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D. A. Mooley

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D. A. MOOLEY

Abstract

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.

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D. A. Mooley

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An examination of the severe cyclonic storms which formed over the Bay of Bengal and those which struck the coast during the period 1877–1977 brings out a higher mean annual frequency, and a higher percentage of storms intensifying into severe storms, during the period 1965–77. By and large, the formation and landfall of these systems are random events in time continuum, consistent with the Poisson stochastic process. The probabilities of one, two or three severe storms striking the coast in a specific period on the basis of the Poisson probability model could be used for planning funds to instigate the sufferings of the people resulting from a severe storm striking the coast.

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J. Shukla and D. A. Mooley

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We have examined 46 years (1939–84) of observed data to study synoptic and statistical relationships between the summer monsoon rainfall over India, the Southern Oscillation, and the midtropospheric circulation over India.

The change in Darwin pressure from January to April and the latitudinal position of the April 500-mb ridge along 75°E are taken as two quasi-independent predictor parameters to develop a regression equation to predict the summer monsoon rainfall. Verification of predictions on independent data shows that the root-mean-square error for predicted rainfall is 36 mm, which is less than half of the standard deviation (82 mm) and only about 4% of the mean rainfall (857 mm).

Since the observations needed to define the predictor parameters would be available well before the monsoon season, and since the performance of the empirical prediction formula is reasonably good, this method can be of some possible use for long-range forecasting of seasonal mean rainfall over India.

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B. Parthasarathy and D. A. Mooley

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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 and B. Parthasarathy

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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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D. A. Mooley and D. A. Paolino

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This is primarily a statistical study based on linear correlation analysis. The mean monthly surface thermal field over India based on a fairly well-distributed network of 119 stations has been examined for March-May during the period 1901–75 for the relationship with rainfall during the following monsoon season. The study brings out three areas (the fist and the third from the central portion of western India, the second from southern peninsular India) for which the relationships between area average of mean monthly minimum temperature for April (for the first and the second area ) or for May (for the third area), and Indian monsoon rainfall are stable and consistently significant for 20 to 30-yr periods after 1940. The best of these relationships is that with the May minimum temperature over the third area, significant at the 0.1% level. The relationships with mean April minimum temperature over the first and the second areas are just significant at the 5% level. The mean May minimum temperature over the third area is significantly related to the 500 mb April ridge, tendency in Southern Oscillation Index (SOI), and the tendency in eastern equatorial Pacific sea surface temperature (SST). In combination with the ridge, the mean minimum May temperature gives forecasts of Indian monsoon rainfall which are as good as those given by tendency in SOI in combination with the ridge. The parameters SOI tendency and the May minimum temperature are found to be equally useful.

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D. A. MOOLEY and G. APPA RAO

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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.

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H. N. Bhalme and D. A. Mooley

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An objective numerical index has been used to obtain, on the dryness side, the family of Drought Area Indices (DAI) and on the wetness side, the family of Flood Area Indices (FAI) for India for the period 1891–1979. Three series of the DAI family are the percentage areas of India corresponding to mean monsoon index for a given year: ≤ −1 (mild drought or worse), ≤ −2 (moderate drought or worse), and ≤ −3 (severe drought or worse). Likewise, on the wetness side, three series of the FAI family are the percentage areas of India corresponding to mean monsoon index for a given year: ≥ +1 (mild flood or worse), ≥ +2 (moderate flood or worse), and ≥ +2 (severe flood or worse). Power spectrum analysis of the DAI series shows a high-frequency peak which is probably associated with the quasi-biennial oscillation. Spectrum and cross-spectrum analysis of the FAI series and sunspot numbers of the double (Hale) sunspot cycle reveal that a highly significant ∼22-year cycle in the FAI is nearly in phase with the double sunspot cycle and that they are interrelated. Harmonic dial analysis shows that all of the large-scale flood events over India occurred consistently in the major sunspot cycle, suggesting an association of large-scale flood recurrence over India with the double sunspot cycle. The strong evidence of the relationship between areal extent of flood over India and the double sunspot cycle reported here shares in the kind of relationship reported for the western United States, but in the opposite sense of weather characteristics, i.e., for flood rather than drought.

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H. N. Bhalme and D. A. Mooley

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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.

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