Break Monsoon: Why the Rain Stops in the Middle of the Season

A question I am asked frequently, particularly by those whose work depends upon rainfall, is why the monsoon appears to stop for a fortnight in the middle of the season. The answer lies in the position of the monsoon trough, and understanding it explains a great deal about why some seasons succeed and others do not.

Active and break phases

The monsoon is not a continuous flow of rain from June to September. It is an oscillating system which alternates between active spells of vigorous, widespread rainfall and break periods during which rainfall over much of the country reduces markedly or ceases altogether.

The determining factor is the position of the monsoon trough — the elongated zone of low pressure extending from north-west India south-eastward towards the head of the Bay of Bengal.

The active phase

When the trough lies in its normal position across the Gangetic plain, moist south-westerly flow converges into it and rises, producing widespread rainfall across central and northern India. This is the monsoon behaving as expected.

The break phase

During a break, the trough shifts northward towards the foot of the Himalaya. The consequences are distinctly uneven:

• Central India, the Gangetic plain and peninsular India become largely dry, with rising temperatures and increasing humidity
• The Himalayan foothills, Uttarakhand, Himachal Pradesh and the north-east receive very heavy rainfall, frequently accompanied by landslides and flash flooding
• The west coast, contrary to expectation, may also see reduced rainfall despite continuing westerly flow

What causes a break

Several mechanisms contribute, and they frequently operate together.

The intra-seasonal oscillation. The monsoon exhibits an internal rhythm with a period of roughly thirty to fifty days, in which a zone of enhanced convection forms near the equator and migrates northward across the subcontinent over two to three weeks before dissipating, after which the cycle repeats. Breaks correspond to the suppressed phase of this oscillation.

The Madden–Julian Oscillation. This eastward-propagating disturbance circles the tropics over thirty to sixty days, alternately enhancing and suppressing convection as it passes. When its suppressed phase is positioned over the Indian Ocean, break conditions become considerably more likely.

Absence of Bay of Bengal systems. Much of the rainfall over central India is delivered by low-pressure systems forming over the north Bay and tracking inland. Extended periods without such formation contribute directly to break conditions.

Mid-latitude influence. The intrusion of dry mid-latitude air from the north-west, associated with the position of the subtropical westerly jet, can suppress convection over northern India.

Why breaks matter more than onset

The seasonal rainfall total is important, but its distribution in time is frequently more important still, and this is where breaks exert their influence.

Consider the kharif crop cycle. Sowing follows the establishment of the monsoon. The crop then passes through germination, vegetative growth, flowering and grain filling, each with differing water requirements. A break of ten days is generally tolerable during the vegetative stage. A break of the same length during flowering may reduce yield substantially, irrespective of how much rain falls before or afterwards.

It is entirely possible for a season to record a normal aggregate rainfall total and still produce a poor agricultural outcome, because the rain arrived at the wrong times. Equally, a season with slightly below-normal total but well-distributed rainfall may prove entirely satisfactory.

This is why I would encourage caution when interpreting seasonal rainfall percentages reported in the press. The headline figure describes quantity. It says nothing about timing, and timing is frequently what determines consequence.

Recognising a developing break

Several indications typically precede or accompany a break, and they may be observed in publicly available data:

• Northward displacement of the monsoon trough in analysis charts
• Weakening of the low-level westerly flow across the Arabian Sea
• Rising daytime temperatures across central and northern India, frequently accompanied by oppressive humidity
• Reduction in cloud cover visible in satellite imagery, with the deep convective zone shifting north or towards the equator
• Increasing rainfall reports from the Himalayan foothills concurrent with drying elsewhere

Duration and recovery

Breaks commonly last from three days to two weeks. Those extending beyond two weeks, particularly during July and August when the crop water requirement is greatest, are the ones which give rise to genuine concern.

Revival ordinarily occurs through the southward return of the monsoon trough, frequently assisted by the formation of a fresh low-pressure system over the Bay of Bengal. The transition can be rapid, with widespread heavy rainfall resuming within a day or two of the trough re-establishing itself.

Current rainfall distribution across India may be followed on our Live Weather Dashboard, and the IMD publishes daily monsoon reports and trough position analyses throughout the season.