Physical Oceanography: The Motion of the Ocean
By Alex Brylske
If you dive for very long in South Florida, you soon come to learn that there is no more welcomed friend than the Gulf Stream. Countless times, I've ridden seaward on dive boats while praying to the weather gods for two gifts: Let the seas be flat and let the Gulf Stream be near shore. As it passes through the Florida Straits and up the Gold Coast, the meandering of this famous River in the Sea, to a great extent, dictates local diving conditions. It determines the visibility, the temperature and even the marine life that we encounter. It also explains why, during summer, tropical species such as Angel and Butterfly fishes are often sighted as far north as Long Island. Clearly, East Coast divers have no better or more influential benefactor.
However, given the influence that the Gulf Stream has on our lives as divers, it has long puzzled me as an educator why divers aren't more curious about it. No one questions its existence, or even its power, but rarely am I asked the fundamental question: Why does it exist? I suppose that the unquestioning acceptance of the great current shows how much we accept that some phenomenon are just too big or too complex to comprehend. Indeed, much of the motion of the ocean does appear random, chaotic or certainly incomprehensible. But the reality is quite different. The apparently random movement that we witness, such as currents or waves, is belied by an underlying order; and discerning this order from the enormity and chaos of the ocean is the realm of
physical oceanographers. One of the four primary disciplines of oceanography, physical oceanography is the study of the ocean's currents, air-sea interactions, waves, tides and global water circulation. But covering all of this is a tall order for one article, so we'll confine our discussion to aspects that are particularly relevant to divers, wind-driven circulation and major ocean currents.
Rivers in the Sea
Surface currents are driven by what are termed primary and secondary forces. The primary force starts the water moving and determines its velocity, while secondary forces influence the direction and nature of flow. Primary forces include wind, thermal expansion/contraction of the water and differences in density between water layers. Secondary factors include gravity, friction, pressure gradients, the shape of the ocean basin in which the current circulates and the mysterious Coriolis effect.
Let's look first at how contraction and expansion of water due to heating and cooling drives sea currents. Solar heating can cause water to expand by about 4 percent, and while that may not sound like much, on a global scale this is very significant. One consequence of thermal heating and expansion is that, near the equator, sea level is about 3 inches (8 cm) higher than the sea level in the temperate zone. And the difference is even greater in the cold polar regions due to thermal contraction. The net result of this phenomenon is that the sea surface isn't really level but has a slight "slope," with warm equatorial water flowing downhill toward the poles. However, because of the slow speed and tremendous distance, the Earth's eastern rotation moves the water toward the western boundary of ocean basins, such as the eastern coast of North America. (Remember, the east coast of a continent is the western edge of an ocean basin.)
While this temperature difference between polar and tropical regions is a factor, it's not as important as the effect of wind. Most of Earth's surface wind energy is concentrated in bands of winds termed the Trade Winds and the Westerlies. (See Figure 1.) As wind blows across the sea surface, it transfers some of its energy to the water by friction. A general rule is that the friction of wind blowing for at least 10 hours will cause surface water to flow downwind at about 2 percent of the wind speed.
Because the water flow will eventually encounter a continent (except in the southern ocean where it completely circumnavigates Antarctica), the moving water piles up in the leeward ocean basin. This means that the weight of the water column will be higher on the "piled up" side of the basin, and gravity will act to pull the water back down the slope in the direction it came. But remember, rather than simply moving back and forth in response to the force of wind versus gravity, the Coriolis effect causes the flow to be deflected either to the right (in the Northern Hemisphere) or to the left (in the Southern Hemisphere). Continents and other topographic features on the sea bottom deflect the moving water into a circular pattern called a gyre (a Greek word meaning "circle").
All ocean basins contain gyres, but perhaps the most famous is the North Atlantic Gyre (depicted in Figure 2). Physical oceanographers divide the North Atlantic gyre into four separate but interconnected currents. Each has its own distinctive flow characteristics and temperatures. All ocean gyres are similar, but the North Atlantic provides a useful model. Notice how the east-west component of the North Atlantic Gyre tends to flow to the right of the driving winds (Westerlies in the North Atlantic and Trade Winds in the near the equator). When these currents encounter continents, the current turns clockwise, either north or south.
Hills in the Ocean
Certainly, as every seasick diver comes to realize, the sea is rarely flat. But the waves and swells aren't the only "hills" in the ocean. In fact, many are flabbergasted to learn that "sea level" is only a concept. When examined carefully, as is now possible through satellite imagery, the sea surface possesses a distinct, though transient, topography. It warps into expansive mounds and depressions like the hills and valleys you might encounter if you walked across a rolling pasture. The hills are caused by converging currents piling the sea surface upward, while the valleys are the result of diverging currents caused by the water moving apart.
While the height differences between the sea's hills and valleys rarely differ by more than a meter or so, they have a tremendous effect on the circulation of surface currents. The reason is because of the development of pressure gradients. These gradients arise as a consequence of the horizontal variation in the height of the water surface (the hills and valleys). Specifically, water piled up in a hill creates a zone of high pressure, due to an increase in the height of the water column. In response, water flows down the pressure gradient. The steeper the pressure gradient, the faster the water will flow, just as a ball will roll down a steeper slope faster than a gentle one.
As the gigantic gyre of the North Atlantic makes its way around its circuit, the converging flow forms a "hill" of water about 6.5 feet (2 m) high. This hill is centered in the Atlantic in the area of the Sargasso Sea, and is maintained by wind energy. The hill also explains, in part, the movement of a particle of water, labeled B in Figure 1. It cannot continue turning to the right because it would have to move uphill against gravity and the pressure gradient. But it also cannot move left because that would defy the Coriolis effect. The only choice is to continue westward and then clockwise around the entire Atlantic gyre. Imagine this happening to all of the particles within the gyre and one can begin to comprehend the dynamic balance between the downhill force of the pressure gradient and the uphill tendency of the Coriolis deflection.
This motion also holds the current along the edges of the ocean basin. While most oceanic gyres operate as explained, there is one notable exception. Oddly, the greatest current on Earth is technically not a gyre because it does not flow around an ocean basin. The West Wind Drift, or Antarctic Circumpolar Current, flows endlessly eastward around the entire Antarctic continent driven by unceasing and very powerful westerly winds. (See Figure 3.) This king of currents is never deflected by a continent, which explains the monstrous seas, often in excess of 30 feet (9 m), so common in the Southern Ocean.
Go West, Young Man
The gyres that give rise to the major surface currents of the world are classified as western boundary currents, eastern boundary currents or transverse currents. As anyone who has ever witnessed the awesome power of the Gulf Stream might guess, the fastest and deepest currents are found at the western boundary of ocean basins (the eastern boundary of continents). These are fast, narrow and deep, moving warm water from the equator toward the poles. In addition to the Gulf Stream, there are four other major western boundary currents including the Kuroshio current in the North Pacific, the Brazil current in the South Atlantic, the Agulhas current in the Indian Ocean and the East Australian current in the South Pacific. They're depicted in Figure 3.
Our own Gulf Stream is the largest of the western boundary currents, and one of the longest studied. In fact, it was none other than the patriot-scientist Benjamin Franklin who first attempted a detailed mapping of the famous current. This river in the sea moves past the south Florida coast, on average, at a rate of 5 miles (8 km) per hour all the way to the depth of more than 1,500 feet (450 m). The volume of water it transports is almost unfathomable (how's that for a pun?). Because of the incredible quantity of water moved by ocean currents, a special unit of measurement, the sverdrup (sv), was devised early in the last century by the famous oceanographer Harald Sverdrup. One sverdrup equals 3.5 million cubic feet (1 million cubic meters) of flow per second. The Gulf Stream flows at more than 55 sv, which is equal to 300 times the flow of the world's greatest river, the Amazon.
Western boundary currents can move incredible distances within relatively well-defined borders. Just ask divers off Florida's East Coast and you'll hear exactly how important and identifiable the "Stream," as it's known locally, is to making this region the most popular diving destination in the world.
But it would be a misconception to view western boundary currents as having perfectly well defined limits. Friction with the adjacent water interferes with the current flow, forming swirling countercurrents called eddies. These meandering loops can sometimes break off from the major current forming large circular loops termed cold core or warm core gyres (the temperature depends upon the direction of flow, clockwise or counterclockwise). In the North Atlantic, these eddies are widely distributed, and can be up to 620 miles (1,000 km) in diameter while retaining their characteristics for more than three years. In fact, scientists have learned recently that the influence of these eddies may reach all the way to the sea floor, and be the source of what oceanographers term "abyssal storms." Such storms would explain the mysterious but commonly observed ripple marks in deep sediments.
Just as there are five western boundary currents, there are also five eastern boundary currents. They include: the Canary Current in the North Atlantic, the Beneguela Current in the South Atlantic, the California Current in the North Pacific, the West Australian Current in the Indian Ocean and the Peru or Humbolt Current in the South Pacific, also illustrated in Figure 3. Eastern boundary currents are the mirror image of their western boundary cousins in every way. Rather than carrying warm water towards the poles, they carry cold water towards the equator. Furthermore, rather than deep and narrow, they are shallow and broad, sometimes as wide as 620 miles (1,000 km.). Unlike their western boundary counterparts, their limits are not well defined, and eddies tend not to form from their flow. Their flow rate is also significantly less. For example, the Canary Current carries only 16 sv of water at about 1.2 miles (2 km) per hour. And unlike the Gulf Stream, which is highly noticeable to any observant mariner, the Canary Current often goes unnoticed.
Why, then, are western boundary currents so intense while their counterparts to the east are so mild? The factors are numerous and complex, some of which are beyond the scope of this discussion. But one reason has to do, once again, with the wind. Note from Figure 1 that the north and south Trade Winds straddle the equator and their combined effect causes currents along the equator to converge. This concentrates and pushes water toward the west where it "piles up" at the western edge of the ocean basin before quickly returning poleward. Here's where the famous Coriolis effect intercedes.
As the current moves towards the poles, the Coriolis effect increases. (The Coriolis effect is greatest at the poles and nil on the equator.) In the North Atlantic, to compensate, the "hill" of water (depicted in Figure 4) becomes steeper on the western side, thereby increasing the pressure gradient. Thus, the increasing tendency of the Coriolis effect to turn the water to the right is balanced by the stronger pressure gradient on the western side of the hill, and the net result is a rapid northerly flow (or rapid southerly flow in the Southern Hemisphere).
Another reason for this intensification of the current is the rotation of the earth itself. This rotation displaces the hill of water from the center of the ocean basin, toward the west. Therefore, the water must squeeze closer to the western boundary to pass around the hill. The combined effect of these phenomena, as well as a few that are too complex for the scope of this article, result in what is termed "western intensification."
The exact opposite phenomenon occurs to the east. As you see from Figure 1, the westerlies in each hemisphere are widely separated, and do not converge, as do the trade winds. So, the water driven by the westerlies is not concentrated along a line of convergence as happens at the equator. Deflection from the Coriolis effect can also move some of the eastbound water toward the equator before reaching the eastern boundary of the basin, thus dissipating some of the intensity of the current flow (phenomena illustrated in Figure 2).
The Coriolis effect also influences water well below the surface. This occurs because water tends to flow in what can be imagined as "layers" (laminar flow). As depicted in Figure 5, the angular deflection of the Coriolis effect is initially quite small but each "layer" of water is affected by the one above it. The result is a spiraling effect oceanographers term the "Ekman Spiral," named for the Swedish oceanographer who first explained the mathematics of this process. Note that the water does not spin like a whirlpool; instead, each "layer" of water moves the one immediately below it, but only slightly, toward the right. At a given depth, called the friction depth, dependent upon the force of the wind, the Coriolis effect and frictional forces balance out to zero. Amazingly, because of this phenomenon, the water flow just above the friction depth actually moves in a direction opposite that of the surface current.
The net motion of the water column down to the friction depth (usually about 330 feet (100 m ), called Ekman Transport, is the amount of water affected by surface winds; and it behaves as an entity separate from the rest of the water column below it. The direction of the net transport (average of all the speeds and directions of the Ekman Spiral) is 90 degrees to the right (in the Northern Hemisphere) or left (in the Southern Hemisphere) of the wind direction. The phenomenon is depicted in Figure 5. These great gyres, which exist due to the balance between gravity and the Coriolis effect, are called geostrophic (earth turning) gyres. Six oceanic gyres are responsible for six of the great current circuits of the world ocean. Two occur in the Northern Hemisphere and four in the Southern. They are shown in Figure 3. While the currents we observe are an obvious form of water movement, they only involve about 10 percent of the ocean, a surface layer extending down to about 1,300 feet (400 m) . Below this depth, wind has little or no effect. Deep currents are driven primarily by pressure gradients formed by differences in the density of sea water.
East Side, West Side
As we've seen, most of the power of the world's ocean currents is derived from the trade winds, which form near the margins of the tropics, and the mid-latitude westerlies. These constant global features are responsible for what are termed "transverse currents", currents that flow in an east-west direction and link the eastern and western boundary currents. In the equatorial regions of the Atlantic and Pacific, the wind-driven Equatorial current is relatively shallow and broad, transporting about 30-sv westward. As indicated, the trade winds, combined with the convergent flow at the equator, result in a piling of water along the western boundary of ocean basins. For example, on the Atlantic side of Panama (the western boundary of the Atlantic basin) the water level is normally about 8 inches (20 cm) higher than across the isthmus on the Pacific side (eastern boundary of the Pacific basin). Moreover, because of the Pacific's much vaster expanse, the difference is even greater. In fact, in the western Pacific the height differential between there and the Pacific's eastern basin edge is about 3.3 feet (1 m).
In the middle latitudes, the westerlies drive the eastward moving transverse currents. And, as explained, because there is no convergent flow as there is near the equator, eastern flowing currents tend to be wider and flow more slowly. Finally, in the Northern Hemisphere, transverse currents eventually encounter continents and are diverted. But not so in the Southern Ocean. As the current flow is unencumbered by any land masses, the intense westerly winds blowing over the Southern Ocean drive the Antarctic Circumpolar Current, or the West Wind Drift, that we discussed previously. Carrying more water than any other current on earth, this granddaddy of all ocean currents transports more than 100 sv, twice the flow of the Gulf Stream, through the Drake Passage between the tip of South America and the Palmer Peninsula of Antarctica.
Under and Over
An interesting phenomenon related to equatorial currents is that countercurrents often accompany them. As the name implies, these are currents that flow in the opposite direction from the other adjacent current. This happens near the equator, in part, because of the "doldrums", the region just north of the equator where the trade winds cease. Without the winds to continually drive the current westward, a backflow develops sending the piled-up water in the western basin eastward.
An even more intriguing phenomenon was discovered in 1956, involving another form of countercurrent in the tropical Pacific. This current actually flows beneath the surface currents. The phenomenon is termed an undercurrent, and the one in the tropical Pacific is called the Cromwell Current. (It was named in honor of its discoverer, Townsend Cromwell, who discovered it while investigating long line fishing techniques.) The Cromwell Current is quite considerable, flowing at an average velocity of 3 miles (5 km) per hour at a depth of 330-660 feet (100-200 m). It carries about half the volume of the Gulf Stream, and has been tracked for more than 8,700 miles (14,000 km), all the way from New Guinea to Ecuador. Since its discovery, undercurrents have been found underneath most major currents.
Undercurrents can significantly affect local land masses. For example, the ocean around the Galapagos Islands is relatively cold, which may sound strange given that these islands straddle the equator. It was initially believed this cold water came from Antarctica by way of the Humbolt Current. But later research proved that the Humbolt turns west long before reaching the Galapagos region. Instead, the cold water surrounding the Galapagos is a result of upwelling from the Cromwell Current as it nears its eastern end. As it upwells to the surface, the cool water moves westward as part of the South Equatorial Current, and directly in the path of the islands made so famous by Charles Darwin. Exactly what causes undercurrents is not clear.
Baby It's NotCold Outside
It's the world's ocean currents that distribute tropical heat. Without their moderating function, the extremes between the tropics and polar regions would be even more pronounced, and the resulting storms would be on a cataclysmic scale. The amount of heat that's transferred is greatest at mid-latitudes, where about 10 million billion calories, that's 10,000,000,000,000,000, of heat are transferred each second! That's more than one million times the power consumed by the world's human population. No better example of the effect of ocean currents can be seen than the climate of the United Kingdom. Given its moderate weather, it's easy to forget that it's at the same latitude as Canada's iceberg-infested province of Labrador. The moderate maritime climate of the UK owes its existence to the Gulf Stream's amazing ability to disperse heat from the tropics throughout the north Atlantic.
With all the emphasis on the lateral movement of water, it's easy to forget that some wind-driven currents flow vertically. This movement is termed upwelling or downwelling, depending upon the direction of flow. Special circumstances contribute to upwelling in equatorial regions far offshore. This explains why there are regions of high biological productivity in the tropical Pacific, even though tropical waters are typically noted for their inability to support life (something we'll discuss in a future installment on biological oceanography). But, particularly to divers, the most important implication of upwelling and downwelling results when it occurs near the shore.
Coastal upwelling is a result of wind. It can occur when wind blows parallel to the shore or offshore. (The mechanism for this is discussed in the previous section.) First, friction caused by wind blowing across the ocean surface creates a current. Next, the Coriolis effect deflects the current to either the right or left (depending upon the hemisphere) and Ekman Transport moves the water mass offshore. This is depicted in figure 6. Upwelling occurs when the surface water is replaced by deeper offshore water, and has significant implications. The replacement water, which often contains high levels of nutrients, can result in significant increases in biological productivity, the sea comes alive. On a large scale, this explains the highly productive fishing grounds found off places like the west coast of South America. On a more local scale, upwelling can have drastic effects. Off the coast of Southern California, for example, the offshore Santa Ana winds create an upwelling that often brings cold but clear nutrient-rich water to the coast, creating some excellent diving conditions.
By contrast, when water is driven toward a coastline it's forced downward, and flows back seaward along the bottom. This is downwelling, and is one mechanism that helps supply the deep ocean with nutrients and other essential components. But unlike upwelling, downwelling has little effect on productivity, although it is an essential long-term process.
Currents are fascinating phenomena, but to divers, understanding, or not understanding, them can have profound practical consequences. In a sense, they're the sea's version of weather. And just as no prudent diver would consider ignoring the weather, ignoring or not understanding currents is good way to ruin your day.
What exactly is this Coriolis effect that we hear so much about? Named for Gaspar Gustave de Coriolis, the Frenchman who worked out the mathematics, the effect explains the motion of an object in a rotating system. As it relates to the earth's movement, geophysics, it explains the lateral displacement of a north-south moving object (clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere). If you want to see the effect in action, and you have a globe of the Earth, try this: Spin the globe in an eastward direction (like the way the Earth turns). As it's spinning, take an erasable marker and attempt to draw a straight line from the North Pole to the equator. (It's easiest if you use the globe's support bar containing the latitude scale as a straight edge). Note that even though the line you drew was straight, the mark on the globe curves clockwise. If you repeat the exercise starting from the South Pole, the line deviates to the left (counterclockwise). The same thing happens to water, air or any object as it moves over the earth.