Sound speed c as a function of depth in various ocean regions.

Near the ocean surface, c decreases as a result of the temperature decrease with depth. At greater depth, temperature changes are small and c increases linearly with pressure (depth).

The effect of salinity on variations of c with depth are negligible, but salinity determines the overall magnitude of c: Salinity is low in the Pacific Ocean, higher in the Sargasso Sea (Atlantic Ocean) and highest in the Mediterranean Sea.

The Weddell Sea is an example of a polar region: Temperature is uniform over the entire depth range and extremely low, so c is also very low and shows only a vertical dependence on pressure (depth).

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