The Surface Currents

An Introduction to the World’s Oceans Sverdrup et al. - Chapter Nine - 8th Ed.

Surface Currents• The Ekman spiral and Ekman transport

– Coriolis force– Frictional coupling– Wind and surface water transport

• Ocean gyres– Wind patterns and Ekman transport

• Geostrophic flow– Convergent and divergent Ekman transport– Force balance: Coriolis force and pressure

force– Using subsurface density distribution to

deduce volume transport in the geostrophic flow

Fig. 9.1

Fig. 9.2

Fig. 9.3

Oceanic gyres.Oceanic gyres.

Fig. 9.4

TOPEX/Poseidon satellite eye view of the North Atlantic “hill”

TOPEX/Poseidon satellite eye view of the North Atlantic “hill”

Wind-Driven Ocean Currents

Wind-driven currents and continuity

currents: see map in Figure 9.5

•Pacific Ocean currents

•Atlantic Ocean currents

• Indian Ocean currents

•Arctic Ocean currents

Fig. 9.5

Fig. 9.6

Current Flow

• Current speed– Relation to wind speed

– Acceleration through narrow gaps

– Volume transport by ocean currents

9.3 Current Flow, continued

• Western intensification– Stronger, deeper, and narrower currents on western

side of oceans

– Factors that play key roles in western intensification:• Eastward turning of the Earth• Increase of Coriolis effect with latitude• Changing strength of wind with latitude• Friction between land masses and ocean water currents

– Stronger geostrophic currents implies steeper sea surface slope

Eddies• Fast-moving current moves into slower-

moving water• Currents develop meanders that can break off

to form eddies– Trapped warm or cold water.

• Warm eddy circulates clockwise (to the right.• Cold eddy circulates counterclockwise (to the left)• Can last for years (3 or so) and disturb the abyssal

plain.

• Eddies and main flow energy dissipation• Measuring eddies

– Satellites measure sea surface height to map geostrophic eddies

– Floats monitor deep water eddies

Fig. 9.7

Gulf Stream with eddies:

3. warm core

4. cold core

Gulf Stream with eddies:

3. warm core

4. cold core

Fig. 9.9

Convergence and Divergence

• Langmuir cells

– Caused by strong winds

– Mix surface waters

– Organize the distribution of organic matter in convergent and divergent zones

• Permanent zones (see map)

– Convergences (low in nutrients and aquatic life)

• The tropical convergence

• The subtropical convergences

• The Arctic and Antarctic convergences

– Divergences (upwelling delivers nutrients to surface waters)

• Two tropical divergences

• The Antarctic divergence

Fig. 9.10

Convergence and Divergence

•Seasonal zones – Examples from the west coast of

North America: •Southerly alongshore winds produce

onshore Ekman transport and downwelling in winter

•Northerly alongshore winds produce offshore Ekman transport and upwelling in summer

Changing Circulation Patterns

• Global currents– Major changes in ocean circulation associated with

glacial and interglacial periods - Milankovitch Cycles

– Ice cores and sediment cores record changes

• North Pacific Oscillations– Pacific Decadal Oscillation (PDO)

• Ocean: current shifts and changes occur in water temperature

• Atmosphere: changes in surface pressure, winds, and precipitation

• North Atlantic Oscillations– Complex interaction between the ocean and

atmosphere• Pools of anomalously cold and low-salinity surface water

associated with harsh winters in Europe

• Life spans of anomalies: 4-10 years

Fig. 9.14

Fig. 9.16

9.7 Measuring the Currents • Current measurements fall into two groups:

– Following water parcels•Drifting buoys are tracked using Global

Positioning Systems (GPS) and satellites– Measuring speed and direction at a fixed point:

•Current meters•Doppler effect method•Satellites measure sea surface height to

deduce geostrophic flows

9.8 Practical Considerations: Energy from the Currents

•Massive reservoir of energy (280 trillion watt-hours)

• Indirect source of solar energy•No cost effective method of

harnessing energy at present time

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