Chapter 3 - The Physical Setting

3.2 Dimensions of the Ocean

The ocean and adjacent seas cover 70.8% of the surface of Earth, which amounts to 361,254,000 km². The areas of the named parts vary considerably (Table 3.1) , and the Pacific is the largest.

Oceanic dimensions range from around 1500 km for the minimum width of the Atlantic to more than 13,000 km for the north-south extent of the Atlantic and the width of the Pacific. Typical depths are only 3-4 km. So horizontal dimensions of ocean basins are 1,000 times greater than the vertical dimension. A scale model of the Pacific, the size of an 8.5 in × 11 in sheet of paper, would have dimensions similar to the paper: a width of 10,000 km scales to 10 in, and a depth of 3 km scales to 0.003 in, the typical thickness of a piece of paper.

Because the ocean is so thin, cross-sectional plots of ocean basins must have a greatly exaggerated vertical scale to be useful. Typical plots have a vertical scale that is 200 times the horizontal scale (Figure 3.4). This exaggeration distorts our view of the ocean. The edges of the ocean basins, the continental slopes, are not steep cliffs as shown in the figure at 41°W and 12°E. Rather, they are gentle slopes dropping down 1 meter for every 20 meters in the horizontal.

Figure 3.4 Cross-Section of the South Atlantic along 25°S showing the continental shelf offshore of South America, a seamount near 35°W, the mid-Atlantic Ridge near 14°W, the Walvis Ridge near 6°E, and the narrow continential shelf off South America. Upper Vertical exaggeration of 180:1. Lower Vertical exaggeration of 30:1. If shown with true aspect ratio, the plot would be the thickness of the line at the sea surface in the lower plot.

The small ratio of depth to width of ocean basins is very important for understanding ocean currents. Vertical velocities must be much smaller than horizontal velocities. Even over distances of a few hundred kilometers, the vertical velocity must be less than 1% of the horizontal velocity. We will use this information later to simplify the equations of motion.

The relatively small vertical velocities have great influence on turbulence. Three dimensional turbulence is fundamentally different than two-dimensional turbulence. In two-dimensions, vortex lines must always be vertical, and there can be little vortex stretching. In three dimensions, vortex stretching plays a fundamental role in turbulence.

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