Unit 10 Ocean Currents

1. Unit 10 Ocean Currents Bill Nye The Science Guy on Ocean Currents Bill Nye The Science Guy on Ocean Currents Intro video

2. Ocean currents • Surface currents • Affect surface water within and above the pycnocline(top 10% of ocean water) • Driven by major wind belts of the world • Deep currents • Affect deep water below pycnocline (bottom 90% of ocean water) • Driven by density differences • Larger and slower than surface currents

3. Measuring surface currents • Direct methods • Float meters • Intentional • Inadvertent • Propeller meters • Indirect methods • Pressure gradients • Satellites • Doppler flow meters The great Nike sneaker spill

4. Surface currents closely follow global wind belt pattern • Trade winds at 0-30º blow surface currents to the west • Prevailing westerlies at 30-60º blow currents to the east Atlantic Surface Currents

5. Wind-driven surface currents

6. Current gyres • Gyres are large circular-moving loops of water • Subtropical gyres • Five main gyres (one in each ocean basin): • North Pacific • South Pacific • North Atlantic • South Atlantic • Indian • Generally 4 currents in each gyre • Centered at about 30º north or south latitude

7. Current gyres • Gyres (continued) • Subpolar gyres • Smaller and fewer than subtropical gyres • Generally 2currents in each gyre • Centered at about 60º north or south latitude • Rotate in the opposite direction of adjoining subtropical gyres

8. Ekman spiral • Ekman spiral describes the speed and direction of flow of surface waters at various depths • Factors: • Wind • Coriolis effect Ekman spiral

9. Ekman transport • Ekman transport is the overall water movement due to Ekman spiral • Ideal transport is 90º from the wind • Transport direction (right or left) depends on the hemisphere

10. Geostrophic flow and western intensification • Geostrophic flow causes a hillto form in subtropical gyres • The center of the gyre is shifted to the west because of Earth’s rotation • Western boundary currents are intensified Western Intensification

11. Western intensification of subtropical gyres • The western boundary currents of all subtropical gyres are: • Fast • Narrow • Deep • Western boundary currents are also warm • Eastern boundary currents of subtropical gyres have opposite characteristics

12. Currents and climate • Warm current • warms air • high water vapor • humidcoastal climate • Cool current • cools air • low water vapor • drycoastal climate Warm and cool currents

13. Upwelling and downwelling • Vertical movement of water • Upwelling = movement of deep water to surface • Hoists cold, nutrient-rich water to surface • Produces high productivities and abundant marine life • Downwelling = movement of surface water down • Moves warm, nutrient-depleted surface water down • Not associated with high productivities or abundant marine life

14. Coastal upwelling and downwelling • Ekman transport moves surface water away from shore, producing upwelling • Ekman transport moves surface water towards shore, producing downwelling

15. Equatorial Upwelling • Tradewinds blow equatorial waters north and south • Upwelling from the deep replaces water moving north and south Equatorial upwelling

16. Antarctic surface circulation • Surface currents circle Antarctica moving to the east • Driven by prevailing westerlies

17. The Gulf Stream and sea surface temperatures • The Gulf Stream is a warm, western intensified current • Meanders as it moves into the North Atlantic • Creates warm and cold core rings (eddys) Gulf Stream Current

18. Gulf Stream

19. The Gulf Stream Benjamin Franklin’s Map

20. The Sargasso Sea

21. North Pacific Garbage Patch • Video - The Great Pacific Garbage Patch

22. Other surface currents • Longshore currents • Run parallelalong coastlines • Responsible for formation of barrier islands and transport of sand along beaches • Rip currents • Occur when coastal waves or currents converge • Dangerous for swimmers because they flow off the beach towards the open ocean

23. El Niño-Southern Oscillation (ENSO) • I am El Nino! • El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmas time • La Niña = opposite of El Niño • Southern Oscillation = change in atmosphericpressure over Pacific Ocean accompanying El Niño • ENSO describes a combined oceanic-atmospheric disturbance

24. Normal conditions in the Pacific Ocean

25. El Niño conditions (ENSO warm phase)

26. La Niña conditions (ENSO cool phase; opposite of El Niño)

27. The 1997-98 El Niño • Sea surface temperature anomaly map shows warming during severe 1997-98 El Niño • Internet site for El Niño visualizations • More animations • Current state of the tropical Pacific

28. El Niño recurrence interval • Typical recurrence interval for El Niños = 2-12 years • Pacific has alternated between El Niño and La Niña events since 1950

29. Effects of severe El Niños

30. Deep currents • Deep currents: • Form in subpolar regions at the surface • Are created when high density surface water sinks • Factors affecting density of surface water: • Temperature (most important factor) • Salinity • Deep currents are also known as thermohaline circulation

31. Deep ocean characteristics • Conditions of the deep ocean: • Cold • Still • Dark • Essentially no productivity • Sparse life • Extremely high pressure

32. Identification of deep currents • Deep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined

33. Global conveyer-belt It takes 1,000 years for water to make a complete circuit

34. The Conveyor Belt and CO2 • At the ocean surface, CO2 is dissolved by water and photosynthetic plankton convert it to oxygenand carbonate (shells) • CO2 in the water is also circulated on the conveyor belt into the deep oceans where it is sequestered • In effect, the oceans and conveyor belt current acts as a carbon “sink”, removing it from the atmosphere

35. Global Conveyor Belt Links • NOS current site • Animations • NASA Video • The Next Ice Age?