Chapter 5: Ocean Currents

Planet Water

The Earth’s oceans make up nearly 71% of the Earth’s surface. There is a lot more water on Earth than there is earth on Earth. Perhaps we should rename planet Earth, planet Ocean. The shear size of the ocean means that the patterns of water movements and the variations in ocean temperature have a tremendous impact on global weather patterns and the Earth’s climate.

This chapter is primarily directed towards understanding what forces put the oceans in motion and how those motions affect life at a most basic level. The ocean’s primary production is intimately connected to the movement of currents, and impacts most living things in the ocean. As such, oceanographers and marine biologists are interested in the causes, effects, and patterns of ocean currents.

There are two major types of ocean circulation. The first is known as thermohaline circulation, the second as wind driven circulation. Thermohaline circulation primarily affects the deep waters of the ocean, whereas wind driven circulation moves waters at the surface.

Thermohaline Circulation

Thermohaline circulation is the flow of water induced by differences in temperature (thermo-) and salinity (haline). These differences in water properties leads to differences in densities. Bottom water currents form when sea water entering polar regions cools and freezes. This process occurs with the greatest intensity along the southern coast of Greenland and north of Antarctica Peninsula. At these locations, water is cooled, and seasonal ice is formed, creating cold, salty, and very dense water that sinks to the ocean bottom. Sinking surface water draws on surrounding waters and creates a convectional force that drives surface water flow in the North Atlantic. Once it sinks, the motion of these deep waters is often compared to a “oceanic conveyor belt,” moving cold waters along the ocean bottom. North Atlantic Deep Water, once formed off of Greenland, moves southward through the entire length of the Atlantic Ocean towards Antarctica. Here it mixes with the sinking, super dense, Antarctic Bottom Water and eventually enters the Indian and Pacific basins. This water is then up-welled and returned in a surface circulation. It can take a thousand years for water from the North Atlantic to find its way into the surface waters of the North Pacific. Click to watch a nice 3D animation of Thermohaline Circulation on YouTube.

The displacement of this cold water has a major impact on global temperature. Cold is transported away from the poles and moves it into the deep and eventually towards the equator. The resulting current helps to draw warmer waters from the mid-latitudes towards the poles. The overall effect is to moderate much of the worlds climate.

Wind Driven Currents

To understand the movement of water at the surface we have to first understand the movement of air. Individual molecules of nitrogen and oxygen gas that make up the air, respond to heating with movement. Warm air moves faster than cold. Warm air, tends to rise, cool air tends to sink. The rising of warm air creates pocket of low pressure, and the sinking cool air creates pockets of high pressure. High pressure compresses the air underneath it and forces air to move outward. The rotation of the Earth puts a spin onto this outward motion (clockwise in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere.) The circular motion of high pressure cells is often referred to as anti-cyclonic rotation. This outward moving air moves from high pressure areas to areas of low pressure. Low pressure cells tend to spin wind in a cyclonic rotation (counterclockwise in the Northern Hemisphere and clockwise in the Southern.) The folks at NOAA have put together a great explanation of how wind works and this is a great YouTube Explanation of wind.

As wind moves across the water, the collision of air molecules and water molecules transfers some energy from the air to the water. As a result of this energy transfer, water moves at about 3–4% of the speed that the wind is blowing. As surface currents in the ocean are formed by interactions between wind and water, they are greatly influenced by the earth’s rotation, and ocean basin geography. These interactions form fairly stable patterns, called currents, that you see in the map below. Click to see an animation of the Ocean’s Circulation Patterns on YouTube.

Sometimes currents can pinch off sections and create circular currents of water called an eddy. These spinning pockets of water are often important to marine life and eddies of significant size will actually be named by NOAA in the same way the weather service names storms.

Currents effect global climate

Surface currents play an enormous role in Earth’s climate. Western boundary currents, such as the Gulf Stream and Kuroshio Currents, are warm, deep, and fast moving surface currents that transport a lot of water and heat from the tropics to higher latitudes. The Kuroshio Current, for example, can travel between 25 and 75 miles a day, 1 – 3 miles per hour, and extends some 3,300 feet down into the ocean’s depths. As the warm waters of the Gulf Stream move across the North Atlantic towards the British Islands, it has a significantly moderating affect on the temperatures of of Northern Europe. The Gulf Stream’s warm waters raise temperatures in the North Sea, which in turn raises the air temperatures over land between 3 to 6°C (5 to 11°F). London, U.K., for example, is at about six degrees further south than Quebec, Canada. However, London’s average January temperature is 3.8°C (38°F), while Quebec’s is only -12°C (10°F). In contrast, cold water currents, such as the Labrador Current, draw cold away from the poles and bring it deep into the mid-latitudes, affecting the weather as far south as New England. Like warm surface currents, they are driven mainly by atmospheric forces and are influenced by the earth’s rotation. The California, Peru, Canary, Oyashio, and Benguela Currents are just a few examples of these cold water currents. If it were not for these warm and cold oceanic surface currents the tropics and polar regions of the Earth would have climates that were much more extreme than they are today. The Arctic would be significantly colder, and the tropics much warmer.

The Coriolis Force

Air at the equator is constantly heated which causes it to rise. As heated air in the upper atmosphere cools, it tends to sink. This creates large global loops called Hadley Cells. The rising and falling is also effected by the rotation of the Earth from West to East. The Earth is 40,000 km (24,900 miles) around at its widest part, the equator. Because it spins on its axis once in 24 hours, a point on the Earth’s equator is traveling about 1,700 km per hour (1,000 miles per hour) relative to its axis. However, the closer you get to the poles, the smaller the distance any one point takes in its rotation around the Earth’s access. Anchorage sits close to 60° north latitude. At that latitude the distance traveled around the Earth’s axis is only half the distance that it is at the equator. Because the Earth is rotating as one round mass, that means Anchorage moving only half as fast as Quito, Ecuador (which sits right on the equator.) Air moving from high latitudes towards the equator then tends to lag behind, and a person on the surface would feel a wind blowing out of the east towards the west. On the other hand, air moving north from the equator to high latitudes is deflected eastward. The deflection of air masses is called the Coriolis Force (YouTube). Water is similarly effected by the Coriolis Force.

Oceanic Gyres

Oceanic Gyres are large circular rotations of water created by the interaction of wind, the Coriolis Force, and the edges of the continents. For example the interactions between the Kuroshio Current to the west, the North Pacific Current to the north, the California Current to the east, and the North Equatorial Current to the south create a circular motion known as the North Pacific Gyre. The Pacific and the Atlantic both have north and south gyres. A fifth oceanic gyre is found in the Indian Ocean. Gyres will actually create small “hills” of water in the ocean by constantly pushing water in. Floating debris and trash often gets trapped in these Gyres. As we have dropped more and more plastic pollution into the oceans, more and more of it has accumulated in these gyres and incorporated into the food chain by seabirds and fish.

Ekman Transport

In the early part of the 20th century, a Norwegian scientist, Fridtjof Nanson, noted that icebergs in the North Atlantic moved to the right of the wind. This force driving these icebergs was described by the 19th-century French engineer-mathematician Gustave-Gaspard Coriolis.

Nanson’s student, Walfrid Ekman, demonstrated that the earth’s rotation caused this effect and in particular, that the Coriolis Force was responsible. One of the primary results of the Coriolis Force is that the net movement of water, forced by large-scale winds, are to the right of the wind in the Northern Hemisphere (and the left in the Southern Hemisphere). Ekman was able to show that these effects moved downwards in the water column due to friction. So called Ekman Transport, imagines the water column as a series of thin layers, each layer of water is moved by friction from the layer above it. Each layer moves slower than the layer above it, and shifted to the right, creating a spiral as much as 100 meters down.

Coastal Up-welling and Down-welling

As a result of Ekman Transport winds blowing along a coastline, there is a phenomenon known as up-welling or down-welling. The waters moved offshore by the wind are replaced by waters from the depths below. For example a wind blowing from the north along a western coastline will cause water to be pushed out to sea. To replace the water moving offshore waters are brought to the surface from the ocean bottom. These waters are normally very cold and rich in nutrients. Areas of coastal up-welling are typically areas of high productivity. Click to watch an animation of coastal up-welling.

Primary Production

It is difficult to separate a discussion of the currents, particularly up-welling currents, from a discussion of primary production. Currents move water, but they also move nutrients.