From UPSC perspective, the following things are important :
Prelims level : NE Monsoon
Mains level : Factors affecting Monsoon
- The southwest, or summer, monsoon, finally withdrew from the country, having overstayed and delayed its retreat by a record time.
- The same day, the northeast, or winter, monsoon made its onset, on time.
- The northeast monsoon does not have anything to do with the Northeast region of the country, though a part of the system does originate from the area above it.
- The northeast monsoon derives its name from the direction in which it travels – from the northeast to the southwest.
- On the other hand, the summer monsoon, at least the Arabian Sea branch of it, moves in exactly the opposite direction – from the southwest to the northeast.
- That is why it is also called the southwest monsoon.
Role of ITCZ
- The reversal of direction in the lower-atmosphere moisture-laden winds happens primarily due to the southward movement of Inter Tropical Convergence Zone (ITCZ) during the withdrawal phase.
- The ITCZ is a dynamic region near the Equator where the trade winds of the northern and southern hemispheres come together.
- The intense sun and warm waters of the ocean heat up the air in this region and increase its moisture content.
- As the air rises, it cools, and releases the accumulated moisture, thus bringing rainfall.
- During the monsoon season, this ITCZ is located over the Indian landmass.
- By September, as the temperature in the northern hemisphere begins to go down, the ITCZ starts moving southwards, towards the Equator, and further into the southern hemisphere where the summer season begins to take shape.
Mechanism of NE Monsoon
- The months of October, November and December are supposed to comprise the northeast monsoon season, though the normal date for the onset of this monsoon is only around October 20.
- The southern peninsular region receives rains in the first half of October as well, but that is attributable to the retreating summer monsoon.
- The summer monsoon season ends on September 30 but the withdrawal does not happen overnight.
- From the beginning of the season, as it starts its northward journey over the Indian landmass, the monsoon takes a month and a half to cover the entire country.
- The southward withdrawal takes place over a period of three to four weeks.
- It usually starts around the second week of September and continues till about the second week of October, bringing rain as it retreats.
El Niño impact
- Like the southwest monsoon, the northeast monsoon is also impacted by the warming and cooling of sea surface waters in the central Pacific Ocean. But the impact is the opposite.
- The northeast monsoon is known to receive a boost from El Niño, when the sea surface temperatures in the equatorial Pacific Ocean, off the western coast of South America, are warmer than usual.
- And, when the opposite phenomena La Niña happens, rainfall during the northeast monsoon is known to get depressed.
- This year the El Niño Southern Oscillation, or ENSO, is in neutral state and is likely to remain like that for the rest of the year.
Another winter rains
- Many other parts of the country, like the Gangetic plains and northern states, also receive some rain in November and December but this is not due to the northeast monsoon.
- It is caused mainly by the Western Disturbances, an eastward-moving rain-bearing wind system that originates beyond Afghanistan and Iran, picking up moisture from as far as the Mediterranean Sea, even the Atlantic Ocean.