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Climate part 3

 

The Indian monsoon:-            

The climate of India is strongly influenced by the wind system of India and this wind system is called ‘monsoon wind system’.

Where did word monsoon come from?

Sailors who came to India were benefited from the reversal of the wind system as they helped the sailing ships. These were the people who first noticed the phenomenon of monsoon. The Arabs who also sailed to India named this seasonal reversal of winds as ‘monsoon’.

The monsoon is experienced in tropical area roughly between 20°N 20°S.

Let’s now look at the factors playing a major role in occurrence of this mechanism:-

    I.        The differential heating and cooling of land and water creates a low pressure on the landmass of India while the seas around experience comparatively high pressure.

 II.        The shift of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain.

III.        The presence of the high-pressure area, east of Madagascar approximately at 20°S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.

IV.        The Tibetan plateau gets intensely heated during summers, which results in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level.

 V.        The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summers. 

 

ENSO:-

Changes in the pressure conditions over the southern oceans also influence the monsoons. Normally when the South Pacific Ocean experiences high pressure, the Indian Ocean experiences low pressure. In certain years there is a reversal in the pressure conditions and the South Pacific Ocean experiences a lower pressure as compared to Indian Ocean. This periodic change in the pressure conditions is referred to as southern oscillation (SO). Have we ever wondered why does monsoon set late sometimes? The pressure difference between Tahiti (a place in Pacific Ocean) and Darwin (a place in Indian Ocean near Australia) determines the intensity of the monsoon. If the pressure difference is negative, the monsoon would set late and intensity would be below average. There is another phenomenon called El Nino in which a warm ocean current replaces the cold ocean current that flows over the Peruvian coast every 2 to 5 years. The changes in pressure conditions are connected to El Nino. Combined, this phenomenon is called ENSO.

 

The onset of the monsoon:-

Now we are going to learn about how monsoon commences and reaches different parts of the country.

Burst of monsoon:- The monsoon winds are pulsating in nature and these are affected by the atmospheric conditions they encounter on their way. The duration of monsoon is about 100-120 days from early June to mid September. Around the time it arrives, the normal rainfall increases and persists for several days. This is called burst of the monsoon and can be distinguished from the pre monsoon showers.

How does it pass through ?

The monsoon reaches the southern tip of India by the first week of June. Next, it precedes into two branches- the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch reaches Mumbai near about June 10. The Bay of Bengal branch also advances rapidly and reaches Assam in the first week of June. The lofty mountains (Himalayas) cause the winds to deflect west over the Ganga plains. By mid of June the Arabian Sea branch reaches Saurashtra- Kuchchh and central region of the country. Both the branches merge over the north-western parts of Ganga plains. Delhi generally receives the monsoon from the Bay of Bengal branch which reaches there by end of June. By the first week of July, Punjab, western UP, Haryana and eastern Rajasthan experience monsoon. By mid of July, Himachal Pradesh and rest of the country experience monsoon.

Withdrawal of monsoon :-

The retreat or withdrawal of monsoon is a more gradual process. It begins in the north-eastern part of the country by early September and by mid October; half of the northern peninsula is free of monsoon. By early December, the monsoon has withdrawn from whole of the country.

Fun fact: - The island groups are the first to receive the showers and last one to get withdrawn. They receive their first shower in last week of April from south to north. The withdrawal takes place from north to south from first week of December to first week of January. By this time rest of the country is already under the influence of winter monsoon.  

 

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Location: Amritsar, Punjab, India

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