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 clicks are not the rapid echolocation signals sperm whales use to locate squid in darkness. Nor are they the short, patterned sequences known as codas, which play an important role in the species’ social communication. Instead, these signals are low-frequency clicks produced at repetition rates of approximately 0.1 to 0.3 hertz, meaning that the interval between clicks can last from roughly three to ten seconds. Despite the long pauses, male whales maintain the timing with remarkable consistency.

An international research team analysed slow clicks recorded in waters around the Seychelles, Sri Lanka, Norway and Scotland. Their findings, published in Annals of the New York Academy of Sciences, suggest that these sounds represent a distinct form of long-range communication. The study examined both the physical properties of the clicks and the environmental conditions that determine how far they can travel. The researchers found that the combination of extreme source level, low frequency, long range and precise rhythm is what makes the signals especially unusual.

The biological purpose of the clicks remains uncertain. Because the signals appear to be produced primarily by males, one possibility is that they function as an acoustic advertisement. A large male sperm whale has a much larger sound-producing organ than a smaller male, and the size of that organ can influence the frequency and power of the sound. In this scenario, a deep, intense click could provide information about the caller’s body size, potentially attracting females or discouraging rival males. The sound might effectively announce the presence of a powerful animal across a vast area of ocean.

Sperm whales possess the largest known sound-producing organ of any animal. This enormous nasal complex can weigh more than five tonnes in a fully grown male and may account for about one-third of the whale’s body length. Inside it are structures known as phonic lips, which work somewhat like a biological valve. When the whale forces air through the nasal system, the phonic lips are driven together, generating acoustic pulses. The whale’s surrounding tissues and air sacs then help shape and amplify the resulting sound.

The most powerful measurements came from a male sperm whale recorded off the Seychelles. Researchers lowered a hydrophone, an underwater microphone, approximately four metres below the surface from the research vessel R/V Odyssey. The whale was within about 200 metres of the instrument, and the team used distance measurements to estimate the sound level at a standard range of one metre. From a 30-minute recording containing 65 slow clicks, the strongest signal reached 226 decibels referenced to 1 micropascal at one metre.

That figure cannot be directly compared with decibel levels measured in air. Underwater acoustics uses a reference pressure of 1 micropascal, while airborne sound is normally referenced to 20 micropascals. Water also transmits sound differently from air because it is denser and has different acoustic properties. Nevertheless, within the scale used by marine scientists, a source level above 200 decibels is extraordinarily powerful—comparable in some respects to intense artificial sources such as powerful sonar systems and seismic surveying equipment.

The estimated 70-kilometre active space is also highly dependent on ocean conditions. Temperature and salinity determine how sound bends through the water, while depth and the shape of the seabed influence reflections and transmission paths. Background noise is another major factor. Commercial shipping, construction and other human activities can mask biological sounds, reducing the distance over which whales can detect one another. A signal capable of travelling dozens of kilometres in a quiet deep-ocean environment may become detectable over only a few kilometres in a noisy area.

The rhythm of the slow clicks raises another intriguing question: how do the whales keep time when each beat may be separated by several seconds? The researchers speculate that sperm whales could use echoes from the seafloor or other environmental features as an acoustic timing reference. A powerful click sent downward might return after a predictable delay, depending on the depth of the seabed and the speed of sound in the water. The whale could potentially use that returning echo as an environmental stopwatch, although there is currently no evidence proving that this is how the rhythm is maintained.

The next step will be to determine how other sperm whales respond. Researchers hope to use playback experiments involving underwater loudspeakers and sound-recording tags attached to whales with suction cups. They could test whether males answer the clicks, approach or avoid the source, and whether females react differently. Changes in frequency, intensity or repetition rate may reveal what information the signals carry. For now, the evidence supports the existence of a powerful and highly rhythmic communication system—but not a whale “language.” The slow clicks may be advertisements, territorial warnings or signals with an entirely different function, offering scientists a new way to study how sound connects animals across the immense spaces of the deep sea.

Subject of Research:
Male sperm whale slow-click communication and its acoustic properties.

Article Title:
Extreme Rhythm Keeping in Long-Range Slow Click Communication of Sperm Whales

News Publication Date:
18-Jun-2026

Web References:
https://doi.org/10.1111/nyas.70289

References:
Annals of the New York Academy of Sciences, “Extreme Rhythm Keeping in Long-Range Slow Click Communication of Sperm Whales.” DOI: 10.1111/nyas.70289

Image Credits:
Simone Videsen, Aarhus University

Keywords:
Sperm whales, whale communication, slow clicks, underwater acoustics, marine biology, bioacoustics, echolocation, whale behavior, ocean noise, animal communication