How Do Surface Currents Develop

Winds Produce Currents

If winds blow constantly from the same direction on the ocean's surface for long durations, ocean surface currents tin can exist produced. Currents are like to rivers of water moving in the ocean. Currents range in size from relatively pocket-size longshore currents nigh a beach, to currents that span ocean basins. Prevailing winds are examples of prolonged winds that produce large-scale body of water bowl currents.

The simplified map in Fig. three.xiii shows the surface winds that menstruation from regions of high atmospheric pressure over the globe's oceans. These are winds that drive the system of surface currents in the ocean. Surface currents are just fifty to 100 meters deep (Table 3.1). Though shallow, they are extremely important in determining the globe's weather condition and climates, and in distributing the ocean'south 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. three.13. Oceanic high-pressure centers and their simplified wind patterns. Individual surface currents are identified in Tabular array three.1. Fig. 3.14. Major surface currents of the world bounding main. Individual surface currents are identified in Table 3.one.

**Table 3.1.** Key to the major surface currents shown in Fig. 3.13
Abbreviation	Proper name of Current	Abbreviation	Name of Current
Ag	Agulhas Current	K	Kuroshio Current
Al	Alaska Current	Fifty	Labrador Current
Exist	Benguela Current	N	Norwegian Current
Br	Brazil Current	NA	North Atlantic Current
Cal	California Current	NE	N Equatorial Current
Tin can	Canary Current	NP	N Pacific Electric current
EA	East Australian Current	O	Oyashio Current
EC	Equatorial Countercurrent	P	Republic of peru (Humbolt) Electric current
EG	East Greenland Current	SE	South Equatorial Current
F	Florida Electric current	SP	South Pacific Electric current
G	Guinea Current	WA	W Australian Current
GS	Gulf Streem	ACC	Antarctic Circumpolar Current

The Ekman Spiral

Body of water currents are produced by friction created past wind bravado over the water surface. Even so, the direction and speed of water currents exercise not match those of the wind currents higher up them. A 20 km/h eastward wind does not produce a twenty km/h eastward current. Ocean currents are much slower than winds due to friction. Wind-produced sea currents motion at an bending to the direction of the wind. The rotational motion of the earth influences ocean currents.

Fig. 3.15. The Ekman spiral describes the move or
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Image past Byron Inouye

Surface h2o flows at a 20–45° angle to the right of the air current in the Northern Hemisphere and 20–45° to the left of the air current in the Southern Hemisphere (Fig. three.fifteen). This deflection of water motility is due to the Coriolis effect from the world'due south rotation (Fig. 3.8).

The Coriolis outcome influences the surface bounding main too as deeper ocean water layers, which are created by slight differences in temperature and salinity. Forces between h2o molecules and friction between water layers cause deeper layers of h2o to motion when surface water moves. Each successively deeper layer of h2o deflects further to the right of the wind in the Northern Hemisphere and to the left of the air current in the Southern Hemisphere. As depth increases the speed of each layer decreases. As current moves down the water cavalcade, some water flows in a direction opposite to the surface electric current (Fig. 3.xv). This current pattern is called the Ekman screw. An Ekman spiral is the effect of drag created from increasing ocean depth, when ocean water encounters wind at the surface (Fig. 3.xv).

Averaging the motility of all of the layers of water affected past the Ekman screw, water in a wind-driven current moves about ninety° to the right of the wind in the Northern Hemisphere and 90° to the left of the wind in the Southern Hemisphere (Fig. iii.16). Water move in surface currents is called Ekman transport. For example, if the wind blows from the south to the north, the electric current flows 90˚ to the correct—directly eastward.

Fig. 3.sixteen. The average deflection of h2o in wind-driven currents is (A) 90° to the correct in the Northern Hemisphere and (B) 90° to the left in the Southern Hemisphere.

In the open ocean, turbulent mixing of surface h2o or surface waves often disrupt the Ekman spiral. In deep water, the Ekman spiral stops "working" at virtually 150 to 300 k depth. If the seafloor is shallower than this depth the internet direction of water movement is non as deflected compared to the air current.

Gyres

Ocean surface currents tend to form ring-similar circulation systems called gyres. A roll 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. Nevertheless, to explicate how a ringlet 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, merchandise winds drive currents westward, forming a Due 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 N Pole, forming the western-ocean boundary currents. Western purlieus currents are very strong. Ii examples are the Gulf Stream (GS) that runs in the Atlantic body of water basin and the Kuroshio Current (G) in the Pacific ocean basin (Fig. three.xiv). They are narrower, simply deeper and swifter, than the other currents in the coil. For example, speeds of ii chiliad/sec have been measured in the Gulf Stream. These currents, every bit deep as 1 km, more often than not remain in deeper water beyond the continental shelf. Western-ocean purlieus currents bear warm h2o from the equator north.

Eventually, the western boundary currents fall nether the influence of the westerly winds and brainstorm flowing to the east, forming the North Atlantic Current (NA) and Due north Pacific Current (NP). When they arroyo the eastern-bounding main boundaries of continents, they turn and flow due south, forming the eastern-body of water boundary currents. Eastern-sea boundary currents are shallower and slower than western-sea boundary currents. They flow over the continental shelves, close to shore, carrying colder waters from the northward to the southward. Two examples are the California Current (Cal) in the Pacific body of water basin and the Canary Current (Can) in the Atlantic sea bowl.

The North Equatorial Current (NE) and the Southward Equatorial Current (SE) menstruum in the same direction. The SE turns south and behaves the reverse 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. Information technology takes about 54 months for h2o to travel the circuit of the Due north Pacific gyre, while only fourteen months in the North Atlantic coil.

One major current, the equatorial Countercurrent (EC), appears to be an exception to the circulation blueprint set upward by the gyres. This countercurrent forms simply northward of the equator in the region between the due north equatorial current and the south equatorial electric current and flows in the opposite direction.

Measuring Air current Currents

Large-calibration body of water surface currents can be predictors of conditions trends or of how marine life move around unabridged ocean basins (Fig. 3.14). However, smaller regional- or local-scale currents likewise occur that impact ocean travelers and marine life. Early ocean voyagers relied on their knowledge of sea weather, including large and smaller scale currents, to travel safely from port to port. Understanding currents is of import 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 on mariners observed drifting objects and measured the distance they traveled over time to obtain speed. Modernistic methods also rely on this principle. Some of the common methods for measuring currents are shown in Table 3.2.

**Table 3.ii.** Table of methods and devices used to mensurate 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 exist correlated with current speed. Some meters tin can besides study the direction of water flow. Flow meters are useful in smaller bodies of water.
Clod Cards	Clod cards are minor blocks of plaster (or a similar type of textile) used to mensurate relative menses rate between sites. Equally 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.
Shallow Water Drifter	Shallow water drifters float near the surface of the water and are pushed past the predominant surface current. The distance traveled, fourth dimension, and management of a drift tin can exist measured by an observer or GPS device. The movie is a Davis out-of-stater, which uses underwater sails moved by current flow.
Deep Ocean Drifter	Deep bounding main 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 so transmit data back to scientists when they surface. Deep body of water drifters are useful for long-term deployment in deep water as they can surface and sink through many cycles.
Audio-visual Doppler Current Profiler (ADCP)	An ADCP emits sound pulses underwater then measures the frequency of the sound bouncing back off of the water particles. If the water particles are moving abroad from the ADCP, and so the frequency will be longer. If the particles are moving toward the ADCP, so the frequency will be shorter. ADCPs are useful for measuring flow in trophy. They can also be mounted on the bow of ships.
Shore Based Current Meters	Shore-based current meters transport out sound signals and measure the frequency of sound bouncing back. These instruments bounciness audio off current of air induced surface currents. Shore-based electric current meters are useful when measuring surface currents. When multiple meters are used at once, current velocity maps can be generated.

Activity

Activity: Current Observation Methods

Explore unlike ways of measuring water currents and empathise the effects of currents on stationary objects.

Activity

Activity: Build a Drifter

Drifters are used to measure current speed below the surface of the water. Speed tin be calculated by measuring the time it takes a out-of-stater to travel a known distance. Build a drifter to measure water flow at a embankment, river, lake, or manmade channel.