Explain the process of arriving monsoon in India

The southwest monsoon arrives in India around June 1. This happens when high temperatures over the Indian landmass creates a low pressure zone. This attracts the rain bearing wind from the Indian ocean. This wind reached Kerala and the north east during the first week of June, bringing with it heavy rainfall. Over the next one and a half months, the monsoon reaches the entire country. The monsoon rains continue till the middle of September after which the phase of retreating monsoon starts.

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As May begins, the countdown to the onset of the southwest monsoon in India has started. As per the standard timetable, it is normally expected to reach the Andaman and Nicobar Islands region about two weeks from now. Usually, by May 15, monsoon reaches the south Andaman Sea. By May 20, it covers almost the entire Andaman and Nicobar Islands. By May 25, it covers most of Sri Lanka, some parts of Myanmar and by June 1, it arrives in Kerala, most of Tamil Nadu and southeast Andhra Pradesh. 

However, it’s not that easy for the monsoon to follow a strict timetable, just like we find it hard to do so in our daily lives. There are many factors which determine not only monsoon’s performance, but also its timetable in a given year. The monsoon onset dates (shown in the image above) have been calculated by India Meteorological Department (IMD) based on rainfall recorded decades ago.

People often say that when they were children, monsoon used to arrive over the mainland of India on time, but they have been seeing variability in the onset of monsoon over the last few years. As a result of the natural variability in the monsoon system, these onset dates rarely hold true now (the southwest monsoon used to reach central India around June 10 earlier, but it doesn’t anymore). Hence, there is a strong need to follow new monsoon onset dates or a new timetable. Weather factors are also responsible for the variations in the onset date. 

Monsoon arrived late over the Andaman and Nicobar Islands in 2005, 2011, 2012. But it also arrived before time in this region in 2007, 2008 and 2010. Let us assume the southwest monsoon is like a train going from the Andaman and Nicobar Islands towards Pakistan, stopping at several stations on the way. We have seen how a train reaches its final destination on time even if it has turned up late at the intervening stations by making up for lost time. The same thing happened in 2012 when the monsoon was “running” late near the Andaman and Nicobar Islands, but it reached Pakistan on July 11, before the normal date of July 15. 

But sometimes, the monsoon tends to stall at some places on its way across the Indian subcontinent. An on time or early arrival at the Andaman and Nicobar Islands doesn’t guarantee a timely arrival in the rest of the country. 

It has been observed that low pressure systems (such as cyclones) in the Arabian Sea and the Bay of Bengal during May and June can interfere with the onset of the monsoon. The position, intensity and timing of such low pressure systems determine whether their presence would help the monsoon or not. Monsoon’s bizarre progress in 2014 due to Cyclone Nanauk is a classic example. It helped the monsoon progress along the west coast of India, but couldn’t help its progress in the interiors. 

There are so many possibilities every year that the accuracy of a long-term forecast of monsoon onset dates is always low. 

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