This year, the monsoon in India is likely to withdraw 40 days later than normal. The season started with 33% deficit rainfall but is ending with 10% higher than normal rainfall, with heavy spells of rain resulting in devastating floods in many states.
These trends, however, did not feature in the pre-monsoon bulletin of the Indian Meteorological Department, the country’s primary weather forecasting body that functions under the Ministry of Earth Sciences. The bulletin, released in May, had predicted a normal monsoon.
In an interview with Scroll.in, Mrutyunjay Mohapatra, the director general of the department, explained why the forecast was not accurate. He said the number of heavy rainfall days was increasing because of climate change, which was making predictions more difficult.
Excerpts from the interview:
What explains the prolonged monsoon till October 10?
The monsoon had a few special characteristics this year. The monsoon started late on June 8 against the normal date of June 1. And it was delayed by about 10 to 15 days in the central part of the country and in eastern India. But it covered the entire country by July 19 against the normal date of July 15. It reached in time in the extreme northwestern part of the country.
Usually, the monsoon withdraws from September 1 from extreme northwest India, that is West Rajasthan, and by September 15, it withdraws from the entire country. But this year, the monsoon has not withdrawn so far. Rajasthan is still receiving rain. We are expecting that monsoon will withdraw in the next three days around October 9 or 10.
The monsoon has been quite active in the month of September because of various factors. One important factor is the low-pressure systems that dropped over Bay of Bengal moved towards Rajasthan and under its influence, an East-West oriented low-pressure zone came about. This sustained the monsoon for quite a long time.
Thereafter, a depression formed over the northeast Arabia Sea and it crossed the Gujarat coast and it moved in that direction sustaining the monsoon features.
The monsoon started with an El Nino condition. And El Nino conditions are not good for the monsoon. But it vanished and became a neutral condition from August and September.
Another factor is that the positive Indian Ocean Dipole developed. This is the warming near the western part of the Indian Ocean near the Somalia coast and cooling near the Indonesian coast. So that helps in creating the activity over the Indian region for monsoon.
These are the three basic reasons that sustained the monsoon flow.
What explains 10% higher rainfall than long term average?
The above-normal rainfall can be largely attributed to the large scale conditions like the El Nino being replaced by the neutral conditions, the negative Indian Ocean Dipole being replaced by a positive one and the third one is the low-pressure systems.
In June, we did not get the low-pressure systems. We were 33% deficient. In July, we got a few and the same process continued in August and September. The low-pressure systems developed but the number of days for which it persisted also increased in Madhya Pradesh, Rajasthan and its neighbourhood for 12 days in September.
Why was IMD not able to predict the higher rainfall or the prolonged season?
If you look at our long-range forecast issued around April 15 and May 31, we reiterated that this year will be a normal monsoon year, unlike many international and national agencies. They claimed it would be a below-normal monsoon. In that sense, our performance has been better compared to others. But still, we could not predict above-normal rainfall.
The rainfall pattern this year was deficient in June, then it became normal and later above-normal. There are large intra-seasonal variations that could not be captured. Seasonal forecasts mainly deal with long scale processes but these models do not take care of intra-seasonal variability.
So along with the long-range forecast, we issue an extended range forecast. This is a forecast that indicates the expected rainfall for the coming four weeks. It is provided for each meteorological subdivision. The seasonal forecast is provided for the country as a whole. The extended range forecast augments the seasonal forecast. The performance of this extended range forecast is good for up to two weeks.
Even though the magnitude of 110% could not be predicted but if you look at the trend, we predicted that rainfall in June will be less because of El Nino conditions and that happened. We also said that rainfall in July, August and September would increase.
We said that there would be a neutral El Nino conditions and that happened. We also predicted the positive Indian Ocean Dipole. We could predict the large scale processes and the trends but we could not predict quantitatively.
Monsoon forecasting was always tough but is it becoming tougher now, and if so, why?
The long-range forecast we issue is for planners, it is not meant for the general public. It cannot be applied at the district or state level. It will not benefit a farmer. We do not have the capacity for that. It is for the country as a whole for food supply chains. In that sense, there are challenges to improve the long-range forecast so that it can be provided at the state level. We are planning that in the next five years. There has been an improvement in the long-range forecast that we have been issuing.
The accuracy of our extended range forecast that is up to four weeks is not so good. We need to improve on that. We plan to issue it on a district basis.
The short to medium range forecast is for up to seven days. We earlier issued warnings up to three days and no we are issuing warnings up to five days. Our warning accuracy has increased in the last five years. Now, heavy rainfall can be predicted with 74% accuracy. It was 74% in this monsoon season.
Still, there is 26% missing and there could be false alarms also. These are our challenges. Our target is to improve [accuracy for warnings] by 10%-15% in the next few years.
What is the impact of climate change on the monsoon?
It has been found by the Ministry of Earth Sciences that there is an impact of climate change on the occurrence of rainfall in different spatial and temporal domains. But there is no trend in the seasonal rainfall of monsoons in India, it is random in nature.
The country is divided into 36 different meteorological subdivisions. And all of them show different trends. It has been found that the number of days with light to moderate rainfall is decreasing in Odisha, the Gangetic [parts of] West Bengal, Gujarat, Maharashtra, Uttar Pradesh and Bihar. Moderate rainfall is less than 7 cm in 24 hours. But the number of days with extremely heavy and very heavy rainfall in these states is increasing. This is rainfall more than 15 cm. This has been attributed to climate change.
Further studies show that the prediction of such heavy rainfall events is becoming difficult. This means that if we have the accuracy of three days, then it will become one and a half days under the influence of climate change. It is a challenge now. We have to improve our mechanisms to detect heavy rainfall early so that the common man can use it to save life and property.
How do we address this? We need better observation networks and radars. Once we get these observations, they need to be ingested in the numerical models. The computing power needs to be improved. We have to explain these physical processes in the presence of climate change. We also need more studies on this.
How do you see the IMD improving on its weather predictions, particularly to help farmers who need more localised forecasts?
For farmers, we have a special bulletin called the Farmers’ Weather Bulletin. This is issued twice a week and it provides a forecast for the next five days. Along with that, the Ministry of Agriculture provides information on agriculture so it goes as a joint bulletin. It provides the expected weather parameters, its impact on different crops and crop stages, and expected impact on pests.
There are also various advisories on the spraying of pesticides. This is at the district level. But on a pilot basis, we are trying to do this at the block level. We have issued advisories to 700 blocks for five days. Our target in the next five years is to cover 7,000 blocks.
We are also disseminating text messages to around 40 million farmers registered with us.