1. Ocean Currents

2. Ocean Currents There are two types of ocean currents:
Surface Currents: are wind driven and affect about 10% of the ocean water, but cover the entire ocean surface.
Thermohaline Currents: (heat/salt) = density driven currents. They occur deeper in the ocean and affect ~ 90% of the ocean water.

3. Surface Currents Surface currents are mostly horizontal currents.
These currents follow the pattern of the wind e.g. the Westerlies/ Trade winds
They occur from the surface down to a few hundred meters ~ 660ft.
Generally travel clockwise in the northern hemisphere and counter clockwise in the southern hemisphere.

5. Significance of surface currents
Surface currents cause the general “mixing” of the worlds oceans and waters
move nutrients from more productive areas to less productive areas of the oceans.
Important role in migration and the movement of many less active (or passive) organisms (plankton, jellyfish).

6. Deep currents
Thermohaline currents are driven by density and temperature changes in water.
Thermo- = heat -haline = salt
These currents are also known as the “ocean’s conveyor belt”
They flow under the surface of the ocean and move significant amounts of ocean water vertically.

7. “The Great Conveyor Belt”
A global "conveyor belt" is set in motion when cold water forms in the Northern Atlantic, sinks, moves south, and circulates around Antarctica, and then moves northward to the Indian, Pacific, and Atlantic basins.
It can take a thousand years for water from the North Atlantic to find its way into the North Pacific.

8. Conveyor Belt

9. Significance of deep currents
Form upwelling: the drawing up of cold water from deeper regions to shallow waters throughout the Ocean
Result: all ocean nutrients don’t end up settling on the bottom: surface organisms can benefit from decomposers, cycle nutrients and sustain life.
So what is downwelling? 

10. DENSITY DRIVEN WIND DRIVEN sunlight Upwelling Downwelling 0m
Surface Mix Layer
Deep Water Layer
4000m m 0m
Upwelling Downwelling
WIND DRIVEN
sunlight
DENSITY
DRIVEN

11. Upwelling & Downwelling Convergence: water masses collide
and sink ® downwelling
Divergence:
Surface waters move APART.
Deep water rises up to fill
the gap ® upwelling
Upwelling brings NUTRIENTS
(from DECAYED organic
matter) back up to the surface.
Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.3

12. Upwellings will occur along the coast to fill void
When wind blow from the land toward the ocean, it pushes water away from the coast
Upwellings will occur along the coast to fill void
NE Tradewinds/ California Coast Upwellings

13. Density Stratification
Thermocline – Heat
Halocline – Salinity
Pycnocline – Density
Pycnoclines can prevent mixing between layers

14. Scripps Video - Pycnoclines
Scripps Video - Pycnoclines

15. REVIEW
Winds move water a net of 90 degrees to right of the wind (surface 45 degrees to rt)
Ekman spiral – top 600ft water moved by wind – in spiral shape as water pulls water
Where water converges due to wind = downwellings
Where water diverges due to wind = upwellings

16. REVIEW Upwellings and Downwellings necessary to mix oceans
Bring cold, nutrient rich waters back to surface
Take warm, nutrient depleted waters down to ocean bottom
Currents N and S of equator tend to move in circular patterns called GYRES

17. Gyres
Deflected by the continents and driven by surface winds, they bend and create large current loops called circulation gyres.
The centers often collect floating mats of seaweed and garbage. (Sargasso Sea and the N. Pacific gyre garbage patch.)

18. Gyre Formation
CLOCKWISE
Warm
Cool
COUNTER CLOCKWISE

19. Five major gyres and the Antarctic circumpolar current

20. North Atlantic Gyre Notice how the winds are helping the currents.
This also shows the effects of the Ekman spiral

21. Sargasso Sea “Sea of Weeds”

22. Sargasso Sea…home to many

23. North Pacific Garbage Patch

24. Comparing Currents… Are all currents similar? NO!
Eastern or Western side of ocean
Warm or Cold
Fast or Slow
Deep or Shallow
Wide or Narrow
High latitudes toward Low Latitudes or vice versa

25. Why are some currents FAST and other currents SLOW
Why are some currents FAST and other currents SLOW? What is the difference between Eastern AND Western Boundary Currents?

26. What happens when you push in on a plastic container of water?

27. The container moves first, and water moves a
fraction of a second later.

28. Water sloshes up again the left-hand side, creating a pile

29. As Earth rotates the continents smack into the oceans
Earth’s rotation
The Americas hit the
Atlantic Ocean
Asia, Australia hit the
Pacific Ocean

30. A West to East Cross Section of the mound of water
Continents crash into the mound of water.
The mound is asymmetrical.
This leads to Western Intensification – water piles up on the coast of North America
West
East
North
America 1m
Only ~ 1m high, but that’s enough to create BIG
differences in the currents.

31. Western Intensification
East
North
America
Gravity is trying to pull this down and out to flatten the ocean.
The flow is constricted on the WEST side and spread out on the EAST

32. Western Intensification
Gulf Stream
(Flowing into screen)
Narrow and fast
East
West
Canary Current
(Flowing OUT of screen)
diffuse, slow
North
America
Think about constricting flow out of a garden hose by covering half
the opening with your thumb. The constricted flow moves FASTER.
WHY?
SAME AMOUNT of water forced to move through smaller opening