Although monsoons are mentioned in our old scriptures like the Rig Veda and in the writings of several Greek and Buddhist scholars, the credit for first scientific studies of the monsoon winds goes to the Arabs. Near about the tenth century, Al Masudi, an Arab explorer from Baghdad, gave an account of the reversal of ocean currents and the monsoon winds over the north Indian Ocean. Date of commencement of monsoons at several places was reported by Sidi Ali in 1554 A.D.
In summer the sun shines vertically over the Tropic of Cancer resulting in high temperature and low pressure in Central Asia while the pressure is still sufficiently high over Arabian Sea and Bay of Bengal. This induces air flow from sea to land and brings heavy rainfall to India and her neighbouring countries.
In winter the sun shines vertically over the Tropic of Capricorn. The north western part of India grows colder than Arabian Sea and Bay of Bengal and the flow of the monsoon is reversed (Fig. 5.1).
is influenced by the shape of the continents, orography, and the conditions of air circulation in the upper troposphere.
The southeast trade winds in the southern hemisphere and the northeast trade winds in the northern hemisphere meet each other near the equator. The meeting place of these winds is known as the Inter-Tropical Convergence Zone (ITCZ).
Jet stream is a band of fast moving air from west to east usually found in the middle latitudes in the upper troposphere at a height of about 12 km. The wind speeds in a westerly jet stream are commonly 150 to 300 km p.h. with extreme values reaching 400 km p.h. Jet stream is the latest theory regarding the origin of the monsoons and has earned world wide acclaim from the meteorologists.
This plateau is surrounded by mountain ranges which rise 6,000 – 8,000 m above sea level. It gets heated in summer and is 2°C to3°C warmer than the air over the adjoining regions.
This is the season of outblowing surface winds but aloft the westerly airflow dominates. The upper westerlies are split into two distinct currents by the topographical obstacle of the Tibet Plateau, one flowing to the north and the other to the south of the plateau. The two branches reunite off the east coast of China (Fig. 5.7).
With the beginning of summer in the month of March, the upper westerlies start their northward march, but whereas the northerly jet strengthens and begins to extend across central China and into Japan, the southerly branch remains positioned south of Tibet, although weakening in intensity.
These happenings are closely related to the Indian monsoon. S. Rama Rattan opined that the development of monsoon winds is deeply connected with the jet stream in addition to the differential heating of land and sea.
Recent studies have revealed that there seems to be a link between meteorological events which are separated by long distances and large intervals of time. They are called meteorological teleconnections. The one which has aroused considerable
interest among the meteorologists is the difference between an El Nino and the Southern Oscillation. El Nino (EN) is a narrow warm current which appears off the coast of Peru in December. In Spanish, it means The Child Christ because it appears around Christmas. In some years this warm current is more intense than usual.
Another major programme is the Indian Middle Atmospheric Programme (IMAP) initiated by the Department of Space. This programme has been launched to augment the existing weather prediction scheme. This is expected to improve scientific understanding of climatic changes that take place in Indian tropical region and the area along the Tropic of Cancer when the monsoon winds
2. El Nino in previous year
3. Northern India (March)
4. East coast of India (March)
5. Central India (May)
6. Northern hemisphere (Jan. and Feb.)
8. 50 hPa ridge-trough extent (Jan. and Feb.)
9. 10 hPa (30 km) westerly wind (Jan.)
10. Tahiti-Darwin (Spring) 11. Darwin (Spring)
12. South America, Argentina (April) 13. Indian Ocean Equatorial (Jan.-May)
14. Himalayan (Jan.- March) 15. Eurasian (Previous December)