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Surface Currents Surface Currents Chapter 11 Pages 11-3 to 11-25

Surface Currents Understanding what causes currents and where they flow is fundamental to all marine sciences. It helps explain how heat, sediments, nutrients, and organisms move within the seas. Surface Currents Chapter 11 Page 11-3

Causes of Currents Deep and shallow areas of the sea have currents. Surface currents are generally from 0 to about 400 meters (1,300 feet), although some go much deeper. Deep currents are those whose upper portions remain below the ocean surface. Different phenomena drive surface and deep currents. Wind. If the wind blows long enough in one direction, it will cause a water current to develop. The current continues to flow until internal friction, or friction with the sea floor, dissipates its energy. Surface Currents Chapter 11 Pages 11-3 to 11-4

Causes of Currents Changes in sea level. Sea level is the average level of the sea’s surface at its mean height between high and low tide. The ocean’s surface is never flat, ocean circulation cause slopes to develop. The steeper the “mound” of water, the larger and faster the current. The force that drives this current is the pressure gradient force. Surface Currents Chapter 11 Pages 11-3 to 11-4

Causes of Currents Variations in water density. When the density of seawater in one area is greater than in a neighboring area, the weight pressing down on deeper waters will be different, too. The horizontal pressure gradient between the two areas initiates a current that flows below the surface as more dense water sinks below less dense water. Surface Currents Chapter 11 Pages 11-3 to 11-4

Causes of Currents Persistent winds set surface currents in motion. The trade winds and the westerlies account for most the Earth’s wind energy. If you compare a map of average wind direction at the Earth’s surface and a map of surface ocean currents, you’ll notice similarities. You’ll also notice differences. For example, ocean boundaries force currents to turn; they can’t just keep going in a particular direction. Surface Currents Chapter 11 Pages 11-3 to 11-4

Causes of Currents Earth’s Major Surface Wind Patterns in the Atlantic Ocean Surface Currents Chapter 11 Pages 11-3 to 11-4

Gyres The combination of westerlies, trade winds, and the Coriolis effect results in a circular flow in each ocean basin. This flow is called a gyre. Surface Currents Chapter 11 Pages 11-5 to 11-6

Gyres There are five major gyres – one in each major ocean basin: 1. North Atlantic Gyre 2. South Atlantic Gyre 3. North Pacific Gyre 4. South Pacific Gyre 5. Indian Ocean Gyre Surface Currents Chapter 11 Pages 11-5 to 11-6

Gyres The flow of currents in all parts of the ocean is a balance of various factors, including the pressure gradient force, friction, and the Coriolis effect. Surface Currents Chapter 11 Pages 11-5 to 11-6 The North Atlantic Gyre

Ekman Transport You’ve already learned that currents have a tendency to flow to the right in the Northern Hemisphere and to the left in the Southern Hemisphere because of the Coriolis effect. This results in an interesting phenomenon called Ekman transport. The Ekman transport is an interesting phenomenon discovered in the 1890s by Fridtjof Nansen. Surface Currents Chapter 11 Pages 11-6 to 11-8

Ekman Transport The wind and the Coriolis effect influences water well below the surface because water tends to flow in what can be imagined as layers. Due to friction, the upper water currents push the deep water below it. This deep layer pushes the next layer below it. The process continues in layers downward. Each water layer flows to the right of the layer above causing a spiral motion. This spiraling effect of water layers pushing slightly to the right from the one above (to the left in the Southern Hemisphere) is called the Ekman spiral. Surface Currents Chapter 11 Pages 11-6 to 11-8

Ekman Transport There is a net motion imparted to the water column down to friction depth. This motion is called the Ekman transport. The net effect, averaging of all the speeds and directions of the Ekman spiral, is to move water 90° to the right of the wind in the Northern Hemisphere, or to the left in the Southern Hemisphere. Surface Currents Chapter 11 Pages 11-6 to 11-8

Ekman Transport Ekman Spiral Surface Currents Chapter 11 Pages 11-6 to 11-8

Western Ocean Boundary Currents and Eastern Ocean Boundary Currents Satellite images show that the ocean is really “hilly,” not calm or flat. These images show that water piles up where currents meet. Where currents diverge, “valleys” form. Surface Currents Chapter 11 Pages 11-8 to 11-15

Western Ocean Boundary Currents and Eastern Ocean Boundary Currents There is a dynamic balance between the clockwise deflection of the Coriolis effect (attempting to move water to the right) and the pressure gradient created by gravity (attempting to move the water to the left). The balance keeps the gyre flowing around the outside of the ocean basin. Surface Currents Chapter 11 Pages 11-8 to 11-15

Western Ocean Boundary Currents and Eastern Ocean Boundary Currents Geostrophic currents are created by the Earth’s rotation. This current results from the balance between the pressure gradient force and the Coriolis effect. Characteristics of western and eastern ocean boundary currents. Surface Currents Chapter 11 Pages 11-8 to 11-15

Western Ocean Boundary Currents and Eastern Ocean Boundary Currents Western boundary currents are found on the east coasts of the continents and are stronger and faster than eastern boundary currents due to western intensification. Western boundary currents flow through smaller areas than eastern boundary currents. Surface Currents Chapter 11 Pages 11-8 to 11-15 Formation of eddies.

Western Ocean Boundary Currents and Eastern Ocean Boundary Currents Trade winds blow along the equator pushing water westward, causing it to “pile up” on the western edge of ocean basins before it turns to the poles. The Earth’s rotation tends to shift the higher surface level in the center of the gyre westward. The higher surface level is now west of center and forces the current to “squeeze” through a narrower area. Total water volume balances out. Western ocean boundary currents handle the same volume, but through smaller areas, so water must move more rapidly. Surface Currents Chapter 11 Pages 11-8 to 11-15

Countercurrents Ekman spirals are not the only way water flows in directions that differ from the major ocean currents. Countercurrents are associated with equatorial currents. As the name implies, a countercurrent runs in the opposite direction of its adjacent current. The North Equatorial Current (NEC) and South Equatorial Current (SEC) flow west until they encounter continents. Surface Currents Chapter 11 Pages 11-16 to 11-17

Countercurrents An undercurrent flows beneath the adjacent current instead of beside it. One significant undercurrent is the Pacific’s Cromwell Current, named for Townsend Cromwell, who discovered it in 1956. The Cromwell Current flows more than 14,000 kilometers (8,700 miles) from New Guinea to Ecuador at a depth of approximately 100 to 200 meters (300 to 600 feet). It flows at an average speed of 5 kilometers (3 miles) per hour and carries a volume about half that of the Gulf Stream. Since the discovery of the Cromwell Current, undercurrents have been found beneath most major currents. Surface Currents Chapter 11 Pages 11-16 to 11-17

Countercurrents Surface Currents Chapter 11 Pages 11-16 to 11-17

Upwelling and Downwelling Upwelling is an upward vertical current that brings deep water to the surface. Downwelling is a downward vertical current that pushes surface water to the bottom. Surface Currents Chapter 11 Pages 11-17 to 11-20

Upwelling and Downwelling Coastal upwellings occur when the wind blows offshore or parallel to shore. In the Northern Hemisphere this wind blowing southward will cause an upwelling only on a west coast. The same wind on the east coast in the Northern Hemisphere sends surface water toward shore causing a downwelling. Surface Currents Chapter 11 Pages 11-17 to 11-20

Upwelling and Downwelling Surface Currents Chapter 11 Pages 11-17 to 11-20 Coastal Upwelling

Upwelling and Downwelling These currents have strong biological effects: Upwelling tends to bring deepwater nutrients up into shallow water. Upwellings also relate to significant weather patterns. Downwellings are important in carrying and cycling nutrients to the deep ocean ecosystems and sediments. They remove organic nutrients from the surface. Downwelling effect may be a reduction of productivity of some surface species and an increase in productivity of some bottom species. Surface Currents Chapter 11 Pages 11-17 to 11-20

Upwelling and Downwelling Surface Currents Chapter 11 Pages 11-17 to 11-20 Equatorial Upwelling

Heat Transport and Climate Currents play a critical role by transporting heat from warm areas to cool areas and affects climate by moderating temperatures. Without currents moving heat, the world’s climates would be more extreme. The Earth’s cold regions would be colder and the warm regions would be warmer. Winters in northern Europe would be significantly colder without the Gulf Stream bringing heat from the tropics. Southern California owes its mild climate to the moderating effects of the Pacific Ocean. The southerly current along the California coast brings cool water from the north, keeping southern California cooler than it would otherwise be in the summer. Surface Currents Chapter 11 Page 11-20

El Niño Southern Oscillation (ENSO) El Niño tremendously affects world weather patterns. This brings low pressure and high rainfall in the Western Pacific. The opposite happens in the Eastern Pacific with high pressure and less rainfall. Surface Currents Chapter 11 Pages 11-21 to 11-24

El Niño Southern Oscillation (ENSO) For reasons still not clear, every 3 to 8 years a rearrangement of the high- and low-pressure systems occur. Surface Currents Chapter 11 Pages 11-21 to 11-24 Normal El Niño

El Niño Southern Oscillation (ENSO) High pressure builds in the Western Pacific and low pressure in the Eastern Pacific. Trade winds weaken or reverse and blow eastward – the southern oscillation. This causes warm water of the west to migrate east to the coast of South America. The loss of upwelling deprives the water of nutrients. A normally productive region declines with the collapse of local fisheries and marine ecosystems. Over the eastern Pacific, humid air rises causing precipitation in normally arid regions. Flooding, tornados, drought and other weather events can lead to loss of life and property damage. Surface Currents Chapter 11 Pages 11-21 to 11-24

El Niño Southern Oscillation (ENSO) Surface Currents Chapter 11 Pages 11-21 to 11-24 The Pacific ‘see-saw.”

El Niño Southern Oscillation (ENSO) Surface Currents Chapter 11 Pages 11-21 to 11-24 Effects of ENSO.

Deep Currents Deep Currents Chapter 11 Pages 11-26 to 11-32

Deep Circulation and Water Masses You’ve learned that water density differences are one of several causes of currents. In the deep ocean layers, water density variation, not wind, is the primary cause of currents. Deep circulation is water motion caused by mixing water of differing densities. Deep Currents Chapter 11 Pages 11-26 to 11-28

Deep Circulation and Water Masses Deep Currents Chapter 11 Pages 11-26 to 11-28 Layers in the Sea

Deep Circulation and Water Masses Deep circulation drives most of the vertical motion of seawater and the ocean’s overall circulation. Deep circulation begins when water density increases due to cooling and increased salinity. When water becomes denser than the water below it, the denser water sinks. The cold dense water stays on the bottom until mixing brings it back to the surface. Tides and internal waves keep deep water mixed. The ocean stratifies into density layers. Deep Currents Chapter 11 Pages 11-26 to 11-28

Deep Circulation and Water Masses A Temperature-Salinity Diagram Deep Currents Chapter 11 Pages 11-26 to 11-28

Deep Circulation and Water Masses Each water layer has specific temperature and density characteristics. Recall that these water layer masses don’t easily mix with water layers of differing density characteristics. The reason for these differences is that water mass characteristics mainly develop at the surface. Deep Currents Chapter 11 Pages 11-26 to 11-28

Deep circulation and Water Masses Based on density stratification, there are five generally recognized primary water masses: To about 200 meters (600 feet) - surface water To the main thermocline (depth varies with latitude) - central water To about 1,500 meters (5,000 feet) - intermediate water Below intermediate water, but not in contact with the bottom, to about 4,000 meters (13,000 feet) - deep water In contact with the seafloor - bottom water Deep Currents Chapter 11 Pages 11-26 to 11-28

Deep circulation and Water Masses The Flow of Atlantic Deep Water Deep Currents Chapter 11 Pages 11-26 to 11-28

How Deep Water Masses Form Since water mass characteristics form at the surface, you may wonder how the deeper layers get to the bottom. The answer is that the intermediate, deep, and bottom water masses form primarily, but not entirely, at high latitudes (around 70° North and South). Two deep masses, Antarctic Bottom Water and North Atlantic Deep Water, make up most of the world’s deep water. Pacific Deep Water and Mediterranean Deep Water are also important. The densest ocean water forms in the Antarctic during the winter. This Antarctic Bottom Water has a salinity of about 34.65‰ and temperature of -0.5˚C (31˚F). At the surface, its specific density is 1.0279 grams per cubic centimeter, or almost 3% higher than pure fresh water. Deep Currents Chapter 11 Pages 11-28 to 11-29

How Deep Water Masses Form This high density is due to low temperature and high salinity. As seawater freezes, it leaves salt behind. The water that remains therefore becomes saltier, explaining why Antarctic Bottom Water has such high salinity. Deep Currents Chapter 11 Pages 11-28 to 11-29

How Deep Water Masses Form According to estimates, about 8 million cubic meters of Antarctic Bottom Water form every second. This very dense water descends to the bottom, spreads along the Antarctic deep-sea continental shelf and creeps northward. Antarctic Bottom Water is thought to reach as far as about 40˚ north latitude, taking somewhat less than 1,000 years to get there. Antarctic Bottom Water is a primary source for both the deep and bottom water layers. Deep Currents Chapter 11 Pages 11-28 to 11-29

How Deep Water Masses Form In the North Atlantic, high-salinity surface water cools and sinks. Pacific water that forms in the Northern Hemisphere is not as dense as deep bottom water, so it forms Pacific Intermediate Water. Intermediate water also develops in the North Atlantic, South Atlantic and South Pacific at latitudes that are not quite as cold as the Arctic or Antarctic. Mediterranean Deep Water forms due to evaporation rather than cooling. Deep Currents Chapter 11 Pages 11-28 to 11-29

How Deep Water Masses Form This illustration shows a cross-section of the entire Atlantic Ocean looking eastward. Note how the different water masses float or sink, depending on their relative density. Circulation patterns are also indicated. Deep Currents Chapter 11 Pages 11-28 to 11-29

Deep Water Flow Patterns The enormous water quantities sinking at the poles and in the Mediterranean are the source of the deep water masses and circulation. Dense water descends into low areas and bottom water upwell to compensate. The rising warm water enters wind-driven currents and is carried to the poles. There it cools, becomes more dense, and sinks again, repeating the process. Deep Currents Chapter 11 Page 11-30

Deep Water Flow Patterns Deep Circulation. An idealized model of deep circulation in the world ocean. Deep Currents Chapter 11 Page 11-30

The Ocean Conveyor Belt When we look at both deep water and surface currents, we can see how they influence the Earth’s climate. The interconnected flow of currents that redistribute heat is called the ocean conveyor belt. Some oceanographers call the system the Earth’s air conditioner. Deep Currents Chapter 11 Pages 11-30 to 11-32

The Ocean Conveyor Belt Deep Currents Chapter 11 Pages 11-30 to 11-32

The Ocean Conveyor Belt Deep water forms primarily at high latitudes. Water from the North Atlantic Deep Water flows south along the Atlantic bottom, merging with Antarctic Bottom Water. From there it flows eastward, with some flowing into the Indian Ocean, but most flowing to the South Pacific and on to the North Pacific. As the water mixes, it rises, warms, and eventually reaches the surface. From there it is pushed by the trade winds around the ocean. It carries heat from the equatorial regions north and south toward the poles. There it cools and descends, starting the cycle all over. Deep Currents Chapter 11 Pages 11-30 to 11-32

The Ocean Conveyor Belt This description is very generalized. If you were to follow a single water molecule, in theory it would take 1,000 to 2,000 years to complete a cycle on the ocean conveyor belt. As you may guess from previous discussions about how currents move heat, the ocean conveyor belt is important because it has a great effect on the world’s climate. Deep Currents Chapter 11 Pages 11-30 to 11-32

The Ocean Conveyor Belt Scientists hypothesize that some of the coldest intervals within ice ages have resulted from disruption of the ocean conveyor belt. They hypothesize that dilution of the North Atlantic Ocean with excess fresh water decreases the sinking of North Atlantic Deep Water. If this thinking is correct, then global warming may, ironically, lead to an ice age. Deep Currents Chapter 11 Pages 11-30 to 11-32

The Ocean Conveyor Belt The hypothesized cause is at global warming increases the melting of glaciers and ice caps. This dilutes the seawater, preventing the high-density, salty water from forming. Without this high-density water, there’s no downwelling to feed North Atlantic deep water currents. This would disrupt the ocean conveyor belt by shutting down the transport of relatively warm water to the far North Atlantic. Such events would cause large parts of the Northern Hemisphere, especially Europe, to become much colder. Scientists think events did occur during the last ice age, but the possibility of their happening now is speculative. Deep Currents Chapter 11 Pages 11-30 to 11-32

Studying Ocean Currents Chapter 11 Page 11-33 to 11-37

Three Distinct Approaches There are three main approaches to study currents: Lagrangian method, also called the float method. Studying the current by tracking a drifting object. This involves floating something in the current that records the information as it drifts. Eulerian method, also called the flow method. Studying the current by staying in one place and measuring changes to the velocity of the water as it flows past. This method uses fixed instruments that meter/sample the current as it passes. Studying Ocean Currents Chapter 11 Page 11-33

Three Distinct Approaches Use of altimeter satellites to measure the highs and lows of the sea surface. Because geostrophic currents flow around highs and lows, satellite altimeters can produce ocean current maps everywhere on the ocean surface. This has revolutionized our knowledge of currents and tides. All oceanographers today use satellites during their studies. Studying Ocean Currents Chapter 11 Page 11-33

Instrumentation and Methods There are five examples of instruments or methods that scientists apply for studying currents. For Lagrangian study methods researchers use a drogue. The advantage over a simple surface float is that the “holey sock” ensures that the current and not the wind determine where it drifts. Studying Ocean Currents Chapter 11 Page 11-34 to 11-37

Instrumentation and Methods Scientists are now using floats that transmit data to satellites. For example, the Argo float drifts at depths typically ranging from 1,500 to 2,000 meters (5,000 to 6,500 feet) before periodically rising to the surface to transmit to a satellite a temperature and salinity profile of the water it rose through. Then, it sinks back to its drifting depth. Studying Ocean Currents Chapter 11 Page 11-34 to 11-37

Instrumentation and Methods Argo “Floater” Collecting information on subsurface currents. Argo floats periodically adjusts its buoyancy to rise to the surface. On the surface, it transmits its position to a communication satellite. Once the uplink is complete, Argo returns to deep water to track current flow for ten days. Studying Ocean Currents Chapter 11 Page 11-34 to 11-37

Instrumentation and Methods For Eulerian study methods researchers use: Various types of flow meters. These devices use impellors and vanes to measure and record current speed and direction. The information gathered is either transmitted immediately or stored for retrieval later. A more sophisticated device is the Doppler Acoustic Current Meter. This instrument determines current direction and speed. Studying Ocean Currents Chapter 11 Page 11-34 to 11-37

Instrumentation and Methods Studying Ocean Currents Chapter 11 Page 11-34 to 11-37 Researchers Using A Flow Meter Doppler Acoustic Current Meter

Instrumentation and Methods Along with ocean-based instruments, oceanographers use altimeter satellites to measure surface geostrophic currents. The Jason II satellite, launched in 2008, measures sea level height around the world. The satellite image is updated every two weeks. Studying Ocean Currents Chapter 11 Page 11-34 to 11-37

Instrumentation and Methods Altimeter Satellite Studying Ocean Currents Chapter 11 Page 11-34 to 11-37 Jason II Satellite