..this post is under construction...
India (South Asia) lies south of 30oN latitude. As seen in the figure, this region lies in the zone of North Easterly Trade Winds
(winds are named from the direction they flow, rather than to the
direction they are going). Since solar insolation at Equator is maximum,
there is high temperature and low pressure. Air in this region rises up
and air from tropics (30oN and 30oS)
rushes here. Now, shouldn't the arrows in the diagram then be straight
rather than curving, i.e. the wind should come from absolute North or
South rather than from North-East or South-West? This is because of an
apparent force, Coriolis force.
Suffice to say that according to it, things moving in Northern
Hemisphere are deflected towards right and in Southern Hemisphere
towards left of their direction of motion. This apparent force is nil at
the equator and increases as we move towards poles.
These pressure belts are not static, i.e. they largely follow the Sun's apparent movement over earth. This is because of rotational motion of earth on its inclined axis (23.5o). When the sun is in Northern Hemisphere (i.e. summer season in North), these pressure belts also tend to migrate northwards. Since Sun is now directly above northern tropical latitudes, they receive maximum insolation, and hence are the areas of low pressure. For a very crude understanding, Monsoon can be thought of as winds flow into this low pressure from South. But that leaves the following questions:-
1. Where are these winds originating from?
2. What drives these winds and gives them energy? What is the source of rainfall?
3. Why don't the winds from North come into this low pressure region?
4. When do the winds strike (or technically, Monsoon breaks)?
5. Till how long will these continue to flow over the subcontinent?
The thermal concept is a very old concept that cannot answer these questions. In fact, Monsoon is a much more complex phenomenon than the thermal concept makes it look.
Let's get into details.
In the diagram above, there is ITCZ (Inter Tropical Convergence Zone) at the Equator. It is the zone where trade winds from Northern and Southern Hemisphere converge. Now this zone is characterised with a central westerly air flow, called equatorial westerlies in some parts. With the shift of sun northwards, the ITCZ also migrates northwards. The northern region, N-ITCZ, is now the zone where south-east trade winds after crossing the equator, reach and rise. At this point, it's technically wrong to call it a convergence zone as no convergence occurs here (as there is nothing coming from the North).
The low pressure areas that develop over the sub-continent are not stationary and may change their position suddenly depending on upper air circulations. This proves the fact that these areas are not merely thermally induced. Moreover, high temperatures grip the areas, especially north-western part starting mid-April, reaching maximum in May. However, monsoon does not even reach the southern coast until first week of June. Further, monsoonal rainfall is not wholly orographic (meaning mountain induced, where moist winds are forced to rise over mountain slopes and saturate in process, giving precipitation), rather they are convectional as well as cyclonic.
The questions above have been answered to a large extent by the recent concepts proposed after MONEX (Monsoon Expedition) and subsequently refined with satellite observations. In brief, there are two phenomenon - Jet Streams and Tibetan Plateau heating- which EFFECT the monsoon along with shifting of ITCZ; and there are several phenomenon, like El-nino and Indian Ocean Dipole (IOD) which AFFECT the monsoon.
In summers, Tibetan plateau, a huge granitic barren landmass, 4000m high undergoes immense heating, receiving 2-3oC
more insolation than neighbouring areas. It acts as a barrier as well
as a heat source. As a heat source, immense heating leads to rising of
air. This warm air collects in upper troposphere and upper air
anti-cyclonic conditions (high pressure conditions, imagine massive
amount of air collecting at a point) are formed. Anti-cyclones being
clockwise in Northern Hemisphere (remember ANC - Anti-Cyclones North
Clockwise), the air circulating at the southern end would be from east
to west (easterly). A distinct branch of this anti-cyclonic flow lies
over India, extending from North-East to South-West. On intensification,
this arm extends up to Indian Ocean and intensifies the already
existing High Pressure cell, so as to finally move as South-West
Monsoon. Thus, in essence, its role is to strengthen the winds crossing
the equator so that they reach the Indian shores.
As a barrier, Tibetan plateau's role is related to jet streams.
Monsoon
is coming and along with it comes all the baggage of unpredictability
and way-off-the-mark forecasts. Why is it that even with so advanced
technology and simulation models at our disposal we just can't get the
prediction right? How do Americans or Europeans get the forecasts in
their country right, while we, with so much progress in 'rocket science'
are not able to? In this blog post I'll help to deconstruct Monsoon.
The complexity of this still-to-be completely understood phenomenon will
answer our questions.
Monsoon (from Arabic Mausim
meaning season) refers to seasonal reversal in direction of winds. It
is said that Al-Beruni coined this term while he was on Gujarat coast.
To understand this reversal, we must know about the Global Pressure Belt
system.
These pressure belts are not static, i.e. they largely follow the Sun's apparent movement over earth. This is because of rotational motion of earth on its inclined axis (23.5o). When the sun is in Northern Hemisphere (i.e. summer season in North), these pressure belts also tend to migrate northwards. Since Sun is now directly above northern tropical latitudes, they receive maximum insolation, and hence are the areas of low pressure. For a very crude understanding, Monsoon can be thought of as winds flow into this low pressure from South. But that leaves the following questions:-
1. Where are these winds originating from?
2. What drives these winds and gives them energy? What is the source of rainfall?
3. Why don't the winds from North come into this low pressure region?
4. When do the winds strike (or technically, Monsoon breaks)?
5. Till how long will these continue to flow over the subcontinent?
The thermal concept is a very old concept that cannot answer these questions. In fact, Monsoon is a much more complex phenomenon than the thermal concept makes it look.
Let's get into details.
In the diagram above, there is ITCZ (Inter Tropical Convergence Zone) at the Equator. It is the zone where trade winds from Northern and Southern Hemisphere converge. Now this zone is characterised with a central westerly air flow, called equatorial westerlies in some parts. With the shift of sun northwards, the ITCZ also migrates northwards. The northern region, N-ITCZ, is now the zone where south-east trade winds after crossing the equator, reach and rise. At this point, it's technically wrong to call it a convergence zone as no convergence occurs here (as there is nothing coming from the North).
The low pressure areas that develop over the sub-continent are not stationary and may change their position suddenly depending on upper air circulations. This proves the fact that these areas are not merely thermally induced. Moreover, high temperatures grip the areas, especially north-western part starting mid-April, reaching maximum in May. However, monsoon does not even reach the southern coast until first week of June. Further, monsoonal rainfall is not wholly orographic (meaning mountain induced, where moist winds are forced to rise over mountain slopes and saturate in process, giving precipitation), rather they are convectional as well as cyclonic.
The questions above have been answered to a large extent by the recent concepts proposed after MONEX (Monsoon Expedition) and subsequently refined with satellite observations. In brief, there are two phenomenon - Jet Streams and Tibetan Plateau heating- which EFFECT the monsoon along with shifting of ITCZ; and there are several phenomenon, like El-nino and Indian Ocean Dipole (IOD) which AFFECT the monsoon.
In summers, Tibetan plateau, a huge granitic barren landmass, 4000m high undergoes immense heating, receiving 2-3oC
more insolation than neighbouring areas. It acts as a barrier as well
as a heat source. As a heat source, immense heating leads to rising of
air. This warm air collects in upper troposphere and upper air
anti-cyclonic conditions (high pressure conditions, imagine massive
amount of air collecting at a point) are formed. Anti-cyclones being
clockwise in Northern Hemisphere (remember ANC - Anti-Cyclones North
Clockwise), the air circulating at the southern end would be from east
to west (easterly). A distinct branch of this anti-cyclonic flow lies
over India, extending from North-East to South-West. On intensification,
this arm extends up to Indian Ocean and intensifies the already
existing High Pressure cell, so as to finally move as South-West
Monsoon. Thus, in essence, its role is to strengthen the winds crossing
the equator so that they reach the Indian shores.As a barrier, Tibetan plateau's role is related to jet streams.
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