Independence of monthly and bimonthly rainfall over southeast Asia during the summer monsoon season

Independence of monthly and bimonthly rainfall has been investigated for 39 well-distributed and long-record stations in the field of the Asian summer monsoon. The study reveals: (1) monthly rainfall is pairwise independent but is not tripletwise or quadrupletwise independent and (2) rainfall during the first half of the summer monsoon season is independent of rainfall during the second half. The implications of these results are discussed

Gamma distribution probability model for Asian summer monsoon monthly rainfall

Using data from 39 well-distributed and long-record stations over the area, we found gamma distribution to be the most suitable probability model from among the Pearsonian models that show good fit to monthly rainfall in the Asian summer monsoon. We show that the monthly rainfall distribution is not Gaussian and the simple square-root, cube-root, and logarithmic transformations are of limited utility for normalizing the rainfall distribution. A Craig type chart indicates that the rainfall distribution is a Type I distribution or a special or limiting case of this distribution; these distributions are fitted to monthly rainfall, and the goodness-of-fit is tested by the chi-square test. The gamma distribution (Pearson's Type III), which is a limiting case of Type I distribution and next, to the Gaussian distribution in simplicity, gives a good fit to monthly rainfall at all the stations in each of the summer monsoon months; the Kolmogorov-Smirnov test and the variance ratio test confirm this good fit. The Type I distribution shows good fit to June rainfall at 26 stations, July rainfall at 31 stations, August rainfall at 24 stations, and September rainfall at 23 stations. Type IX, a special case of Type I, shows good fit to June rainfall at four stations, July rainfall at two stations, August rainfall at four stations, and September rainfall at three stations. In cases where the gamma and other Pearsonian distributions show good fit, the gamma distribution is found to be the most suitable. The spatial distribution of the scale and shape parameters of the gamma distribution applied to monthly rainfall over the area is examined and the chief features of the distribution are indicated and explained. Deciles of the mixed gamma distribution applied to monthly rainfall are tabulated; these can be used to obtain the monthly rainfall probabilities required by any user

Computation of the average precipitation over the western part of the peninsular India during summer monsoon from the continuity equation for atmospheric water vapour

Water vapour fluxes computed across different walls of the triangular volume of peninsular India, bounded by Trivandrum, Bombay and Nagpur, were used to compute the net flux convergence on a monthly mean basis for the months June through September for the years 1967-72. The precipitation rates over the region were computed by using the flux convergence values and the equation of continuity for water vapour and were compared with the actual rainfall. The agreement between the computed precipitation and actual rainfall was found to be fairly close

Large-scale droughts/ floods and monsoon circulation.

An objective numerical drought index based on monthly monsoon rainfall and duration has been developed for assessment of drought intensity. The drought intensity equation serves the dual purpose of assessing the intensity of drought as well as flood. The Drought Area Index (DAI) is defined as the percentage area of India having a mean monsoon index ≤ −2 (i.e., moderate or higher drought severity). Likewise, the Flood Area Index (FAI) is the percentage area of India with mean monsoon index ≥ +2 (i.e., moderate or more severe wetness), where the mean monsoon index is the mean drought index for the four monsoon months. A year is defined as a large-scale drought or flood year when DAI or FAI ≥ 25. Using the evolved criteria, years of large-scale drought and flood over India have been identified during the period 1891–1975. The method adopted for defining large-scale drought or flood does bear out the actual experience. Power spectrum analysis reveals a weak triennial cycle in DAI series and a highly significant quasi-periodicity of 20 years in the FAI series—nearly a double sunspot cycle. The FAI series is in phase with the double sunspot cycle and large-scale floods have been more frequent in the high-amplitude maximum phase of sunspot cycle. Weaker meridional pressure gradients, larger northward seasonal shifts of the monsoon trough, larger numbers of days of breaks in the monsoon, smaller frequencies of depressions and shorter westward extents of depression tracks appear to be the major factors associated with large-scale droughts, opposite features have been observed for large-scale floods. The height of the 200 mb surface in May is found to be abnormally low in the latitude belt 15–30°N, along 70°E during large-scale drought years, in contrast to abnormally high levels during flood years. The 200 mb surface during May seems to have the potential for prediction of extreme abnormality in the following monsoon season

Distribution function for seasonal and annual rainfall over India

Distribution functions for seasonal (southwest monsoon) and annual rainfall at 53 long-record stations in India have been obtained. It was found that the frequency distributions are right skewed. Tests for normality show that while normal distribution gives a good fit to seasonal and annual rainfall at stations in some parts of India it does not give a good fit to seasonal and annual rainfall at stations over the major portion of the country. Tests of goodness of fit of the Gamma distribution, however, clearly indicate that this distribution provides a good fit to seasonal and annual rainfall at stations in different parts of the country

Synoptic climatology of the daily 700 - mb summer monsoon flow pattern over India

The daily (mean of 0000 and 1200 GMT) 700 mb contour patterns over India are classified in five broad types for each summer monsoon month by using a chart-to-chart correlation method. Certain characteristics of these patterns, such as mutual transitions, persistence, preferred periods of occurrence and interrelationships are studied. Statistical probabilities of two threshold 24 h rainfall amounts (2.5 and 10 mm) being equated or exceeded for each type are computed for 107 stations, more or less uniformly distributed over India. This knowledge of the spatial distribution of precipitation probabilities associated with various circulation types can be used in forecasting probabilities of precipitation over the country if the circulation patterns can he forecast by numerical methods. These probabilities are then compared to the climatological and conditional probabilities of obtaining threshold rainfall amounts on different days of the subsequent 5-day period—given that the threshold rainfall occurred on the current day. The results, if averaged for all types and months, show that persistence is superior to the synoptic climatology developed in this study for forecasting precipitation probability for the next day over an regions and for forecasting precipitation probability up to 2–4 days—depending on region and threshold rainfall criteria. Synoptic climatology is superior to persistence as an aid for forecasting precipitation probability after 4 days over all the regions. Some shortcomings of the present study and future plans are described briefly

Worst summer monsoon failures over the Asiatic monsoon area

The summer monsoon (June-September) is the period in which 50-90 percent of the annual rain over southeast Asia including India occurs

Probability model for droughts in ancient China

A statistical analysis has been made of the occurrence of drought over cast-central China during the periods 725 BC-426 BC and 200 BC-1 BC as reported by Wang (1979) from ancient Chinese chronicles. The occurrence of drought is random and the number of droughts in a decade follows the Poisson as well as the binomial distribution. Changes in the mean frequency of drought are not found to be significant

Relationship between all-India summer monsoon rainfall and southern oscillation/eastern equatorial Pacific sea surface temperature

The interannual variability of all-India summer monsoon (June to September) rainfall and its teleconnections with the southern oscillation index (SOI) and sea surface temperature (SST) anomaly of the eastern equatorial Pacific ocean have been examined for the period 1871-1978 for different seasons (i.e., winter, spring, summer and autumn). The relationship (correlation coefficient) between all-India summer monsoon rainfall and SOI for different seasons is positive and highly significant. Further examination of 10-, 20- and 30-year sliding window lengths' correlations, brings out the highly consistent and significant character of the relationships. The relationship between all-India monsoon rainfall and SST for different seasons is negative and is significant at 1 level or above. Drought years are characterised by negative anomalies of SOI and positive anomalies of SST and vice versa with flood years. The relationship between SOI and SST is negative and significant at 0.1 level. The relationships between all-India summer monsoon rainfall, SOI and sst are expected to improve our understanding of the interannual variability of the summer monsoon