May 25 – The annual Asian monsoon has arrived, touching the western coast of India last Saturday with 5 centimeters of rainfall in Thiruvananthapuram, the State Capital of Kerala in 24 hours. The monsoon, which refers to the high seasonal winds blowing from the Indian Ocean and Arabian Sea in the southwest, moves eastwards. It brings heavy rainfall and unsettled conditions to much of Southeast Asia during the summer months, and extends its influence from India to Japan.
The summer monsoon has been gaining strength in recent decades, although its origins commence in the uplifting of the Tibetan Plateau following the collision of India and Asia about 50 million years ago. This has been verified from the study of wind-blown dust in the Loess Plateau in China, and plant fossils and sediment records from the South China Sea. Recent changes in weather patterns attributed to global warming have also seen an increase in the summer monsoon, with wind velocity increasing and rains intensifying. A warming of the Indian Ocean due to the closing of an undersea channel near Indonesia from earthquake activity has resulted in a reduced flow of colder waters from the Pacific and is also thought to be having an effect.
The summer monsoon occurs from late May until September. The Great Indian Desert (Thar Desert) and adjoining areas of the northern and central Indian subcontinent heat up considerably during the hot summers, causing low pressure areas over the northern and central Indian subcontinent. To fill this, moisture-laden winds from the Indian Ocean rush in to the subcontinent. These winds, rich in moisture, are drawn towards the Himalayas, creating winds and blowing storm clouds towards the subcontinent. However the Himalayas act like a high wall and do not allow the winds to pass into Central Asia, forcing them to rise. With the gain in altitude of the clouds, the temperature drops and rainfall occurs. Some areas of the Indian subcontinent receive up to 10,000 millimeters of rain.
Three separate monsoons can be identified from this. The Arabian Sea branch of the Southwest Monsoon first hits the coastal state of Kerala in India and hence is the first location to receive rain from the Southwest Monsoon. This is what happened on Saturday, and was three days earlier than expected this year. This monsoon affects Mumbai in particular.
Around mid-June, the Bay of Bengal branch of the Southwest Monsoon flows over the Bay of Bengal heading towards North-Eastern India and Bengal, picking up more moisture from the Bay of Bengal. It then hits the Eastern Himalaya and provides a huge amount of rain to the regions of North-East India, Bangladesh and West Bengal. Mawsynram, situated on the southern slopes of the Eastern Himalaysa in Shillong, India, is one of the wettest places on Earth. After striking the Eastern Himalayas, the monsoon turns towards the west, travels over the Indo-Gangetic Plain at a rate of roughly one to two weeks per state, pouring rain across the land. This monsoon also affects Burma and Southwestern China, bringing rains to Yunnan and Guangxi in the summer, accounting for much of the region’s fertility and high crop yields. In India, the Bay of Bengal Monsoon accounts for 80 percent of the rainfall in the country. Indian agriculture – which accounts for 25 percent of the GDP and employs 70 percent of the population – is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy.
From September, the so-called Retreating Monsoon starts to take effect. With the suns heat beginning to wane, the northern land mass of the Indian subcontinent begins to cool off rapidly. With this air pressure begins to build over northern India. The Indian Ocean and its surrounding atmosphere still hold its heat. This causes cold wind to sweep down from the Himalayas and Indo-Gangetic Plain towards the vast spans of the Indian Ocean south of the Deccan peninsular. While traveling towards the Indian Ocean, the dry cold wind picks up some moisture from the Bay of Bengal and pours it over peninsular India. Cities like Chennai, on the South-East coast, which get less rain from the Southwest Monsoon, receive rain from the Retreating Monsoon. About 50 percent to 60 percent of the rain received by the state of Tamil Nadu is from the Northeast Monsoon. This monsoon is what brings the autumn rains to Sri Lanka, Thailand, Vietnam and Indo-China, however it is not as powerful as the Bay of Bengal Monsoon.
China is also affected by the monsoons, with a southeast monsoon from the western Pacific Ocean occurring in Eastern China in the spring (Shanghai’s “Plum Rains”), and the southwest monsoon from the Bay of Bengal reaching into the Chinese mainland. These monsoons are the main cause of rainfall. Starting in April and May, the summer rainy season monsoons hit the southern provinces of Guangdong, Guangxi, and Hainan from the Pacific Monsoon, while from June, the Bengali Monsoon replaces these with the rains blowing northward to North China in July and August, decreasing in September and finishing in October. Eastern China experiences many climatic changes from monsoons, while the northwest area of Xinjiang remains a non-monsoonal region – protected from rains by the Himalayas, defining it as part of Central Asia.
Monsoons can cause a great deal of flood damage, particularly in India, where cities such as Mumbai have to battle both with archaic infrastructure and an increase in summer monsoon intensity. High tides can also create havoc. Although the infrastructure development in Mumbai is improving dramatically following the severe floods of two years ago, high tides remain for now a threat, coinciding of course with the new moon each month. These fall on May 26, June 25, July 24 and August 21 this year and if heavy monsoonal rain occurs on these days, flooding of up to 25 feet of water could occur, not receding until the next high tide. Travelers to Mumbai are advised to try and avoid meetings around these dates.
Meanwhile, as global warming appears to be intensifying summer monsoonal activity, the spread of monsoonal related anti-flooding infrastructure is likely to develop in key Asian cities from Mumbai to Shanghai over the coming years, and the impact of the summer monsoons to increase across Asia.