The study, published in the journal Discover Oceans, was led by C. M. Jimna Janardhanan of Cochin University of Science and Technology together with P. Anand and R. P. Raju of India’s Naval Physical and Oceanographic Laboratory, along with colleagues from the National Institute of Oceanography and Kerala University of Fisheries and Ocean Studies. Using two decades of temperature and salinity measurements spanning 2000 to 2020, drawn from the World Ocean Database 2018 and the international Argo float program, the team mapped where inversions form, how strong they become, and what they do to the sound speed structure of the upper ocean.

The North Indian Ocean is uniquely suited to producing these inversions. It is bounded to the west by the Arabian Sea and to the east by the Bay of Bengal, and its upper layers are transformed each year by the monsoon cycle. During the southwest monsoon from June to September, and again in the early post-monsoon period, enormous volumes of freshwater pour into the basin from monsoonal rainfall and from great river systems, chief among them the Ganges–Brahmaputra and the Irrawaddy. This freshwater dilutes the surface, creating intense salinity stratification: light, fresh water floats atop dense, salty water and resists mixing.

That stratification has a remarkable consequence. It isolates the shallow surface mixed layer from the deeper thermocline, forming what oceanographers call a barrier layer. When winter cooling strips heat from the surface, the isolated surface water cools faster than the water trapped just beneath it, and a warm layer becomes sandwiched between cool surface water and the cold thermocline below. This is the surface layer temperature inversion. The research team found the most dramatic examples in the northern Bay of Bengal, where the temperature difference across the inversion layer reaches about 5 degrees Celsius, and in the South Eastern Arabian Sea, where it approaches 3 degrees. Such strong inversions, the authors note, are unique to the North Indian Ocean; comparable shallow inversions observed in the Pacific, such as in the Oyashio–Kuroshio frontal zone, do not reach these gradients.

The seasonal evolution follows a clear rhythm. In October, localized inversions first appear in the Krishna–Godavari basin of the western Bay of Bengal, with temperature differences up to 2 degrees Celsius and inversion layer thicknesses of 40 to 50 meters. By November, inversions spread to the northeastern bay, hugging the coast. December brings organized, basin-scale inversions to both the northern Bay of Bengal and the South Eastern Arabian Sea, with temperature differences of roughly 5 and 3 degrees respectively and layer thicknesses of 20 to 80 meters. January sees inversions across nearly the entire bay, often farther offshore, before the pattern retreats and weakens through February and March.

What makes this study distinctive is its classification scheme. Rather than simply cataloging inversions, the researchers grouped ten representative stations across the basin into six inversion types according to the dominant physical mechanisms driving them. Four processes can create an inversion: net heat loss from the sea surface, low-salinity river water influx, advection of cold low-salinity water over warmer salty water, and penetrative solar radiation that deposits heat below the surface. In the northern and eastern Arabian Sea, surface heat loss alone does the work. In the northeastern Arabian Sea, heat loss combines with runoff from the Indus River. In the South Eastern Arabian Sea, inversions form when the East India Coastal Current carries cold, fresh water from the Bay of Bengal over the warm, saline local waters, aided by solar radiation. Along India’s east coast, heat loss, river runoff, and advection all conspire. In the head of the Bay of Bengal, the immense freshwater discharge of the Ganges–Brahmaputra and Irrawaddy systems dominates, producing an intensely stratified surface layer shallower than 10 meters.

The acoustic payoff comes from how these structures reshape the sound speed profile. Because sound speed in seawater increases with temperature, pressure, and salinity, a warm inversion layer can deepen the sonic layer depth, the depth to which sound speed increases near the surface and within which acoustic energy becomes trapped in a surface duct. Using the UNESCO equation of state for sound speed and a ray-based propagation model called cTraceo at a frequency of 3000 hertz, typical of anti-submarine warfare sonars, the team simulated transmission loss with and without inversions at each station, placing the sound source both inside the inversion layer at 10 meters depth and below it, over a sand-silt-clay seabed.

The results split cleanly along mechanistic lines. Where river water influx is not the dominant cause, inversions deepen the sonic layer and extend its reach. At stations representing inversion types I, III, IV, and VI, transmission loss remained below 80 decibels uniformly from the surface down to 50 meters out to ranges of 20 kilometers or more during inversion conditions, whereas in non-inversion conditions the same threshold was breached in patches beyond just 5 kilometers. In other words, the inversion turns the upper ocean into an efficient acoustic waveguide. Where freshwater dominates, however, the opposite occurs: intense haline stratification prevents the sonic layer from deepening, it shoals, and sound propagates less effectively. At the northeastern Arabian Sea station, the region with transmission loss below 60 decibels shrank from 10 kilometers in non-inversion conditions to 5 kilometers during inversion; at the freshwater-dominated Bay of Bengal stations, the well-illuminated depth shoaled from 60 meters to about 30 meters. When the acoustic source sits below the inversion layer, none of these changes matter much, because the duct lies out of reach.

The researchers also quantified the duct’s behavior through its cut-off frequency and limiting ray angle, parameters that determine which sound frequencies get trapped and which ray paths escape. Notably, the cut-off frequency decreased during inversions at stations near India’s east coast and Sri Lanka, where advection and heat loss dominate, broadening the range of frequencies that can duct. Because diffraction leakage from a 50-meter surface duct is negligible at 3000 hertz, the ray model’s conclusions are robust at operational sonar frequencies, though the authors caution that at lower frequencies, below the cut-off, ducting fails entirely and inversion-driven variability becomes irrelevant.

The implications reach beyond naval acoustics. Any system that relies on predictable underwater sound, from tsunami early warning networks to acoustic tomography of ocean heat content, must contend with the seasonal emergence and decay of these warm layers across the North Indian Ocean. The authors emphasize that their mechanism assignments rest on climatological data and previous process studies, and that profile-specific attribution would require concurrent observations and numerical simulations. Their proposed next step is an integrated ocean-acoustic modeling framework capable of capturing how inversions steer sound in three dimensions. For now, the message is clear: every winter, a hidden thermal architecture spreads across the northern Indian Ocean, quietly redrawing the map of where sound can and cannot travel, and anyone listening beneath the waves must read that map to be heard.

Subject of Research: Surface layer temperature inversions in the North Indian Ocean and their effects on underwater acoustic propagation

Article Title: Distribution of surface layer temperature inversion in the North Indian Ocean and associated acoustic propagation characteristics

Article References: Jimna Janardhanan, C. M., Anand, P., Raju, R. P., Sabu, A. K., Krishnan, A. R. A., Sajeev, R., & Thadathil, P. (2026). Distribution of surface layer temperature inversion in the North Indian Ocean and associated acoustic propagation characteristics. Discover Oceans, 3(1), Article 50. https://doi.org/10.1007/s44289-026-00163-z

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00163-z

Keywords: surface layer temperature inversion, North Indian Ocean, Bay of Bengal, Arabian Sea, underwater acoustics, sonic layer depth, surface duct, barrier layer, sonar propagation, monsoon freshwater, sound speed profile, ocean stratification

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