Exploring Our Fluid Earth

Winds Produce Currents

If winds blow constantly from the same direction on the ocean’s surface for long durations, ocean surface currents can be produced. Currents are similar to rivers of water moving in the ocean. Currents range in size from relatively small longshore currents near a beach, to currents that span ocean basins. Prevailing winds are examples of prolonged winds that produce large-scale ocean basin currents.

The simplified map in Fig. 3.13 shows the surface winds that flow from regions of high atmospheric pressure over the world’s oceans. These are winds that drive the system of surface currents in the ocean. Surface currents are only 50 to 100 meters deep (Table 3.1). Though shallow, they are extremely important in determining the world’s weather and climates, and in distributing the ocean’s heat and nutrients. Winds are described by the direction from which they blow, whereas water currents are described by the direction toward which they flow.

Fig. 3.13. Oceanic high-pressure centers and their simplified wind patterns. Individual surface currents are identified in Table 3.1.

Image by Byron Inouye

Fig. 3.14. Major surface currents of the world ocean. Individual surface currents are identified in Table 3.1.

Image by Byron Inouye

The Ekman Spiral

Ocean currents are produced by friction created by wind blowing over the water surface. However, the direction and speed of water currents do not match those of the wind currents above them. A 20 km/h eastward wind does not produce a 20 km/h eastward current. Ocean currents are much slower than winds due to friction. Wind-produced ocean currents move at an angle to the direction of the wind. The rotational motion of the earth influences ocean currents.

Fig. 3.15. The Ekman spiral describes the motion or “Ekman transport” of water influenced by wind and the Coriolis effect.

Image by Byron Inouye

Surface water flows at a 20–45° angle to the right of the wind in the Northern Hemisphere and 20–45° to the left of the wind in the Southern Hemisphere (Fig. 3.15). This deflection of water motion is due to the Coriolis effect from the earth’s rotation (Fig. 3.8).

The Coriolis effect influences the surface ocean as well as deeper ocean water layers, which are created by slight differences in temperature and salinity. Forces between water molecules and friction between water layers cause deeper layers of water to move when surface water moves. Each successively deeper layer of water deflects further to the right of the wind in the Northern Hemisphere and to the left of the wind in the Southern Hemisphere. As depth increases the speed of each layer decreases. As current moves down the water column, some water flows in a direction opposite to the surface current (Fig. 3.15). This current pattern is called the Ekman spiral. An Ekman spiral is the result of drag created from increasing ocean depth, when ocean water encounters wind at the surface (Fig. 3.15).

Averaging the movement of all of the layers of water affected by the Ekman spiral, water in a wind-driven current moves about 90° to the right of the wind in the Northern Hemisphere and 90° to the left of the wind in the Southern Hemisphere (Fig. 3.16). Water movement in surface currents is called Ekman transport. For example, if the wind blows from the south to the north, the current flows 90˚ to the right—directly east.

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Fig. 3.16. The average deflection of water in wind-driven currents is (A) 90° to the right in the Northern Hemisphere and (B) 90° to the left in the Southern Hemisphere.

Image copyright and source

Image by Byron Inouye

In the open ocean, turbulent mixing of surface water or surface waves often disrupt the Ekman spiral. In deep water, the Ekman spiral stops "working" at about 150 to 300 m depth. If the seafloor is shallower than this depth the net direction of water movement is not as deflected compared to the air current.

Gyres

Ocean surface currents tend to form ring-like circulation systems called gyres. A gyre is a circular ocean current formed by a combination of the prevailing winds, the rotation of the Earth, and landmasses. Continents interfere with the movement of both surface winds and currents. Gyres form in both the northern and southern hemispheres. However, to explain how a gyre is formed and operates, we will examine gyres of the Northern Hemisphere. The names of the currents shown in Fig. 3.14 are listed in Table 3.1 and will be referred to in the following discussion.

In the Northern Hemisphere near the equator, trade winds drive currents westward, forming a North Equatorial Current (NE), which moves at about 1 m/sec. At the western boundary of an ocean basin, the water turns and flows towards the North Pole, forming the western-ocean boundary currents. Western boundary currents are very strong. Two examples are the Gulf Stream (GS) that runs in the Atlantic ocean basin and the Kuroshio Current (K) in the Pacific ocean basin (Fig. 3.14). They are narrower, but deeper and swifter, than the other currents in the gyre. For example, speeds of 2 m/sec have been measured in the Gulf Stream. These currents, as deep as 1 km, generally remain in deeper water beyond the continental shelf. Western-ocean boundary currents carry warm water from the equator north.

Eventually, the western boundary currents fall under the influence of the westerly winds and begin flowing to the east, forming the North Atlantic Current (NA) and North Pacific Current (NP). When they approach the eastern-ocean boundaries of continents, they turn and flow south, forming the eastern-ocean boundary currents. Eastern-ocean boundary currents are shallower and slower than western-ocean boundary currents. They flow over the continental shelves, close to shore, carrying colder waters from the north to the south. Two examples are the California Current (Cal) in the Pacific ocean basin and the Canary Current (Can) in the Atlantic ocean basin.

The North Equatorial Current (NE) and the South Equatorial Current (SE) flow in the same direction. The SE turns south and behaves the opposite of the gyres in the Northern Hemisphere. Gyres in the Northern Hemisphere travel in clockwise directions while gyres in the Southern Hemisphere travel in counter-clockwise directions. It takes about 54 months for water to travel the circuit of the North Pacific gyre, while only 14 months in the North Atlantic gyre.

One major current, the equatorial Countercurrent (EC), appears to be an exception to the circulation pattern set up by the gyres. This countercurrent forms just north of the equator in the region between the north equatorial current and the south equatorial current and flows in the opposite direction.

Measuring Wind Currents

Large-scale ocean surface currents can be predictors of weather trends or of how marine life move around entire ocean basins (Fig. 3.14). However, smaller regional- or local-scale currents also occur that affect ocean travelers and marine life. Early sea voyagers relied on their knowledge of sea conditions, including large and smaller scale currents, to travel safely from port to port. Understanding currents is important for navigating traffic in harbors and shipping lanes. There are several methods that can be used to study the direction and speed of currents. Early mariners observed drifting objects and measured the distance they traveled over time to obtain speed. Modern methods also rely on this principle. Some of the common methods for measuring currents are shown in Table 3.2.

Table 3.2. Table of methods and devices used to measure currents.

Device	Description	Picture
Flow Meter	Flow meters are small, often handheld devices used to measure current flow. Water current spins a propeller as it moves past the meter. The amount the propeller spins can be correlated with current speed. Some meters can also report the direction of water flow. Flow meters are useful in smaller bodies of water.	Image Image courtesy of Hannes Grobe, Wikimedia Commons
Clod Cards	Clod cards are small blocks of plaster (or a similar type of material) used to measure relative flow rate between sites. As water current flows over the blocks, they dissolve. The faster the water flow, the more the clod cards dissolve. Clod cards are useful tools for measuring water flow near ocean bottom.	Image Image by Matthew Lurie
Shallow Water Drifter	Shallow water drifters float near the surface of the water and are pushed by the predominant surface current. The distance traveled, time, and direction of a drift can be measured by an observer or GPS device. The picture is a Davis drifter, which uses underwater sails moved by current flow.	Image Image courtesy of National Oceanographic and Atmospheric Administration (NOAA)
Deep Ocean Drifter	Deep ocean drifters flow with the current below the surface. They are programmed to descend to a predetermined depth for several days and then rise to the surface. While underwater, they record their position and then transmit information back to scientists when they surface. Deep ocean drifters are useful for long-term deployment in deep water as they can surface and sink through many cycles.	Image Image courtesy of National Oceanographic and Atmospheric Administration (NOAA)
Acoustic Doppler Current Profiler (ADCP)	An ADCP emits sound pulses underwater and then measures the frequency of the sound bouncing back off of the water particles. If the water particles are moving away from the ADCP, then the frequency will be longer. If the particles are moving toward the ADCP, then the frequency will be shorter. ADCPs are useful for measuring flow in bays. They can also be mounted on the bow of ships.	Image Image by Wusel007, Wikimedia Commons
Shore Based Current Meters	Shore-based current meters send out sound signals and measure the frequency of sound bouncing back. These instruments bounce sound off wind induced surface currents. Shore-based current meters are useful when measuring surface currents. When multiple meters are used at once, current velocity maps can be generated.	Image Image courtesy of National Oceanographic and Atmospheric Administration (NOAA)