Chapter 3 - The Physical Setting

3.7 Important Concepts

1. If the ocean was scaled down to a width of 8 inches it would have depths about the same as the thickness of a piece of paper. As a result, the velocity field in the ocean is nearly 2-dimensional. Vertical velocities are much smaller than horizontal velocities.



2. There is only one ocean with three official parts (basins).



3. The volume of ocean water exceeds the capacity of the ocean basins, and the ocean overflow on to the continents creating continental shelves.



4. The depths of the ocean are mapped by echo sounders which measure the time required for a sound pulse to travel from the surface to the bottom and back. Depths measured by ship-based echo sounders have been used to produce maps of the sea-floor. The maps have poor horizontal resolution in some regions because the regions were seldom visited by ships and ship tracks are far apart.



5. The depths of the ocean are also measured by satellite altimeter systems which profile the shape of the sea surface. The local shape of the surface is influenced by changes in gravity due to sub-sea features. Recent maps based on satellite altimeter measurements of the shape of the sea surface combined with ship data have depth accuracy of ±100 m and horizontal resolutions of ±3km.



6. Typical sound speed in the ocean is 1480 m/s. Speed depends primarily on temperature, less on pressure, and very little on salinity. The variability of sound speed as a function of pressure and temperature produces a horizontal sound channel in the ocean. Sound in the channel can travel great distances. Low-frequency sounds below 500 Hz can travel halfway around the world provided the path is not interrupted by land.

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