Salt, Sound, and Strategy
Why is the Ocean Water Salty ? - YouTube
How the Three-Dimensional Distribution of Ocean Salinity Affects Sonar Propagation, Range Prediction, and ASW
By Stephen — April 2026
Bottom Line Up Front
Ocean salinity—its spatial distribution across latitude, depth, and proximity to geological and hydrological features—is a critical but underappreciated variable in naval acoustic propagation modeling. While temperature and pressure have long dominated sound velocity profile calculations, salinity is the decisive variable precisely where it matters most: at the axis of the deep sound channel, where temperature is nearly constant and pressure is linear, leaving salinity as the only factor that can shift the channel geometry on which SOSUS, its successors, and long-range low-frequency sonar systems depend. Salinity gradients produced by differential evaporation, freshwater inflows, hydrothermal venting, water mass intrusions, and climate-driven changes to thermohaline circulation are reshaping the undersea acoustic environment in operationally significant ways. As climate change weakens the Atlantic Meridional Overturning Circulation, melts Arctic sea ice, and alters salinity stratification worldwide, the acoustic assumptions underpinning U.S. and allied antisubmarine warfare doctrine are eroding. China's massive oceanographic survey campaign—42 research vessels and hundreds of seabed sensors deployed across the Pacific, Indian, and Arctic oceans—demonstrates that peer competitors already treat three-dimensional salinity mapping as a strategic imperative. The U.S. Navy must invest in persistent, real-time salinity observation through expanded glider and float networks, integrate dynamic salinity data into tactical decision aids, and treat the changing ocean as a contested environment whose acoustic properties can no longer be assumed from historical climatology.The Invisible Variable
Every naval officer who has completed the undersea warfare qualification pipeline learns the foundational equation: the speed of sound in seawater is a function of temperature, salinity, and pressure. The MacKenzie equation, still widely used since its publication in 1981, renders this relationship with empirical precision:
C = 1448.96 + 4.591T − 0.05304T² + 0.0002374T³ + 0.0160Z + (1.340 − 0.01025T)(S − 35),
where
C is sound speed in meters per second,
T is
temperature in degrees Celsius,
S is salinity in practical salinity
units, and
Z is depth in meters.
The approximate sensitivities are well established: a 1°C rise in temperature increases sound speed by roughly 4.5 m/s; a 100-meter increase in depth adds about 1.7 m/s; and a 1 PSU increase in salinity contributes approximately 1.3 m/s.
That hierarchy of influence—temperature first, depth second, salinity a distant third—has led to a decades-long practice of treating salinity as operationally negligible across most of the open ocean. The expendable bathythermograph, or XBT, measures only temperature as a function of depth and remains the primary tool aboard U.S. Navy surface combatants and patrol aircraft for constructing the sound velocity profile. Salinity is typically supplied from historical climatological databases rather than real-time measurement. In the deep open ocean, where salinity varies by only 1 to 2 PSU across the entire water column, this simplification is often defensible.
But the open ocean is not where the hardest acoustic problems live. In the littorals, near river mouths, under melting polar ice, in semi-enclosed seas with intense evaporation, and along ocean fronts where water masses of radically different provenance collide, salinity gradients create acoustic environments that temperature and pressure alone cannot predict. The halocline—the layer of rapid salinity change with depth—can refract, trap, or scatter sound energy in ways that degrade sonar performance unpredictably. In these environments, the assumption that salinity is a static background parameter is not merely imprecise. It is tactically dangerous.
Where Salt Comes From—and Why Its Distribution Is Not Uniform
The ocean's average salinity of approximately 35 parts per thousand—roughly 35 grams of dissolved solids per liter—is the product of geological processes operating over billions of years. Weakly acidic rain dissolves minerals from continental rock and delivers them to the sea via rivers. Hydrothermal vents along mid-ocean ridges cycle the entire ocean volume through superheated rock approximately once every 10 million years, adding some dissolved minerals while stripping others. Submarine volcanism, with an estimated one million underwater volcanoes on the ocean floor, contributes chloride and other volatiles directly. In all three cases, evaporation removes water molecules but leaves dissolved ions behind. The ocean accumulates salt because it has no drain.
What matters for naval acoustics, however, is not the global average but the three-dimensional distribution. That distribution is governed by the interplay of evaporation, precipitation, river discharge, ice formation and melt, and thermohaline circulation—and it varies dramatically with geography.
In the subtropical gyres, where solar heating drives intense evaporation and precipitation is scant, surface salinities reach 36 to 37 PSU. In equatorial regions, heavy rainfall dilutes the surface layer. At high latitudes, the melting of sea ice injects large volumes of nearly fresh water into the upper ocean, creating sharp haloclines. Major river systems—the Amazon, the Congo, the Ganges-Brahmaputra, the Mississippi—generate freshwater plumes that extend hundreds of kilometers offshore and create salinity gradients as steep as several PSU over short horizontal distances. The Mediterranean Sea, a semi-enclosed basin where evaporation vastly exceeds precipitation and river input, sustains salinities of 38 to 39 PSU in its eastern basin—and exports dense, salty water through the Strait of Gibraltar that can be detected as an intrusion layer at approximately 1,000 meters depth across much of the eastern Atlantic.
None of these features are acoustically neutral.
Salinity and the Sound Velocity Profile
The sound velocity profile—the SVP—is the fundamental tool of tactical acoustic prediction. From it, a sonar operator or acoustic prediction algorithm determines whether sound rays will refract upward or downward, whether surface ducts or deep sound channels exist, and whether convergence zones will form at tactically useful ranges. The shape of the SVP determines whether a submarine can hide in a shadow zone, whether a towed array will achieve long-range detection, and whether a sonobuoy pattern will produce results or merely burn expendable stores.
In most open-ocean scenarios, the SVP is dominated by the vertical temperature structure: warm surface water yields high sound speeds near the surface; the thermocline produces a negative gradient that refracts sound downward; and below the thermocline, the pressure effect dominates, driving sound speed upward again with depth. The minimum sound speed at the base of the thermocline defines the axis of the deep sound channel, or SOFAR channel, where low-frequency sound can propagate for thousands of kilometers with minimal loss.
Salinity perturbs this standard structure in several operationally significant ways. Near freshwater inflows, the low-salinity surface layer depresses sound speed in the upper water column, altering the depth and character of the surface duct. Research conducted in estuarine environments has demonstrated that a halocline gradient as modest as 0.25 PSU per meter can significantly modify high-frequency sonar propagation patterns, creating anomalous refraction that changes predicted detection ranges. In the littoral waters where rivers meet the sea, the combination of salinity stratification, shallow bathymetry, and variable bottom composition produces what one standard Navy reference calls "a complicated and unpredictable set of sound paths." This is the acoustic environment of the world's most contested waterways—the South China Sea, the Persian Gulf, the Baltic, the waters around Taiwan—and it is precisely where adversary submarines are most likely to operate.
The Deep Sound Channel: Where Salinity Becomes the Decisive Variable
If there is a single domain where the conventional hierarchy of acoustic influence—temperature first, salinity last—breaks down most consequentially, it is the deep sound channel. The SOFAR channel, discovered independently by Maurice Ewing and J. Lamar Worzel at Columbia University and by Leonid Brekhovskikh at the Lebedev Physics Institute in the 1940s, is the horizontal layer of minimum sound speed that acts as an acoustic waveguide, trapping low-frequency sound energy and propagating it across ocean basins with minimal loss. The original demonstration was dramatic: a one-pound TNT charge detonated near the Bahamas was detected 2,000 miles away near West Africa. The Navy immediately recognized the implications, and by 1950 had launched SOSUS—the Sound Surveillance System—a chain of bottom-mounted hydrophone arrays positioned at the deep sound channel axis, designed to detect Soviet submarine transits at ranges of hundreds of miles.
The channel axis lies at the depth where the temperature-driven decline in sound speed is exactly balanced by the pressure-driven increase. In mid-latitudes, this typically occurs between 700 and 1,200 meters. But here is the critical point for this analysis: at those depths, the ocean is well below the thermocline. Temperature is nearly constant—varying by fractions of a degree over hundreds of meters of depth. Pressure is a linear function of depth and entirely predictable. What remains as the variable that can shift the axis depth, alter the minimum sound speed value, and change the width and refractive geometry of the channel is salinity. In the deep isothermal layer, salinity is no longer the distant third factor in the sound speed equation. It is, functionally, the only variable factor.
This matters operationally because even small perturbations in the channel axis depth or minimum sound speed translate directly into changes in the geometry of SOFAR propagation. The channel axis varies geographically from approximately 750 meters in the North Pacific to 1,200 meters near Bermuda, and it shoals to the surface at high latitudes above 60° where it disappears entirely. A 1980 Naval Ocean Systems Center study of a great circle acoustic path from Perth, Australia to Bermuda documented how the channel axis shifted from 1,200 meters at both endpoints to a shallow 200-meter secondary channel near the Antarctic Convergence, where the interaction of cold, fresh Antarctic water with warmer, saltier subantarctic water created radically different stratification. At every transition point along that path, salinity gradients were the primary driver of the channel geometry change.
For SOSUS and its successor Integrated Undersea Surveillance System, these variations are not academic. The arrays were positioned on continental slopes and seamounts at the axis of the deep sound channel and oriented perpendicular to expected threat axes. Their sensitivity was extraordinary—capable of detecting acoustic power of less than a single watt at ranges of hundreds of kilometers. But that performance depended on sound energy arriving through the channel with predictable propagation characteristics. When salinity anomalies shift the channel axis, alter the critical depth, or create secondary channels that split the acoustic energy, the arrival-time structure that SOSUS processing relied upon—the characteristic SOFAR signature building to its sharply defined finale—changes in ways that degrade localization accuracy.
The same physics shapes the acoustic world of the ocean's other great users of the deep sound channel: baleen whales. Payne and Webb proposed in 1971 that fin and blue whales exploit SOFAR channel propagation for long-distance communication, broadcasting at frequencies of 15 to 25 Hz that couple efficiently into the channel and can be detected by conspecifics thousands of kilometers away. Subsequent research has confirmed whale detections at ranges exceeding 1,000 kilometers. A 2022 study by Affatati and colleagues modeled how climate-driven changes in sound speed profiles would affect North Atlantic right whale vocalizations and found that future warming and freshening of the North Atlantic would create a new near-surface acoustic duct, altering propagation paths and potentially increasing background noise levels that mask whale calls. The whales and SOSUS face the same fundamental problem: any change to the salinity structure at channel-axis depths changes the acoustic waveguide that both depend on.
The Mediterranean Outflow Water provides a concrete example of how a salinity anomaly can restructure the deep sound channel across an entire ocean basin. This dense, salty water mass—38 to 39 PSU, far above the Atlantic average—exits through the Strait of Gibraltar and spreads as an intrusion layer at approximately 1,000 meters across the eastern Atlantic. Where this intrusion intersects the SOFAR channel axis, it creates a local sound speed maximum that splits the channel, generating a secondary channel above and the primary channel below. The acoustic effect is to divide what should be a single waveguide into two, redistributing energy between them and altering convergence zone ranges for any sonar system operating in the region. A SOSUS array calibrated for a single-channel SOFAR profile in the western Atlantic would see a fundamentally different arrival structure in the eastern Atlantic where the Mediterranean outflow intrudes—and the position and intensity of that intrusion varies with the strength of Mediterranean evaporation and Gibraltar exchange, both of which are changing under climate forcing.
The Mediterranean Exception—and What It Teaches
The Mediterranean Sea has long served as a natural laboratory for understanding how salinity shapes acoustic propagation. With surface salinities consistently above 38 PSU in the eastern basin, the Mediterranean's sound velocity profile differs fundamentally from that of the open Atlantic. The elevated salinity raises sound speed throughout the water column, effectively lifting the deep sound channel axis to shallower depths. This, in turn, permits convergence zone propagation in a basin that would otherwise be too shallow to support it. Norman Friedman's research on Cold War ASW notes that in the Mediterranean, summer evaporation raises salinity sufficiently to enable convergence zones that are not available in winter—a seasonal acoustic effect driven almost entirely by salinity rather than temperature.
Research conducted by the Naval Postgraduate School using World Ocean Database and Generalized Digital Environmental Model data has documented significant inter-annual variability in Mediterranean transmission loss. Convergence zone paths proved relatively stable, but surface duct, bottom bounce, and deep sound channel propagation showed marked variability—driven in substantial part by salinity changes associated with the warming and salinification trend that has been accelerating since the 1990s. The Mediterranean is warming at approximately 0.06°C per year and salinifying at roughly 0.012 PSU per year in the intermediate water layers. These are not hypothetical future changes. They are measured facts that are altering the acoustic battlespace today.
The Mediterranean also illustrates the importance of salinity-driven water mass intrusions. Mediterranean Outflow Water, dense and salty, exits through the Strait of Gibraltar and spreads across the eastern Atlantic at mid-depths. Where this intrusion creates a sound speed maximum, it generates secondary sound channels and alters the geometry of the primary deep sound channel. A sonar operator working the eastern Atlantic who ignores the Mediterranean outflow layer is working with an incomplete picture of the acoustic environment.
The Arctic: Where Salinity Becomes Primary
The Arctic Ocean inverts the standard acoustic hierarchy. Surface temperatures are near freezing year-round, producing little thermal contrast. Instead, the dominant oceanographic structure is the cold halocline—a layer of cold, relatively fresh water that caps the warmer, saltier Atlantic water intruding from below. In the Arctic, salinity is the primary driver of density stratification and, consequently, of the sound velocity profile.
The Arctic's SVP is unique: sound speed is lowest at the surface, where both temperature and salinity are at their minimum, and increases with depth as salinity and pressure rise. This produces upward-refracting conditions that trap sound energy near the surface and at the ice-water interface. Under thick, ridged ice, this half-channel propagation can extend detection ranges considerably—but it also means that ice noise, generated by the mechanical stress of pack ice, is efficiently propagated and can mask contacts.
Climate change is transforming this environment with consequences that are only beginning to be understood operationally. The Office of Naval Research's Canada Basin Acoustic Propagation Experiment (CANAPE) deployed oceanographic and acoustic sensors to measure how melting sea ice alters the underwater acoustic environment. The findings confirmed what physical oceanography predicts: the influx of fresh meltwater sharpens salinity gradients and makes sound velocity profiles more unpredictable. Increased open water generates wave-driven ambient noise that degrades passive sonar performance. The net effect is an Arctic acoustic environment that is simultaneously more accessible to surface and subsurface operations and more difficult to predict or exploit with confidence.
NATO has recognized this operational reality. In June 2024, the NATO Centre for Maritime Research and Experimentation deployed the research vessel NRV Alliance to the Arctic specifically to characterize the changing sonar environment. Portugal's first-ever under-ice submarine patrol, Operation ARCTIC 2024 aboard the SSK NRP Arpão, confirmed that meltwater-driven salinity changes had made sonar optimization significantly more difficult. The Portuguese Navy reported that the addition of fresh water from melting ice had rendered salinity gradients and sound velocity profiles substantially less predictable for sonar operators.
Climate Change as an Acoustic Threat
The operational implications of climate-driven changes to ocean salinity extend well beyond the Arctic. A 2023 study by researchers at the Netherlands Organization for Applied Scientific Research (TNO) and Utrecht University used coupled climate and acoustic propagation models to project how climate change would alter sound propagation in the North Atlantic through the end of this century. The results were striking. Under both moderate and high-emission scenarios, the AMOC—the Atlantic Meridional Overturning Circulation, the great density-driven conveyor belt that transports warm, salty water northward—was projected to weaken by 34 to 45 percent.
As the AMOC weakens, reduced delivery of salty subtropical water to the subpolar North Atlantic freshens the upper ocean and cools it. This produces a decrease in near-surface sound speed of up to 40 m/s in the upper 125 meters, creating a sub-surface acoustic duct that does not exist under present conditions. In that new duct, shipping noise and other low-frequency sounds would propagate over distances exceeding 500 kilometers with markedly reduced transmission loss. The study found a strong correlation between AMOC slowdown and increased sound pressure levels in the upper 200 meters, with a projected increase of up to 7 dB at the end of the century.
For naval operations, the implications are profound. A 7 dB increase in ambient noise at operationally relevant depths degrades passive sonar signal-to-noise ratios for every platform in the North Atlantic. A new sub-surface duct that guides low-frequency energy over hundreds of kilometers could reveal submarines that historically operated in the acoustic shadow below the thermocline. Convergence zone ranges would shift. The entire tactical geometry of ASW in the GIUK gap and Norwegian Sea—the choke points that define NATO's undersea defense of the North Atlantic—would change. As the researchers noted, naval forces that rely heavily on acoustic sensors in the North Atlantic and Norwegian Sea would see their performance "severely affected."
The Geological Dimension: Black Smokers and Submarine Volcanism
Salinity variation is not only a surface and upper-ocean phenomenon. Along the approximately 65,000 kilometers of mid-ocean ridge that wind through every major ocean basin, hydrothermal vent systems inject superheated, mineral-laden fluid into the deep ocean. Black smoker vents discharge fluid at temperatures exceeding 400°C, laden with dissolved metals and sulfides. When this plume meets ambient seawater near 2°C, the resulting precipitation of minerals and the sharp thermal and chemical gradients create localized anomalies in water properties.
Researchers at the University of Washington demonstrated in 2006 that black smoker vents produce measurable acoustic energy—broadband noise with narrowband tonal components indicative of resonance within the chimney structures. The Cabled Observatory Vent Imaging Sonar, deployed on the Juan de Fuca Ridge, has used acoustic backscatter to map plume velocity and distribution, confirming that hydrothermal activity creates localized perturbations in both the ambient noise field and the physical properties that determine sound speed.
For most tactical sonar applications, individual vent fields are too small and too deep to affect operational propagation paths. But the cumulative effect of hydrothermal circulation is oceanographically significant: the entire ocean volume cycles through these systems roughly once every 10 million years, and the dissolved material they add and subtract shapes the baseline salinity and chemical composition of deep water masses. More operationally relevant, the plumes from particularly vigorous vent fields can rise hundreds of meters above the seafloor, creating anomalous water mass signatures that affect the deep sound channel. Submarine volcanic eruptions produce more dramatic effects: the 1952 eruption of Myōjin-shō south of Japan was detected on SOFAR hydrophones in California, demonstrating that submarine geological events generate acoustic signatures that propagate across ocean basins.
China's Oceanographic Campaign: Salinity as Strategic Intelligence
If there remained any doubt that the three-dimensional distribution of ocean salinity constitutes actionable military intelligence, China's behavior has resolved it. A March 2026 Reuters investigation, corroborated by ship-tracking data spanning more than five years, revealed that at least 42 Chinese research vessels have been conducting systematic oceanographic surveys across the Pacific, Indian, and Arctic oceans. The campaign is focused on waters of direct military relevance: the seas near Taiwan and Guam, approaches to the Malacca Strait, and the Western Pacific operating areas where U.S. submarine forces would contest Chinese naval power.
The Dong Fang Hong 3, operated by Ocean University of China, exemplifies the pattern. Between 2024 and 2025, it conducted repeated survey missions in three distinct zones, sailing in the tight grid patterns characteristic of seabed mapping. In October 2024, it inspected Chinese seabed sensors near Japan capable of detecting underwater objects and returned to the same location in May 2025 for what analysts assess was recalibration or data retrieval. In March 2025, it executed dense survey tracks between Sri Lanka and Indonesia.
The data being collected includes seabed terrain, water temperature, salinity, and current patterns—precisely the variables required for acoustic propagation modeling. Rear Admiral Mike Brookes, commander of the U.S. Office of Naval Intelligence, testified to a congressional commission that the survey data "enables submarine navigation, concealment, and positioning of seabed sensors or weapons" and that China's collection effort represents "a strategic concern."
China's "transparent ocean" initiative, proposed by oceanographer Wu Lixin in 2014, aims to deploy networks of sensors, buoys, and subsea monitoring systems across key maritime corridors. Hundreds of sensors have reportedly been positioned east of Japan, near the Philippines, and around Guam. The goal is persistent environmental awareness—including real-time salinity profiles—across the underwater battlespace. Peter Scott, a former chief of Australia's submarine force, told Reuters that the movement of sound waves and submarine operations are directly affected by water temperature, salinity, and currents. Jennifer Parker, a former Australian ASW officer, assessed that the scale of China's effort indicates intent to develop "an expeditionary blue-water naval capability that also is built around submarine operations."
The U.S. and Allied Response: Gliders, Floats, and the Data Gap
The United States is not without oceanographic collection capability, but the scale and persistence of its effort have historically been uneven. The Naval Oceanographic Office (NAVOCEANO) operates six Pathfinder-class survey ships and a growing fleet of unmanned underwater vehicles, buoyancy gliders, and drifters. NAVOCEANO's Littoral Battlespace Sensing gliders measure temperature and salinity profiles in real time and have been contributed to NOAA's hurricane forecasting mission since 2018, demonstrating the dual-use value of persistent ocean observation. During the 2025 Atlantic hurricane season alone, 79 glider missions across six partner institutions delivered over 90,000 temperature and salinity profiles.
The international Argo program provides the broadest sustained ocean observation capability, with approximately 3,800 free-drifting profiling floats measuring temperature and salinity in the upper 2,000 meters on a 10-day cycle. Argo data flow through the U.S. Navy's Fleet Numerical Meteorology and Oceanography Center and directly support acoustic propagation modeling. The program has been expanded with Deep Argo floats capable of profiling to 6,000 meters and Biogeochemical Argo floats carrying additional sensors. Argo data are critical for maintaining the Hybrid Coordinate Ocean Model (HYCOM) and other numerical ocean prediction systems that feed tactical decision aids.
The Royal Navy, recognizing the growing importance of persistent ocean data, signed a contract in early 2026 with Teledyne Marine for ocean gliders and profiling floats capable of diving to 1,000 meters. The systems collect temperature, salinity, current, and acoustic-relevant environmental data in direct support of ASW operations. The Royal Navy's information warfare meteorology and oceanography specialists interpret this data to provide tactical environmental advice to commanders, enabling friendly forces to exploit local conditions for improved sensor performance while degrading adversary detection capability.
Yet gaps remain. XBTs, which measure only temperature, are still the primary real-time environmental sampling tool aboard most surface combatants. Historical salinity databases, while useful for climatological planning, cannot capture the mesoscale variability—ocean fronts, eddies, freshwater plumes, seasonal halocline changes—that drives tactical acoustic uncertainty. The Anti-Submarine Warfare Tactical Decision Aid and its successors ingest temperature profiles and derive salinity from historical water mass data, adjusting for temperature inversions to maintain a stable water column. This approach was adequate when the ocean's thermohaline structure was relatively stable. In an era of accelerating climate change, where the AMOC may weaken substantially, where Arctic haloclines are freshening unpredictably, and where Mediterranean salinification is altering intermediate water masses, historical climatology is an increasingly unreliable proxy for the real ocean.
Recommendations
The Navy should take four steps to close the salinity data gap and restore confidence in acoustic predictions.
First, accelerate the deployment of persistent, autonomous salinity-sensing platforms in operationally critical areas. Ocean gliders and profiling floats that measure conductivity, temperature, and depth should be deployed at sufficient density to capture mesoscale variability in the Western Pacific, the North Atlantic, the Arctic, and key littoral chokepoints. NAVOCEANO's goal of 100 simultaneously deployed gliders should be a floor, not a ceiling.
Second, integrate real-time salinity data into tactical acoustic prediction systems at the platform level. The next generation of ASW tactical decision aids must ingest CTD-quality salinity profiles, not just XBT temperature casts supplemented by historical databases. Sound velocity profilers that derive salinity from time-of-flight acoustic measurements—as described in the TEOS-10 compliant equations published by Allen and colleagues in 2025—offer a path to faster, more accurate environmental characterization without the calibration burdens of conductivity sensors.
Third, fund sustained research into the acoustic effects of climate-driven salinity change. The correlation between AMOC weakening and the emergence of new acoustic ducts in the North Atlantic, documented by TNO and Utrecht University, demands operational validation through at-sea acoustic experiments. The Navy should establish long-duration acoustic monitoring stations at multiple latitudes in the North Atlantic, analogous to the RAPID-MOCHA array that monitors AMOC transport at 26.5°N, to track acoustic propagation changes in real time.
Fourth, treat the oceanographic environment as a contested domain. China's transparent ocean initiative demonstrates that comprehensive environmental knowledge is being weaponized for undersea advantage. The U.S. Navy should expand counter-oceanographic intelligence efforts, monitor and characterize adversary environmental sensing networks, and develop capabilities to deny or degrade adversary access to real-time ocean data in contested waters.
Conclusion
Salt enters the ocean through rain dissolving continental rock, through hydrothermal vents cycling seawater through the superheated mantle, and through submarine volcanic outgassing. It stays because evaporation purifies the water that leaves but abandons the ions that arrived. This geochemical reality, operating over four billion years, has produced an ocean whose dissolved salt content varies in three dimensions with latitude, depth, season, proximity to rivers and ice, and the slow pulse of thermohaline circulation.
For the undersea warrior, this variation is not a curiosity of physical oceanography. It is an operational variable that shapes the speed of sound, the geometry of acoustic propagation, the range at which a submarine can be detected, and the depth at which it can hide. Ignoring salinity in the open ocean was once a defensible approximation. In the littorals, under the ice, along ocean fronts, and in a climate-disrupted North Atlantic, it is an approximation that adversaries are already prepared to exploit. The Navy that knows its salt will own the sound channel. The Navy that does not will search in silence.
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