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