Every year, whenever the time for monsoon comes, we see the Indian Meteorological Department (IMD) getting too busy with various types of models and most they talk about in the recent years is about one parameter, the El Nino.
This gives rise to a question, ‘What does an ocean current, thousands of miles away from Indian Coasts, and which is not even in Indian Ocean, got to do with Monsoon in India?’
The answer lies in the mechanism of Indian Monsoon. Let’s see how.
Mechanism of Indian Monsoon:
After numerous experiments like MONEX and Indo-Soviet Joint exercise to understand the mechanism of monsoon a number of models were prepared based of weightage given to the individual parameters. The neo-classical theory says that the mechanism behind formation of Monsoon is practically the culmination of three individual phenomena. These are:
- Shifting of ITCZ and creation of low-pressure region over Northern India during May-June.
- The Mascarene Highs and the Easterly Jet Stream
- Meteorological Teleconnection or El Nino effect
In the recent times, the effect of Indian Ocean dipole has also been associated with the phenomena. Let’s try to understand these phenomena one by one.
1) Shifting of ITCZ and creation of the low-pressure belt over Northern India
In winters, the Westerly Jet-stream flows over the Indian Sub-Continent. The Westerlies flow at an average height of 6,000 meters at a speed of 90-400km/hour from western direction to eastern direction. Here we need to take into consideration that the average height of Himalayas is 6,500 meters. Hence Himalayas act as a barrier to the Westerly Jet stream and forces it to move either through the south of it or through north of it but not over it.
However, in the summers, we all know that the wind pressure belts shift further north in the Northern Hemisphere. As a result the Westerly Jets Streams too gets shifted to the north of Himalayas. Whereas the area over the plains of Northern India becomes the
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‘Climatic Equator’ or Inter-Tropical Convergence Zone’ as it receives direct sun light (at a 90oangle) and the wind pressure belts of the two hemispheres converge here. That is why it is called the Inter-Tropical Convergence Zone.
Now since this region receives the direct sun rays hence, the air-mass just above gets heated faster and in the process expands, becomes lighter in density and starts rising above according the natural laws of expansion of physics. This causes creation of low-pressure belt of the vast expanse of Northern India. This phenomenon happens at other places of the world as well but there relatively cooler wind starts coming from both the north and southern high-pressure regions. In case of Northern India this is not possible as the Himalaya acts as a barrier for the chilled winds from the Tibetan highlands and never allow them to traverse to India. As a result, relatively cooler wind from the southern region, especially the winds dispersing from the high-pressure zone of Mascarene Highs attempts to fulfill this low-pressure region.
We need to take into consideration that the location of Mascarene Highs is just at the South-East of Africa on the high seas, hence the wind dispersing from there and traversing across the Indian Ocean to reach Indian Sub-continent is expected to be moisture laden and cause rainfall in India. This factor is further enhanced by the second phenomena, ie. the Easterly Jet Stream.
2) The Mascarene Highs and the Easterly Jet Stream
There is a plateau or Highland called Tibetan Highland just at the north of Himalayas. The average height of Tibetan Highland is 4,000m. It is enclosed by Himalayas in the South with an average height of 6,500m and the Qunlun-Tienshan Mountain Ranges in the north with an average height of 5,500m. Hence one can easily understand that the highlands of Tibet would be heated faster than in comparison to the two hill ranges situated at the North and South of it, a phenomenon which would make it look like a heat-island. This relatively faster heating would be creating a relative low-pressure belt over Tibetan Heat-island, to fulfill which winds would be gushing into from the two mountain ranges at the north and south of it. Which would in-turn provide a larger push to the Westerly Jet Streams flowing over the region and shift it to the band of 8,000m height. At this height the Himalayas would not be a barrier to the Jet Streams and a section of it would convert it-self into Easterly Jet Stream and gush towards the Mascarene Highs to provide more source of air for the High Pressure region.
This phenomenon increases the potentiality of high-pressure in the Mascarene Highs and feed into the south west monsoon coming towards the Indian Sub-continent. So, one can say that hotter the Tibet gets more rainfall we’ll receive.
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Now this is also only one part of the jig-saw puzzle which partially explain the phenomena of higher than expected rainfall at times. Then what about the deficiency? This is answered by the meteorological teleconnection of El Nino effect.
3) Meteorological Teleconnection or El Nino effect
At times during the summers in southern hemisphere in December, a hot current appears at the Peruvian Coast. This hot current causes heavy rainfall in the region but since it is an anomaly or abnormal hence it kills majority of the fishing stock of the region, hence even the local people pray against the occurrence of this phenomena. In the larger picture, this hot current creates a low-pressure region around Peruvian Coast, which causes this high rainfall. In turn, a counter high-pressure region gets created just at the east of Australian coast in the Western Pacific. This high-pressure belt in turn creates a counter low-pressure belt in the west coast of Australia in the eastern most part of Indian Ocean. Till date we have heard only of one low-pressure belt in Indian Ocean region caused by the ITCZ, now if a counter low-pressure belt is also created then one can understand that this low-pressure belt will attract winds dispersing from the Mascarene Highs and weaken the Indian Monsoon. So more powerful the El Nino is, more powerful would be the counter-pressure regions and weaker would be Indian Monsoon.
However, this phenomenon is not regular hence the variation we witness in the amount of rainfall received in India. Even in the years of El Nino, not every year we witness deficient rainfall in India, it primarily depends on the intensity of the pressure zones El Nino creates.
By the way at times, an opposite phenomenon called La Nina or ‘little girl’ is also witnessed. La Nina is a cold current occurring at times (not regularly, just like El Nino) in the middle or western Pacific. As a result of La Nina the water body across the Western Pacific till the early eastern Indian Ocean, ie. throughout the Northern Coastal region of Australia experiences cooler high-pressure. As a result, instead of pulling winds dispersing from Mascarene Highs it adds an extra impetus to the Monsoon in India and adds on to its strength.
It is because of these reasons that, these parameters and their intensity become so important to the Indian Meteorological Department in predicting the Monsoon in India.
 Source: One life is not enough by K. Natwar Singh