Mysteries of Indian monsoon probed in Bay of Bengal
A hurricane-hunter plane and research ships aim to gather the most-detailed observations yet of ‘intraseasonal’ variations in rainfall.
doi:10.1038/nindia.2018.85 Published online 27 June 2018
Heavy rains and seven-metre-high waves pummelled the research vessel Thomas G. Thompson in the Bay of Bengal this month, routinely drenching the oceanographers on deck. But that was just fine with the scientists. Their entire plan involved getting as wet as possible, in order to directly measure what happens where the air and the sea meet in a summer storm.
The team is part of a multinational group of researchers who are descending on the Indian Ocean this summer to study its seasonal monsoon. They intend to gather the most detailed observations yet on the wet and dry periods that alternate roughly every 10–50 days during the monsoon season, which lasts from June to September.
If modellers could better predict these varying patterns — called monsoon intraseasonal oscillations, or MISOs — then officials could better prepare for the monsoon each year. That includes timing the planting of crops in concert with the rains, storing water behind dams for hydropower, and preparing for floods and other natural disasters.
“One billion people on the Indian subcontinent depend on planning for water resources,” says Harindra Fernando, a mechanical engineer at the University of Notre Dame in Indiana and one of the project’s leaders. “When people are waiting for rain, it’s important to know when you will get it and when you won’t.”
MISOs represent the monsoon’s ‘active’ and ‘break’ periods, in which weeks of heavy rainfall give way to brilliant sunshine before starting all over again. The patterns of rainfall generally track northward over the Bay of Bengal, and sometimes veer towards the Indian subcontinent — where they can cause serious damage. In 2017, a powerful MISO brought torrential rain and landslides to Sri Lanka, killing more than 200 people.
Weather and climate models have not been able to accurately predict MISOs1. Strong and frequent interactions between the atmosphere and the ocean seem to help get them started2, as warm ocean waters feed energy into the air above (see 'Stormy skies'). A study published this year suggests that certain ocean processes, such as a type of wave that helps warm the topmost waters, could play a big role in kicking off many MISOs3. Having direct measurements from within a MISO will help modellers to pinpoint the exact conditions that drive them, says Jason West, an atmospheric scientist at the University of Colorado Boulder who led that study.
The Bay of Bengal project aims to measure the microphysics of how energy flows between the ocean and the atmosphere once MISOs are under way. It builds on a long history of international field campaigns to understand the intricacies of the Indian Ocean monsoon, so that neighbouring countries can better prepare for it4. Funders of the five-year study include the Indian Ministry of Earth Sciences and the US Office of Naval Research, working with institutions such as Sri Lanka’s National Aquatic Resources Research and Development Agency (NARA).
In the thick of it
The project’s aerial component started on 15 June from an air base in Colombo, Sri Lanka. Scientists have been flying aboard the US Air Force’s hurricane-hunter C-130 plane, which carries equipment to measure properties of clouds and the atmosphere. It is releasing instrument packages called dropsondes that measure temperature, pressure and wind speed as they plummet towards the sea.
The Thompson, meanwhile, left Chennai, India, on 4 June for the first of two research legs to gather data on ocean conditions. Scientists on board — mostly students and early-career researchers — deployed a variety of instruments to measure temperature, salinity, currents and other factors at different depths and locations across the Bay of Bengal. They also released radiosondes, which are instrument packages carried upward by weather balloons to gather meteorological data.
“We want to observe the conditions across the air–sea interface cleanly, which is a challenging thing to do,” says Emily Shroyer, an oceanographer at Oregon State University in Corvallis who led the first leg. The second leg will take another group of scientists on board and will wrap up by 22 July. This team will travel farther to the south, while an associated group will take measurements near Sri Lanka with the NARA research vessel Samudrika, says NARA oceanographer Priyantha Jinadasa.
With enough data and analysis, MISO forecasts could improve in perhaps five or ten years, says team member Debasis Sengupta, a monsoon expert at the Indian Institute of Science in Bangalore. "It is a fascinating and deep problem in tropical climate," he says.
Project scientists are already planning a second, longer season of field observations for next summer. Details have not yet been finalized, but the team will continue to target how energy flows between the air and the sea during the monsoon. “There’s constantly this game going on between the atmosphere and the ocean,” says Amit Tandon, an oceanographer at the University of Massachusetts in Dartmouth. “It’s every bit as exciting as a World Cup match between two nations.”
This article was first published in Nature.