Current Affair 1:

The Bay of Bengal branch of the southwest monsoon has been stalled since May 31, a day after the onset of the monsoon simultaneously over Kerala and Northeast India. That is a stalling period of 19 days as of June 19, one of the longest in recent time, according to data from the India Meteorological Department (IMD).

The Arabian Sea branch of the monsoon has been stalled since June 10, which is a stalling period of nine days. The stalled monsoon has led to an all-India deficit in monsoon rainfall of 20 per cent between June 1 and June 18.

It is also one of the reasons for the late heatwaves in most of north India and many parts of west, central and eastern India since the beginning of June.

Why this year stalling is different from previous years?

‘Break periods’ are a normal characteristic for the monsoon. But some of the recent breaks have been much longer than normal. The season is usually said to be on a break when the monsoon trough moves towards the foothills of the Himalayas.

During this period, it starts raining heavily in the Himalayan and the northeastern states while the rest of the country remains dry, especially what the IMD defines as the ‘core monsoon zone’. The core monsoon zone stretches from Gujarat in the west-to-West Bengal and Odisha in the east.

The monsoon trough is an elongated low-pressure region that causes the rainfall during the monsoon season over the country. This normally happens once or twice during the season once the monsoon trough has settled into its normal position.

Currently, the monsoon trough has not even reached its normal position. Therefore, this stalling is different from the regular break period of the monsoon.

 

How does the Indian monsoon develop?

Every year, around the first week of June, a vast expanse of roiling grey clouds advances from the Arabian sea and makes landfall in Kerala.

From June to September, the southwest or summer monsoon moves across India, bathing the country in rain – during this period, India receives 70-90% of its annual rainfall. In the cooler months, from October to November, the retreating monsoon or the Northeast monsoon sets in, and brings rain to the eastern coast of India, especially Tamil Nadu.

As per the modern ‘energetics’ theory:

The physics of the Indian summer monsoon is not only affected by the amount of energy available from the sun, but also how much water vapour is available in the air and how well the water vapour can be lifted upward to form clouds.

The tilt in the Earth’s axis causes different parts of the Earth to receive direct rays from the sun during different times of the year.

During summer in the northern hemisphere, the Tropic of Cancer receives direct rays from the sun, and the continental land masses in this hemisphere heat up considerably more than the oceans, creating a low-pressure zone over India and Central Asia. This causes the intertropical convergence zone (or ITCZ) – an area of low pressure that forms a band girdling the Earth – to shift northwards from the Equator towards the Tropic of Cancer.

This zone is formed at the meeting of the southeast and northeast trade winds, which are winds close to the Earth’s surface that blow from east to west just north and south of the Equator, due to the Earth’s rotation from west to east.

When this shift occurs, the ITCZ shifts northwards from below India to run directly through the Indian subcontinent and strengthens the low pressure forming over this area. At the same time, the southeast trade winds, which cross the Equator due to this movement, become deflected towards the east due to the Coriolis effect (a force that causes fluids like air and water to curve as they travel across the Earth’s surface).

These deflected trade winds now blow towards India from the southwest, picking up large amounts of moisture from the Arabian sea. As they hit the Indian peninsula, they cause the southwest or Indian summer monsoon.

The summer monsoon winds split into two arms:

The summer monsoon winds split into two arms with one traveling over the Arabian sea, while the other moves over the Bay of Bengal.

The Arabian Sea arm causes rainfall all along India’s western coast. The Bay of Bengal arm over the eastern coast and moves over the Bay of Bengal to strike against the Bengal coast and brings rain to the southern slopes of the Shillong plateau. The Himalayas, which act as a barrier towards the further inland movement of this arm, herd it towards northern India. The two arms converge over Punjab and Himachal Pradesh by mid-July.

Apart from ITCZ, there are many other events:

These events involve jet streams, which are bands of narrow, meandering, and fast-moving winds (usually 100-200 Km/h but can go up to 400 Km/h) in the upper levels of the atmosphere (between 9 km and 16 km above sea-level).

There are three jet streams that are thought to affect the Indian summer monsoon – the subtropical westerly, the tropical easterly, and the Somali or cross-equatorial jet stream.

What are the subtropical, tropical easterly, and Somali jet streams? How do they affect the southwest monsoon?

The subtropical jet stream is formed when warm air from the equator meets the cool air from the polar regions and flows from west to east.

During summer in the northern hemisphere, as the Tropic of Cancer begins to receive the sun’s direct rays, two things happen. One, in response to a northward shift in heating patterns during the Indian summer, the subtropical jet stream moves northwards, right over the Tibetan plateau from its position over central India.

Due to this, the second event occurs – a seasonal jet stream, the tropical easterly, is set up. As the Tibetan plateau begins to heat up, the air rises to meet the subtropical westerly jet stream; the intermingling of these two currents is affected by the Coriolis force, which deflects the newly formed tropical jet stream towards the west.

The tropical jet stream flows from east-to-west (10-12 km above the Gangetic plains) across India, and subsides above the Indian Ocean, where it then lends extra energy to and ‘pushes’ the southwest monsoon towards India.

The Somali jet stream is set up due to the intense heating of the air over northern Bay of Bengal from moist convection, which attracts winds from the equatorial Indian Ocean toward the Indian subcontinent forming the low-level westerlies (prevailing winds from the west toward the east in the middle latitudes) over the Arabian Sea. These westerly winds bring moisture over Indian land, thus further enhancing the convection.

Therefore, the monsoon itself is thought to intensify the movement of the southwest winds of the lower atmosphere. The accumulation of water vapour of in the atmosphere is held responsible for the ‘burst’ or sudden onset of the Indian summer monsoon in early June, and for the rapid movement of the summer monsoon across India.

What other factors affect the Indian monsoons?

The Indian monsoon is an extremely complex climate pattern that is affected by many factors, of which the most well-known are the El Nino and La Nina, the Indian ocean dipole (IOD), and the EQUINOO (Equatorial Indian ocean oscillation).

 

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