Why Do Anti-Trade Winds and the Tropical Easterly Jet Form? The Hidden Engine of the Indian Monsoon

 

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Introduction

• In one of our previous blogs, we discussed how the Earth's geoid shape influences the unequal distribution of solar energy (insolation) across the globe, thereby creating the fundamental pattern of "global temperature distribution." We also examined another equally important controlling factor—the seasonal migration of the Sun's vertical rays between the Tropic of Cancer and the Tropic of Capricorn that explains the consequences of the apparent position of the Sun shifting northward and southward during the year,  causing the angle of solar incidence to change continuously. Consequently, the Northern and Southern Hemispheres experience alternating periods of maximum heating and cooling in an approximately six-month cycle.
• The combined effect of these two factors—

1. the Earth's geometry, and 
2. the seasonal migration of solar heating—forms the thermal foundation of the global atmospheric circulation system. 

• However, differential heating alone cannot explain the complete behaviour of the atmosphere. The atmosphere continuously attempts to restore "thermal equilibrium" through- 

1. large-scale movements of air, 
2. giving rise to complex circulation patterns extending from the Earth's surface to the upper troposphere.

• Building upon this foundation, the present blog explores one of the most important yet often misunderstood aspects of atmospheric dynamics: 

1. the generation of the Anti-Trade Winds, and 
2. the Tropical Easterly Jet (TEJ). 

• Rather than treating these as isolated wind systems, we shall examine how they emerge naturally from -

1. seasonal thermal contrasts, 
2. pressure gradients, 
3. the Hadley circulation, and  
4. the exceptional summer heating over the Indian subcontinent and the Tibetan Plateau.

• Understanding these upper-air circulation systems is essential because they play a decisive role in the development, intensity, and spatial distribution of the Indian Summer Monsoon, making them one of the key components of tropical atmospheric dynamics.

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Origin of the Anti-Trade Winds and the Tropical Easterly Jet

• The infographic above illustrates one of the most fundamental principles of atmospheric circulation. When a land surface experiences intense solar heating-

1. the air above it becomes warmer, 
2. expands, 
3. becomes less dense, and 
4. begins to rise. This upward movement creates a "thermal low-pressure zone" at the Earth's surface. As air ascends, the amount of air available near the ground decreases, producing a pressure gradient.

• Now, as we understand, according to the basic principle of wind movement, wind always flows from a region of relatively higher pressure towards a region of relatively lower pressure ( Point 03 in the infographics). Consequently, surface winds begin converging from the surrounding high-pressure regions towards the heated low-pressure area. This explains the origin of surface winds.
• However, an important question naturally arises. If air continuously moves away from the surrounding high-pressure regions towards the low-pressure centre, will those high-pressure regions eventually lose their air? The answer is no. The atmosphere functions as "a continuous three-dimensional circulation system." The air rising over the low-pressure region does not disappear; instead, it spreads outward in the upper troposphere. Thus, while the surface is characterized by "horizontal convergence," the upper atmosphere is characterized by "horizontal divergence." This upper-level divergence reflects the following consequences

1. replenishes distant regions, and  
2. maintains the global balance of atmospheric mass.

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Now, look at the following infographics-

      

• As discussed in our previous blog, during May, June, and July, the Thar Desert frequently records surface temperatures of about 45–48°C, generating one of the strongest thermal low-pressure systems over the Indian subcontinent. The heated air rises vigorously and, in the lower troposphere, spreads outward towards the comparatively higher-pressure region over the southern Indian Ocean. This upper-level return flow is commonly referred to as the Anti-Trade Wind because it flows in the opposite direction of the near-surface trade winds.
• The circulation becomes even more significant when the Thermal Axis of the Indian Monsoon is established. As explained earlier, this thermal axis is formed through the combined influence of three major heat sources:

1. The intense heating of the Thar Desert.
2. The elevated heating of the Tibetan Plateau, whose average elevation of about 4,500 metres places it deep within the troposphere.
3. The reduced summer snow cover over the Himalayas, which lowers surface albedo, allowing greater absorption of solar radiation and strengthening atmospheric heating.

• Together, these three factors produce exceptionally strong upward convection over northern India and the Tibetan Plateau. The rising air penetrates much higher into the upper troposphere, reaching approximately 12–13 km, where it diverges southward. This upper-level easterly flow develops into the "Tropical Easterly Jet (TEJ)" during the Indian summer monsoon season.
• Both the Anti-Trade Winds and the Tropical Easterly Jet eventually move towards the relatively higher-pressure region over the southern Indian Ocean, where large-scale subsidence occurs. The descending air contributes to the maintenance of the subtropical high-pressure belt situated at 10-15 degrees south of the equator known as "the Mascarene High." 
• From this high-pressure region, surface winds once again flow towards the intense thermal low- the Indian Thermal Axis- over the Indian subcontinent as the Southwest Monsoon, thereby completing the atmospheric circulation loop. The Anti-Trade Winds and the Tropical Easterly Jet (TEJ) are among the most important upper-air circulation systems controlling the velocity and intensity of the Indian Summer Monsoon, because when these upper-level winds are strong accelerates the southwest monsoon winds, resulting in a stronger and more active monsoon. Conversely, when the system is weaken cause the monsoon winds to slow down and reduces the intensity and distribution of rainfall.

• Thus, the "Anti-Trade Winds" and the "Tropical Easterly Jet" are not isolated atmospheric phenomena, but integral components of a continuous circulation system driven by-

1. differential heating, 
2. the thermal axis of the Indian monsoon, 
3. upper-air divergence, and 
4. large-scale conservation of atmospheric mass. Together, they sustain the seasonal monsoon circulation over the Indian subcontinent.

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Note- 

In the next blog, we are going to discuss the "Classical Theories" related with origination of the Indian Monsoon system

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