So far we have been discussing the different kinds of waves that exist in and on the oceans of the world. Our story would not be complete, however, without some mention of the large, persistent currents that plow through the oceans. We know some of them by name—the Gulf Stream, for example. In this short chapter we’ll touch on the global system of currents that are maintained by a corresponding system of global winds. We’ll also recall why these currents are important for the earth’s climate.
Christopher Columbus is often credited with having discovered the so-called trade winds that blow across the northern Atlantic Ocean. In his first voyage (1492) he took advantage of the northeast wind that blows from Portugal to the Canary Islands. From there the northeast wind carried him west and south to the Bahamas in a mere 36 days. (Remember that winds are labeled by the direction from which they blow, not the direction to which they are headed.)
If Columbus had tried to return to Europe along the same track, against the wind, he would have faced months of arduous tacking and might have run out of food and water before making landfall. Instead, the canny (and lucky) captain sailed north to the latitude of the Azores (about 39 degrees), where he found a providential prevailing westerly wind that carried him home. In this way he discovered an antiparallel wind system across the North Atlantic basin. His voyage opened a century of exploration and trade.
He may not have recognized, however, that these trade winds drive a circular pattern of currents in the Atlantic. The south-flowing Canary Current is one segment of the pattern; the Gulf Stream is another part. The circle is closed by the North Equatorial Current that flows westward. These currents are massive rivers in the ocean.
Fig. 12.1 Major ocean currents of the world. (Wikipedia.)
As you can see in figure 12.1, every ocean basin contains such a circular pattern of currents. Some currents have familiar names: the Gulf Stream, the California Current, the Peru and Brazil Currents. Others, like the North and South Equatorial Currents, or the Antarctic Circumpolar Current, are less well known to the public, at least in North America.
The circular patterns of currents, called gyres, rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. These major currents are permanent features of the global ocean, and they are caused by prevailing global winds. In figure 12.2 we see how the global winds change direction from one latitude band to another. So, for example, the 0-to 30-degree bands in opposite hemispheres contain the northeast and southwest trade winds. Similarly the bands between 30 and 60 degrees latitude contain the prevailing westerlies in opposite hemispheres.
If you compare the two maps, you can see how, for example, the westerly wind in the North Atlantic drives the North Atlantic Drift and the northeast trade wind drives the North Equatorial Current. The North Atlantic gyre is completed by the Gulf Stream and by the Canary Current on the western coast of Africa. Gyres in the other ocean basins are generated by associated winds in the same way.
Fig. 12.2 Global wind directions vary in latitude. (Wikipedia.)
Each gyre is a slowly rotating mass of water that fills most of an ocean basin. A gyre is really a hill of water, whose center is higher than its circumference by a meter or so. Two forces combine to maintain the height difference. The Coriolis force acts on the circulating currents at the periphery of the gyre, driving water toward the center of the gyre. The water piles up at the center and tends to run downhill because of gravity. Over the long term, balance is maintained along each radius of the gyre between the Coriolis force and gravity. This steady state is called geostrophic flow.
Wind-driven currents flow at about one-hundredth of the speed of the wind, or 0.1–0.5m/s (0.25–1.0 knots). The volume of water a current can carry is really remarkable. The Gulf Stream, for example, transports about 55 million m3/s, or 55 “sverdrups” (a measure of volume transport in the oceans). This amounts to 500 times the flow of the Amazon River. Even this giant stream is dwarfed by the Antarctic Circumpolar Current.
These major currents redistribute the heat the oceans receive from the sun, an important factor in regulating the climates of the world. The Gulf Stream carries warm water from the tropics toward Europe, which would otherwise suffer the climate of Labrador. The Canary Current returns the cooled water down south again. Similarly, the Brazil Current transports warm water south, and the Peru Current carries cold water north.
The Sargasso Sea is a special sea, unique in having no coastline. It is bounded by the Gulf Stream, the North Atlantic Current, the Mid-Atlantic Ridge on the east, and the North Equatorial Current on the south. In ancient times sailors would bring home tales of ships being trapped there and being assaulted by horrific sea monsters. But the sea is actually benign and has rich populations of specialized plants and animals. Unfortunately, it has become a floating garbage dump, filled with all kinds of plastic trash that ships have spilled. (Another eyesore like this exists in the Pacific Ocean, the Great Pacific Garbage Patch.)
The Gulf Stream and the Kuroshiru Current are prime examples of a current on the western coast of an ocean basin. Such currents are stronger and faster than their mates on the eastern coasts of the basin, a phenomenon known as the western intensification of currents. In chapter 10 we saw how Rossby waves contribute to this effect.
The second oddity is the Antarctic Circumpolar Current. Sir Edmond Halley, the British astronomer of comet fame, discovered the current while surveying the Southern Ocean during the 1699–1700 expedition of the HMS Paramore. The Circumpolar Current is unique in having no continent to interrupt its headlong flow. It is driven ever eastward by the powerful Westerlies in the Southern Ocean. Wind speeds average between 15 and 24 knots (7.7–12.4m/s) depending on latitude, while the current flows at less than 20cm/s. Despite its slow speed the current transports as much as 150 million m3/s of water, larger than any other current. Its slow speed is compensated for by a huge cross section, which extends down to 4,000m and northward by as much as 2,000km. The current is nearly in geostrophic balance, meaning that the Coriolis force pushes water north into a small heap, while gravity pulls the water back south, as in a gyre.
The Circumpolar Current would continue to accelerate eastward under the force of the wind if physical mechanisms did not cause it to reach a dynamic equilibrium. The nature of these mechanisms is still a matter of debate. Around 1951, Walter Munk proposed that friction with deep underwater ridges would limit the current speed at the surface. Other scientists suggest that the Coriolis force creates northward meanders of the current, so that fast water is mixed with slower water. More complex schemes have also been proposed, involving the upwelling of deep water at the southern border of the current and downwelling at the northern border.
Satellites have recently revealed a new feature of the Antarctic Circumpolar Current, a circumpolar wave. The wave travels eastward at a slower speed than the average current and completes a circuit of the globe in 8–9 years. It has two crests and two troughs. The crests are associated with pools of water 2–3°C warmer than elsewhere in the Circumpolar Current, and the troughs are 2–3°C cooler. The pools can be thousands of kilometers long.
How these waves are generated is uncertain, but they probably influence the temperature of the overlying atmosphere. There are preliminary indications that the alternation of warm and cold pools correlates well with 4-to 5-year rainfall cycles found over areas of southern Australia and New Zealand. Some scientists believe that the Antarctic Circumpolar Wave may be more important than El Niño in governing rainfall over these regions.
Now how are these major currents generated by the winds? To explain the coupling of wind and water, oceanographers have introduced the concept of wind stress, the horizontal force (per unit of area) that the wind exerts on the water’s surface. It is a shearing force that tends to accelerate the surface of the water faster than the deeper layers.
The stress is found to increase with the square of the wind speed near the surface, as you might expect, and also depends on an observational constant. For the main purpose of describing the origin of a current, oceanographers avoid the messy details of how much the water surface is roughened by waves and bury all the details in the constant. In actuality, the so-called constant increases with wind speed, so the stress varies as the cube of the wind speed.
With this description of the driving forces and with observations of the seasonally changing wind pattern, it is possible to predict the observed pattern of global currents. In 1947, Harald Sverdrup (Scripps Institute) demonstrated mathematically how this could be done. He was drawn to the problem when he noticed that the Equatorial Counter Current in the Pacific flows against the prevailing winds (see fig. 12.1). The details are complex; suffice it to say that he helped to found the basic theory of ocean currents.
Of course, the major currents generate eddies on a smaller scale, so that even in the middle of a gyre, the flow patterns become quite complicated. The U.S. Ocean Prediction Center uses a sophisticated computer program, along with satellite and buoy observations, to generate daily maps of these smaller motions. They are freely available on the Web.
