In India, during October-November, with the apparent movement of the sun towards the south, the ______ over the northern plains becomes weaker.
Monsoon trough
Understanding the Monsoon Trough in India
The question asks about a specific atmospheric feature over the northern plains of India that weakens during October-November as the sun apparently moves towards the south. This period is significant as it marks the transition from the monsoon season to the winter season in India.
Let's analyze the key terms:
- October-November: This is the post-monsoon season in India, often referred to as the retreating monsoon season.
- Apparent movement of the sun towards the south: After the summer solstice, the sun's direct rays shift towards the Southern Hemisphere. This leads to decreasing temperatures and solar radiation in the Northern Hemisphere, including India.
- Northern plains: This region in India experiences significant weather changes driven by large-scale atmospheric systems.
- Weakens: The atmospheric feature loses its intensity or influence.
Analyzing the Options
We need to identify which of the given troughs weakens during this transition period.
Here's a brief look at the options:
- Upper level trough: These are troughs located in the upper atmosphere (e.g., jet streams) and are associated with weather systems like Western Disturbances, which become more common in winter, not necessarily weakening in October-November in this specific context over the plains.
- Monsoon trough: This is a seasonal low-pressure area that lies roughly parallel to the Himalayas across the northern plains during the summer monsoon season (June to September). It is a key driver of monsoon rainfall.
- Lee trough: A trough formed on the leeward (downwind) side of a mountain range due to the barrier effect on airflow. While leeside troughs can occur, the major feature weakening over the northern plains in October-November is not typically described as a Lee trough in this context.
- Inverted trough: A trough of low pressure that curves poleward from a low-pressure center, typically associated with tropical cyclones or other specific weather patterns, not the broad seasonal feature weakening over the northern plains post-monsoon.
Why the Monsoon Trough Weakens
During the summer monsoon season, intense heating over the northern plains of India creates a strong thermal low-pressure area. This low-pressure area, along with its extension known as the monsoon trough, attracts moisture-laden winds from the Indian Ocean (the southwest monsoon). The strength and position of this trough are critical for the monsoon.
As October-November arrives, the sun's apparent movement southwards causes the temperatures over the northern plains to decrease significantly compared to the monsoon months. This reduction in heat weakens the thermal low-pressure system.
Furthermore, the monsoon trough is linked to the Inter-Tropical Convergence Zone (ITCZ), which also shifts southwards following the sun's apparent movement. As the ITCZ and the thermal low over the plains weaken and shift south, the monsoon trough over the northern plains becomes less pronounced and eventually dissipates or moves southwards.
Therefore, the atmospheric feature that weakens over the northern plains of India during October-November due to the sun's southward movement is the Monsoon trough.
Conclusion
The transition from the summer monsoon to winter in India is marked by the retreat of the monsoon. A key characteristic of this retreat is the weakening and southward shift of the monsoon trough over the northern plains. This weakening is directly linked to the decrease in temperature caused by the sun's apparent southward movement.
| Atmospheric Feature | Description | Relevance in Oct-Nov over Northern Plains |
|---|---|---|
| Upper level trough | Trough in the upper atmosphere, influencing weather patterns like Westerlies. | Not the primary feature weakening; Western Disturbances associated with these strengthen in winter. |
| Monsoon trough | Seasonal low-pressure trough over northern plains during monsoon. | Weakens significantly and shifts south as temperatures decrease post-monsoon. |
| Lee trough | Trough formed downwind of mountains. | A localized phenomenon, not the dominant seasonal weakening feature over the plains. |
| Inverted trough | Poleward curving trough from a low center. | Different structure and context from the seasonal monsoon trough. |
Revision Table: Key Concepts
| Term | Explanation |
|---|---|
| Monsoon Trough | A low-pressure area extending over the Indo-Gangetic plain during the summer monsoon season. It is the surface manifestation of the ITCZ over India. |
| Retreating Monsoon | The period from October to December when the southwest monsoon withdraws from India. |
| Apparent Movement of Sun | The perceived movement of the sun across the sky over the year, due to Earth's tilt and orbit. Leads to seasonal changes. |
| Thermal Low | A low-pressure area created by intense heating of the ground surface. |
Additional Information: Monsoon Retreat and Weather Changes
The weakening of the monsoon trough and the southward shift of the ITCZ are the main reasons for the retreat of the monsoon from northern India. This retreat leads to distinct weather changes:
- Clear skies become more common.
- Daytime temperatures remain high, but nights become cooler.
- Rainfall decreases significantly in the northern plains.
- In southern India, particularly the southeastern coast, the retreating monsoon winds picking up moisture from the Bay of Bengal can bring rainfall (the northeast monsoon).
- The transition period can also see the formation of tropical cyclones in the Bay of Bengal and Arabian Sea due to the changing atmospheric conditions.
Understanding the behavior of the monsoon trough is essential for comprehending the Indian climate system and its seasonal variations.
Important Questions from Climatology
In the context of climate, what is the full form of ITCZ?
With reference to the climatic conditions of the Prairies, which of the following statements is correct?
Land and sea breeze are caused due to _____________
Which island emerged in the Bay of Bengal in the after math of the Bhola cyclone in 1970?
______ are a narrow belt of high altitude (above 12,000 m) westerly winds in the troposphere. Their speed varies from about 110 km/h in summer to about 184 km/h in winter.
