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in the arctic. The deep ocean waveguide typically traps sound that propagates within a ±15° cone around the horizontal. In shallow-water cases, say tens or hundreds of meters water depth, the waveguide is established pri- marily by the surface and seafloor boundaries (Figure 7c). Last, another aspect of waveguide propagation is that as the range increases so do the number of possible acoustic paths that connect the source and receiver. The
  Figure 6. An example of the depth and time structure of temperature fluctuations over a month in the Philippine Sea. The temperature fluctuations are from the lifting and falling of density surfaces (black lines), primarily caused by internal waves with periods from a few minutes to a little over a day.
 Figure 7. Examples of sound speed profiles (left) and acoustic paths (right) for midlatitude (a), Arctic (b), and shallow- water (c) environments. Green stars on the sound speed profiles indicate the source depth. See text for details.
    entire ocean volume and ride on the more gentle, stable water-column density gradient.
Figure 6 shows an example of internal waves in the Philip- pine Sea that compare well with the GM model (Colosi et al., 2019). Internal waves are seen to change the tempera- ture (and therefore the sound speed) of water parcels as they rise and fall vertically with the waves. Internal waves following the GM spectrum are found nearly everywhere in the world’s oceans, although anomalous places with departures from the GM spectrum include regions of abrupt topography, the Arctic, and the Mediterranean Sea.
One other critical factor is that the propagation occurs in a relatively strong waveguide. In ocean waveguides, sound does not travel in straight lines but along curved trajectories that oscillate around the horizontal as the wave moves down range (see Figure 7). In the deep ocean basins (average water depth of roughly 4,400 m), a vol- umetric waveguide is formed by decreasing the sound speed from the surface as the water temperatures drops and increasing the sound speed with depth in the isother- mal abyssal ocean as pressure increases. In midlatitudes (Figure 7a), the sound speed minimum (sound-channel axis) is near 1,000 m depth.
As the latitude increases (Figure 7b), the minimum moves to shallower depths until it is near the surface
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