The study, published in the journal Ocean Dynamics, was carried out by researchers at the Naval Physical and Oceanographic Laboratory of India’s Defence Research and Development Organisation in Kochi, together with a colleague from the Kerala University of Fisheries and Ocean Studies. Their central finding is deceptively simple: the upper 50 meters of the Bay of Bengal has warmed over the past several decades, while the layer between 100 and 200 meters has, in contrast, cooled. That vertical seesaw matters enormously for acoustics, because the speed of sound in seawater depends on temperature, pressure, and salinity, and the fine-grained structure of those variables determines where sound bends, bounces, and fades.

Sound speed in the ocean increases with both temperature and pressure, so a warm, well-mixed surface layer typically acts as a duct that traps near-surface sound and channels it over long distances with comparatively little loss. Below that, the thermocline produces a minimum in sound speed, and the depth at which that minimum occurs defines the sonic layer depth, a critical parameter for anyone predicting how a sonar signal will behave. When the research team examined long-term changes in the isothermal layer depth, the mixed layer depth, the barrier layer thickness, and the sonic layer depth, they found substantial variability across the record. The in-layer and below-layer sound-speed gradients, the quantities that acousticians use to characterize how rapidly sound speed changes with depth, also shifted appreciably, signaling that the surface-duct characteristics of the bay are not fixed features but moving targets.

Perhaps the most striking physical result is the consistent increase in upper-ocean heat content. This echoes a broader pattern documented across the tropical Indian Ocean, which has emerged as one of the fastest-warming ocean basins on the planet, driven by a combination of greenhouse forcing, weakening of evaporative cooling, and changes in monsoon-driven mixing. In the Bay of Bengal, the peculiar stratification created by massive freshwater discharge from the Ganges-Brahmaputra river system already produces a thick barrier layer that isolates the mixed layer from deeper waters. Long-term warming of the surface and cooling of the 100 to 200 meter layer intensifies this stratification, which the authors note can sharpen the acoustic transitions at the base of the sonic layer and alter how much sound leaks out of the surface duct into the deeper ocean.

Temperature, however, is only half of the story. The researchers also assembled surface pH observations from the World Ocean Database and from the RAMA/BOBOA mooring array, a long-running buoy stationed in the bay that has tracked the carbonate chemistry of the surface ocean. Their analysis reveals a declining tendency in surface pH in recent decades, mirroring the rise in atmospheric carbon dioxide. As carbon dioxide dissolves in seawater it forms carbonic acid, lowering pH in a process known as ocean acidification, and the Bay of Bengal appears to be tracking this global signal, with regional studies suggesting that atmospheric pollutant deposition may even accelerate acidification along its coasts.

Why would a chemist’s measure of acidity interest an acoustician? The answer lies in the peculiar chemistry of sound absorption at mid-to-high frequencies. Below roughly 10 kilohertz, the dominant absorbers of sound in seawater are not water molecules themselves but two dissolved solutes: boric acid, which governs absorption below about 1 kilohertz, and magnesium sulfate, which dominates above it. The boric acid relaxation that converts acoustic energy into heat depends critically on pH. As pH falls, the boron chemistry shifts, and the absorption coefficient drops, particularly at frequencies above 1 kilohertz. This counterintuitive consequence of acidification, first highlighted by researchers at the Monterey Bay Aquarium Research Institute more than a decade and a half ago, means that a more acidic ocean is literally a noisier one, because sounds that would once have been absorbed over tens of kilometers now carry farther.

Using standard sensitivity calculations based on the widely applied Francois-Garrison absorption model, which was derived from careful laboratory and field measurements of sound absorption in natural seawater, the team showed that the observed decrease in pH reduces the calculated sound-absorption coefficient in the bay, with the effect most pronounced above 1 kilohertz. They also found that salinity variability alone produces appreciable changes in absorption, a reminder that in a bay whose surface salinity swings dramatically with the monsoon and river discharge, even the salt content of the water can leave a measurable fingerprint on acoustic losses.

To translate these chemical and hydrographic changes into a concrete acoustic prediction, the researchers turned to BELLHOP, a well-established ray-tracing propagation model used throughout the underwater acoustics community. Running the model under contrasting pH conditions representative of the observed long-term trend, they found lower transmission loss under the lower-pH scenario, meaning that sound would persist over greater distances before being absorbed. Critically, the difference between the two scenarios grew with propagation range under the prescribed acoustic environment, because absorption is a cumulative process: small per-kilometer changes compound over tens and hundreds of kilometers into substantial end-to-end differences in signal strength.

The authors are careful to emphasize the limits of what they have shown. All of the acoustic results in the study are model-derived, and the team states plainly that quantitative validation will require direct field measurements of acoustic absorption and transmission loss in the bay itself. Model predictions of how acidification reshapes absorption are only as good as the environmental inputs and the underlying absorption formulations, and the Bay of Bengal’s complex, strongly stratified water column, laced with internal waves and monsoon-driven variability, presents a formidable challenge to any propagation model. Nonetheless, the direction of the effect aligns with the global literature, and the bay’s particular combination of warming, intensified stratification, and falling pH makes it a natural laboratory for studying the ocean-climate-acoustics nexus.

The implications extend well beyond the technical literature. Navies and ocean-monitoring agencies that calibrate sonar performance using historical acoustic conditions may find their predictions drifting as the water column changes beneath them. Climate models and acoustic forecasting systems may need to incorporate carbonate chemistry alongside temperature and salinity to remain accurate. And for the whales, dolphins, and other sound-dependent animals that navigate and forage in these waters, a modestly more transparent ocean could subtly alter the reach of both their own calls and the growing din of human activity. What this study makes clear is that climate change is not only warming the ocean’s surface and souring its chemistry; it is also, almost invisibly, changing the very medium through which the sea speaks.

Subject of Research: Long-term temperature and pH variability in the Bay of Bengal and its implications for underwater acoustic propagation.

Article Title: Long-term variability of temperature and pH in the Bay of Bengal and its potential implications for underwater acoustics

Article References: Long-term variability of temperature and pH in the Bay of Bengal and its potential implications for underwater acoustics. (n.d.). https://doi.org/10.1007/s10236-026-01851-2

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01851-2

Keywords: Bay of Bengal, ocean warming, ocean acidification, underwater acoustics, sound absorption, mixed layer depth, sonic layer depth, transmission loss, Indian Ocean, carbonate chemistry, EN4 dataset, BELLHOP model