In the context of climate change, it is pertinent to ascertain whether the characteristics of Indian summer monsoon are also changing. The Indian summer monsoon(June–September) rainfall is very crucial for the economic development, disaster management, hydrological planning of the country.In spite of growth in the service sector, economy of India is still dependent on agriculture. Crop failure, drought and more extreme cases like famine due to weak or deﬁcient monsoons becomes very critical to the country. Therefore, it is important to monitor closely the rainfall variation across the country on daily, weekly, monthly and seasonal time scale.
Sources of rainfall recording in India:
India Meteorological Department (IMD) after its establishment in 1875, took initiative to install more and more rain gauges around the country to capture the complex variation of rainfall across the country. Blanford(1886–1888) the ﬁrst head of the IMD took personal effort to collect rainfall data from all the provinces and had brought out the detailed analysis of rainfall pattern over India.All India monthly, seasonal and annual rainfall series were constructed based on the area weighted rainfall of all the 36 meteorological subdivisions of the country.
The main source of monthly rainfall data of the 306 stations is National Data Center, IMD Pune. However, rainfall data of some of the 306 stations were not readily available in the published form or in the form of soft copy with IMD Pune. Hence, attempts were made to fill up the missing rainfall values from the different data sources i.e. Irrigation Dept., Revenue Dept., Agriculture Department of State Government,Regional Meteorological Centre of respective stations, Indian Daily Weather Reports(IDWR) and Monthly Climatic Data for the World.
Trends in Indian Monsoons:
For the country as a whole, the summer monsoon rainfall do not show any significant trend. However, there may be large variation in the regional scale. In order to study the long term changes in regional as well as sub divisional scale, we have carried out linear trend analysis using monsoon season rainfall of five homogeneous regions and
30 meteorological subdivisions for the period 1871-2016 and recent period 1981-2016. The significance of trend has been assessed by Mann Kendall rank statistic test.
The long-term monsoon trend has been getting much attention recently. The seasonal total rainfall which has decreased by about 10% since the 1950s till the beginning of the 21 st century, is reported to be ‘recovering’. This may sound like good news but the details may matter as to how the rainfall will be distributed in terms of frequency and intensity. Heatwaves, droughts and floods are serious socioeconomic hazards for India and a monsoon recovery will not necessarily mean a reduction in these hazards.
Factors influencing Monsoons in India:
The following are a few factors which affect Monsoons in India :
i. The differential heating of the landmass of Asia and the Indian Ocean.
ii. The existence of the Himalayan ranges and the Tibetan Plateau.
iii. The occurrence of heavy-light snow over the Tibetan Plateau.
iv. The existence and circulation of upper air jet streams in the troposphere.
The origin of India monsoon has traditionally been ascribed to the differential heating and cooling of the continent of Asia and the Indian Ocean and Monsoon were considered large scale land and sea breezes. This simplistic model fails to explain many of the complex issues associated with monsoon.
According to recent dynamic theories, the Monsoon is a highly complex phenomenon that happens due to seasonal migration of planetary winds and pressure belts following the sun and is closely influenced by the middle and upper tropospheric circulation. The occurrence and changing position of jet stream influence the origin and development of monsoons.
The presence of Himalayan ranges and Tibetan Plateau also have a close bearing on the origin of Monsoon. The jet streams that lie about 12 km in the troposphere are bifurcated by the Himalayan Mountains and the Tibetan Plateau. Moreover other global climatic phenomenon like El Nino and La Nina also affect rainfall pattern in Monsoon.
However , human activities are also affecting monsoons in a bad way. Afforestation , Reduction of Global warming etc… can help in regulating uncertainities in Indian monsoons.