Furthermore, the acoustic propagation range is not “hundreds of thousands of kilometers”; given that Earth’s circumference is approximately 40,000\text{ km}, transmission spans across ocean basins range between several thousand and 15,000\text{ kilometers} (e.g., the 1991 Heard Island Feasibility Test).
The hydrodynamic and acoustic interactions between oceanic currents and the sound corridor occur via the following mechanisms:
1. Depth of the SOFAR Channel and Current Stratification
- Corridor Geometry: The central axis of the SOFAR channel (the c_{\min} line corresponding to the local minimum sound velocity) is situated 600 – 1200\text{ meters} below the surface in temperate and tropical latitudes. In polar regions, due to cold surface waters, this channel ascends directly to the ocean surface.
- Ambient Currents: This depth layer is not isolated from oceanic currents. On the contrary, within this stratum:
- Thermohaline Circulation (Deep Ocean Conveyor): Massive, slow water mass transfers driven by density differentials in temperature and salinity.
- Mesoscale Eddies: Rotating water vortices spanning 50 – 200\text{ km} in diameter that extend hundreds of meters down from the surface.
- Deep Western Boundary Currents: Deep-sea currents flowing along continental margins, attaining velocities of 0.1 – 0.5\text{ m/s}, traverse directly through the SOFAR corridor.
2. Physical Effects of Currents on the Sound Corridor
Oceanic currents directly influence acoustic wave propagation through three primary mechanisms:
A. Vector Velocity Addition (Effective Sound Speed)
As an acoustic wave propagates through a moving fluid, the ambient current velocity vector (\vec{v}) adds vectorially to the intrinsic speed of sound: \vec{c}_{\text{effective}} = \vec{c}_s + \vec{v}_{\text{current}}
In a scenario where the average speed of sound is 1500\text{ m/s} and the current velocity is 0.1 – 1\text{ m/s}, sound traveling in the direction of the current arrives earlier, whereas sound propagating against the current experiences latency.
This differential travel-time latency forms the foundation of Ocean Acoustic Tomography, formulated by physicist Walter Munk, wherein the velocity and direction of internal ocean currents are deduced by measuring the Time Difference of Arrival (TDOA) of acoustic signals.
B. Water Mass Transport (Advection) and Corridor Deflection
Currents horizontally transport water masses characterized by distinct temperature and salinity profiles:
- Warm-Core Eddies: The intrusion of warm water pockets elevates local sound velocity, depressing the SOFAR channel axis to deeper depths.
- Cold-Core Eddies: Cold water advection lowers sound speed, pulling the channel axis upward toward the surface.
- This phenomenon causes an acoustic wave traversing the corridor to deviate off-axis, alters refraction angles, and leads to energy leaking outside the waveguide into acoustic shadow zones (leakage/shadow zones).
C. Internal Waves and Acoustic Scattering
When currents traverse stratified layers of varying densities or strike submarine topography (bathymetric features/seamounts), they generate massive underwater internal waves. These waves disrupt the phase coherence of low-frequency whale vocalizations (< 1000\text{ Hz}), inducing acoustic scattering and amplitude fluctuations across the signal.
3. Utilization Strategy of Currents by Whales
Large cetacean species (e.g., Blue whales, Fin whales) generate infrasonic vocalizations within the 15 – 40\text{ Hz} band:
- Low-frequency signals undergo minimal viscous dissipation in water, enabling them to traverse transoceanic distances.
- Whales utilize ocean currents both to conserve metabolic energy along migratory corridors and by diving into stable regions where currents do not disrupt the SOFAR axis, thereby broadcasting acoustic signals across global scales.
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