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I am currently learning at school about the climatic conditions (mainly winds, difference in pressure, Coriolis effect, etc.) that cause and affect the monsoons received by India.

One of the factors mentioned is the shift of the Inter tropical Convergence Zone (ITCZ) towards the Ganga plain in the summers. I tried reading up on Wikipedia to understand this better (here especially), but couldn't understand much.

Could someone explain, in simple terms,
(a) What the ITCZ and monsoon trough are? Are they just regions of low pressure?
(b) What causes this to shift away from the equator?
(c) Does 'convergence' have anything to do with all of this?

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    $\begingroup$ Great going for a 14 year old kid to ask questions of such depth ! I am trying to recall what I did as a 14 year old... $\endgroup$ – gansub Jan 23 '15 at 1:44
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    $\begingroup$ @gansub It is there in the class 9 NCERT geography book, chapter 4, page 30. Online copy is there at ncert.nic.in/NCERTS/textbook/textbook.htm?iess1=4-6 $\endgroup$ – ghosts_in_the_code Jan 23 '15 at 15:03
  • $\begingroup$ Yay!!! Gold badge! $\endgroup$ – ghosts_in_the_code Jun 4 '17 at 15:01
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On a broader scale the ITCZ and monsoons are related. This is because the global circulation shifts as a result of the tilt of the earth's axis relative to the orbit around the sun. In northern hemisphere summer, the northern hemisphere is tilted towards the sun and receives more radiation (energy) than the southern, and vice versa for northern winter, The southern hemisphere is affected in the same way. Without the tilt we would not experience any seasons.

In the tropics the largest amount of energy reaches the region where the sun is at the zenith. In northern summer this region is north of the equation, where as in winter it is at the south. At the spring and fall equinox it coincides with the equator. Hence the region of maximum energy input wanders seasonally to the north and south of the equator.

Climatologically the region with maximum energy input sets up a circulation in the form of so-called Hadley cells where warm air rises where it is heated the most and then transported at elevation to the north and south. At the location of upward air flow formation of convective clouds yielding precipitation becomes the norm. This zone to which ground winds flow towards the region of maximum energy influx is the ITCZ and because the zone of maximum solar energy influx wanders, the ITCZ flows. The zone is fairly wide so the regions around the equator sees the effects of tropical precipitation more or less without seasonality, actually two rain periods and two drier periods as the ITCZ moves once to the north and once to the south over the region each year. Away from the equator the resulting period of precipitation becomes more and more marked as one moves to the north and south resulting in strongly seasonal rains.

This movement of the large scale circulation on the earth produces several associated effects and the different monsoons are such phenomena. There are several monsoons on the earth created by weather phenomena typical for the specific region. The Asian monsoon is perhaps the most known. The specific conditions leading up to this monsoon is found in the high elevation Tibetan plateau. During the northern winter the plateau sees a larger high pressure forming which generates winds flowing south over India and causing a dry climate. During the northern summer large parts are heated generating a low pressure over land which draws in moist air from the sea yielding strong precipitation. Hence the monsoon follows the same seasonality as is seen in the wander of the ITCZ but the cause for the regional change in seasonal wind and precipitation pattern has a different cause.

So to understand the large scale weather systems and their seasonal patterns, you must start by understanding how solar energy drives the atmospheric circulation. This causes a specific and typical pattern which is locally disrupted by the distribution of land and oceans on the earth. Land masses can be heated more and more quickly than oceans on a seasonal basis, land masses also includes topography that can impede the atmospheric circulation, so it it perhaps obvious that land masses contribute on regional and local scales.

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Answers:

A) ITCZ and Monsoon Trough are the zones around Earth’s equator where winds of both hemispheres meet. Winds blow from areas of high pressure (cold air masses) towards areas of low pressure (warm air masses). That means in a simple way, that winds blow from the poles towards the equator and their direction is affected by Earth’s rotation. Obviously the sun’s energy warms Earth more around the equator than at the poles. The difference between the ITCZ and the Monsoon Trough is simply the direction of the winds from both hemispheres when they meet. Usually, winds from northern hemisphere blow from NW to SE and those of the southern blow from SW to NE. During the northern hemisphere summer, the winds from the northern hemisphere sometimes change direction from NE to SW. When this occurs you have a Monsoon Trough. Due to the new movement of winds they favor the genesis of vorticity around very low pressures which in turn could form more easily tropical cyclones.

B) The shifting of ITCZ is the result of the Earth’s rotation, axis inclination and the translation of Earth around the Sun. Seasons are the result of this. ITCZ moves toward the hemisphere with most heat, wich are either hemisphere summers.

C) “Convergence” means that the winds from every hemisphere blow towards the same point, or more accurately, the same line in this case the equator. That’s precisely why it is called the Intertropical (between the tropics), Convergence (winds blow towards equator) Zone (it is a wide area along the equator obviously). It should be pointed out that the term “Monsoon” alone refers simply to the rains produced by the shift nortwards or southwards of the ITCZ during summer of either hemisphere as in northern India, making the rains seasonal.

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According to me ITCZ is basically the trough where both the north eastern and south western winds converge or meet. You must know that winds move from high pressure to low pressure, therefore when the winds meet an area of low pressure is created on the landmass which in turn brings in winds with moisture causing rainfall. This is how the ITCZ affects the Indian monsoon.

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  • $\begingroup$ This answer would be better if it cited references. $\endgroup$ – Fred Dec 28 '15 at 6:11
  • $\begingroup$ Initially they do not meet over landmass. And it is north eastern and south eastern winds that converge not south western winds. $\endgroup$ – gansub Dec 28 '15 at 9:51

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