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Not as simple as rainfall

TreeTake is a monthly bilingual colour magazine on environment that is fully committed to serving Mother Nature with well researched, interactive and engaging articles and lots of interesting info.

Not as simple as rainfall

To understand the changes in monsoon pattern, it becomes imperative to first understand the entire mechanism behind monsoon winds.

Not as simple as rainfall

Although the rainy season has been a part and parcel of the Indian subcontinent, its unpredictable nature has baffled scientists and meteorologists for quite some time now. Recent studies show how monsoon pattern is undergoing haywire changes due to climate change and other factors which may not be a good sign for the future. In this article, we examine the changing monsoon pattern along with its recent trends and an analysis of the impact this is projected to create in the long term...

Himanshi Shukla

As those little drops pour down on earth, first slowly and then all at once, they bring with them smiles and relief on numerous faces across the subcontinent. While the Western countries deride rains, monsoons in India are welcomed as they bring relief from the scorching summer heat and also spell boom and bounties for agricultural produce as Indian agriculture is heavily dependent on the rains. Derived from Arabic word ‘Mausim’ which literally means weather, monsoons have a special place in hearts of Indians. From countless Bollywood songs to even more countless paper boats which kids anxiously wait for, and finally to the farmers who depend on monsoon rain especially for paddy, the rainy season has something to offer to everyone.

Understanding the mechanism behind Monsoon winds

To understand the changes in monsoon pattern, it becomes imperative to first understand the entire mechanism behind monsoon winds. As per Professor Dhruv Sen, Department of Geology, University of Lucknow:  “Generally described as ‘seasonal reversals in the direction of winds’, monsoons are winds that typically hit the Indian mainland landmass at around 1st of June.  First they arrive in Kerala (mainland) and even before that they hit the Lakshadweep Islands.”

As per Dr D R Khullar, MA/HES, Former Principal, Government PG college, Kalka (Haryana):  “Many scholars still believe that the main mechanism of monsoon is differential heating of land and sea in the summer season. The landmass, particularly in the Northwest India gets intensely heated during summer season causing a very low pressure area there. The Indian Ocean remains comparatively cooler leading to high pressure condition there. This causes northward shift in the position of Inter-tropical Convergence Zone (ITCZ) and south-east trade winds across the equator and after deflection these winds enter India as Southwest monsoon winds. Intertropical Convergence Zone establishes itself about 25 degree north latitude over India and Pakistan. ITCZ is a low-pressure zone located at equator where trade winds converge and the air tends to ascend. The shift in the position of the ITCZ is also related to the withdrawal of the westerly jet stream from the south of the Himalayan range. The easterly jet stream is caused by the summer heating of the Himalayan and the Tibetan highlands. These highlands occupy a vast area of 4.5 million square kilometre with an average altitude of more than 4000 metres as these islands are heated in the summer due to the northward shift of the sun, a clockwise circulation of wind takes place in the middle of the troposphere. This results in two main streams of air from this land mass. One of these streams goes towards the equator while the other is deflected towards the pole.  The equatorward flow enters India as easterly jet stream while the poleward stream goes to East Central Asia as westerly jet stream. Monsoon sets in over Kerala coast by 1st of June and reaches Mumbai and Kolkata by 10 to 13th of June; it spreads all over the subcontinent by 15th of July.”

The monsoon rainfall is never continuous but comes as wet spells followed by the dry spells. Pickle depressions from the Bay of Bengal causes rainfall in the Northern plains of India while the West Coast of India receives rainfall from the Arabian Sea current of the monsoon. Intensity of rainfall on the West coast of India depends upon the offshore meteorological conditions and the position of the equatorial jet stream along the eastern coast of Africa. The frequency and the tracks of Bay of Bengal depressions depend upon the position of ITCZ which is generally known as the monsoon trough. A change in the position of ITCZ causes a change in the axis of the tropical depression which brings about a change in the intensity and distribution of rainfall. The amount of rainfall decreases from west to east-north-east over the West coast and from east-south-east towards north-west over the Indian plane and the northern Peninsula.

Understanding 'break in the monsoon' or dry spell

“When the Southwest monsoon fails to bring rainfall for two or more weeks and is a dry spell in the rainy season, it is called the break in the monsoon. This break may be caused either by failure of the tropical depression or by dislocation of monsoon trough (ITCZ) over the north India. Over the West coast, we experience dry spell when the winds blow parallel to the coast. In the western part of Rajasthan, the inversion of temperature prevents rain bearing winds from rising up, a dry spell is experienced. The Southwest monsoon starts retreating by 1st of September from the Western strip of Rajasthan, and by 15 September, it retreats from most parts of Punjab, Haryana, Rajasthan and Gujarat. It retreats from most of the parts of India by mid October, except the southern Peninsula. The retreating monsoon picks up moisture from the Bay of Bengal and establishes itself over Tamil Nadu coast in the month of December and causes rainfall there,” explains Prof Khullar.

The other factors that influence the monsoon are El Nino, La-Nina and Southern Oscillation (ENSO). As per Prof Khullar: “El Nino narrow warm current which appears off the coast of Peru in December. In Spanish, it means the ‘Child Christ’' because it appears around Christmas. It is a temporary replacement of the cold Peruvian and Humboldt Current which normally flows along the coast. It appears once in every 327 years and is responsible for widespread floods and droughts in the tropical regions of the world. Sometimes it becomes more intense and increases the surface water temperature of the sea by 10 degree Celsius. This warming of tropical Pacific water affects the global pattern of pressure and wind system including the monsoon winds in the Indian Ocean. The El Nino phenomena, which influence the Indian monsoon, reveals that when the surface temperature goes up in the southern Pacific Ocean, India receives deficient rainfall and vice versa.”

“El Nino has a great impact on the Indian monsoon and is used in India for forecasting long-range monsoon rainfall. Meteorologists believe that the severe drought of 1987 was caused by El Nino. In 1990 to 1991, there was a wild El Nino event and the onset of Southwest monsoon was delayed over most parts of the country ranging from 5 to 12 days. After El Nino weather conditions return to normal.  However, sometimes trade winds become so strong that they cause abnormal accumulation of cold water in the central and eastern Pacific region.  This event is called La Nina which in effect is the complete opposition of El Nino and La Nina also marks and active hurricane season. But in India the presence of La Nina portends exceptionally good news as it is the harbinger of heavy monsoon showers in India. El Nino Southern Oscillation (ENSO) is the name ascribed to the curious phenomena of sea shore pattern of meteorological changes observed between the Pacific and Indian oceans. It has been noticed that whenever the surface level pressure is high over the Indian Ocean, there is low pressure over the Pacific Ocean and vice versa. When the winter pressure is high over Pacific Ocean and low over the Indian Ocean, the Southwest monsoon in India tend to be stronger. In the reverse case, the monsoons are most likely to be weaker. Different indices have been used to measure the intensity of southern oscillation but the most frequently used is the Southern Oscillation index. This is the difference in the preferred between Tahiti in French Polynesia representing the Pacific Ocean and between the Northern Australia representing the Indian Ocean. The positive and negative values of these are so that is the SOI that is Tahiti minus Port Darwin pressures are pointers towards good or bad rainfall in India,” he adds.

How is the monsoon forecasted in India?

As per Prof Khullar: “After the drought of 1987 a model has been developed to forecast monsoon rainfall by utilising 16 indicators. The Indian meteorological Department IMD is using these indicators for forecasting the behaviour of the monsoons some of the indicators are global while others are regional these indicators are divided into four categories, namely- temperature, pressure, wind pattern, and snow cover.”

What trends do the recent studies indicate about the changing pattern of the monsoons and what role does climate change play in this ‘Great Game’?

According to a recent study entitled ‘Remote local drivers of the Pleistocene South Asian Summer monsoon precipitation: A test for future predictions’ by Kaustubh Thirumalai (Department of Geosciences, Tuscon, Arizona, USA) and others: “South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. South Asian precipitation and runoff are strongly coherent with, and lag atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.”

The study reconstructs South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. The research explicitly recognises the climate change factor in the changing trends: “Anthropogenic aerosols have weakened the dynamical monsoon circulation and associated precipitation in past decades. Nevertheless, models indicate that the thermodynamic impact of increased greenhouse gases (GHGs), as well as consequent increased atmospheric moisture content, will result in increased summer monsoon precipitation overall and extreme precipitation events.”

This conclusion, even before being published, by researchers is being actually felt by the people whose lives are disrupted owing to ‘extreme events’ caused by climate change. Thus, in the past, Kerala floods (2018), Jammu and Kashmir floods (2017) and even the Mumbai floods that occur on an annual basis should be viewed in this light. On the same note, however a dichotomy exists because dry spells too are visible in various parts. An Indian Meteorological Department (IMD) study released in March 2020, which analysed data of over 30 years pertaining to changes in Monsoon pattern, states: “Parts of India are facing a severe impact of the climate crisis with reduced rainfall and increase in dry, or no rain, and heavy rain days during the monsoon season.” The study found there were “significant decreasing trends” in the south-west monsoon from 1989 to 2018 in the Ganga basin states like Uttar Pradesh, Bihar and West Bengal, which are known to have among the most fertile lands in the country. Meghalaya, where Cherrapunji is known as one of the places with the highest precipitation globally, Nagaland and Arunachal Pradesh recorded a decreasing trend along with Himachal Pradesh. The study found a significant increase in heavy rainfall (6.5 cm or more) days in Gujarat’s Saurashtra and arid regions of Kutch and south-eastern Rajasthan, as well as northern Tamil Nadu, northern Andhra Pradesh, south-west Odisha, parts of Chhattisgarh, south-west Madhya Pradesh, parts of West Bengal, Manipur and Mizoram, Konkan, Goa and Uttarakhand.

A recent study published in Scientific Reports by Nature Publishing Group, IIT Kharagpur researchers, led by Professor Rajib Maity, reveals that the pattern of monsoon and daily extreme rainfall is changing in India with more extremes expected over southern India and Himalayan foothills. They further predicted a continued southward shift of precipitation extremes over South Asia. The researchers have studied data of Indian Summer Monsoon precipitation for close to five decades, 1971 – 2017 (base period: 1930-1970). The data shows more increase in the magnitude of extreme precipitation over South India (~18.5 mm/day for the worst climate change scenario) compared to Northern and Central India (~2.7 mm/day). “Our analysis of precipitation from the Coordinated Regional Downscaling Experiment indicates a southward shift of precipitation extremes over South Asia. For instance, the Arabian Sea, South India, Myanmar, Thailand, and Malaysia are expected to have the maximum increase to about 18.5 mm per day for an RCP8.5 scenario in mean extreme precipitation. However, north and central India and Tibetan Plateau show relatively less increase (~2.7 mm/day for RCP8.5 scenario). Analysis of air temperature at 850 mb and precipitable water (RCP4.5 and RCP8.5) indicates an intensification of Indian Ocean Dipole in future, which will enhance the monsoon throughout India. Moisture flux and convergence analysis (at 850 mb) show a future change of the direction of south-west monsoon winds towards the east over the Indian Ocean. These changes will intensify the observed contrast in extreme precipitation between south and north India, and cause more extreme precipitation events in the countries like Myanmar, Thailand, Malaysia, etc,” says lead researcher Prof Rajib Maity from the Dept of Civil Engineering.

What do the experts say about the link between climate change and increase in extreme monsoon events?

Professor Vibhuti Rai, Environmental Geologist, University of Lucknow, states: “I would say ‘Climatic Fluctuation’ is the correct term to define these sudden changes or disaster events in India and in the world. Cyclones on the Western coast of India are rare, yet we’ve had at least three in the past five years. Thus the effects of climate change are showing an erratic pattern. Hence no proper modelling has been done. In the past five to ten years there has been no absolute pattern. In Belgium, in Germany there were huge floods and also in many parts of Europe. In Mumbai we have excessive flooding every year. This needs to be evaluated or understood in the backdrop of climate change. ‘Climatic chaos’ that ensues then waves off different energy patterns. I would say there have been more and more extreme patterns. Mega prediction needs to be done based on proper modelling system which will indicate a major shift in the climate and that has its effect on the monsoon as well.”

Shiraz Wajeh, President, Gorakhpur Environmental Action Group, states: “When we take the example of eastern Uttar Pradesh, overall rainfall has decreased but the intensity of rainfall has increased. Thus, overall the annual rainfall remains the same. This one simple change has led to unprecedented problems for the area. Uneven spread of rainfall along with increased intensity causes water logging which harms the crops and brings about many diseases. Nowadays, as we notice, the summer sets in a bit early, that is by March onwards. This causes a decreased intensity of jet streams to weaken and this in turn causes changes in the monsoon pattern. Temperature regime too is showing a dichotomy of maximum and minimum temperatures at the same time. These extremes are causing coldwave days and heatwave days. Humidity is increasing as well.”

Professor CP Rajendran, adjunct professor at the National Institute of Advanced Studies, Bengaluru, states: “Extreme rainfall events in India are on the rise with intensification by the end of the century. Increased frequency of El Nino events and accelerated air pollution increased warming of the Indian Ocean and land use changes at local levels will contribute to this alternating cycles of droughts and flood. These events can spiral into environmental disasters in India due to lack of preparedness and resilience at the local level.”

Professor Dhruv Sen, Geology department, University of Lucknow, states: “We are at the transitional stage of climate change. It can be said that we are at the crossroads and don’t know whether the future direction will be left or right. It is very difficult to predict that. Based on the past trends, there has been alternate warming and cooling of the earth surface that caused and then reversed Ice Ages. This time-period can be seen as warming. When warming will reach its zenith, it will be followed by cold period. But again, this has been the past trends and won’t necessarily be happening in the future. No one can predict that correctly. Climate, as opposed to weather, is a long term phenomenon which is assessed retrospectively using such criteria as temperature, pressure, wind direction, cloudiness, humidity, etc. The parameters used are based on past trends. Monsoons, being ‘seasonal reversal in the direction of the winds’, are a combination of a myriad of geological factors. If not for the Himalayas, the monsoons won’t stay in the Indian subcontinent and India would have been a cold desert, owing to winds from the Siberia region. Thus, Monsoons became relevant in the Indian context only after the Himalayas were formed which was 65 crore years ago. Now, even if we have accurate- long term high resolution data of the past 100 years, we won’t be able to correctly predict the shifts that have been taking place and based on that, the changes that may take place.”

Impact, implications and the way forward

The study in the Science Advances states: “The South Asian (including Indian) monsoon region is home to a fifth of the world’s population that relies heavily on seasonal precipitation for residential, agricultural, and industrial purposes.” Professor Dhruv Sen, states: “India is an agrarian economy not because its contribution to GDP is high- over 30%, but because it generates over 70% employment in the country. This has led economists to coin the term ‘disguised unemployment’. As per a British economist who wishes to remain anonymous: “Indian agriculture is a gamble with Monsoon.” That’s how our sector and hence a major portion of economy is dependent on monsoon. Any extremes- whether a dry spell or excessive rains- thus harm the crops and cause trouble to the farmers and the country as a whole.

Shiraz Wajeh states: “Changes have led me to the conclusion that the temperature regime of the crops-especially paddy- will have to be regulated as crops are becoming more and more sensitive to these changes.” About the floods that are caused by heavy rainfall, especially in the urban areas, he points out that drainage problem immediately ensues and so does the transport get affected. “Roads and other such infrastructure are still being made keeping in mind the average rainfall and hence when this pattern goes haywire, you can clearly see chaos all over. Therefore, my suggestion is to make infrastructure as resilient to extreme events as possible.”

Gaurav Singh, (NET) TGT Geography at Delhi Public school, Lucknow, sums up the long term and short term changes due to highly unpredictable nature of summer monsoon. “The short term changes include- unusual and unprecedented spells of hot weather, crop failures due to lack of irrigation over flooded land and thus hardship to many of the farmers. Long term changes include extremely wet monsoon, more frequent droughts and floods, increase in warming an increase in sea level along the coastal regions of India.”

Professor Vibhuti Rai states: “There can be no way forward unless we take up small steps to combat climate change. We are living in a commercial world where everything is advertised to cater to consumerist aspirations of the people. Earlier, when there was no plastic or minimum use of plastic, there was less waste generated; lesser ACs and cars meant lesser air pollution. All this has a spill-over effect that is being increasingly felt in the monsoon pattern as well. Changes have to begin at the micro level if something at the macro level is to be achieved.”

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