Extreme Weather Events Could Be The New Normal: IMD Chief Mohapatra

Monsoon arrives early, causing havoc in the northeast with disastrous floods and landslides, while northwestern states of Rajasthan and Punjab battle scorching heat. 

And in the national capital region of Delhi, frequent thunderstorms turn the month of May surprisingly pleasant. India's weather, as we knew it, is changing. 

In an exclusive conversation, India Meteorological Department Director General Dr Mrutyunjay Mohapatra explains the multiple factors at work and what these mean for weather forecasting and related policy intervention. 

Here is the first part of a two-part interview of the IMD head:

Edited excerpts:
Q: The early onset of monsoon this year has devastated northeastern states with floods and landslides even as large parts of north India were reeling under severe heat. At the same time, frequent thunderstorms have left the Delhi-NCR region with a surprisingly pleasant month of May. What is happening with the weather in India?
A: Towards the end of May, there have been very active monsoon conditions with early onset and advance of monsoon over the country, especially along the west coast including Kerala, Karnataka, Maharashtra, Goa, and Mumbai city. There has also been highly intense rainfall activity over northeastern states, which has been disastrous, leading to loss of life. At the same time, temperature has been shooting off above 40°C in parts of northwest India, which has also been disrupted by thunderstorm activity in different intervals. As a result, the month of May has been quite pleasant for the country with heat conditions almost absent in eastern India and being limited only to northwest India – Rajasthan and parts of Gujarat, Punjab, and Haryana. The monsoon onset and advance this year over Kerala was about eight days before the normal date, about 16 days over Maharashtra, and about 10 days over the northeastern states. 

Q: Why did the monsoon advance so early and rapidly this year?
A: This has been caused by two low pressure systems developing over the Arabian Sea and the Bay of Bengal. The Arabian Sea system became a depression and hit Maharashtra coast near Ratnagiri, amplifying the monsoon current over the Arabian Sea and South Peninsula. Within 2-3 days after advancing over Kerala on May 26, it advanced up to Mumbai by May 28. Similarly, the northeastern states' heavy rainfall was triggered by another depression, which became a deep depression, formed over the Bay of Bengal, and that moved towards northeastern states, crossing the West Bengal coast and Bangladesh. It caused rainfall activity from Odisha to West Bengal and especially northeastern states.

There has been record-breaking rainfall, with one location receiving 41 cm rainfall in the past 24 hours, resulting in widespread flooding and landslides leading to loss of lives and properties. These events are part of the large-scale intra-seasonal variability and inter-annual variability of the monsoon.

Q: Northeastern states have seen deficient rainfall for the last four years, below the historical average. Suddenly this year, monsoon advances rapidly, reaches well before its usual date, and is very heavy. Guwahati got 111 mm of rain in 24 hours, which is unprecedented at this point in time. What explains this sudden change?
A: Last year also saw floods, including in Guwahati city, but not across the entire northeastern states. This year, during the onset phase, it has been more devastating because the advanced monsoon over the northeastern states has been associated with a deep depression. A deep depression during the monsoon season is a rain-bearing weather event that gives copious amounts of rainfall.
We have the eastern Himalayas along Arunachal Pradesh and Jaintia hills along Meghalaya. Whenever the monsoon depression or the local system moves over the region, southerly winds from the Bay of Bengal move with great strength towards these hill ranges. These hill ranges lead to uplifting of the moist air, causing extremely heavy rainfall activity. For example, within a distance of about 3KM, the height of the Garo-Khasi hills can go up to 3 to 4 KM. This type of orographic uplifting of moist air is why places like Cherrapunji and Mawsynram are among the rainiest places in the world. June is one of the most intense rainfall months for the northeastern states.

Q: Moving northward, let’s talk about Delhi where this May has been the wettest since 1950. The month of May for Delhi is seeing an increase in rainy days and a steady decrease in dry days. Is this a trend we are likely to see continue?
A: Yes, you are right, this month of May has been quite rare, not only for Delhi but for the entire country, with rainfall being quite in excess. While monsoon rainfall contributed to excessive rainfall in South India and northeastern states, the frequency of western disturbances caused very good rainfall activity over northwest India and adjoining central India. Usually, the frequency of western disturbances affecting northwest and central India is maximum from December to March. In May, we hardly get western disturbances that cause rainfall activity over the plains of northwest and central India, but this year they have been very frequent and consecutive.

Q: Could you elaborate more on this?
A: Western disturbances are weather systems that develop over the Mediterranean Sea, Caspian Sea, and they move eastwards across Iran, Afghanistan, Pakistan, affecting northwest India. When they reach northwest India, they are sometimes called 'westerly' systems which strengthen the westerly winds. They usually move to the north of Indian latitudes, minimising their impact, and hardly one or two systems affect northwest India. However, if they extend towards the south, these westerly systems can bring moisture from the Arabian Sea, and sometimes Bay of Bengal moisture can also be involved. That is what has happened this year. 

Q: Is that why we have seen an increase in thunderstorms over Delhi?
A: For thunderstorms to occur, you need intense heating, moisture, instability in the atmosphere, and a trigger. This year, the western disturbances acted as the trigger. As a result, the frequency of thunderstorms has been quite high. Thunderstorms lead to rainfall, lightning, hailstorm sometimes, and strong and squally winds. This leads to a fall in temperature for the next 2-3 days, making the month of May quite normal or below normal in terms of temperature this year.

Q: How might climate change be influencing these events?
A: When you consider the impact of climate change, it leads to a rise in temperature. There are rising temperatures in the country during March, April and May, but this trend varies from place to place. For example, over Delhi, there is no trend in the past 30 years, but comparing previous 30 years to recent 30 years shows an increasing trend. Because of the rising temperature over the region, relative humidity in the atmosphere is increasing. This increased humidity supports thunderstorm activity. Another impact of climate change is that temperature is not increasing uniformly across the atmosphere; it's increasing in the lower part but falling at higher altitudes. This creates a larger temperature difference between the lower and upper troposphere, making the atmosphere more unstable. Therefore, all these environmental changes are contributing towards the increase in frequency and intensity of thunderstorms and hence lightning.

Q: While climate change impacts are seen globally, can the weather patterns observed in Delhi this May be directly attributed to global warming?
A: While the environment becomes more favourable for thunderstorm occurrence due to broader changes, it does not mean that a particular location must have that exact environment. Regarding Delhi specifically, we found that the temperature has not increased over Delhi in the past 30 years, so we cannot directly attribute the higher frequency of thunderstorms this May to global warming through the rise in temperature of Delhi itself. However, it could be an indirect impact of global warming. The impact of climate change is visible in terms of increased humidity, leading to more frequent and probable thunderstorm activity.

Q: What else could be the reason for localised weather events like thunderstorms in cities like Delhi?
A: The urban heat island effect is another factor that contributes to thunderstorm activity. Because of the increase in concrete structures, decrease in green cover, and decrease in water bodies, the urban environment becomes more favourable for heating. City centers can be 4-5°C warmer than the outskirts. This warming varies depending on factors like market activity, population density, building density, and forest cover. These create small islands of heat, hence the term 'urban heat island effect'. When weather systems like strong westerly winds move from west to east and encounter tall buildings in a city like Delhi, they converge. This convergence near Delhi leads to the uplifting of air mass, causing thunderstorms and rainfall. The frequency of thunderstorms and rain in a city center will be higher compared to the rural neighborhood. This is a fact proven in different cities across the globe. This effect also leads to wind convergence, moisture convergence, and heating, all contributing to increased rainfall and thunderstorm activity in a city.

Q: So, India’s weather is no longer what it used to be for all these years. Have extreme weather events come to be the new norm?
A: There's a greater degree of unpredictability and instability in the way our weather systems now move. The predictability of weather has reduced. We have scientifically proven that due to the impact of climate change and increased extreme behaviour, there is a direct threat to the predictability of various extreme weather events. For instance, predicting heavy rain has become harder; if it could be predicted three days in advance in normal situations, because of the changing situation, it might only be predictable 1.5 days in advance. The lead period of prediction decreases. 

Also, due to the mechanisms explained, small-scale extreme weather events can develop, like localised heat waves, thunderstorms, or heavy rainfall events where one part of a city might have heavy rain while another has little or none. This warrants certain interventions.

Continued in Part 2