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Monsoon is the rainfall season of India that typically occurs from June to September. During this time, warm, moist air from the Indian ocean flows over the Indian subcontinent, leading to heavy rainfall across much of the country. The monsoon rains are the source of most of the annual precipitation in India.
Following factors affect the monsoon rainfall in India:
- Temperature and Pressure gradient: The monsoon rainfall in India is driven by the temperature and pressure difference between the land and sea surfaces. The stronger the gradient, stronger the monsoon, and higher the rainfall.
- El Nino-Southern Oscillation (ENSO): ENSO is an irregular variation in sea surface temperature in the tropical Pacific Ocean. The warmer than normal pacific (El Nino) causes weaker monsoon winds and reduced rainfall in India.
- Indian Ocean Dipole (IOD): IOD is a phenomenon where the sea surface temperatures in the western and eastern Indian Ocean oscillate between warm and cool phases. Warm water in the western Indian Ocean (positive IOD) leads to the formation of stronger monsoon winds. Conversely, a negative IOD can lead to reduced rainfall.
- Topography: In Northeast India, the hills and valleys trap the winds, causing heavy rainfall in the region and making it one of the wettest regions in the world. On the east coast, the monsoon winds run parallel to the Eastern Ghats, resulting in less rainfall.
- Jet streams: Jet streams play a crucial role in the onset and strength of the Indian monsoon by causing rapid heating of the Tibetan Plateau and creating the necessary pressure gradient. A northward shift of the subtropical westerly jet stream helps to trigger the monsoon, while weaker or stronger than usual jet streams can lead to a delay or weaker monsoon.
The Indian Meteorological Department (IMD) defines normal monsoon as a season when the total amount of rainfall in the country between June and September is within 10% of the long-period average (LPA) rain. But even in a normal monsoon season, many regions suffer drought due to following reasons:
- Uneven distribution of rainfall: The ‘normal monsoon’ does not capture the uneven distribution of rainfall in a season.
E.g., some regions could get flooded while others may not receive rain, the total rainfall could still be within 10% of LTA.
- Delayed onset and early withdrawal of monsoon: If the monsoon season arrives later than usual, it can cause crop failure due to moisture stress. Similarly, if the monsoon season ends earlier than usual, it can cause moisture stress, poor yield, or crop failure, especially in areas of rainfed agriculture.
- Soil moisture deficit: The previous year’s rainfall and soil management practices influence the amount of moisture available for crops during the current season. If there is a deficit in soil moisture, a normal monsoon may not be enough to meet the water requirements of crops.
- Water scarcity: Some regions may not have enough water resources to support agriculture and other activities. This can be due to factors such as overuse of groundwater, high run-off, insufficient infrastructure for water storage and distribution and climate change.
Monsoon rains are the lifeblood of India’s farm-dependent economy. Addressing the variability of monsoon and associated challenges is critical for ensuring that all parts of India have access to sufficient water resources and can thrive in the face of a changing climate.
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