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P. JAGANNATHAN
and
B. PARTHASARATHY

Abstract

Trends and periodicities in the annual rainfall of India were studied using data for 48 stations having record lengths of over 70 yr. Increasing or decreasing rainfall tendencies were found over large continuous areas in India. These trends, however, are not significant over all the stations in the areas but only at a few places distibuted at random. Wherever a trend is significant, it has persistance or a periodicity of more than 40 yr. Quasibiennial oscillation (QBO) is exhibited at several stations in the areas of increasing or decreasing trend. Similarly, the 11-yr cycle (solar cycle) is also exhibited in both areas. The QBO and the solar cycle are both present at only three stations, however. This casts a serious doubt on whether or not there is any physical relation between the QBO and the solar cycle, at least for rainfall.

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D. A. Mooley
and
B. Parthasarathy

Abstract

The Indian economy is very closely linked with the variable performance of the summer monsoon. The incidence of dry and wet conditions over the country has been examined for the period 1871–1978 by examining the normal monsoon rainfall and its variability over the region by means of the Index of Dryness over India (IDI) and the Index of Wetness over India (IWI). These are respectively defined as the country's percentage area characterized by dry and wet conditions. These series are found to be homogeneous, random, highly variable and positively skewed. The year in which IDI (IWI) exceeds the mean by more than two times the mean deviation from the mean is taken as a year of large-scale drought (flood). The occurrence of large-scale droughts or floods is found to be random in time continuum.

The IDI and IWI show consistently significant correlation with the Southern Oscillation Index (SOI) and with sea surface temperature (SST) anomalies of the equatorial eastern Pacific for the concurrent and succeeding seasons. The relationships of the indices of dryness and wetness over the country with SOI and SST anomalies are expected to be useful in understanding the implications of the large-scale anomalies in the performance of the Indian summer monsoon.

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B. Parthasarathy
and
D. A. Mooley

Abstract

A long rainfall series for the contiguous Indian region for the summer monsoon season (June-September), when more than 75% of the annual rainfall occurs over large parts of the country, has been constructed by considering the rainfall data of a very large number of raingages since 1841 to present. The series from 1866–1970 has been found to be homogeneous. The statistical properties of this homogeneous time series have been investigated. The average monsoon rainfall of India is 88.75 cm with a standard deviation of 7.64 cm. Fisher's statistics g 1, g 2 and the chi-square statistic indicate that the that series is normally distributed. The years 1877, 1899, 1918, 1920, 1951 and 1965 were very bad monsoon years when the rainfall was below the 5th percentile of the distribution. The increase of 4.6% in the 30-year average from 1901–30 to 1931–60 is significant at the 5% level. The mean for the period 1931–60 is also significantly higher than the overall mean for the period 1866–1970 at the 5% level. During the period 1870–1920, the decade average was generally very steady. From 1921 onward, the decade mean increased and attained the maximum value of 93.17 cm during the decade 1941–50, and declined thereafter, the highest decline of 4.41 cm being from 1951–60 to 1961–70. The difference between the mean for the decade 1941–50 and the mean for the whole period of the series is close to the 5% significance point. Power spectrum analysis indicates the presence of a quasi-biennial oscillation in the time series. There does not appear to be a significant relationship between Indian monsoon rain and solar activity.

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D. A. Mooley
,
B. Parthasarathy
, and
G. B. Pant

Abstract

Banerjee et al. showed for the first time that the number of Indian subdivisions with normal or above-normal monsoon rainfall is related to the location of the April 500-mb ridge along 75°E. Thapliyal brought out the relationship between monsoon rainfall of peninsular India and this ridge.

A detailed investigation of the relationship between all-India (India taken as one unit) monsoon rainfall, as well as monsoon rainfall of the subdivisions of India, and the location of the April 500-mb ridge along 75°E, and of the stability and consistency of the relationship has been made in this study, which is based on data for 1939–84. The relationship between all-India monsoon rainfall and the ridge location is positive (correlation coefficient = 0.71) and is significant at the 0.1% level and that between the subdivisional monsoon rainfall and the ridge is significant at 5% or above for all the subdivisions lying west of 84°E and north of 12°N. During the years when all-India monsoon rainfall is deficient, the ridge position is south of its normal position and the contribution by these years to the covariance is very high.

The stability and consistency of the relationship between all-India rainfall and the ridge location have been examined over sliding 10-, 15-, and 20-year periods, and it is found that the relationship is significant at the 1% level for all 20-yeat periods. Thus the relationship is characterized by high stability and consistency for periods of 20 years or more.

The contingency table for the two parameters shows that when the ridge is south of the mean position by more than one standard deviation, all-India rainfall is deficient on 80% of such occasions and that when the ridge is similarly located to the north of the mean, all-India rainfall is excessive on two-thirds of such occasions. The regression equation between all-India monsoon rainfall (y, in cm) and the ridge location (x, in degrees), based on data for the period 1939–80, is y = 38.02 + 3.10x. This relationship explains about 53% of the total variance. So far, the ridge location appears to be the only single parameter that explains such a high percentage of the variance of all-India monsoon rainfall. Estimates from this relationship for the independent years 1981–84 are found to be quite good, except for 1983, the year of excessive rainfall.

The southernmost ridge location was 11°N and the northernmost, about 18°N. It is intriguing to find that a variation of 7° in the latitude of the ridge location over western peninsular India during April makes a large difference in all-India monsoon rainfall (from deficient to excessive). Possibly, the ridge locations with relatively high departures from the normal in April have a tendency to persist, resulting in early or delayed transition to the tropospheric conditions from premonsoon season to the monsoon season.

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B. Parthasarathy
,
K. Rupa Kumar
, and
A. A. Munot

Abstract

Detailed correlation analysis of the all-India monsoon rainfall and mean sea-level seasonal pressure at Bombay (19°N, 73°E) up to three lags on either side of the monsoon wren during the last 30 years (1951–80) indicates a systematic relationship. The winter-to-premonsoon (March, April, May–Deceinber, January, February; MAM–DJF) seasonal pressure tendency at Bombay shows a correlation coefficient (CC) of −0.70 (significant at 0.1% level) with the Indian monsoon rainfall.

Further examination of this relationship over a long period of 144 years (1847–1990), using sliding correlation analysis, reveals some interesting features. The sliding CCs were positive before 1870, negative during 1871–1900, positive in the years 1901–40, and again negative later on, showing systematic turning points around the years 1870, 1900, and 1940. In light of other corroborative evidence, these climatic regimes can be identified as “meridional monsoon” periods during 1871–1900 and after 1940, and as “zonal monsoon” periods before 1870 and during 1901–40, similar to the observation of Fu and Fletcher. It is also observed that the relationship between Bombay pressure and Indian monsoon rainfall becomes dominant when the ENSO variance in Bombay pressure is high and falls apart when the ENSO variance is small.

The paper contains a listing of the long homogeneous data series on all-India monsoon rainfall and monthly MSL pressure at Bombay for the period 1847–1990.

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