10.5: Understanding Ocean Motions

Before taking a closer look at individual currents, it’s helpful to learn a little about the planetary, atmospheric, and oceanic factors that generate flows in the ocean. While this is a bit of an advanced topic—and one which students of physical oceanography learn about in great detail—a brief summary for the introductory students provides insights into the machinery that makes the ocean move.

Ekman Transport

Fridtjof Nansen was a Norwegian explorer and oceanographer. He was also a great humanitarian and helped half a million former prisoners of World War I repatriate in countries around the world. For this he was awarded the Nobel Peace Prize in 1922, the only oceanographer ever to receive a Nobel Prize. Nansen’s contributions remain relevant today. He invented the Nansen bottle (still in use for water sampling) and the Nansen closing net, a zooplankton net that opens and closes at depth (still sold today). He is also known for having built a round-bottom ship, the Fram, that could not be crushed if the ocean froze around it. It’s here where we pick up his story.

Nansen dreamed of being the first person to reach the North Pole. He had already achieved fame as the first person to cross the interior of Greenland on skis. But instead of crossing the frozen ocean on skis, he came up with the idea of letting a ship, the Fram, freeze into the sea ice, the movements of which would take him across the North Pole. It was a great idea, in theory. Alas, it was not to be. The drift of the ice and the motions of the underlying currents did not favor a journey over the pole. The Fram only made it to 84°4’N. Still determined to make the pole, Nansen set off on dogsled with the athletic Hjalmer Johansen (1867–1913), shipmate and fellow explorer. They made it as far as 86°13.6’N, the farthest north of any men at the time. Rugged ice and lack of food forced them to head west to Franz Josef Land. Conditions forced them to spend the winter in a small hut, where polar bears, walrus, and seals kept them fed. The following June, they were found and rescued by British explorer Frederick Jackson (1860–1938).

Among Nansen’s many valuable contributions to our knowledge of Arctic waters, one in particular changed our understanding of ocean circulation. While adrift in the Fram, he noted something curious about the interaction of the wind with the ice. The ice appeared to move to the right of the direction of the wind, which he hypothesized was due to Earth’s rotation (Nansen 1902, 369). That conclusion, however, would require mathematical proof.

He mentioned this observation to Norwegian meteorologist and oceanographer Vilhelm Bjerknes (1862–1951), who handed the problem to a Swedish graduate student, V. Walfrid Ekman (1874–1954). The son of a physical oceanographer and well versed in mathematics, Ekman had no trouble deriving the solution (Eliassen 1982; Jenkins and Bye 2006). The rotation of the Earth and the associated Coriolis force explained the drift of the ice to the right of the wind in the Northern Hemisphere. What’s more, the right-directed surface layer dragged the layer beneath it, causing it to move and also deviate to the right, as did the layer beneath that one, and so on. Ekman likened the effect to a spiral staircase. He published his results in 1902 (in Norwegian) and 1905 (in English), and the phenomenon came to be known as the Ekman spiral. Soon it became apparent that the Ekman spiral could explain other water motions as well. And, indeed, the motions of the ocean as driven by the wind-generated Ekman spiral underlie the surface circulation of the world ocean.

The Ekman spiral can be best envisioned as a spiral stack of books. In the Northern Hemisphere, as the wind blows over the surface layer of the ocean—the top book—it slowly moves forward and turns, adopting an angle of 20°–45° to the right of the wind. (In the Southern Hemisphere, the angle is to the left of the wind.) Due to friction, the surface layer drags the layer underneath it—the second book in the stack—and that layer begins to move and turn, adopting an even greater angle relative to the direction of the wind. It also moves more slowly than the surface layer, which moves more slowly than the wind, because the transfer of momentum is never 100 percent efficient. Each successive layer moves a bit more to the right and a bit more slowly. At some point, water layers may even move in the opposite direction of the wind. At the Ekman depth—defined as the depth where the current flow is 37 percent of the surface current flow and in a direction 180° opposite the direction of the wind—the Ekman spiral runs out of steam, so to speak, and the motion ceases. You’ve reached the bottom book.

If we take the average direction of all the water layers from the surface to the Ekman depth, we find that the average direction of water movement is 90° to the right of the wind. So the net transport of water, the net motion caused by a blowing wind, is 90° to the right (or left) of the wind (depending on the hemisphere). The 90° net movement of the ocean by the wind-driven Ekman spiral is called Ekman transport. By way of example, consider that a sustained 20 mph wind will generate a surface current of less than 0.05 mph, or about 236 feet per hour. Slow. That surface current moves in a direction that is 45° to the right of the wind. At depths of roughly 330–500 feet (100–150 m), the much, much slower current moves in the opposite direction—at an angle of 180° to the wind. If we take the average speed and direction of all the layers between the surface and 500 feet (150 m), we find that the net water movement is 90° to the direction of the wind.

Geostrophic Flow

The importance of Ekman transport becomes clear when you consider the effects of the global surface winds on the ocean. You may recall that trade winds blow generally east to west in a region about 20° north and south of the equator. At the same time, the westerlies blow west to east at middle latitudes (30° to 60°). If you consider the flows of water generated by Ekman transport (i.e., 90° to the right or left of the wind, depending on the hemisphere), you might predict that the trade winds will move water toward the poles and the westerlies will move water toward the equator. In theory, water should pile up in the center of the gyres.

Though humans can’t discern changes in the height of the sea surface, satellites can. Indeed, scientists using satellite altimeters observe hills of water—from 3 to 6 feet (0.9–1.8 m) high—in the central gyres. You might say that the ocean surface resembles the bumpy appearance of a pan de muerto (Mexican bread of the dead)—albeit one stretched over thousands of miles. The bumps represent changes in sea surface height, the changes in elevation of the sea surface. Measurements of sea surface height produce maps of sea surface topography—the shape of the sea surface.

As discussed in Chapter 4, satellite altimeters send out a pulse of microwave energy and measure quite precisely the time it takes for that pulse to return to the satellite. In principle satellite altimeters work just like sonar or the laser rangefinders used in golfing, construction, or the military. Knowing the speed of the microwave pulse and dividing the time of pulse and return by two (because the pulse goes down and back up, traveling twice the distance), computers aboard the satellite can calculate the height of the sea surface relative to the height of the satellite. These highly sophisticated instruments can actually detect variations in sea surface height on the order of a few millimeters, which is pretty impressive considering they’re being done in space. Satellite altimetry reveals a great deal about the circulation of the ocean, tides, climate change, El Niño and La Niña, and even the seafloor, because the ridges and trenches and seamounts create bumps and wiggles on the sea surface.

The increased elevation of the sea surface in the center of the gyres creates a pressure imbalance at the surface of the ocean. Just like water poured on a tabletop tends to spread out, the ocean surface tries to remain level. Pressure imbalances, or pressure gradients—differences in pressure between two locations—in the ocean cause currents, just like pressure gradients in the atmosphere cause winds. Where elevations exist, water begins to flow downhill. But the flow of water is subject to the Coriolis force, just like any current in the ocean. As a result, the water will turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Though initially water moves uphill by Ekman transport, it begins to flow back down as a result of the pressure gradient. And as it moves downhill, it is deflected toward the right or left. Eventually, the pressure gradient force and the Coriolis force come in to balance with each other. As a result, the balanced current flows at right angles to the directions of the pressure gradient and the Coriolis force and flows around the sea surface elevation. This balanced current is called a geostrophic current, one under the influence of Earth’s rotation.

The rotation of geostrophic ocean currents around the sea surface elevations in gyres is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The balance between the wind-driven pressure gradient (caused by Ekman transport) and the Coriolis force explains this pattern. Put another way, the major ocean currents are geostrophic. The geostrophic balance provides a satisfactory explanation for the broad surface circulation patterns that we observe in the world ocean.

Western Intensification

Now, there’s one more physical explanation required to explain a difference observed in the surface currents. Don’t worry, it’s a short explanation, and it doesn’t even require you to think.

Along the western boundaries of the gyres—that is, on the western sides of the ocean basins—the surface currents move much more quickly than currents along the eastern sides of the ocean basins. As mentioned earlier, western boundary currents are faster, narrower, and more energetic than eastern boundary currents, which are slower, wider, and less energetic. The difference is explained by something called western intensification, which is caused by latitudinal differences in the Coriolis force. That’s it. That’s the explanation. The Coriolis force, which governs the behavior of geostrophic currents, gets stronger from the equator to the poles. The result is a narrowing and intensification of western boundary currents.