Monsoons came early to India this year as moisture laden winds from the southwest hit the coastal areas of Kerala on May 29, three days ahead of its annual arrival. Squally weather was predicted by the Indian Meteorological Department in parts of Lakshadweep, Kerala and Tamil Nadu and the winds advanced into Comorin, Southeast Arabian Sea, southwest Bay of Bengal and parts of Andaman and Nicobar islands. In the fortnight that has passed since the arrival of monsoon, various states have received more than normal rainfall and the monsoons of 2018 is predicted to bring ample rainfall for the country.
We often use the Hindi term ‘mausam’ to allude to the weather or a particular season, seldom paying attention to its origin. In the sixteenth century, Arab sailors used the term ‘mausim’ to describe the annual alternating winds – blowing six months from northeast and for six months from southwest of the Arabian Sea. As the American Meteorological Society notes, with the passage of time its usage was extended to the rest of the world.The origins of the monsoon season has been the cause of much dispute, but it is generally accepted that monsoon is a result of three physical processes—
■ uneven heating of land and sea causing pressure differential that drives winds from high pressure to low pressure;
■ rotation of the earth forcing winds to veer towards the right in the northern hemisphere and to the left in southern hemisphere; and;
■ change in the state of water from liquid to vapour, which determines the strength and location of the monsoon rains.
The India Meteorological Department (IMD), applies the term monsoon to the seasonal reversals of wind direction along the shores of the Indian Ocean, especially in the Arabian Sea that blow from the southwest during one half of the year and from the northeast during the other half.
India ,interestingly, observes two phases of monsoon. The summer monsoon arrives in India towards the end of May or early June, over the southern tip of the Indian coast (Kerala) and is considered the beginning of the rainy season in the Indian meteorological calendar (Puviarasan, Sharma, Ranalkar and Giri, 2014). Advancing in surges, it covers the entire country by or around July 15 (IMD, 2009). When by late September, southwest monsoon slowly withdraws up to latitude 15°N, persistent surface easterlies blow over Tamil Nadu coast and fairly widespread rainfall occur over coastal Tamil Nadu, south coastal Andhra Pradesh and adjoining areas; the onset of northeast monsoon is declared. Northeast monsoon persists from the month of October to December (RMC, 2017).
Spatial and Temporal variability
Kerala is India’s first state to receive rainfall. While the winds eventually proceed to the rest of the country, rainfall is not necessarily evenly distributed. The country sees large rainfall variability both over space and time. Cherrapunji and Mawsynram, two small towns located on the southern slopes of Meghalaya record an annual rainfall of 11,000 mm a year while areas like western Rajasthan and Tamil Nadu receive less than 200 mm of summer rainfall. Interestingly, in 2016, Meghalaya, where the town of Cherrapunji is located, witnessed a decline of 25 per cent while Rajasthan, witnessed an increase to the tune of 24 per cent, (Purohit and Kaur, 2016). From 1870 to 2016, a period of 146 years, India has witnessed 20 years of excess rainfall and 27 years when rainfall was deficient while the remaining years were normal (Fig. 2).
When the all India monsoon rainfall drops below 10 per cent of its Long Period Average (average rainfall received by the country as a whole during the south-west monsoon for a 50-year period) in a year, IMD declares the year as a drought year (IMD, 2009). During the last century, long term trends were few, but India witnessed a prominent decadal change. As shown in Figure 2, during the decades between 1899-1920 and 1965-1987, monsoon rainfall was insufficient and India faced droughts once every three years. The 21st century itself has witnessed five years when rainfall was in deficit and 2002 was declared by the IMD to be an ‘first-ever all-India drought year’ (Hindu Business Line, 2002). Further, two back to back drought years were recorded in 2014 and 2015 alone. These two years were distressful for farmers with the states of Maharashtra, Andhra Pradesh and Telangana severely drought stricken. Even the well irrigated states of Punjab, Haryana and Karnataka and coastal areas such as Odisha felt the effects of a poor monsoon. The north-western parts of the country, which include Punjab, Haryana, Uttar Pradesh and Rajasthan, had received the least amount of rainfall. Kharif crops were severely impacted in large parts of Odisha due to inadequate rainfall and 33 per cent of 882,000 hectare of cropped area suffered damage (Makkar, Mukherjee and Behera, 2015). With a consecutive drought year of 2014-15, India witnessed a sharp spike in farmer suicides. There were 8007 farmer suicides in 2015 as compared to 5,650 in the former year. The vagaries of monsoon thus played a significant role, especially in the worst hit areas of Maharashtra, Karnataka and Telengana, where the highest numbers of suicide have been recorded (NCRB, 2016).
Causes of short rainfall and drought
Severe droughts in India have been linked with El Niño Southern Oscillation (ENSO) owing their simultaneous occurrence. ENSO anomalies, caused due to unusually high surface sea temperatures, have occurred due to sustained warm spells. The warm spells lead to high pressure areas in the sea that pulls dry air from the landmass, lessening the potency of the positive feedback loop that leads to a normal monsoon. The possibility of droughts depends on the extent to which ENSO conditions raise sea surface temperatures (Kumar, Rajagopalan, Hoerling, Bates and Cane, 2006). However, although six leading droughts since 1871 have been accompanied with severe ENSO occurrences, a correlation between an extreme ENSO event and a drought is not always necessary. The 2002 drought, for example, occurred even when ENSO was moderate. Similarly, a severe ENSO event recorded in 1997 – the 20th century’s strongest – was not accompanied by a drought in India (ibid).
Rising Sea Surface Temperatures
Apart from the ENSO phenomenon, the warming of the Indian Ocean has also been related to precarious rainfall in the Indian subcontinent. The western tropical Indian Ocean has particularly been warming for more than a century – at a rate faster than any other region of the tropical oceans – and has turned out to be the largest contributor to overall trend in the global mean sea surface temperature (SST). During 1901 to 2012, the Indian Ocean warm pool (the central-eastern Indian Ocean characterised by sea surface temperatures greater than 28°C) witnessed an increase of 0.7°C, while the western Indian Ocean experienced anomalous warming of 1.2°C in summer SSTs (Mathew, Kapoor, Pascal and Masson, 2014). This warming has drastic potential to change Asian monsoon circulation and rainfall.
In the northeastern part of the Indian Ocean, – near the Bay of Bengal, monsoon depressions (MDs) bring rain to the nearby regions. MDs are depressions formed within the monsoon trough, frequently used to describe weak cyclonic disturbances that form over the Bay of Bengal and generally track northwestward over the Indian subcontinent. They are known for producing copious amounts of precipitation and are responsible for the majority of rainfall during the summer monsoons. In recent years, there has been a significant reduction in MDs caused due to anomalous moisture convergence over the western Indian Ocean, which in turn associated with the rapid warming of the sea surface. This reduces the moisture transfer into the Bay of Bengal, adversely affecting the formation and intensification of monsoon depressions (Vishnu, Francis, Shenoi and Ramakrishna, 2016).
Break in Monsoons in India
Break in monsoon is an intra-seasonal recess in rainfall activity, a natural occurrence after a spell of intense rainfall. It is also seen that the monsoon trough shifts towards the foothills of the Himalayas in the middle of August, which leads to an interim decrease of rainfall over most parts of the country. However, long and dry breaks are often associated with poor monsoon and often determine the fate of rainfed agriculture (Gadgil and Joseph, 2003).
Monsoon prediction and roundup–2018
For 2018, the IMD forecasted the monsoon seasonal rainfall to be 97 per cent of the Long Period Average, with a model error (departure) of ± 5 per cent (PIB, 2018). As of June 13, 2018, ten states have witnessed large variations from normal rainfall. In the states of Gujarat and Arunanchal Pradesh, the departure has been negative, to the extent of –90 per cent and –76 per cent respectively (Fig. 1).
India’s monsoon cycles witness large variations in terms of both space and time. Over the past few years, aberrations have occurred as 2014 and 2015 witnessed droughts, while 2013 received excess rainfall and witnessed the disastrous floods of Uttarakhand. The predictions of 2018 have pointed towards a 54 per cent probability that rains will be normal to above normal; the expected rainfall of 97 per cent of Long Period Average (89 cm), is also an increase compared to 95 per cent in 2017 (Bera and Choudhary, 2018). These signs of a normal monsoon, crucial to push economic growth, portend a year of prosperity
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