Fluctuations in the Drought/Flood Area over India and Relationships with the Southern Oscillation

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.