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CURRENTS: moving masses of wter

MASS FLOW OF OCEAN WATER IS DRIVEN BY WIND AND GRAVITY

Gyres:

• Surface Currents Flow around the Periphery of Ocean Basins

Seawater Flows in Six Great Surface Circuits Mid latitude

• .5 gyres plus antarctic circumpllar current

Coriolis effect

deflection of moving objects when they are viewed in a rotating reference frame.http://en.wikipedia.org/wiki/File:Corioliskraftanimation.gifhttp://www.classzone.com/books/earth_science/terc/content/visualizations/es1904/es1904page01.cfm

The Earth is a spinning globe where a point at the equator is travelling at around 1100 km/hour, but a point at the poles is not moved by the rotation. This fact means that projectiles moving across the Earth's surface are subject to Coriolis forces that cause apparent deflection of the motion.

Northern hemisphere Water down drain goes….Southern hemisphere goes….

Current Animations

Australian animations, all depths of ocean http://www.cmar.csiro.au/currents/animations.htm

NOAA Animationshttp://svs.gsfc.nasa.gov/vis/a010000/a010800/a010841/perpetualocean_cover_1024x676.jpg

Ekman spiral A theoretical model of the effect on water of wind blowing over the ocean. Because of the Coriolis effect, the surface layer is expected to drift at an angle of 45° to the right of the wind in the Northern Hemisphere and 45° to the left in the Southern Hemisphere. Water at successively lower layers drifts progressively to the right (N) or left (S), though not as swiftly as the surface flow. Ekman transport Net water transport, the sum of layer movement due to the Ekman spiral. Theoretical Ekman transport in the Northern Hemisphere is 90_ to the right of the wind direction.

• Midatlantic Hill• Hill of water in the Sargasso Sea due to balance of Coriolis affect and pressure

gradient.

6 great currentsNorthern Hemisphere:North Atlantic Gyre (geostrophic gyre –balance of coriolis and pressure gradientNorth Pacific Gyre

Southern HemisphereSouth Pacific GyreSouth AtlanticIndian Ocean GyreAntarctic Circum Polar Current (Not a gyre

What are Currents, Gyres, & Eddies?

• Even on the calmest days, Earth's oceans are constantly on the move. At the surface and beneath, currents, gyres and eddies play a crucial role in physically shaping the coasts and ocean bottom; in transporting and mixing energy, chemicals and other materials within and among ocean basins; and in sustaining countless plants and animals that rely on the oceans for life—including humans.

• These features are important components of Earth's global ocean circulation that move water mainly horizontally. Their effects can also extend down for miles, in some places reaching the ocean bottom.

Currents are coherent streams of water moving through the ocean and include both long, permanent features such as the Gulf Stream, as well as smaller, episodic flows in both coastal waters and the open ocean.

• They are formed primarily by wind blowing across the surface of the ocean and by differences in the temperature, density and pressure of water and are steered by Earth's rotation as well as the location of the continents and topography of the ocean bottom.

Gyres are spiraling circulations thousands of miles in diameter and rimmed by large, permanent ocean currents.

Eddies are smaller, temporary loops of swirling water that can travel long distances before dissipating.[ MORE ]

Wind, Currents and Coriolis

• Wind is the primary force that creates and moves surface currents; Earth's rotation plays an important role in steering the water's motion. Persistent subtropical high pressure systems centered at about 30 degrees north and south latitude create patterns of strong winds known as the trades and the westerlies.

• Friction between the air and the water sets the sea surface in motion.

As this topmost layer of water moves, it pulls on the water directly beneath it, which in turn pulls on the layer of water beneath that to

create the beginnings of an ocean current.

• The resulting motion is not in line with the wind, however. Earth's rotation causes an apparent force known as the Coriolis effect to deflect straight-line movement across the surface about 45 degrees to the right in the Northern Hemisphere and 45 degrees to the left in the Southern Hemisphere. In addition, each successive layer of water is slightly deflected from the motion of the one above, like a deck of cards fanned out. This forms a phenomenon called an Ekman spiral that was first described by Swedish mathematician Vagn Walfrid Ekman in 1905, but it was not until the late 1980s that a team from WHOI first observed it in the open ocean.

Gyres

• The net wind-driven movement of water, known as Ekman transport, creates a bulge in each ocean basin that is as much as three feet (one meter) higher than mean global sea level. The force of gravity pulling on this large mass of water creates a pressure gradient similar to that in an atmospheric high pressure system which in turn leads to a stable, rotating mass of water.

Five permanent subtropical gyres can be found in the major ocean basins—

• two each in the Atlantic and Pacific Oceans and one in the Indian Ocean—turning clockwise in the Northern Hemisphere and counterclockwise in the Southern. Smaller counterclockwise gyres centered at around 60 degrees north latitude are created by the prevailing winds around permanent sub-Arctic low-pressure systems. Another subpolar gyre, the only one centered on a landmass, circles Antarctica driven by the near-constant westerly winds that blow over the Southern Ocean, unimpeded by land.

Effect of Current on Deep water Horizonn Oil Spill

• http://www.whoi.edu/deepwaterhorizon/chapter5.html

Why Do They Matter?• Currents, gyres and eddies transport water and heat long distances

and help promote large-scale mixing of the ocean. In the process they also transport nutrients, salt and other chemicals and help regulate the planet’s weather, climate and marine ecosystems.

• Strong currents and eddies also influence shipping routes and have been known to damage oil platforms. Powerful offshore currents and weaker coastal currents shape the land by contributing to beach erosion and the movement of barrier islands. Knowledge of how and where these phenomena occur as well as how they might be changing is sought by fishing fleets to locate schools of fish, by the Coast Guard to respond to search-and-rescue emergencies or oil spills, and by policy makers to help formulate marine conservation plans.

Weather and Climate• One of the most important roles of ocean currents is in governing Earth’s

weather and climate. The western boundary currents carry large amounts of heat from tropical waters to the north. This flow is part of the thermohaline circulation, or ocean conveyor and helps distributes heat around the planet. This in turn governs wind patterns, air temperature and precipitation both locally and globally.

• Recent studies have shown that western boundary currents have shifted position slightly over the course decades, leading to changes in wind, temperature and precipitation patterns around the globe more commonly associated with El Niño and the other ocean oscillations. One important question oceanographers are trying to answer is how small changes in the placement, temperature, speed and volume of currents might result in large or abrupt changes in Earth’s long-term climate. Identifying the natural and human factors that could change or disrupt the natural function of ocean currents is also an important part of understanding and predicting future climate changes.

Marine Life• Currents are critical to marine life. Cold water contains large amounts of

the nutrients that feed the base of the food chain. Those places where cold water mixes with warm, nutrient-poor water often contain high levels of biomass (living things) as well as a high degree of biodiversity (different species). Many warm-water animals that favor these boundary zones, such as tuna, swordfish and squid, are particularly important commercial resources, so understanding how and where ocean waters mix gives fishing fleets the ability to locate schools and minimize their time at sea. It also gives marine biologists information they need to help manage fisheries or protect endangered species.

• A gyre’s currents also cause floating debris to slowly drift towards the center of the ocean, forming large patches of floating trash. This can be a hazard to marine life and, as the chemicals in the plastics enter the food chain, of concern to humans, as well.

Physical Processes• Currents shape the coasts in ways that are likely to be obvious to

someone standing on shore. They also physically shape the ocean basins in ways that are much more subtle, but no less important to oceanographers. Much as a slow-moving river will have a silty bottom and a fast-moving stream will have a rocky bed, ocean currents transport and deposit material on the ocean bottom in identifiable ways. By understanding the relationship between the size, composition and distribution of particles found on the bottom with the motion of the water column above, scientists who study long cores of ocean sediment can tell how currents have changed or moved over time. This in turn helps explain how factors such as fresh water from melting ice or changes in global wind patterns might lead to large-scale changes in ocean circulation or climate in the future.[ LESS ]

What is El Niño?

• El Niño is a warming of surface waters in the eastern tropical Pacific Ocean. Together with, La Niña, these make up two of the three states of the constantly changing El Niño/Southern Oscillation (ENSO) that can affect weather patterns around the globe.

• ENSO is just one of many oscillations in Earth’s ocean and atmosphere that occur naturally over different time and geographic scales. El Niño, Spanish for "the little boy" or "the Christ child," was named by Peruvian fishermen when they noticed changes in anchovy populations around Christmas more than 100 years ago caused by uncharacteristically warm surface water in the eastern tropical Pacific Ocean. Much later, scientists realized that El Niño was part of a much larger, recurring phenomenon that can bring about abnormal and often severe changes in temperature and precipitation throughout the tropics.

• In a "normal," or ENSO-neutral year, a low atmospheric pressure center forms over northern Australia and Indonesia and a high pressure center forms on the other side of the Pacific over Peru. At the same time, the trade winds blow steadily east to west along both sides of the equator to move warm surface waters from the eastern to the western Pacific and cause cold, nutrient-rich bottom water to well up off the coast of South America.

La Niña, Spanish for "the little girl."

• As the name suggests, conditions this phase of ENSO are generally the reverse of El Niño. Where eastern tropical Pacific waters are warmer than normal during El Niño, they are much colder during a La Niña phase.[ MORE ]

What are other oscillations?

• Many other naturally occurring ocean-atmosphere oscillations in the Pacific, Atlantic, and Indian Oceans have been recognized and named. Some of them have much more of an impact on climate and weather patterns in the U.S. and elsewhere than ENSO. Many of these, as during ENSO, ocean and atmosphere interact as a coupled system, with ocean conditions influencing the atmosphere and atmospheric conditions influencing the ocean. However, not all exert as strong an influence on global weather patterns, and some are even less regular than ENSO.[ MORE ]

Why are they important?• When ocean and atmospheric conditions in one part of the world change as a

result of ENSO or any other oscillation, the effects are often felt around the world. The rearrangement of atmospheric pressure, which governs wind patterns, and sea-surface temperature, which affects both atmospheric pressure and precipitation patterns, can drastically rearrange regional weather patterns, occasionally with devastating results.

• Because it affects ocean circulation and weather, an El Niño or La Niña event can potentially lead to economic hardships and disaster. The potential is made worse when these combine with another, often overlooked environmental problem. For example, overfishing combined with the cessation of upwelling during an El Niño event in 1972 led to the collapse of the Peruvian anchovy fishery.

• Extreme climate events are often associated with positive and negative ENSO events. Severe storms and flooding have been known to ravage areas of South America and Africa, while intense droughts and fires have occurred in Australia and Indonesia during El Niño events.[ MORE ]

What is the Ocean Conveyor?

• A fundamental element of today’s climate system is a conveyor-like ocean circulation pattern that distributes vast quantities of heat and moisture around our planet. This global circulation is propelled by the sinking of cold, salty—and therefore dense—ocean waters.

• In today’s ocean, warm, salty surface water from the Caribbean, the Gulf of Mexico, and the equatorial Atlantic flows northward in the Gulf Stream. As the warm water reaches high North Atlantic latitudes, it gives up heat and moisture to the atmosphere, leaving cold, salty, dense water that sinks to the ocean floor. This water flows at depths, southward and beneath the Gulf Stream, to the Southern Ocean, then through the Indian and Pacific Oceans. Eventually, the water mixes with warmer water and returns to the Atlantic to complete the circulation.

• The principal engine of this global circulation, often called the Ocean Conveyor, is the difference in salt content between the Atlantic and Pacific Oceans. Before the Isthmus of Panama existed, Pacific surface waters flowed into the Atlantic. Their waters mixed, roughly balancing the two oceans’ salinity.[ MORE ]

Boundary Currents Have Different Characteristics

Western boundary Currents

• Gulf Stream-Largest- 5 mph/ 450 m deep, 43 mi wide

• Japan or Kuroshio Current in No pacific• Brazil current in so Atlantic• Agulhas Cucrrent in indain ocean• East Australian current in so pacific

Eastern Boundary Currents

• Canary current in No. Atlantic• Benguela Current in So. Atlantic• California current in No. Pacific• West Australian Current in Indan Ocean• Peru or Humboldt Current in So Pacific

The Gulf Stream

SverdrupVolume transport sverdrup (sv) = 1 x 10 6 m3 per second

Types of Currents

• Transverse – flow east to west and west to each.

Trade Winds-

– Atmospheric circulation and the Coriolis effect create global wind patterns including the trade winds and westerlies.

Atmospheric circulation and the Coriolis effect create global wind patterns including the trade winds and westerlies.

• In the Northern Hemisphere, warm air around the equator rises and flows north toward the pole. As the air moves away from the equator, the Coriolis effect deflects it toward the right. It cools and descends near 30 degrees North latitude. The descending air blows from the northeast to the southwest, back toward the equator (Ross, 1995). A similar wind pattern occurs in the Southern Hemisphere; these winds blow from the southeast toward the northwest and descend near 30 degrees South latitude.

• These prevailing winds, known as the trade winds, meet at the Intertropical Convergence Zone (also called the doldrums) between 5 degrees North and 5 degrees South latitude, where the winds are calm. The remaining air (air that does not descend at 30 degrees North or South latitude) continues toward the poles and is known as the westerly winds, or westerlies. The trade winds are so named because ships have historically taken advantage of them to aid their journies between Europe and the Americas (Bowditch, 1995).

Countercurrents and Undercurrents Are Submerged Exceptions to Peripheral Flow

SURFACE CURRENTS AFFECT WEATHER AND CLIMATE

WIND CAN CAUSE VERTICAL MOVEMENT OF OCEAN WATER

Nutrient-Rich Water Rises near the Equator

Wind Can Induce Upwelling near Coasts

Wind Can Also Induce Coastal Downwelling

Langmuir Circulation Affects the Ocean Surface

THERMOHALINE CIRCULATION AFFECTS ALL THE OCEAN'S WATER

Water Masses Have Distinct, Often Unique Characteristics

Different Combinations of Water Temperature and Salinity Can Yield the Same Density

Thermohaline Flow and Surface Flow: The Global Heat Connection

Thermohaline circulation caused by heating in lower latitudes and cooling in higher latitudes

Water Layers and deep circulation of Atlantic Ocean.

The Formation and Downwelling of Deep Water Occurs in Polar Regions

Water Masses May Converge, Fall, Travel across the Seabed, and Slowly Rise

Upwelling

Coastal Upwelling

Equatorial Upwelling

Deep, cold water moves up. Nutrient Rich

Water moving westward on either side of equator is deflected poleward

caused by winds parallel to shore or offshore

Thermohaline Circulation –Conveyor Belt

• Driven by the sun's heat absorbed by tropical oceans and impacted by variations in salt content in the water, the Thermohaline Circulation or THC is a powerful force on the world's climate system. As heat from the tropics is carried by the Gulf Stream into the North Atlantic where it is vented into the atmosphere, a deep convection of ocean waters is caused by surface cooling, with the flow of water then sinking to depths and then upwelling back to the surface at lower latitudes.When frozen, water releases salt, and thus when it melts it is salt-free. This factor and the heavier density of salty water is particularly important in polar regions where the convergence of fresh and saline waters influences ocean currents.

The THC is dynamic and has been known to dramatically shift, as it appears to have done just after the last Ice Age and perhaps during episodes of abrupt climate change. Because such shifts in the THC's "conveyor belt" of ocean heat can cause major changes in climate over relatively short-time scales (10-20 years) which in turn can have enormous impact on human activities, understanding the THC has become a major focus for climate research. As we examine in Climate Science 10,000, scientists are looking at how influxes of freshwater into the North Atlantic can impact the THC. See Gordon, 1986 and Broecker, 1987 for more on the Thermohaline Circulation system.

HOW TO MEASURE CURRENT

• Float Methods-drift with current– Surface currents near coast• Drogues are tracked.• Swallow floats use sonar pings

– Flow Methods - at a fixed position• Eckman flow meters-have rotating vanes• Slocums AOV • Acoustical Tomography

– (Low frequency sound:– T, salinitiy, movement of water below surface)