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Oceanic Circulation
• Current = a moving mass of water
Oceanic CirculationSurface Currents
– horizontally flowing water in the uppermost 400m of the ocean
– driven by wind
Thermohaline Circulation– slower, deeper circulation– due to the action of gravity on water masses of
different densities
Surface Currents• Wind
– primary force responsible for surface currents
• Friction drags water along• Coriolis effect• Continents prevent continuous flow and
deflect water…Gyre
– the circular flow around the periphery of an ocean basin
Fig. 8-1, p. 172
Fig. 8-4, p. 173
Figure 7.4
Figure 7.7 Ekman Transport
Figure 7.8
Geostrophic Gyres
• Gyres in a balance between the pressure gradient (due to gravity) and the Coriolis Effect
Six great surface current
• North Atlantic Gyre
• South Atlantic Gyre
• North Pacific Gyre
• South Pacific Gyre
• Indian Ocean Gyre
• West Wind drift or Antarctic Circumpolar Current
Fig. 8-3, p. 173
Flow within Gyres
• Western Boundary Currents (ex: Gulf Stream)– westward intensification - more concentrated
due to water piling up due to eastward rotation of earth and the Coriolis Effect
– narrow, fast ( 3-10 km per hour or 2-6 miles per hour), deep currents that carry warm water toward poles
Figure 7.8
Fig. 8-13b, p. 180
Flow within Gyres
• Eastern Boundary Currents (ex: Canary Current)– broad, slow, shallow currents that carry cold
water toward equator
Table 7.2
Flow within Gyres
• Transverse Currents – Northern or Southern Border currents
• (ex: North Atlantic Current)
– Equatorial currents• (ex: North Equatorial current)
– currents that flow from east to west or west to east
Flow within Gyres
• Currents affect climate:– North Atlantic current warms England– California current cools San Francisco in the
summer
Figure 7.9
Sea Surface Temp in August
And in February
Vertical Circulation
• Upwelling– upward movement of water
• Downwelling– downward movement of water
Figure 7.12a
Vertical Circulation
• Coastal Upwelling– cold, deeper water upwells to replace the
surface water– leads to increased nutrients & productivity and
cooler climates
Figure 7.10
Vertical Circulation
• Equatorial Upwelling– westward flowing equatorial currents are
deflected poleward– deeper water comes up to replace the surface
water
Figure 7.13- Causes of upwelling
Vertical Circulation
• Downwelling– water driven toward the
coast will be forced down– Brings down dissolved
gases
Figure 7.11
ENSOEl Nino Southern Oscillation
• Southern Oscillation = Atmospheric pressure changes in the Pacific every 3 – 8 years – Becomes high in west & low in east– TradeWinds weaken, cease or reverse and blow
west to east
• El Nino = warm water moves east toward North & South America
Figure 7.21
Fig. 8-19 (a-b), p. 186
Fig. 8-19 (c-e), p. 187
Figure 7.22
El Nino 1997-98
La Nina2000
ENSOEl Nino Southern Oscillation
• Eastern Pacific (along west coast of South America):– Upwelling ceases– Sea-level rises in eastern Pacific– Water temp increases– Increased precipitation
Figure 7.23
Deep CirculationThermohaline Circulation
• Driven by density differences
• water masses do not mix easily but flow above or beneath each other
Some Water Masses in the Deep Atlantic
• Antarctic Bottom Water• North Atlantic Deep water• Mediterranean Intermediate Water• Antarctic Intermediate Water
Thermohaline Circulation
• Sinking of water masses is offset by slow, gradual rising across warmer temperate and tropical zones
Fig. 8-26, p. 192
Thermohaline Circulation
• Much slower than surface circulation– 10-20 km per year or 6-12 miles per year– Would take water a year to move as far as
surface water would move in an hour
Figure 7.28- The Ocean Conveyor Belt Model