What causes the monsoon?

What causes the monsoon?

The monsoon, which is essentially the seasonal reversal in wind direction, causes most of the rainfall received in India and some other parts of the world. The primary cause of monsoons is the difference between annual temperature trends over land and sea. The apparent position of the Sun with reference to the Earth oscillates from the Tropic of Cancer to the Tropic of Capricorn. Thus the low-pressure region created by solar heating also changes latitude. The northeast and southeast trade winds converge in this low-pressure zone, which is also known as the Intertropical Convergence Zone or ITCZ. This low-pressure region sees the continuous rise of moist wind from the sea surface to the upper layers of the atmosphere, where the cooling means the air can no longer hold so much moisture resulting in precipitation. The rainy seasons of East Asia, sub-Saharan Africa, Australia and the southern parts of North America coincide with the shift of ITCZ towards these regions.
What causes the Indian monsoon?

The Thar desert and adjoining areas of the northern and central parts of the Indian subcontinent heat up during the summer. Because of the rapid solar heating, mainly between April and May, a low-pressure zone develops over the subcontinent. To fill up this void, the moisture-laden winds from the Indian Ocean rushes in. The ITCZ also shifts northwards towards the subcontinent causing monsoon rains, which typically reach the mainland in the last week of May or the first week of June. The met-department declares the onset of monsoon over Kerala if 60% of the 14 enlisted stations falling in the southern states report a rainfall of 2.5mm or more for any two straight days after May 10. Beyond Kerala, the monsoon gets divided into two parts: the Arabian Sea branch and the Bay of Bengal branch. By the first week of July, monsoon winds reach almost all over India.

What are the ways to forecast monsoons?

There are three main methods used for forecast of the southwest monsoon here. The first is the statistical method, which uses the historical relationships between the monsoon and various global weather parameters. The data is then used to forecast the onset and the intensity of the monsoon. The second is the empirical method, which uses time series analysis of past rainfall data to predict monsoon. The third is the dynamical method, which uses general circulation models of the atmosphere and oceans to predict the southwest monsoons. Prediction models based on the statistical approach have so far yielded the most accurate results for the Indian monsoon. However, none of the models can claim 100% accuracy. Given the plethora of factors involved, its necessary to keep developing new models and refining existing ones.

 Which method is used by our Met department?

Prior to 2002, the IMD used to issue annual forecasts using a model based on 16 parameters. Since 2003, two new models were introduced, which used 8 and 10 parameters instead of 16 to predict monsoon. The forecast system was also made a two-stage one — the first forecast was issued in mid-April and the second stage by June-end. This model also gave false predictions for 2004.

Since 2007, a new forecast system using the ensemble method is in use. The parameters have been reduced to 8. They include the surface temperatures of the North Atlantic, the equatorial southeast Indian Ocean and central Pacific Ocean and the air temperature of northwest Europe. Apart from this, the warm water volume of the equatorial Pacific, air pressure over the North Atlantic and East Asia as well as wind patterns over the north central Pacific are the parameters used to predict the monsoon. Instead of relying on one model, the ensemble method uses inputs from the forecast of all models to calculate the final result.

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