Onset of Southwest Monsoon in India
Context:
The Southwest Monsoon has commenced over Kerala and progressed into most parts of Northeast India. It has also made its way in most parts of southwest and west-central Arabian Sea and the Lakshadweep area.
Relevance:
GS-01 (Physical Geography)
Details:
• The Southwest Monsoon has advanced into the remaining parts of the southwest Arabian Sea, parts of the west-central Arabian Sea, most parts of the southeast Arabian Sea and Lakshadweep area, most of Kerala, Mahe, parts of southern Tamil Nadu, and the remaining parts of Maldives and Comorin area.
• Additionally, it has advanced into parts of the northeast Bay of Bengal and most of Northeast India, covering entire Nagaland, Manipur, Mizoram, Arunachal Pradesh, and most parts of Tripura, Meghalaya, and Assam. Consequently, the Southwest Monsoon set in over Kerala on May 30, 2024.
• A cyclonic circulation is present over northeast Assam and its surrounding areas in the lower tropospheric levels. Another cyclonic circulation is over northwest Uttar Pradesh and its neighborhood, with a trough extending from this region to west Bangladesh in the lower tropospheric levels.
Monsoon in India
• India is characterized as ‘monsoon’ type which is predominantly found in South and Southeast Asia. Among the four seasonal divisions in India, the monsoon occupies two distinct periods:
○ Southwest Monsoon Season: Occurring between June and September, this period is marked by seasonal rainfall brought by the southwest monsoons.
○ Retreating Monsoon Season: Taking place in October and November, this season is defined by the retreat of the monsoon.
Factors Influencing the Formation of the Southwest Monsoon:
• Inter Tropical Convergence Zone (ITCZ) Shift: During summer, the ITCZ shifts over the Ganga plain, forming the monsoon trough, usually positioned about 5°N of the equator.
• High-Pressure Area near Madagascar: Located approximately at 20°S over the Indian Ocean, this high-pressure zone’s position and intensity significantly influence the Indian monsoon.
• El Nino/Southern Oscillation (ENSO): Typically, the eastern South Pacific Ocean has high pressure while the tropical eastern Indian Ocean experiences low pressure. However, in some years, this pattern reverses, leading to lower pressure in the eastern Pacific compared to the eastern Indian Ocean, a phenomenon known as the Southern Oscillation (SO).
• Differential Heating and Cooling: The land in India heats up and cools down more quickly than the surrounding seas, creating low pressure over the landmass while higher pressure persists over the seas.
• Jet Streams: The northward movement of the westerly jet stream above the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer play crucial roles.
• Heating of the Tibetan Plateau: Intense summer heating of the Tibetan plateau generates strong vertical air currents, resulting in low pressure at about 9 km above sea level.
• Tropical Easterly Jet (African Easterly Jet): This wind system also impacts the monsoon dynamics.
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