Role of Jet Streams in Indian Monsoons

Role of jet streams in Indian Monsoons

In the winter season, the sub-tropical westerly jet stream takes position over northern India, and the presence of the Himalayas and the Tibetan Plateau leads to a division of this jet stream into two distinct branches.

The northern branch situates itself to the north of the Tibetan Plateau, while the southern branch finds its place over northern India, south of the Himalayas.

At higher altitudes within the troposphere, a system of high pressure forms, marked by anticyclonic conditions featuring a clockwise air circulation. This high pressure zone materializes to the south of the southern branch of the sub-tropical jet stream, spanning across Afghanistan and the north-western reaches of Pakistan.

As a result, the prevailing winds descend over the north-western parts of India, ushering in a period of atmospheric stability and arid conditions.

In the summer months of April, May, and June, a shift in the sun's overhead position contributes to the emergence of surface-level low pressure areas near Pakistan and north-west India.

The downward motion of winds from the upper air high pressure dampens the upward movement of winds originating from the surface-level low pressure. This dynamic gives rise to warm and parched weather conditions.

Interestingly, despite the high temperatures and significant evaporation, the months of April and May remain dry.

In contrast, the upper air conditions foster the creation of low pressure above the eastern Himalayan region due to the influence of seasonal easterly jet streams. Consequently, winds originating from southern Myanmar are compelled to ascend, resulting in the precipitation observed in Myanmar, Bangladesh, and the North East India region.

Following the initial week of June, the southern branch of the sub-tropical westerly jet stream recedes, leaving only the northern branch operational, positioned to the north of the Tibetan plateau.

This shift leads to the development of a dynamic depression over the north-western segment of Indo-Pakistan. As this dynamic depression interacts with the prevailing thermal depression in the area, the monsoon bursts forth.

The Influence of Jet Streams on Monsoons

Jet streams, which are narrow corridors of high-speed winds flowing from west to east in the upper levels of the troposphere, play a significant role in shaping climate and weather patterns. These fast-moving "rivers of wind" have the ability to direct air masses and impact various atmospheric phenomena.

M.T. Yin's theory highlights the connection between monsoon bursts and upper-level air circulation, with jet streams serving as key factors. The sub-tropical westerly jet stream and the equatorial easterly jet stream are the two main jet streams that influence monsoon winds.

Sub-tropical westerly jet stream: This jet stream prevails during winter in the upper troposphere of northern latitudes. It can be found over the Indian subcontinent at an altitude of approximately 12 km. The Himalayan mountain system divides this jet stream into two segments. The portion south of the Himalayas descends over the northwestern part of India, contributing to atmospheric stability.

Equatorial easterly jet stream: This jet stream is a prominent feature of upper-level air circulation during the Indian monsoon season. It appears as a band of strong easterlies extending from Southeast Asia across the Indian Ocean and Africa to the Atlantic.

The Role of Western Disturbance

A Western disturbance (WD) is an extra-tropical storm originating in the Mediterranean region. It travels from west to east within the extra-tropical zones. In this context, "disturbance" refers to an area of lowered air pressure. Western disturbances bring precipitation in the form of rainfall, snowfall, and fog to northern India. This pattern is not related to the monsoon and is influenced by westerly winds. Western disturbances are characterized by cloudy skies, elevated nighttime temperatures, and unusual rain patterns.

Western disturbances are of great importance to Indian agriculture, particularly for Rabi crops like wheat in the northern and northwestern regions. However, excessive precipitation can lead to crop damage, landslides, floods, and avalanches. In some instances, these disturbances also trigger cold wave conditions and dense fog in northern India.

Formation of Western Disturbances

Western disturbances originate as extra-tropical cyclones in the Mediterranean Sea. Moisture is drawn from the Mediterranean Sea and/or the Atlantic Ocean. A high-pressure zone over Ukraine and nearby areas reinforces the influx of cold air from Polar Regions into a relatively warmer region with higher moisture content. This dynamic creates favorable conditions for cyclogenesis in the upper atmosphere, resulting in the development of an eastward-moving extra-tropical depression. These disturbances journey across the Middle East, passing through Iran, Afghanistan, and Pakistan before entering the Indian subcontinent.

The Jet Stream: Nature's High-Speed Air River

The jet stream, a powerful band of winds located about 5 to 7 miles above the Earth's surface, flows vigorously from west to east.

How Does the Jet Stream Operate?

The jet stream courses high in the sky, influencing wind patterns and pressure at that altitude. These effects extend to the surface, influencing factors like high and low-pressure regions, ultimately shaping the weather we experience. At times, the jet stream flows in a straight, smooth manner, resembling a swift river. However, it can also bend and twist, much like the meandering of a river, leading to slower movement and less predictable shifts in areas of low pressure.

Additionally, the jet stream can impact the strength of low-pressure systems. It acts akin to a vacuum cleaner, drawing air from the top and intensifying the system, which results in lowered pressure. Generally, lower pressure within a system correlates with stronger winds and stormier conditions. Conversely, a more relaxed and undulating jet stream can promote the dominance of high-pressure areas, typically leading to calm weather, gentle breezes, and dry skies.

What Triggers the Jet Stream?

Earth's division into two hemispheres prompts a perpetual movement of air, redistributing heat and energy from the equator to the poles. Three major air circulation cells exist within each hemisphere in the troposphere, the lowest atmospheric layer. Consequently, the jet stream arises due to variations in temperature. In the northern hemisphere, this disparity results in cold air to the north of the jet stream and warmer air to the south.

Seasonal changes also impact the jet stream's position. During winter, the temperature gap between the equator and the poles widens, leading to a stronger jet stream that crosses over the UK. This explains the prevalence of wetter weather. In contrast, a lesser temperature discrepancy characterizes summer, causing the jet stream to usually position itself north of the UK, ushering in calmer and drier conditions.

Implications for Air Travel

Meteorologists at the Met Office are among a select few worldwide producing weather charts for global aviation. These charts outline the forecasted jet stream's location, elevation, strength, and associated turbulence.

Although the jet stream's position and altitude vary, it generally aligns with the cruising altitude of transatlantic aircraft. Flying with the jet stream's flow translates to quicker travel and fuel savings. However, arriving too early may necessitate circling while awaiting landing clearance. Weak jet streams can lead to flight delays, and traveling against the stream incurs higher fuel costs and potential tardiness. Flight planning thus demands a certain skill.

Jet streams can introduce turbulence, especially when wind speed changes or the stream's trajectory isn't linear. This turbulence resembles the eddies in a river's flow, occasionally disrupting smooth air travel.

Technically, flying in the cold air side of the jet stream enhances engine efficiency and fuel economy.

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