Arrival of Monsoon Rains in West Bengal including the City of Joy

Gautam Kumar Das

Young love for monsoon in the City of Joy cares not for consequences. They should not have mooned away the precious time as the monsoon arrives in Kolkata on Friday, the 12th June, 2020 and its surroundings. Residents of the Kolkata city enjoy monsoon very much, though sometimes they face distress in the water logging situation due to heavy rainfall during this wet season. Monsoon 2020 has delayed just one day of its normal arrival as the official date for the arrival of monsoon in south Bengal is revised to June 11 from June 10 earlier this year. Monsoon arrives in the City and most parts of south Bengal on Friday as the conditions have tuned favourable for the onset of rains with monsoon clouds having started to float into Kolkata and its surroundings, while south Bengal receives moderate rainfall over next 24 hours, though north Bengal could be lashed by huge rainfall.

North Bengal, naturally, receives huge precipitation than south Bengal due to its geographical locations as the clouds hit the mountains, heavy rainfall follow, which is not happened to the districts of south Bengal. Winds and clouds are blowing into the City of Joy from the south on Thursday, June 11 indicating monsoon currents are at doorstep. The monsoon is on set in the city of Joy and across most parts of south Bengal on Friday, 12th June, 2020. Before arrival of the monsoon, the conditions has gradually become favourable as depicted by India Meteorological Department (IMD) in their radar imagery on the 12th June stating the low pressure area over north coastal Andhra Pradesh and adjoining coastal Odisha and neighbourhood with associated cyclonic circulation extending up to 7.6 km above mean sea level, tilting south westwards with height persists. Conditions are becoming favourable for further advance of monsoon into West Bengal. Ultimately monsoon rain lashed the City of Joy with heavy showers on the 12th instant of June this year.

Southwest monsoon entered the country in the normal date of 1st June, 2020 along the coastal stretch of Kerala. IMD announced on 1st June that the south west monsoon set in over Kerala on 1st June, 2020 coinciding with its normal date having 1st day of monsoon rainfall in Tribandram - 50 mm, Kojhikod - 90 mm, Kollam - 150 mm, Kochin - 75 mm and Goa - 70 mm. The Director General of the IMD explained that the arrival of monsoon is generally evidenced by the southwest direction of wind flows shown by radar, intensity of southwest winds is normally in the range of 15-20 knots, and the vertical depth of the cloud is 4-5 km. The Director General of the IMD states that all these conditions are satisfied for Kerala on the day of southwest monsoon set in the country on 1st June.

Forecasting on the amount of monsoon rainfall over the country, the IMD displays a weather bulletin stating as a whole for the 2020 southwest monsoon season (June to September) is likely to be normal (96% to 104%) of Long Period Average (LPA); Quantitatively, monsoon season rainfall for the country is likely to be 102% of the LPA with a model error of ± 4%. Normal rainfall over the country is predicted by the IMD as because, currently El Nino Southern Oscillation (ENSO) Neutral conditions are prevailing over the equatorial Pacific and Neutral IOD (Indian Ocean Dipole) conditions are prevailing over the Indian Ocean.

A positive IOD is associated with a stronger than normal monsoon and such IOD conditions are likely to continue during the monsoon season. However, a few other climate models indicate possibility of development of weak La Nina conditions in the later part of the season or thereafter. Strong La Nina conditions favour the normal rainfall by the southwest monsoon over the country. Naturally, the latest experimental forecasts based on Monsoon Mission Coupled Forecasting System (MMCFS) suggests that there is high probability for the 2020 monsoon (June to September) rainfall to be above normal to excess (more than 104% of LPA). In comparison to such forecasts by the IMD, reflected in reality, heavy rain and thundershower are going on since morning of the Friday, on the day monsoon reached Kolkata. Alipore receives 51.1 mm for 24 hours and Dum Dum receives 41 mm rain in last 6 hours on 12 June, 2020, though in the last year (2019), Kolkata received 1187 mm of rainfall as against the normal of 1326 mm from June to September i.e. 10% below normal. The onset of monsoon in 2019 was delayed in arrival in the City of Joy on 21st June i.e. 10 days later than normal as the onset of monsoon in the country was delayed by a week in Kerala.

In 2020, Kolkata metropolis has recorded 472 mm of rainfall i.e. more than double the normal of 216 mm over the pre-monsoon season. The City of Joy has received more than 100 mm rainfall during the first 10 days of June so far, against its monthly average rainfall of 300.6 mm, even before the official onset of the monsoon. Ultimately, the southwest monsoon arrived in West Bengal, hitting most parts of the the state and bringing moderate rainfall. The arrival is facilitated by a low pressure belt formed over coastal Andhra Pradesh and Osisha. The districts which received light to moderate rainfall in south Bengal are Purba Bardhaman, Purba Medinipore, Paschim Medinipore, Nadia, Hooghly, North and South 24 Parganas and Kolkata. In north Bengal, Cooch Behar, Jalpaiguri, Alipurduar, Darjeeling, Kalimpong and some parts of South Dinajpur received light to moderate rainfall. The monsoon rainfall is very much important for the state’s economy as its quantity will determine the kharif crops sown across the 23 districts of West Bengal.

Rainfall, Relative Humidity and Temperature data on 12th June, 2020, the day of arrival of monsoon in West Bengal

   Source: IMD

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Jun 15, 2020