1. Essentials of Oceanography
Ocean Currents
Essentials of Oceanography

3. Wind-driven Ocean Circulation
Surface ocean circulation
mixed layer above top 100 m
controlled by winds + coriolis
Overhead wind patterns
Wind blows against surface- friction sets water into motion
Continents interfere with the winds and redirect airflow
Result- circulation cells within each ocean basin

4. Gyres
Gyre -
closed, circular flow of water around an ocean basin
5 gyres:
North Atlantic
South Atlantic
North Pacific
South Pacific
Indian Ocean
Plus circulation around Antarctica- closed circuit
wind and water can freely flow around Antarctica

5. Surface Ocean Currents

6. Why do surface currents only flow around the outside of ocean basins?
Why doesn't the Coriolis Effect turn the currents into the center of the gyres?
The observed surface current is called geostrophic flow
balance b/w Coriolis and gravity
keeps circulation around the periphery.
Types of geostrophic currents
Transverse currents
Western boundary
Eastern boundary

7. Transverse Currents
Driven toward east or west by Trades and Westerlies.
Trades strongest, most consistent, so equatorial currents strong.
Equatorial currents
ITCZ displaced to north of the equator = caloric equator
Due to land distribution.
Equatorial Counter Current
Returns flow at the doldrums
No wind to prevent back flow
ITCZ = Intertropical Convergence Zone

8. Western Boundary Currents
Flow from equator to pole along western margin of basins
strong, fast, narrow, focused flow
Transports heat to higher latitudes

9. Cause of Strong Western Boundary Current
Convergence of trade winds
all water along the equator is being driven to the west
Water piles up on western margin
Westerlies don't converge
Rotation of the earth
"hill" offset to west, so flow confined to narrower channel.

10. Gulf Stream Good example of a Western Boundary
Current that flows like a river- amount of water carried = 100x discharge from all rivers!
First mapped by Ben Franklin
Major mechanism for transport of heat to North.
Climate in England vs. Newfoundland

11. Eastern Boundary Currents
Eastern flow more diffuse, wider, slower
Cold water currents

12. Divergence and Convergence
Where currents or current and land split apart or come together.
Convergence leads to downwelling
Divergence leads to upwelling- brings cold, nutrient-rich water up from about 500 m
2 important areas of upwelling
Pacific equatorial region
Near shore
along shore winds force water off the coast - creates low water pressure
eastern margins of ocean basins - Calif. coast, Peru

13. Divergence and Convergence

14. El Niño- Southern Oscillation (ENSO)
Represents interactions between:
atmospheric circulation
ocean circulation
climate
Begins in equatorial Pacific, but has global effects
Cause is not well understood
El Nino refers to changes in ocean circulation
Named for anomalous warm current off Peru that occurs at Christmas time.
Normally - cold current off of Peru due to upwelling

15. El Niño- Southern Oscillation (ENSO)
Refers to changes in atmospheric conditions
Oscillation in the distribution of high and low pressure systems across the equatorial Pacific.
Affect wind patterns, which affects surface ocean circulation.
ENSO = combined effects of atmosphere and oceans

16. Normal
El Nino

17. ENSO Comparison Normal years El Nino years
Lower pressure over Indonesia
Higher pressure over eastern equatorial Pacific
Driven by strong trade winds
Weak equatorial counter current
Strong upwelling near Peru (and Calif)
W. Pac ~ 8º warmer than E Pac.
Rain in western Pacific, dry in eastern Pacific
El Nino years
Higher pressure over Indonesia
Lower pressure over eastern Pacific
Decreased pressure gradient across the equatorial Pacific weakens trade winds
Stronger countercurrent transports warm water to the east
Reduced upwelling
Shift in rainfall to the east

18. Periodicity ENSO periodicity 2-7 years
~1 event every 4 years for past century and 1 strong event per decade
But duration and extent variable (each unique)
Appear to be becoming more frequent over past few decades
prolonged ENSO conditions
Natural variability vs. global warming effects

19. Effects of ENSO
Largest effect is on global precipitaion patterns

20. Oceanic Deep-water Circulation
Subsurface currents arise from the density differences between water masses
Produced by the variations in water temperature (thermal effect) and salinity (haline effect)
Collectively referred to as thermohaline circulation

21. Thermohaline Circulation
Evaporation and lower temperatures cool surface waters from ~ 45º N and ~ 45º S latitude to the poles
Cold (and therefore dense) polar water sinks and then drifts equatorward, below warmer, less dense surface water
Cold water descends to a depth of corresponding density, 'sliding' under less dense water and over more dense water
Deep waters slowly return to the surface (after ~1000 years) through upwelling along the equator and in coastal regions

22. Deep Water Formation Northern hemisphere deep water formation
Winter cooling and evaporation
Produces cold, moderately saline waters
Primarily in Greenland and Norwegian Seas
Evaporation in the Mediterranean Sea creates very saline intermediate water
Combine to form North Atlantic Deep Water (NADW)
Southern hemisphere bottom water formation
Formation of sea ice
Produces of cold, saline waters
Densest water in the global ocean!!
Created off the shores of Antarctica
Called Antarctic Bottom Water (AABW)

23. Global Circulation
NADW sinks and flows southward along the western side of the Atlantic Ocean
NADW and AABW mix in the Antarctic Circumpolar Current
Mixed water mass of NADW and AABW flows northward into the Indian and Pacific Oceans
Upwells in the N. Pacific and Indian Oceans and returns to the south as warm shallow waters

24. Summary Surface circulation is driven by global wind patterns
El Nino is a warming of the west coast of South America and causes a disruption of global precipitation
Deep water circulation is driven by gravity through density changes caused by temperature and salinity