To better understand how air-sea interactions over the Indian Ocean influence monsoon dynamics, Debarshi Sarkar (PhD candidate from the Tandon Lab) and Ersen’S Joseph (full-time engineer at WHOI and master’s student in the Tandon Lab) participated in the third cruise of the ASTraL (Air-Sea Transition Layers) /EKAMSAT (Enhancing Knowledge of the Arabian Sea Marine Environment through Science and Advanced Training) field program.

Research Vessel Thomas G. Thompson

This expedition was conducted in two legs: the first (Leg-1) from May 3 to May 15, and Leg-2 (which recently ended) from May 19 to June 19, with additional time allotted for mobilization and demobilization. Debarshi and Ersen’S was part of Leg-1, and for both legs, the R/V Thomas G. Thompson (R/V TGT) departed from the deep-sea port at Phuket, Thailand. The leg-1 focused on a region spanning latitudes 11°N to 15°N and extending longitudinally from 85.5°E to the edge of India’s Exclusive Economic Zone (EEZ).

Track of R/V TGT for Leg-1 ASTraL/EKAMSAT 2025

The key objective of Leg-1 was to locate a ‘mini warm pool’ in the Bay of Bengal (A region with warming temperatures) using satellite data. This would serve as the focal point for the field experiment. This region would need to be in a zone of weak current and away from the EEZ to minimize the drift of the OARBITER buoy, which was scheduled for deployment by NRL during Leg 2.

The science crew (from left): Orson Hyde (engineer from University of Notre Dame), Charlotte Begouen Demeaux (PhD student from University of Maine), Carlyn Schmidgall (PhD student from University of Washington, Seattle), Chief Scientist Dr. J. Thomas Farrar (Woods Hole Oceanographic Institute), Dr. Laurent Grare (Engineer from Scripps Insititute of Oceanography), Ersen’S Joseph & Debarshi Sarkar

During Leg-1. We deployed 3 sea gliders and 4 wave gliders to collect high-resolution upper-ocean data. Drifters, which were capable of measuring winds, waves, and SST, were released across a wide area to ensure a large spatial distribution. Apart from the drifting assets, R/V TGT was equipped with a flux tower for measuring meteorological conditions, a ceilometer for cloud and aerosol properties, a LIDAR for vertically profiling wind structure, an ADCP for subsurface currents, and a thermosalinograph for temperature and salinity at a single depth. These instruments were kept on throughout the time we were outside international waters. The science team also took turns operating the Underway CTD (UCTD) system to measure the vertical profiles of temperature and salinity along the cruise track. The second objective of the cruise was to calibrate NASA’s PACE satellite measurements using in-situ data radiometry, optical, flow cytometry, and pySAS data.

Wave Glider Ida right after deployment

Chief Scientist Dr. J. Tom Farrar inspects a sea glider moments before deployment.

Sunset over the Bay of Bengal, with wave gliders ready on deck the evening before deployment.

The Leg-1 experienced calm ocean surface conditions (characterized by swells and weak wind waves) and mild atmospheric conditions (with weak winds and patchy non-convective cumulus clouds). Aside from a few brief convective downpours, the overall sea state remained favorable, enabling smoother operations and more consistent data collection.

Dr. Laurent Grare demonstrates to the science crew how to remotely operate the wave gliders.

Being the smaller science team, each member juggled multiple responsibilities. UCTD schedulers made a concerted effort to ensure everyone got time to rest. Debarshi’s primary roles included overseeing data from the ship’s sensors, wave gliders, and UCTD operations. In addition, he processed satellite winds, SST and sea level data to assist the chief scientist in optimizing UCTD deployment waypoints. Ersen’S assisted Dr. Laurent Grare on the engineering side, helping to assemble and ensure the smooth operation of the wave gliders during the cruise.

Debarshi and Dr. Laurent Grare on deck manually operating an underway CTD profiler during their shift.

Looking Ahead

This cruise was made possible through the collaborative efforts of a broad team of scientists from the U.S. and India, whose intellectual contributions helped shape EKAMSAT’s scientific vision. Special thanks go to the captain, crew, and port staff of the R/V TGT for their professionalism and support. Observations from both legs of the campaign will be crucial for understanding how air-sea coupling influences the formation, evolution, and eventual dissipation of the Indian Ocean mini warm pool, a poorly sampled and less understood oceanic feature that is thought to influence the Indian Monsoon season.