Weather Bee: How the arrival of monsoon is declared

What does the arrival of monsoon mean? How does IMD decide that monsoon has arrived at a particular place?

The India Meteorological Department (IMD) announced on May 13 that the southwest monsoon had advanced to the Nicobar Islands on the day, apart from some parts of the south Bay of Bengal and parts of the Andaman Sea. What does the arrival of monsoon mean? More importantly, how does the IMD decide that monsoon has arrived at a particular place? Here are some charts and maps that try to answer these questions.

The southwest monsoon season, which runs officially from June to September, is the rainiest period for India as a whole and most of its regions. (AP)

What does the arrival of the monsoon mean?

The southwest monsoon season, which runs officially from June to September, is the rainiest period for India as a whole and most of its regions. The reason why these four months are rainy is that large scale atmospheric patterns support a rainy weather. These patterns usually develop on the Kerala coast on June 1, cover the entire country by July 8, and retreat from north-western regions of India on September 30. This is why the season officially runs from June to September. However, the actual date on which these patterns develop/dissipate over particular regions is the day on which monsoon is said to have arrived over/retreated from that region.

To be sure, while the monsoon usually arrives at the Kerala coast – usually its first port of call on the Indian mainland – on June 1, its usual date of arrival for Andaman and Nicobar Islands is May 22. Therefore, the monsoon arrival is early at these islands by only around a week. Moreover, it does not follow from this that the atmospheric patterns also develop over Kerala a week ahead of schedule. The IMD has forecast that monsoon will arrive at the Kerala coast on May 27, with a possible error of four days in the forecast.

How does the IMD decide that monsoon has arrived at a particular place?

As expected from the explanation above, monsoon is said to arrive at a place when the atmospheric patterns of the season develop. What are these patterns? The IMD lists three measurable patterns for Kerala. While not all three are always measured for the rest of the country, the department has indeed listed all three as a reason for declaring monsoon onset over the Nicobar Islands. Here is what the three conditions are.

One important condition is suggested by the name of the season itself: the southwest monsoon. The season is called so because an important feature of the season is winds – they have their origin in the Mascarene Islands (the islands of Réunion, Mauritius, and Rodrigues) in the southern hemisphere – blowing from the southwest of India. IMD declares monsoon’s arrival over Kerala when such winds cross some measurable thresholds. One threshold is that the zonal component (the component parallel to the equator) of wind speed at a height of 925 millibar (around 750 metres above sea level) should be 15-20 knots. In addition, such westerly winds (those blowing from west to east) should extend up to the height of 600 millibar (around 4-4.5 km above sea level). This appears to have happened over Nicobar Islands, as can be seen in the accompanying forecast maps for zonal winds at different pressure levels. Since these forecasts were made close to the day of the event, they are likely to be close to the observed trends in winds.

To be sure, winds alone do not make up the monsoon, as westerly winds can develop at other times of the year as well. Such winds should also be accompanied with moisture and clouds in the atmosphere, a necessary requirement for rainfall. This is measured using outgoing longwave radiation. Outgoing longwave radiation is the heat that earth radiates back after absorbing incoming solar radiation. When there are clouds in the atmosphere, this outgoing heat decreases. For declaring the arrival of monsoon, the IMD checks if outgoing longwave radiation has dropped below 200 W/m2 on two consecutive days. As the maps below suggest, the Nicobar Islands have likely met this criterion too.

As is obvious, the final and most important condition for declaring the onset of monsoon is rain itself. For Kerala, the criteria is that 60% of the available stations (from a list of 14) should record at least 2.5 mm rain on two consecutive days. This has not yet happened for Kerala, although regions close to the state appear to satisfy the previous two conditions. On the other hand, weather stations in the Andaman and Nicobar Islands have indeed reported intense rainfall in the past two days. This is why the IMD has declared the onset of monsoon over the region.