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Break-Monsoon Over India

K. RAGHAVANIndian Institute of Tropical Meteorology, Poona, India

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Abstract

Interruption of monsoon rainfall by prolonged spells of sparse rainfall (break-monsoon) during the mid-monsoon months of July and August over the plains of northern India (20°–29°N, 75°–85°E) has been investigated. When a tropical low-pressure system moves from the plains in a northerly direction toward the submontane region of the Himalaya, the monsoon trough of low pressure also moves from the plains to that region. This deprives the plains of northern India of significant ascending motion and normal rainfall and brings about the break-monsoon. During the break-monsoon period, descending motion takes place from the stratosphere to the lower troposphere over the plains, while ascending motion occurs in the regions to the north and south of the plains. As the descending motion persists over the plains, the break-monsoon intensifies and the weather turns mainly dry. A vertical circulation model for the break-monsoon period is proposed. The break-monsoon ceases and normal monsoon conditions are re-established when the monsoon trough returns to the plains and intensifies. This occurs in association with tropical low-pressure systems developing at or near the Bay of Bengal and moving toward the plains. The genesis and persistence of the break-monsoon, therefore, depends mainly on the interaction between the monsoon trough and tropical low-pressure systems of the Indian region.

Abstract

Interruption of monsoon rainfall by prolonged spells of sparse rainfall (break-monsoon) during the mid-monsoon months of July and August over the plains of northern India (20°–29°N, 75°–85°E) has been investigated. When a tropical low-pressure system moves from the plains in a northerly direction toward the submontane region of the Himalaya, the monsoon trough of low pressure also moves from the plains to that region. This deprives the plains of northern India of significant ascending motion and normal rainfall and brings about the break-monsoon. During the break-monsoon period, descending motion takes place from the stratosphere to the lower troposphere over the plains, while ascending motion occurs in the regions to the north and south of the plains. As the descending motion persists over the plains, the break-monsoon intensifies and the weather turns mainly dry. A vertical circulation model for the break-monsoon period is proposed. The break-monsoon ceases and normal monsoon conditions are re-established when the monsoon trough returns to the plains and intensifies. This occurs in association with tropical low-pressure systems developing at or near the Bay of Bengal and moving toward the plains. The genesis and persistence of the break-monsoon, therefore, depends mainly on the interaction between the monsoon trough and tropical low-pressure systems of the Indian region.

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