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ABSTRACT
Southwest monsoon rainfall over India during July 2002 was the lowest since instrumental records of rainfall data have been available. The present study is an attempt to examine some of the probable causes for this unprecedented low rainfall during July. It is found that the strength of the intertropical convergence zone (ITCZ) over the north Indian Ocean and the pre-mei-yu front over the northwest Pacific Ocean during the month of May has significant positive correlation with the July rainfall over India, and it can be used as a precursor for predicting July rainfall over India. The activity of the ITCZ over the north Indian Ocean and pre-mei-yu front in May is an indicator of the strength of first monsoon intraseasonal oscillation (ISO). It was also found that the ITCZ over the north Indian Ocean and pre-mei-yu front were not active during May 2002, probably because of the weak ISO activity during the first half of the monsoon season.
1. Introduction
In spite of growing industrialization, Indian economy still depends upon the rain-fed agricultural production. Moreover, the summer monsoon rainfall is also important for hydroelectric power generation and achieving drinking water requirements. Therefore, performance of the southwest monsoon over India plays a very crucial role in affecting the quality of life in the Indian subcontinent.
The southwest monsoon season (June-September) accounts for 75%-90% of the annual rainfall of the country. Out of these four months, more than 60% of the seasonal rainfall occurs during July and August, July being the rainiest month. Therefore, rainfall during the month of July has a decisive role in determining the overall performance of the southwest monsoon and its subsequent impacts.
Table 1 shows the correlation matrix of monthly rainfall for the monsoon season and all-India summer monsoon rainfall (AISMR) for the period 1979-2002. It may be seen that the correlation between all-India July rainfall (AIJR) and AISMR is 0.80. From the rainfall records of 102 yr (1901-2002), it has been found that in six out of seven years when July rainfall was below 20% of its normal value, seasonal rainfall was below 10% of its normal value and those six years ended as drought years. It may be mentioned that drought is defined when all India monsoon rainfall...