Introduction

An extended low pressure area is observed over the northwest India, Pakistan, and Saudi Arabia, and even up to the northeastern part of Africa during the northern summer. The pressure gradient created between the Mascarene Hgh over the Indian Ocean to this extended low generally develops during the spring season and persists during the monsoon season also. It controls the cross-equatorial flow. The strength of the summer monsoon circulation which brings moist air from the Indian Ocean and causes rainfall over India and its neighborhood depends on this gradient. Several authors1-3 have found that transequatorial pressure gradient measured by Southern Oscillation Index (SOI) is not a useful predictor of the Indian summer monsoon, because it shows significant correlations with the concurrent and the following year's SOI rather than the antecedent SOI. For this reason, the present authors created other pressure indices herein after called Tropical Circulation Indices (TCIs) between pairs of stations, as a measure of pressure gradients in the tropics and trans-Indian Ocean, and examined potential links between Indian monsoon rainfall and TCIs with the hope that some of them might lead to a better correlation than SOIs.

Seven tropical circulation indices (TCIs) using monthly mean values of mean sea level pressure (MSLP) at five selected tropical stations have been computed. We then made a detailed study of the lead/lag teleconnec-tions between Indian summer monsoon rainfall and the TCIs mentioned above, using the data for the period 1953-1982, which may find useful application in long-range prediction of the summer monsoon rainfall over India. Earlier, Hastenrath,4 Parthasarthy et al.,5'6 and Singh and Chattopadhyay7 examined the relationships between the Indian monsoon rainfall and regional/global circulation parameters.

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