Lecture #9 Circulation of the ocean 24 August 2007
Lecture schedule LectureDateTime 1. Introduction to scientific writing 8/20 9:00-10:20 2. How to write a scientific paper in English 8/20 15:00-16:20 3. Earth structure and plate tectonics 8/21 9:00-11:00 4. Circulation of the atmosphere 8/21 13:00-15:00 5. Water and ocean structure 8/22 10:00-11:00 6. Continental margins and ocean basins 8/22 13:00-15:00 7. Sediment 8/23 10:00-12:00 8. Marine resources 8/23 13:00-14:00 9. Circulation of the ocean 8/24 11:00-12:30
Current:mass flow of water • two major types: • surface currents • wind-driven movement of water at or near the ocean’s surface • thermohaline currents • water circulation produced by differences in density • slow deep currents that affect the vast bulk of seawater below the pycnoline
Surface currents • About 10% of the water in the world ocean is involved in surface currents • Occur in the upper 400 m • Driven mainly by wind
Because of Coriolis effect, Winds Surface currents gyre:a circuit of wind-driven current flow around the periphery of an ocean basin
There are six major gyres in the world ocean: • North Atlantic gyre • four currents • note that the two east-west currents flow to the right of the winds
When driven by the wind, the topmost layer of ocean water in the Northern Hemisphere flows at about 45o to the right of the wind direction But what about deeper layers?
100 m Ekman spiral:model of the effect of wind blowing over the ocean surface
In nature, Ekman transport in gyres is <90o • usually about 45oC • due to interaction between the Coriolis effect and the pressure gradient
Hill of water in the North Atlantic Water flow in a gyre is dynamically balanced between the downhill urge of the pressure gradient and the uphill tendency of Coriolis deflection.
Average sea surface height Red – high Green/blue - low • “hill” • about 2 m high • maintained by wind energy
Gyres in balance between the Coriolis effect and the pressure gradient are called geostrophic gyres • There are six great current circuits in the world ocean: • two in the Northern Hemisphere • four in the Southern Hemisphere
Illustration of sea-surface temperature showing the general pattern of surface current flow North Atlantic gyre North Pacific gyre South Atlantic gyre Indian Ocean gyre South Pacific gyre Antarctic Circumpolar Current
Geostrophic currents can be classified based on their position within a gyre: • Western boundary currents • Eastern boundary currents • Transverse currents
1. Western boundary currents • warm, fast, narrow and deep • move warm water poleward 1. Gulf Stream 2. Kuroshio 3. Brazil Current 4. Agulhas Current 5. East Australian Current 1 2 5 3 4
Largest western boundary current is theGulf Stream similar to a river 8 km/hr 5 September 2005 http://www.deos.tudelft.nl/altim/gulfstream/gulf_040905_vel_anot.gif
warm-core eddy cold-core eddy • reach 1000 km in diameter • can last >3 years http://www.nc-climate.ncsu.edu/education/ccms/2003/ben_jon/Pictures/Gulf-Stream2.gif
2. Eastern boundary currents • cold, shallow and broad • move cold water equatorward 1. Canary Current 2. Benguela Current 3. California Current 4. West Australian Current 5. Peru Current 3 1 2 5 4 • no eddies
3. Transverse currents • Flow east-west or west-east
Why are western boundary currents faster, deeper and narrower than eastern boundary currents?
equator Because of the Coriolis effect – which increases as water moves farther from the equator – eastward-moving water on the north side of a gyre is turned sooner and more strongly toward the equator than westward-flowing water at the equator is turned toward the pole. So the peak of the hill is not the center of the ocean basing but close to its western edge.
Slope is steeper on the western side. As a result, the current on the eastern boundary is spread out and slow, and the current on the western boundary is concentrated and rapid. Westward intensification
Question Time! What is the difference between a western boundary current and an eastern boundary current? Name one western boundary current. Name one eastern boundary current.
Surface currents affect weather and climate. The solar heating of Earth varies with latitude
Warm water flows to higher latitudes, transfers heat to the air and cools, moves back to low latitudes, and absorbs heat again; then the cycle repeats.
Winds drive the horizontal movement of surface water. They can also cause vertical movement of surface water. Upwelling Downwelling
Nutrient-rich water rises near the equator Though the Coriolis effect is weak near the equator (and absent at the equator), water moving in the currents on either side of the equator is deflected slightly poleward and replace by deeper water http://www-das.uwyo.edu/~geerts/cwx/notes/chap11/equat_upwel.gif Upwelling is important because it brings nutrient-rich water to the surface.
Wind blowing parallel to the coast can cause coastal upwelling The Coriolis effect deflects the water to the right (in the NH), and the resultant Ekman transport moves it offshore. Deep water then rises (moves vertically) to replace the seaward-moving surface water. Nutrient-rich water
Satellite view of the west coast of the U.S. Although nutrients in surface water are sometimes depleted by the rapid growth of organisms, deep water is often rich in nutrients. When that water rises toward the sunlit surface, biological productivity increases. High biological production near the coast of California
Langmuir circulation Windrows
Question Time! How is the Coriolis effect involved in equatorial upwelling?
In the tropical Pacific, surface winds usually blow from east to west Major wind patterns http://people.hofstra.edu/geotrans/eng/ch1en/conc1en/img/windpatterns.gif
Normal conditions The trade winds blow from the normally high-pressure area over the eastern Pacific (near Central and South America) to the normally stable low-pressure area over the western Pacific (north of Australia).
Sea surface temperature North America equatorial upwelling South America coastal upwelling
North America cooler in east warmer in west
Normal conditions But, for reasons that are still unclear, these pressure areas change places at irregular intervals every 3-8 years.