The 2019 southwest monsoon season ended with surplus rain of 10% and set the record for the heaviest seasonal rainfall ever, a welcome change from the previous year when below-normal monsoon caused drought in as many as seven states. The director general of meteorology, India Meteorological Department (IMD), M. Mohapatra, explains in an interview why the current monsoon season was unique. Edited excerpts:
Despite a deficit of 33% in June, IMD maintained that monsoon would be normal. The season ended with a surplus rainfall of 10%.
We measure seasonal rainfall on the basis of certain processes that cause large-scale inter-annual and intra-seasonal variations. Inter-annual variations were caused by El Niño and the Indian Ocean Dipole (IOD) and they turned out to be, as we predicted. A weak El Niño turned to neutral during the second half and IOD turned positive. This positively impacted rainfall. However, there were some intra-seasonal variations because of the low pressure systems. On average, we get 13-14 low pressure systems in the season from north Bay of Bengal. The number of low-pressure systems was normal this season. However, they persisted for longer, leading to excess rain.
The month of September was the wettest ever and there were more instances of extreme rainfall that led to floods. What precipitated these instances?
There were just three low pressure systems in September, but they persisted for longer and travelled from north Bay of Bengal to Rajasthan and Gujarat. So, both the central part of the country and the southern peninsula received surplus rain. September recorded 52% excess rain, the highest ever. The seasonal rainfall was above normal. However, long-term studies show that both frequency and intensity of heavy rainfall events (>15 cm in 24 hours) is increasing, especially in central India, including UP and Bihar. The atmosphere is becoming more unstable because of global warming.
How does IMD plan to deal with challenge of forecasting extreme rainfall events?
The use of dynamical coupled models along with statistical models has greatly improved our forecasts. This season, we could predict extreme rain events at sub-divisional levels 24 hours in advance with an accuracy of 74%. However, such events occur in pockets on a scale of 10 to 100km, typical of a district. Thus, there is a need to increase our observational network at regional levels so we could detect these convective activities. We plan to increase the number of automated weather stations and automated rain gauges, use more satellite data and bring the entire country under a network of radars. The data will be supported by GPS to improve the quality of the forecast.
The current season saw the most delayed withdrawal of monsoon ever.
The monsoon withdrawal is getting delayed against the normal withdrawal date of 1 September. This is a trend we have observed. Also, this monsoon had no breaks, but rather continuous formation of low-pressure systems. Long-term studies also show an increase in the number of days with heavy rainfall and reduction in days with less to moderate rainfall. Such heavy rainfall is more concentrated in urban areas than in rural areas.
How will the changing climate impact IMD’s forecasting ability?
Our forecasting difficulties will increase. For example, now if we issue a forecast three days in advance, in future it may be just 1.5 days. That means forecasters will have less time to inform people about a heavy rainfall event. We need a denser observational network, hourly updation of models, real-time data, and more frequent forecasts. If we do not take these measures, climate change will limit our predictability. We need impact-based forecasts and risk-based warnings.
Does the dissemination of forecasts still not remain a challenge?
Yes. For this, we are working on an early-warning dissemination system, a kind of multi-hazard warning platform that can disseminate information quickly through all models of channels such as emails, SMS, WhatsApp, graphics, and text. It will be a common alert protocol in which one can send all alerts in one go.
What’s next for IMD?
Apart from improving our forecast for four main natural hazards—extreme rainfall events, thunderstorms, heat waves, and cyclone—we will also step up our sectoral applications. The economy is increasing and each sector is influenced by the weather and climate. At present, we focus on agriculture and disaster management, but we will expand to the energy sector, health, tourism, and transport.