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Introduction
In our previous blogs, we explored the fundamental cause of Earth's thermal differentiation. This unequal distribution of heat primarily results from three factors:
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| Earth's thermal differentiation Factors |
- The Earth's geoid (nearly spherical) shape, which causes solar rays to strike different latitudes at different angles.
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The Earth's axial inclination (23.5°), which changes the latitude receiving the Sun's vertical rays throughout the year.
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The Earth's revolution around the Sun, which, together with axial tilt, produces the seasonal migration of the heat belt.
- As a result, the zone of maximum solar heating continuously shifts between the Tropic of Cancer and the Tropic of Capricorn. As illustrated in the following infographics:
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| Continuous shifting- Zone of maximum solar heating |
- This periodic north-south migration of the thermal belt creates both spatial and seasonal thermal gradients, which form the fundamental energy source driving Earth's atmospheric circulation.
- In the previous chapters of this series, we also examined the major components of the Indian Monsoon system. Their index is provided under the heading "Indian Subcontinent Monsoon."
- In this blog, we begin the study of the Theories of the Indian Monsoon.
- Although these theories differ in their explanations of the monsoon's mechanism, they all share one common foundation:
"The thermal gradient generated by the unequal distribution of solar heating is the primary driving force behind the origin and evolution of the Indian Monsoon."
- From this fundamental concept, various classical and modern theories attempt to explain the timing, intensity, circulation, and variability of the Indian Monsoon. In today's blog, we will discuss the Classical Theories of the Indian Monsoon, and in the next one we will analyse the modern theories.
Halley's
Thermal Concept (1686)
- The first
scientific explanation of the Indian Monsoon was proposed by Edmund Halley in
1686, when he stated that the differential heating of land and sea
is the “fundamental driving force” behind the monsoon.
- During the
Northern Hemisphere summer (June–August), the Sun's vertical rays migrate in
the northern hemisphere, and importantly at the Tropic of Cancer, causing
intense heating over the Indian subcontinent and adjoining Central Asia. As a
result, a strong thermal low-pressure area develops, particularly over northwestern
India, the Thar Desert, and the Tibetan region.
- At the same
time, the relatively cooler Indian Ocean maintains comparatively higher
pressure at the southern hemisphere “subtropical high-pressure belt” situated between 20-30 degrees South latitudes. So, in a cause-and-effect relationship,
the pressure difference generates a strong pressure-gradient force, causing
moist air to flow from the ocean towards the land.
- The
southeast trade winds of the Southern Hemisphere cross the Equator, filling
this thermal low. After crossing the Equator, they are deflected to the right
by the Coriolis force and become the Southwest Monsoon winds, bringing
abundant moisture and rainfall to the Indian subcontinent.
- Thus, "the
mechanism of the seasonal reversal of winds, primarily controlled by the
thermal contrast between the rapidly heated landmass and the relatively cooler
surrounding oceans, was first explained by Edmond Halley through the 'Thermal
Theory of the Monsoon, ' when the theory successfully explained the fundamental
role of differential heating. But it needs to be noted that it is not only a complete
explanation of the Indian monsoon; modern climatology recognizes the
monsoon as a "complex atmospheric system" influenced by multiple dynamic and
thermal factors.
- The reason behind that was the “limitation” associated with
this, such as-
- could not explain the roles of the Tibetan Plateau,
upper-air circulation,
- jet streams,
- the Inter-Tropical Convergence Zone (ITCZ),
or
- ocean–atmosphere interactions, all of which are now recognized as
essential components of the Indian Monsoon system.
Scherhag's
Aerological Concept (1948) – Upper-Air Circulation
- In 1948, the German meteorologist "Richard Scherhag" proposed the "Aerological Concept" as an extension of Halley's Thermal Concept.
- Rather than rejecting the importance of land–sea thermal contrast, Scherhag emphasized that the atmosphere's first layer, the upper troposphere, also plays a crucial role in the development and maintenance of the Indian Monsoon.
- In our previous discussion on the "Thermal Axis" of the Indian Monsoon, we have explained that-
- the Thar Desert,
- the Himalayan region, and
- the Tibetan Plateau together form the "principal thermal engine" of the monsoon system.
- Scherhag's concept builds upon this idea by explaining how intense surface heating influences the above circulations, when during the Northern Hemisphere summer the intense heating over the Indian subcontinent, particularly the Thar Desert, Himalayan region, and the Tibetan Plateau, generates strong upward convection currents.
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| Upward convection currents |
- As we have already discussed, the Tibetan Plateau, with an average elevation of about 4,500 metres, extends deep into the troposphere and acts as an elevated heat source. This intense heating produces vigorous rising air currents, leading to upper-level divergence.
- The divergence of air from the upper troposphere further strengthens the surface thermal low-pressure system, thereby enhancing the inflow of moisture-laden winds from the Indian Ocean.
- In this way, Scherhag demonstrated that the Indian Monsoon is governed by
- Not only by surface thermal contrast, as proposed by Halley, but
- also by upper-air circulation and atmospheric dynamics.
- Therefore, Scherhag's contribution should be regarded as a "scientific refinement" of Halley's Thermal Concept rather than a completely independent explanation of the monsoon's origin, where his work supplemented the thermal theory by providing an atmospheric mechanism that linked intense surface heating with upper-level circulation.
Limitation
- Although Scherhag successfully emphasized the importance of upper-air circulation, his concept did not fully explain-
- the influence of the "Inter-Tropical Convergence Zone (ITCZ),"
- "Subtropical Westerly Jet (STWJ),"
- "Tropical Easterly Jet (TEJ)," or
- ocean–atmosphere interactions such as ENSO and the Indian Ocean Dipole (IOD).
- These aspects were incorporated into the later Dynamic and Integrated theories of the Indian Monsoon.
In the next blog, we are going to discuss the "Dynamic Theories" related to the origination of the Indian Monsoon system.
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