1. Lecture 14 Waves and Currents 3
June 9, 2014

2. Surface Currents Surface currents are driven by wind
Most of Earth’s surface wind energy is concentrated in the easterlies and westerlies.
Due to the forces of gravity, the Coriolis effect, solar energy, and solar winds, water often flows in a circular pattern around the periphery of an ocean basin and is called a gyre.

3. Surface Currents
Water moves clockwise in the Northern Hemisphere gyres and counterclockwise in the Southern Hemisphere gyres.
gyre

4. Wind-driven circulation

5. Major Surface Currents

6. Surface Currents Weather and climate Transport of heat and moisture
Production of biogenic deposit
Planktonic organisms – tracers
Drift of icebergs

7. Current Markers Action of current Side-scan sonar (Fig. 4-15, 4-16)
Winnowing – sorting
Pavement of cobbles, nodules, shells
Scour marks
Side-scan sonar (Fig. 4-15, 4-16)
Current strength (Fig. 4-17)
Ripple formation (Fig. 4-18)

8. Side-scan sonar Activity of nearbottom current
Streaks of coarse material in fine sediments

9. Comet marks
Current action
Acoustic sensing
Sonograph
Dark spots

11. Ripple formation Gentle up-current side Steep down-current slope
Ripple migration
Cross-bedding
Giant ripple
Underwater dune
Tidal ridge

12. Major Surface Currents

13. Surface Currents
Water moves clockwise in the Northern Hemisphere gyres and counterclockwise in the Southern Hemisphere gyres.
gyre

14. Flow Within a Gyre
Gyres can be further subdivided into distinct currents. The four interconnected currents in the North Atlantic Gyre have different flow characteristics and temperatures.
The Ekman Spiral
The motion of the water at the surface is driven by the wind. Each subsequent layer below is driven by the movement of the water above, and slowed down by friction. The net water movement is the Ekman Transport.

15. Flow Within a Gyre
The effect of Ekman spiraling and the Coriolis effect cause the water within a gyre to move in a circular pattern.

16. Ekman spiral

17. Flow Within a Gyre

18. Geostrophic Gyres
Geostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect. Of the six great currents in the world’s ocean, five are geostrophic gyres.

20. Eddie formation in the Gulf Stream

21. Upwelling And Downwelling
Wind induced vertical circulation is vertical movement induced by wind-driven horizontal movement of water.
Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface.
Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases.

22. Coastal vertical flows

23. Equatorial Upwelling

24. Upwelling NW-SW Africa, California, Peru, Chile Coastal upwelling
High production
High supply of organic matter
Depression of oxygen content
Anaerobic sediments (varves)

25. Effects of Surface Currents on Climate
Surface currents distribute tropical heat. Warm ocean currents are shown in red and cold currents in blue.

26. Countercurrents and Undercurrents
Countercurrents flow on the surface in the opposite direction from the main currents.
form due to lack of persistent westward winds
Undercurrents are countercurrents that flow beneath the surface current.

27. The Temperature-Salinity Diagram

28. Thermocline Circulation
The movement of water due to different densities is thermocline circulation. Remember that the ocean is density stratified, with the densest water at the bottom. There are five common water masses:
 Surface water – to a depth ca. 200 m
 Central water – to the bottom of the main thermocline
 Intermediate water – to ca. 1,500 m
 Deep water – water below intermediate water but not in contact with the seafloor
 Bottom water – water in contact with the seafloor

29. Thermohaline circulation

30. Thermocline Circulation Patterns
Diffuse Upwelling
A model of thermocline circulation caused by heating in lower latitudes and cooling in higher latitudes.

31. Water layers and deep circulation of the Atlantic Ocean
Contour currents

32. Thermohaline circulation

33. Water circulation in the World Ocean

34. Ocean Conveyor Belt

35. Deep water circulation
NADW
AABW
AAIW
Halothermal circulation
Eocene ocean
WSDW
Tethys Ocean

36. HEBBLE High Energy Benthic Boundary Layer Experiment
Western Boundary Current
Contour Current
Abyssal or benthic storms
Sediment drift
Contourite

37. Contour current (Deep geostrophic current)

38. Effect of deep current Continental rise 5 km deep No evidence
Scour marks
Lineation
20-30 cm/s
Giant dunes

39. Turbidity Currents Mud-laden gravity-driven current
Continental slope, submarine canyon, deep-sea fans
Strictly episodic
Autosuspension
Downhill running

40. Exchange Currents Marginal, semi-enclosed seas Estuarine circulation
Black Sea, Baltic Sea
Anti-estuarine circulation
Arid zone, excess of evaporation
Mediterranean, Red Sea, Persian Gulf
Application to ocean conveyor

41. End of Lecture 14
Ocean water, circulating in currents, distributes heat from the equator to the poles. Currents have a profound effect on Earth’s weather and climates.