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P. JAGANNATHAN and H. N. BHALME

Abstract

Despite the systematic nature of the monsoon rains over India, large year-to-year variations in the pattern of distribution of rainfall during the season occur. The yearly pattern of rainfall distribution during the monsoon season (May 31–October 2) for each of the years 1901–51 for a network of 105 stations over India is characterized by a set of six distribution parameters. A brief description of the spatial distribution of the different patterns is given to indicate the nature of the component patterns. Polynomial trend analyses of the time series of the distribution parameters indicate oscillatory features. Power spectrum analyses reveal certain significant periods corresponding to the sunspot cycle or some higher harmonics with regional preferences. The variation of distribution parameters in the different parts of the country with the different sunspot epochs is demonstrated. Studies of the distribution of surface pressure anomalies, frequency of storms and depressions, and the frequency of “breaks in monsoon” associated with the contrasting sunspot epochs suggest that the monsoon circulation features as well as the characteristics of the rainfall distribution have a periodicity nearing the sunspot cycle.

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T. N. Krishnamurti and H. N. Bhalme

Abstract

In this paper the elements of a monsoon system are defined, and its oscillations are determined from spectral analysis of long observational records. The elements of the monsoon system include pressure of the monsoon trough, pressure of the Mascarene high, cross-equatorial low-level jet, Tibetan high, tropical easterly jet, monsoon cloud cover, monsoon rainfall, dry static stability of the lower troposphere, and moist static stability of the lower troposphere. The summer monsoon months over India during normal monsoon rainfall years are considered as guidelines in the selection of data for the period of this study. The salient result of this study is that there seems to exist a quasi-biweekly oscillation in almost all of the elements of the monsoon system. For some of these elements, such as the surface pressure field, monsoon rainfall, low-level cross-equatorial jet and monsoon cloudiness, the amplitude of this oscillation in quasi-biweekly range is very pronounced. For the spectral representation of the time series, the product of the spectral density times frequency is used as the ordinate and the log of the frequency as the abscissa. Dominant modes are also found in the shorter time scales (<6 days). A sequential ordering of elements of the monsoon systems for the quasi-biweekly oscillation is carried out in terms of their respective phase angle. The principal result here is that soon after the maximum dry and moist static instabilities are realized in the stabilizing phase, there occur in sequence an intensification of the monsoon trough, satellite brightness, Mascarene high, Tibetan high and the tropical easterly jet. Soon after that the rainfall maximum over central India, arising primarily from monsoon depressions, is found to be a maximum.

In the second part of this paper we offer some plausible mechanisms for these quasi-biweekly oscillations.

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H. N. Bhalme and D. A. Mooley

Abstract

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

Abstract

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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H. N. Bhalme, D. A. Mooley, and S. K. Jadhav

Abstract

An objective numerical drought/flood index has been used to obtain, on the dryness side, the Drought Area Index (DAI) and on the wetness side, the Flood Area Index (FAI) for India for the period 1891–1979. The DAI for a given year is the percentage area of India corresponding to a mean monsoon index with drought intensity ≤ −2 (moderate drought or worse). Likewise, on the wetness side, the Flood Area Index (FAI) for the given year is the percentage area of India corresponding to a mean monsoon index with flood intensity ⩾ +2 (moderate flood or worse), where the mean monsoon index of an area is the mean drought/flood index for the four monsoon months (June-September). A year with DAI/FAI ⩾ 25, i.e., 25% of the country area, is identified as a large-scale drought/good year, respectively. The magnitude 25 used in identifying large-scale drought/flood corresponds approximately to twice the standard deviation of the DAI/FAI series.

The large-scale April Pressure Index (PI) of the Southern Oscillation has been devised with the combination of surface pressure of stations from Australia, India, Indonesia and South America. The fluctuations of PI covering a period of 89 years (1891–1979) and its relation to the DAI and FAI have been examined. The study indicates a significant inverse relationship between the PI and DAI series. This implies that the 1arge negative PI value, significant weakening of the southeast trades over the Indo-Pacific region tends to coincide with a large DAI value, meaning a 1arge area affected by drought during the subsequent monsoon and vice versa. The PI and FAI am significantly positively correlated. This implies that a large positive value of PI, signifying strengthening of the southeast trades tends to correspond to a large value of FAI, meaning a large area affected by flood during the monsoon and vice versa. The spectrum and cross-spectrum analysis of the PI and DAI series suggest that significant correlation between the PI and DAI is mostly due to the oscillations in the range of 3–6 years. The maximum coherence falls over a period of about 3 years. Furthermore, an oscillation of ∼3 years in a climatic element such as DAI arises primarily as a result of the Southern 0scillation. The Southern Oscillation appears to be one possible causal climatic phenomenon for introducing a most common period of anything from 3 to 6 years for the recurrence of tame-scale droughts over India.

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