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Geography-Six things you must know about the Indian Monsoon.


Discovery of the phenomena of monsoon

A Greek text, Periplus of the Erythreaeansea, which describes navigation and trading opportunities, talks about seasonal winds bringing navigators to safety. The writer of this text, Pliny, credits Hippalus with the discovery of these winds and its route across the Arabian Sea to India, around 1st Century BCE.

Who coined the word Monsoon?

The word ‘Monsoon’ is derived from Arabic word ‘Mausim’, meaning seasonal winds. The famous Arab scholar, a world-traveller and a prolific writer, Al- Masudi coined the term Monsoon and gave a good account of these periodic winds of the Herkend (Bay of Bengal). Interestingly Al-Masudi also remarks about using these winds as a source of energy.

southwest monsoon
Photo Courtesy : Indian Metrological Department
  1. Indian Monsoon mechanisms

Monsoon winds are in reality southeast trade winds, which dramatically shifts in summer due to apparent movement of sun towards the northern hemisphere. The dramatic large-scale direction change results in southwest winds and the monsoon. Some of the salient factors that propel the Indian monsoon are listed below.

  • Differential heating and cooling of land and water: This creates low pressure on the Indian landmass while seas around experience comparatively high pressure. As we know, winds move from high-pressure areas to low-pressure areas. This initiates the whole process of Indian monsoon
  • Shift in position of Inter tropical convergence zone (ITCZ)In summer due to apparent movement of sun, ITCZ which is a equatorial trough, normally positioned about 5 degrees N&S of the equator, shifts towards the north, over the Ganga Plain. This puts in place the monsoon trough that initiates the monsoonal circulation.
  • Presence of high pressure region in the east of Madagascar: The intensity and position of this high pressure area plays crucial role in initiating the monsoonal circulation.
  • Intense heating of Tibetan plateau during summer: Tibetan plateau gets intensely heated during summer, which results in vertical air currents and formation of a high-pressure region over the plateau at about 9 km above the sea level.
  • Movement of Westerly Jet Stream to the north of Himalaya: The sub-tropical Westerly Jet Stream, the high-level intense winds, moves to the north of the Himalaya in the summer, leaves a vacuum for southwesterly winds on the surface to complete the wind circulation.
  • Southern Oscillation: Changes in pressure conditions over the Southern Ocean also affect the Indian monsoon. Normally, when the tropical eastern South Pacific Ocean experiences high-pressure, the Tropical Eastern Indian Ocean experiences low pressure. However, in certain years, there is reversal in pressure conditions and the Eastern Pacific region has lower pressure in comparison to East Indian Ocean. This periodic change in pressure condition is known as Southern Oscillation.
  • El-Nino: This is a warm current that flows past Peruvian coast in place of the usual cold Peruvian current. The phenomenon occurs every 2 to 5 years. Hence change in pressure conditions is connected to El-Nino and whole phenomenon of El Niño–Southern Oscillation is referred as ENSO.

4.Timing of Indian Monsoon – how much punctual is it?

An important feature of Indian monsoon is its timing. It comes without a fail every year and strikes the country on nearly same time i.e. June 1.

5.The first beneficiaries of  Indian Monsoon

Kerala is the privileged state, and it experiences the first showers of the monsoon. Monsoon then divides into two branches namely Arabian Sea and Bay of Bengal and approaches the main land mass of India.

6. Indian Monsoon is not an Indian phenomenon only

Monsoon is not restricted to India only. It affects the weather of the entire Indian sub-continent, and Oceana, which comprise Australia and New Zealand. There is also the North American monsoon, which happens once a year, usually in the middle of summer.

Conclusion:

India’s monsoon saga plays out every year. This year, intense summer has been predicted by the India Meteorological Department, and India hopes for a bountiful monsoon.


Analyzing Past Monsoons With Foraminifera


Stable oxygen isotopic composition of foraminifera from the sediments of the Northern Indian Ocean has been used to decipher past changes in the intensity of the South Asian Monsoon. The interpretation of planktonic foraminifera  is mostly based on a combination of sea surface temperature, monsoon runoff and the global ice-volume effect.

During summer, the eastward flowing summer monsoon current carries the high salinity water from the Arabian Sea to the Bay of Bengal, whereas the Bay of Bengal receives fresh water both from direct rains and runoff from the Ganga-Brahmaputra rivers. This low salinity water is transported from the Bay of Bengal to the southeastern Arabian Sea during winter by the winter monsoon current (WMC) driven by the dry northeasterly winds.

The West India Coastal Currents (WICC) carries this low saline water from southeastern Arabian Sea to the northern Arabian Sea. During the winter monsoon, high precipitation occurs over southeastern India (the southern states of Tamil Nadu and southeast coastal Andhra Pradesh), while rainfall at this time is quite low over Kerala compared to the summer monsoon.

Duplessy (1982), analyzing several sediment cores from the Bay of Bengal, showed that the summer monsoon rain was significantly reduced during the Last Glacial Maximum (LGM). Oxygen isotopes of LGM planktonic foraminifera were relatively higher, resulting from reduced freshwater discharge from the major monsoon fed Indian rivers. Likewise, Sarkar et. al. (1990) showed that the winter monsoon rains intensified during LGM by analyzing four different species of planktonic foraminifera from a sediment core from the Eastern Arabian Sea. Kudrass et. al. (2001) showed that the runoff into the Bay of Bengal had steadily increased from 21 ka (LGM) to ~4ka.


 

January 29, 2021