Modern Theories of the Indian Monsoon: Flohn's Dynamic Theory, Easterly Jet Theory & MONEX Explained

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Introduction

• In the previous blog of this India Subcontinent Monsoon series, we discussed the "Classical Theories of the Indian Monsoon," focusing on "Edmund Halley's Thermal Concept (1686)" and "Richard Scherhag's Aerological Concept (1948)." 

• These theories established that thermal differentiation between land and sea is the fundamental driving force behind the Indian Monsoon, while Scherhag further emphasized the role of upper-air circulation in strengthening the monsoon system.
• In today's blog, we move to the Modern Theories of the Indian Monsoon, which provide a more comprehensive explanation of the monsoon's 

1. origin, 
2. circulation, and 
3. seasonal behaviour. 

• Unlike the classical theories, the modern concepts recognize that the Indian Monsoon is governed by the interaction of atmospheric dynamics, upper-air circulation, and global climatic processes.
• We will discuss these three major developments in the evolution of modern monsoon theory:

      

• Let us begin with "Flohn's Dynamic Theory (1951)," which marked a major shift from purely thermal explanations to a dynamic understanding of the Indian Monsoon.

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Flohn's Dynamic Theory (1951)

• In 1951, "Hermann Flohn" proposed the Dynamic Theory of the Indian Monsoon, where, unlike Halley's Thermal Theory, which emphasized land–sea thermal contrast, Flohn focused on the dynamic behaviour of the global atmospheric circulation, particularly the seasonal migration of the "Inter-Tropical Convergence Zone (ITCZ)." Here, the term dynamic refers to the movement and interaction of atmospheric circulation systems rather than merely the motion of air. According to Flohn, the atmosphere responds dynamically to the changing distribution of solar heating over the Earth.

As shown in the following infographics- 
      

1. The Earth's geoid shape, axial inclination (23.5°) and revolution around the Sun create seasonal thermal gradients. 
2. During the Northern Hemisphere summer, the Sun's vertical rays migrate towards the Tropic of Cancer, causing the zone of maximum heating to shift northward. Consequently, the thermal equator, the ITCZ, and the associated pressure belts also migrate northward.
3. As the ITCZ moves northward over the Indian subcontinent, a strong zone of low pressure develops along the monsoon trough, extending from the Thar Desert towards the Bay of Bengal. 
4. Now, to compensate for this northward shift, the Southeast Trade Winds of the Southern Hemisphere cross the Equator and enter the Northern Hemisphere.
5. As a consequence, after crossing the Equator, these winds are deflected to the right by the Coriolis effect (Ferrel's Law) and become the Southwest Monsoon Winds, which, carrying with themselves abundant moisture from the Indian Ocean, produce widespread rainfall over the Indian subcontinent.

• Thus, Flohn did not reject Halley's Thermal Theory, but instead, he provided a "dynamic interpretation" of the same phenomenon by explaining-

1. how the seasonal migration of the ITCZ, and 
2. the global atmospheric circulation control the origin and progression of the Indian Monsoon.

So, in analysing the summary, follow as-

• Formed the conceptual bridge between the Classical Thermal Theory and later Jet Stream theories.
• Connected the thermal gradient with the dynamic movement of pressure belts.
• Introduced the concept of the seasonal migration of the ITCZ.
• Explained the monsoon as a component of the global atmospheric circulation.
• But still not explained the influence of -

1. Subtropical Westerly Jet (STWJ), 
2. ENSO, 
3. the Indian Ocean Dipole (IOD)
4. the Tropical Easterly Jet (TEJ), 

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Koteswaram and P. 

R. Krishnan's Easterly Jet Theory (1952)

• In 1952, P. R. Koteswaram and P. R. Krishnan introduced another important dimension to the understanding of the Indian Monsoon by emphasizing the role of the "Tropical Easterly Jet (TEJ)" in the upper atmosphere. Their theory proposed that the strength and behaviour of the Indian Monsoon are closely linked to upper-tropospheric wind circulation rather than surface processes alone.

• According to them, as ilustrated iin above infographics, in the Northern Hemisphere summer, intense heating over the Indian subcontinent, particularly the Thar Desert, generates vigorous upward convection, and, as the rising air reaches the upper troposphere, it creates strong upper-level divergence, leading to the development of the "Tropical Easterly Jet," an east-to-west flowing wind current at an altitude of about 12–16 km.
• The Tropical Easterly Jet enhances the removal of air from the upper atmosphere, thereby strengthening the "surface thermal low-pressure system" over northern India. This intensified low-pressure area draws larger quantities of moisture-laden air from the Indian Ocean, resulting in a stronger Southwest Monsoon.
• According to this theory, the strength of the Tropical Easterly Jet is directly related to the intensity of the Indian Monsoon, where a stronger TEJ generally favours vigorous monsoon circulation and heavier rainfall, whereas a weaker TEJ is often associated with a weaker monsoon and reduced precipitation.
• Thus, the Easterly Jet Theory demonstrated that the Indian Monsoon is controlled by-

1. not only by surface thermal contrast and the seasonal migration of the ITCZ, but 
2. also by the dynamics of upper-air circulation, making it a major advancement in the modern understanding of the Indian Monsoon.

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MONEX (Monsoon Experiment)

• Although Flohn's Dynamic Theory and Koteswaram's Easterly Jet Theory greatly improved the understanding of the Indian Monsoon, scientists still sought to explain why-

1. the strength of the Tropical Easterly Jet (TEJ) and, 
2. consequently, the intensity of the Southwest Monsoon varied from year to year.

• To investigate these questions, the "Monsoon Experiment (MONEX)" was organized under the "Global Atmospheric Research Programme (GARP)" during the 1970s. The major international observational campaign was conducted in 1978–1979, with the participation of several countries, including India, the USSR (Russia), the United States, Japan, and Australia. Using aircraft, ships, weather balloons, satellites, and upper-air observations, scientists examined the complete atmospheric circulation associated with the Indian Monsoon.
• The observations confirmed that-

1. the Tropical Easterly Jet (TEJ) is not controlled by a single factor. 
2. Instead, it develops through the combined influence of three major thermal forcing regions of the Indian subcontinent:

      

03 regions form the Thermal Axis of the Indian Monsoon

• Together, these three regions form the Thermal Axis of the Indian Monsoon, which acts as the principal thermal engine of the monsoon system. 

1. When this thermal axis is well developed, the Tropical Easterly Jet becomes stronger, enhancing upper-level divergence and supporting a vigorous Southwest Monsoon. 
2. Conversely, if one or more components of the thermal axis weaken, the TEJ also weakens, often resulting in reduced monsoon circulation and below-normal rainfall.

• Thus, MONEX demonstrated that the Indian Monsoon is a complex coupled atmospheric–oceanic system, controlled by the interaction of-

1. thermal forcing, 
2. upper-air circulation, 
3. ocean-atmosphere processes, and 
4. large-scale global circulation rather than by any single mechanism alone.

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• In upcoming "BLOGS" we willl be discussing "the Mechanism of Monsson".

Thanks

Critical Comments are welcome.

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