Chasing the Monsoon in the Bay of Bengal

In ancient Vedic texts, Indra, the king of the gods, battles the vicious monster Vritra who has swallowed all the water in the world and left the people starving without rain for their crops. Armed with a club made from the bones of a powerful sage, Indra challenges Vritra. The sky fills with clouds, thunder and lightning when they fight. When Indra, in a fatal blow, slices open the belly of the beast, the heavens open and life-bringing rain pours down. This cycle would repeat itself every summer (and winter) season and people called it the “monsoon” (from the Arabic word “mausim” for “wind-shift” or “season”). 

Scientists now know that the seasonal monsoon is not caused by gods, but instead by winds, air masses interacting with the Himalayas, and various patterns of heating and cooling air and oceans. There are many things they don’t know, however, about what triggers the onset of the Indian summer monsoon. Nor do they have the full picture of all the dynamic interactions between the atmosphere and the ocean, these two vast reservoirs of fluid that control our climate, and the role they play in the yearly variations in the onset and strength of the monsoon. Hundreds of millions of people live in areas where the seasonal cycle of crops and cattle depend on this burst of life-giving rain, so there is a strong interest in better understanding these processes.

The ASTraL (Arabian Sea Transition Layer) program is the U.S. Office of Naval Research's (ONR) contribution to a joint research initiative called EKAMSAT (Enhancing Knowledge of the Arabian Sea Marine environment through Science and Advanced Training), which partners India and the U.S. The initiative is funded by India’s Ministry of Earth Sciences with U.S. support from ONR, NASA, the National Science Foundation and NOAA. The project involves scientists from a dozen U.S. institutions, including UC San Diego’s Scripps Institution of Oceanography, the University of Washington, NASA, and Woods Hole Oceanographic Institution. Under EKAMSAT, scientists have also been collecting data during fieldwork organized by the Indian National Centre for Ocean Information Services.

About a dozen scientists and engineers from Scripps Institution of Oceanography recently participated in the second of three cruises for this project in the Bay of Bengal east of India. As the project name implies, the cruise was originally meant to take place in the Arabian Sea, west of India, but because of current geopolitical issues, including pirate attacks on cargo vessels, plans had to be adjusted. Although there was initial disappointment in not being able to collect data in an otherwise sparsely sampled part of the ocean, the team pivoted and reset for the Bay of Bengal, a region that has very similar dynamics and processes.

In May 2024, the team boarded the University of Washington’s research vessel Thomas G. Thompson, the sister ship to Scripps’ R/V Roger Revelle, for a total of six weeks at sea. The cruise was split into two legs with most of the Scripps team joining for the four-week-long second leg. Aboard R/V Thompson was a slew of instruments, many from Scripps. Luc Lenain sent three WaveGliders, and Janet Sprintall brought an underway CTD which measures seawater conductivity and temperature at various ocean depths. Luca Centurioni, director of the Lagrangian Drifter Lab, sent six dozen surface drifters. Drew Lucas and Jen MacKinnon of the Multiscale Ocean Dynamics lab (MOD) brought a fast CTD profiling winch system with a turbulence profiler and two WireWalkers. Thus, a well-equipped science party. 

The MOD lab also brought a towed acoustic profiler called the TPADS, something we last deployed from an Italian navy ship in the North Atlantic Ocean. Members of the science party representing various other American and Indian institutions also collected data on the atmosphere through the release of weather balloons six times daily and a slew of atmospheric sensors installed on R/V Thompson. In addition, we collected genetic material in the ocean through water sampling and filtering (a total of 1,000 liters [270 gallons] of seawater was processed during the cruise!) to look for patterns of phytoplankton abundance associated with the monsoon.

As the team for the second leg of the cruise gathered in Chennai, and with the predicted monsoon onset date still two weeks out, the 100℉ air was as thick with heat as with anticipation. When R/V Thompson at last set sail for the open seas, the fresh ocean breeze was much welcomed.

We then spent four weeks mostly in “profiling mode”, that is, running the winch on the back deck with one of our two profilers: either the fastCTD which measures the two most essential ocean variables, temperature and salinity, as we drop it down and winch it back up at four meters per second, or the microstructure profiler called the “epsi”. Scientists care about measuring the microstructure in the ocean, the small, centimeter-scale swirly motions we often refer to as turbulence, as it’s an indicator of ocean mixing. Various important features such as heat, aerosols and energy are distributed in the ocean in this way. For example, heat in the very surface layer of the ocean on a warm day can mix downwards, but depending on how the wind and waves behave, this heat can also be trapped and stored in the ocean for a while and only later be released back into the atmosphere. It can thus be fuel for various atmospheric processes such as convection and rain. In addition to profiling, we also used several autonomous platforms, various instruments that we deploy and then pilot remotely for a few weeks before picking them back up. Some of the autonomous assets were surface ones, like the Wave Glider, and some sampled the subsurface like floats and sea gliders.

Near the halfway mark of the cruise, it was officially declared that the monsoon had begun (defined by a certain amount of rain in the south Indian state of Kerala). This was celebrated by tiny drink umbrellas in sparkling water during the daily science meeting. During these meetings, the atmospheric scientists onboard also gave daily reports on the progress of the weather systems that could impact the start of this year’s monsoon, as well as the development of a tropical depression that passed right above us and made landfall near the Bangladesh-India border as the Severe Cyclonic Storm Remal. That system brought a lot of destruction, killing at least 84 people, leaving millions without power and destroying hundreds of thousands of homes. For us, it meant a few days of heavy squalls and rough waves, but for many others it was a catastrophe that will impact their lives for a long time.

We dodged a bullet, but it also heightened our sense of purpose, knowing that our research could contribute to better prediction of these powerful forces of nature, and potentially helping to mitigate their destructive impacts in the future. So as the rain and thunder danced on in the skies, and the monsoon once again brought life back to the dry lands, we gathered up our last few instruments and set sail for home. Reflecting on the journey, we felt satisfied, our hard drives loaded with terabytes of unique data we’ll spend the next few years analyzing. Furthermore, we got to witness the power of the monsoon and the importance of our work in understanding it better.

Kerstin Bergentz is a fourth-year graduate student in the Multiscale Ocean Dynamics lab and the Lagrangian Drifter Lab working with dynamics in the upper ocean, air-sea interactions, energy transfers, and various types of waves.

The MOD lab is on Instagram and Twitter: @MOD_at_Scripps