Although the northeast monsoon set in over parts of South India on Wednesday, many parts of western and northern India, until recently, reeled under extremely heavy rains and flooding. Weather experts caution that unpredictable weather patterns will be the new norm.
As local variations in rainfall increase, the India Meteorological Department (IMD) grapples with accurately predicting the timing and quantum of rains. This is the result of a combination of inadequate infrastructure and unpredictability of the patterns.
Climatologists say that while rainfall becomes more erratic, the occurrence of extreme rain events leading to floods and devastation is increasing. For example, Chennai, which witnessed torrential rains and flooding in 2015 and inundation again in 2016 owing to a cyclone, went through one of its worst water shortages this summer.
A senior Met department official said the total number of rain days during monsoon is reducing, while the intensity of the spells increases. This means, rains that would normally have otherwise been spread over 10 or 12 days, pours in just two or three days, leaving the region unprepared and unable to cope.
“This is an emerging pattern for India's monsoon—increasing droughts intermittent with heavy rainfall events. Kerala and other parts of the Western Ghats had back-to-back floods in 2018 and 2019. This region also went through back-to-back droughts— with the rest of India— during 2015-16,” said Roxy Mathew Koll, climatologist at the Indian Institute of Tropical Meteorology (IITM) in Pune, and one of the lead authors of the Intergovernmental Panel on Climate Change (IPCC) 2019 report. Since the 1950s, there has been a threefold increase in extreme climatic events.
Interpreting the numbers
Often, while reading rainfall numbers, the averages can make it look normal across the State or the country, while in reality, the local variations are making a bigger impact.
“We are missing the local perspective. Averaging out rainfall for district or State or any region may not be a correct representation of what happens at the ground level and varied experiences community have on this,” said Anjal Prakash, Associate Professor, TERI School of Advanced Studies.
For example, parts of Maharashtra went through a wet period during this monsoon, while other regions in the same State are still reeling under drought. Koll agrees, saying, “Many times, we end up looking at the ‘All India Rainfall’ picture and make assumptions.
There is large regional variability in terms of total rainfall and also how the droughts and floods are manifested. This means we need a local action plan for every region”.
A study of the trends in rainfall pattern over India by M Rajeevan of the Ministry of Earth Sciences and Pulak Guhathakurta of IMD Pune, revealed that rainfall has been increasing over June and August, while the rainfall in July has shown a declining trend. Annual contribution of the June rainfall has increased in 19 sub-divisions, while it has decreased in 17 sub-divisions.
The other problem is the coverage of observation stations. Weather experts say that to accurately cover India, there must be one observation station every 100 sq km. There has to be a denser observation of rainfall in tropical countries as there are variations for every 100 sq km. For this, regions that need more stations must be identified and accordingly covered. The IMD has in excess of 5,000 observation stations, set up since 1901. Of these, it is estimated that there are around only 2,000 working stations.
The last hundred years
Studies of the southwest monsoon and the northeast monsoon (which is Tamil Nadu’s primary source) over the last 100 years have shown a declining trend in rainfall; more severely in some regions.
A study on the trends and variability of the Indian monsoon over 115 years, using Standard Precipitation Index-SPI atmosphere, conducted by Guhathakurta and a team of scientists of IMD Pune, revealed that in Tamil Nadu, a few subdivisions like Cuddalore, the Nilgiris, Perambalur, Pudukkottai, Tiruchy and Villupuram, are showing a decreasing trend in precipitation index. Observing the trend in percentage of area under drought conditions, the report stated Tamil Nadu as falling under the category, ‘Increasing but not significant’. In most of the southern districts of the State, the probability of drought is high and in the range of 15-20 per cent.
Across India, there were 24 years when the variation in rainfall was 10 per cent less than normal. Drought occurrences in eastern UP, Bihar, Assam and Meghalaya are most alarming. Parts of Kerala, Tamil Nadu, Chhattisgarh, Jharkhand, Odisha, Madhya Pradesh, Uttar Pradesh, Punjab and Himachal are showing increased signs of drought occurrence.
The trend over 115 years shows the vulnerability to drought between 10 and 26 per cent, with central and peninsular India having high vulnerability.
A study of the northeast monsoon, which gives Tamil Nadu 48 per cent of its annual rainfall, over 100 years (1910-2010), shows that the variation in average rainfall in the State was one of the lowest at 27.3 per cent, compared to other regions, said Y E A Raj, former head of the Regional Meteorological Centre (Southern states) in his report on the northeast monsoon. Other southern states have recorded a variation of between 30 and 40 per cent. His study found that there were 26 excess rain years and 25 deficient rain years.
However, these variations are extremely local and cannot be extrapolated to a State or a sub-division, say climatologists. This makes interpretation of numbers difficult.
The Intergovernmental Panel on Climate Change report released in September 2019 cautions that extreme El Nino (warm current) and La Nina (cold current) currents will increase in this century. Both these currents have an impact on the Indian monsoon and are likely to impact water availability for drinking and irrigation, thereby affecting food security.
Efficacy of the forecasting model
Is the forecasting model flawed or outdated? IMD uses state-of-the-art forecasting models that are similar to those used by the Met departments of the US and Europe. The weather patterns there are more predictable and easier to track. “Over Europe and North America, the weather systems are thousands of kilometres large and they linger around for over a week or more. Hence the weather forecast there is very reliable, for up to 10 days ahead. In the tropics, including India, the weather systems are small and fast changing. Hence, accurately predicting them using the same models pose a daunting challenge for scientists,” Koll said.
While IMD has accurately forecast cyclones, minimising the damage to lives, its record with predicting quantum of rainfall needs improvement. “The model captures the signal fairly well, but the location or intensity is slightly different,” said Koll. “Research is on to make forecasting more accurate and efficient. We make two-week forecasts from the AgriMet division for the farmers and our forecasting of rain within two to three days is vastly improved,” said Guhathakurta.
News Research Department