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99 Cards in this Set
- Front
- Back
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What powers surface currents?
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winds
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What powers deep currents?
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gravity, through density differences caused by temperature and salinity (thermohaline)
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What ultimately powers all circulation?
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the sun
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What factors help determine the transfer of kinetic energy between the atmosphere and ocean?
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wind speed, water surface tension, and roughness of the water
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What percentage of the wind speed is the current speed?
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1-3%
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What happens to wind-driven current speed at depth?
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It gradually goes down to a depth of 100-200 meters.
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Is wind required for a current to run?
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No; currents can run for a while after the wind has shut off or be pushed by sea surface slopes.
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What factors determine the velocity of a current?
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friction, the Coriolis effect, land (coasts), and pressure gradients
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What does cum sole mean?
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with the sun (to the right in the Northern Hemisphere and the left in the Southern Hemisphere)
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Describe the Ekman spiral.
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Surface currents travel with a 45° angle to the wind, and currents below the surface travel at progressively greater angles but lower speeds. The average transport is 90° to the wind.
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What inhibits the Ekman spiral?
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A high pycnocline or shallow waters can cause the 45° to be lower and the net Ekman transport to be less than 90°.
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How does a pressure gradient force cause downwelling?
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When Ekman transport piles water higher in some places, the entire water column resists this, causing deep water to oppose the current and surface water at the convergence to sink.
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How can sea surface heights help oceanographers determine current speeds?
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The differences in height induce an opposing pressure gradient current at depth with the same magnitude and opposite direction as the surface current.
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How do oceanographers indirectly determine sea surface heights to great precision?
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They make density measurements at many depths, and using a depth where no significant pressure gradients exist, find the height at which the pressure is atmospheric (the surface).
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How do oceanographers directly determine sea surface heights to great precision?
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Synoptic measurements on satellites will soon let them study surface currents in more detail.
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Why do geostrophic currents flow even when the wind is not blowing?
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Ekman transport piles water up in certain locations, storing potential energy in the process.
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What is the main feature of surface current circulation?
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rotation around subtropical gyres in each hemisphere
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How many subtropical gyres are there?
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five, two in the Pacific and Atlantic and one in the Indian
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What circulation occurs in the subpolar regions of the ocean?
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In the Antarctic, the West Wind Drift encircles the continent, while in the subarctic, smaller gyres rotate the opposite direction.
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What is the main role of the continents in surface water circulation?
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They provide western and eastern (and northern in the Indian Ocean) boundaries which make currents change their course.
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What is the general name for poleward-flowing currents of subtropical gyres?
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western boundary currents
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What is the general name for equatorward-flowing currents of subtropical gyres?
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eastern boundary currents
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What are the four legs of a general subtropical gyre?
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western boundary current, west-to-east movement directed by westerlies, eastern boundary current, and Equatorial Current directed by trade winds
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What lies in the center of a gyre?
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a rotating mound of higher water called a subtropical convergence
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Why do geostrophic ocean gyres remain almost always the same regardless of the wind?
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To change the height of the subtropical convergence requires enormous amounts of water transport, so it basically averages out wind speeds over long periods of time.
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Name the western boundary currents.
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Gulf Stream, Brazil Current, Kuroshio, East Australian Current, and Agulhas Current
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Name the eastern boundary currents.
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California Current, Peru Current, Canary Current, West Australian Current, and Benguela Current
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How does western intensification come about?
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The Coriolis effect is stronger near the poles, so the currents around 60° latitude deflect more than the equatorial currents, making the center of the gyre west of the center of the basin.
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What are nine characteristics of western boundary currents as opposed to eastern boundary currents?
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Western boundary currents are narrower, deeper, faster, larger (in terms of volume), more sharply defined, devoid of upwelling, nutrient-poor, lacking fish, and warmer than eastern boundary currents.
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What effects does the distance between the equator and ITCZ (doldrums) have on surface currents?
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Those northwest-blowing trade winds north of the equator produce Ekman transport towards the northeast, creating the Equatorial Countercurrent.
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What is the arrangement of currents around the equator?
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South of 5°N, the South Equatorial Current travels west, with the Equatorial Undercurrent at a depth of 50-300 meters piercing through it in an eastward direction. Between 5°N and 10°N, the Equatorial Countercurrent travels east, and north of that is the North Equatorial Current, travelling west.
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What causes the Equatorial Undercurrent?
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A convergence of water at the western boundary of an ocean creates a pressure gradient that drives the Equatorial Undercurrent.
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Why does the Equatorial Undercurrent travel along the Equator?
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If it drifted north, the Coriolis effect would push it south, back onto the Equator.
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What drives the subpolar gyres?
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the westerlies and polar winds, which are much more variable than the trade winds
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What currents flow in the Southern Ocean?
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The East Wind Drift flows westward around the coast of Antarctica, and the West Wind Drift flows eastward a little further out.
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Name the currents in the North Pacific gyre.
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Kuroshio Current, North Pacific Current, California Current, North Equatorial Current
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Name the currents in the North Atlantic gyre.
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Gulf Stream, North Atlantic Current, Canary Current, North Equatorial Current
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Name the currents in the South Pacific gyre.
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East Australian Current, South Pacific Current, Peru Current, South Equatorial Current
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Name the currents of the South Atlantic gyre.
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Brazil Current, South Atlantic Current, Benguela Current, South Equatorial Current
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What currents are involved in subpolar gyres in the North Pacific?
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the Oyashio Current and Alaska Current
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Which way do subpolar gyres rotate?
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counterclockwise (all are in the Northern Hemisphere)
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Which way do subtropical gyres rotate?
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clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere
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What currents are involved in subpolar gyres in the North Atlantic?
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the West Greenland Current and Labrador Current flow west of Greenland, while the extended North Atlantic Current and East Greenland Current flow east of Greenland.
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When does upwelling occur?
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when surface currents move away from an area, creating a divergence and raising the pycnocline
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When does downwelling occur?
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when surface currents move into an area, creating a convergence and lowering the pycnocline, or when the density of surface water increases by cooling or evaporation
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Why is upwelling important?
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It brings nutrients such as nitrates and phosphates to surface waters where phytoplankton can use them.
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Where are the main open ocean divergences of the world?
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latitudinal bands between the North and South Equatorial Currents and between the West Wind Drift and East Wind Drift
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Where are the main open ocean convergences of the world?
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the centers of subtropical surface current gyres
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How can winds and land cause upwelling or downwelling?
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If the wind blows parallel to the coast, Ekman transport will be offshore or onshore, creating a divergence or convergence and upwelling or downwelling.
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How do boundary currents influence coastal upwelling?
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Western boundary currents inhibit coastal upwelling of cold waters because they are deep and hug the coastline; eastern boundary currents make upwelling easier to find.
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Where do coastal currents flow?
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over the continental shelf when there isn't a shallow current to inhibit them
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When and in which direction do coastal currents flow?
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They flow in direct response to winds and storms, in the direction parallel to the coast that Ekman transport pushes them.
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Where do the strongest coastal currents run?
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windy areas with lots of freshwater input to create a steep pycnocline and reduce friction
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What is an example of a seasonal coastal current?
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The Davidson Current runs northward along the California coast, opposite the California Current, during winter when storm winds blow from the southwest.
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What is "weather" for oceans?
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meandering fronts and eddies
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Why are ocean eddies smaller than atmospheric eddies?
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Ocean currents go slower than winds, so the Coriolis effect can turn them more.
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How do satellites observe eddies?
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Their infrared sensors detect backscattered light of different wavelengths and exaggerate these temperature differences.
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What can be seen from turning satellites toward the Gulf Stream?
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The fronts between the current and surrounding water are sharp, and show that it meanders every which way, sometimes pinching off eddies.
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What are eddies?
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swirling pools of water cut off from their original current source
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Which way do eddies from the Gulf Stream spin?
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Coastal warm-core rings spin clockwise, while cold-core rings in the Sargasso Sea spin counterclockwise.
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How big are eddies and how fast do they rotate?
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100-300 km across and currents of around .9 m/s
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What happens to eddies after they are pinched off?
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They migrate southward and eventually are broken apart.
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Why are eddies important?
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They deliver nutrients to the water just outside of the Gulf Stream by mixing water from both sides of the current.
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What are mesoscale eddies?
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high and low pressure zones in the ocean, between 25 and 200 km in length, drift slowly (km per day), and have current speeds of ~.1 m/s
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What are inertial currents?
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Coriolis effect-guided currents that flow in loops after their power source has gone away
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Why don't inertial currents always flow in a circle?
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There is usually a net drift of water in the area that directs the inertial currents, causing them to spiral in that direction.
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Explain Langmuir circulation.
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Wind causes corkscrews of spiralling water parallel to itself, alternating rotation directions so there are vertical planes of upwelling and downwelling every 25 meters and windrows of debris above the downwelling zones.
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Why is Langmuir circulation important?
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It mixes the upper layers of the ocean, allowing heat and gases to be exchanged between ocean and atmosphere.
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What are the three main density zones of most of the ocean?
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the mixed zone, the pycnocline zone, and the deep zone
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How deep does the pycnocline zone go?
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500-1000 meters
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What causes the sharp increase in density of the pycnocline?
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The thermocline (lowering temperatures) and/or the halocline (salinities) can accomplish this.
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Why do surface waters have higher salinities than deep waters when increasing salinity increases density?
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Surface waters have high evaporation minus precipitation (E-P) and their warmer temperatures more than offset the extra salt.
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What effect does the seasons have on the thermocline?
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In summer and less so in spring and fall, an extra seasonal thermocline develops above 50 meters depth, changing the biology of the area.
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What does the pycnocline do?
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It isolates the surface water and processes from deep water, because it takes a lot of energy to get across it.
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Which oceans have no pycnocline?
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High latitudes except the North Pacific and Arctic lose energy quickly, cooling them to create new deep water.
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How can salinity be increased?
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by evaporation or ice exclusion
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Why do deep water masses retain their identities for so long?
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Vertical mixing between water masses is very slow because it must overcome gravity.
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Where is Antarctic Bottom Water formed?
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the Weddell Sea in the Southern Ocean nearest to the south Atlantic
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What is the densest extensive water mass on earth?
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Antarctic Bottom Water
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Where does North Atlantic Deep Water form?
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the Greenland Sea and Norwegian Sea between the Arctic and North Atlantic
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Why is no bottom water formed in the North Pacific?
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Sills between the Aleutian island chain block off the Arctic from the Pacific, so the North Pacific just doesn't get cold enough.
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What creates a pycnocline in the Arctic?
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An intense halocline develops from the freshwater runoff into the Arctic.
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Besides density, what factors affect deep water flow?
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the Coriolis effect and sea bottom topography, like oceanic ridges
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What properties does Mediterranean water have?
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Mediterranean water is warm and saline due to high E-P in the Mediterranean, and sinks to intermediate depths.
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What removes water from the deep oceans, if it is being formed all the time in some locations?
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Mixing with higher layers, while slow, can dispose of large quantities of water because of the vast area of the oceans.
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Why is deep-ocean circulation important?
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They can cause climate changes, distribute dissolved chemicals (toxins and nutrients), and carry extra carbon dioxide to deep oceans.
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How long does NADW stay in the ocean?
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around 1000 years (residence time)
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Describe the NADW Conveyer Belt.
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Surface water in the North Atlantic sinks, flows south through the Atlantic, around Africa and Australia into the Indian and Pacific Oceans, and mixes towards the surface in both the Indian and North Pacific Oceans, from which it slowly migrates back to the North Atlantic as surface water. This water is cooled through evaporation by westerlies, which dumps the precipitation on Europe.
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How has the NADW Conveyer Belt behaved in the past?
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During the ice age, it was almost turned off, then turned on at the end of the ice age, after the ice had stopped melting, then turned off during the slightly cooler Young Dryas period (11k years ago). More recently, the Greenland Sea Deep Water decreased by 90% in the 1980s for unknown reasons.
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In general, what is used to trace water masses?
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temperature, salinity, and dissolved constituents
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What are conservative properties?
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properties of a water mass not significantly changed by internal (biological, chemical and physical) changes within the water mass
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What are two examples of conservative properties?
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salinity and dissolved Na+ and Cl-
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What happens to conservative properties when water masses mix?
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They are averaged in proportion to the sizes of each water mass.
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What is the result of two water masses mixing on a TS diagram?
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The result lies on a straight line segment connecting the water mass points.
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What is a TS diagram?
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a plot showing temperature on one axis and salinity on the other, with water masses as points and vertical profiles as curved lines
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How do TS diagrams help oceanographers detect water masses?
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Knowing the properties of individual water masses, the TS diagram can show trends towards or away from these points, indicating the presence of those water masses if the curve is sufficiently close.
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What information can dissolved oxygen and carbon dioxide give?
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the relative ages of different water masses (how long since they were at the surface)
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What man-made particles can be used to trace water masses?
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radionuclides such as tritium from nuclear bomb testing of the 1950s and chlorofluorocarbons, which are chemically inert and conservative
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Why is it important to study circulation of man-made particles of the past?
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Carbon dioxide, highly dissolved in seawater, has been released for a long time and needs to be mixed into the ocean to dissolve more.
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