Hidden Warm Layers in the Indian Ocean Reshape How Sound Travels Underwater

Beneath the winter surface of the North Indian Ocean lies a strange thermal sandwich: a layer of warm water resting on top of colder water below, the exact opposite of what oceanographers normally expect. New research shows that these surface layer temperature inversions, or SLTIs, are not just curiosities of physics. They fundamentally bend and channel underwater sound, with direct consequences for sonar performance, submarine detection, and ocean acoustic communication across one of the world’s most strategically important bodies of water.

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