We have just returned to the Chennai harbour after nearly a month in the Bay of Bengal, aboard the ocean research vessels Sagar Nidhi and Roger Revelle. The two ships were on a joint India-United States field campaign, the second since 2013, to understand the influence of the ocean on the wet and dry phases of the summer monsoon. The monsoon is vital to the social and economic well-being of populations across the south Asian region. From farmers to national economic planning agencies, all would benefit from improved monsoon forecasts.
The surface waters of the north Bay of Bengal are among the freshest (i.e. least saline) anywhere in the world, due to monsoon rains and the discharge of fresh water from several mighty rivers. The warm, fresh upper layer of the Bay supplies prodigious amounts of moisture and heat to the atmosphere, forming masses of tall, dark clouds that bring monsoon rain deep inland.
Detailed mapping During the recent campaign, nearly 50 scientists, engineers and students on the two ships mapped the uppermost layer of the Bay of Bengal in unprecedented detail. The commitment of ships, personnel, instruments and other resources represents a major investment by India and the United States in monsoon research. The data from the two ships, as well as moored and drifting instruments, are beginning to reveal how the upper ocean interacts with the atmosphere. Monsoon winds and ocean currents stir the fresh water over large distances, and 200-kilometer wide ocean eddies draw it into thin filaments. Yet the mixing between river water and sea water is very slow. The thin, fresh layer responds quickly to changes on the surface caused by winds and sunlight. At the same time, it helps to trap some of the sun’s heat at subsurface depths, to be released into the atmosphere days and weeks later.
Realistic model simulation of monsoon clouds is a notoriously hard problem in atmospheric science. Given these complex interactions between ocean and atmosphere, even the most sophisticated computer models struggle to provide accurate monsoon forecasts several days to weeks ahead. Errors in monsoon simulation can spread rapidly, potentially degrading global weather and climate forecasts. Therefore, a deeper understanding of monsoon air-sea interaction, the key objective of our joint endeavour, is a prerequisite for improved forecasts.
Training scientists An important part of our cooperative effort is training young scientists to use and build upon modern techniques by working alongside teachers and scientists from both countries. The Sagar Nidhi hosted senior U.S. scientists, and the Roger Revelle hosted six young Indian scientists. We worked with special observation tools including an autonomous robotic boat and glider, underway conductivity-temperature-depth profilers, and acoustic Doppler current profilers, to map several thousand kilometers of upper ocean structure with high resolution. In addition, an air-sea flux mooring, built by U.S. scientists and deployed from the Sagar Nidhi , is making accurate measurements of momentum, heat and moisture exchange across the ocean surface.
As partners in education and science, we are proud of the achievements of this India-United States effort, which has been five years in the planning. Our joint work is part of the active global effort to enhance predictability of weather and climate through deeper understanding.
This field study is one component of the broad India-United States collaboration — The Ocean Mixing and Monsoons and Air Sea Interactions in the Northern Indian Ocean-Regional Initiative (OMM-ASIRI). We thank the Indian Ministry of Earth Sciences’ Monsoon Mission programme and the U.S. Office of Naval Research for their support and leadership of the partnership, and the 17 Indian and U.S. institutions that have made it possible.
(Professor Amit Tandon is at the College of Engineering, University of Massachusetts Dartmouth, and Professor Debasis Sengupta is at the Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bengaluru.)
