 1871-1990 have been examined. The 31-year slidin g CCs showed the systematic turning points of positive and negative CC s around the years, 1900 and 1940. In the light of other corroborative evidences, these turning points seem to delineate 'meridional' monsoo n regime during 1871-1900 and 1940-1990 and 'zonal' monsoon regime dur ing 1901-1940. The monsoon signal is particularly dominant in many reg ional and global circulation parameters, during 1951-1990. Using the t eleconnections of HIM series with 12 regional/global circulation param eters during the recent 36-year period 1951-86 regression models have been developed for long-range prediction. In the regression equations 3 to 4 parameters were entered, explaining upto 80% of the variance, d epending upon the data period. The parameters that prominently enter t he multiple regression equations are (i) Bombay msl pressure, (ii) Apr il 500 mb Ridge at 75-degrees-E, (iii) NH temperature, (iv) Nouvelle m inus Agalega msl pressure and (v) South American msl pressure. Eleven circulation parameters for the period 1951-80 were subjected to Princi pal Component Analysis (PCA) and the PC's were used in the regression model to estimate HIM rainfall. The multiple regression with three PCs explain 72% of variance in HIM rainfall.