1. Ocean Currents
Ocean water circulates in currents caused by wind and by density differences
Currents are the flow of water between areas of different surface levels or different densities

2. Ocean Currents
Ocean water moves constantly sideways, up, down because it is:
Blown by wind
Moved by density differences
Heated unevenly
Evaporated unevenly
Affected by Earth’s rotation (Coriolis)
Pulled by sun & moon
The Ocean: A Driving Force for Weather and Climate

3. CURRENTS Currents are important because they:
moderate climates (transfer heat)
mix nutrients and gases
transport larvae and plankton
Warm air from equator flows toward poles, drives air circulation, makes wind, moves water
Continents deflect east-west water movement, but not air
NASA
NOAA

4. Coriolis Effect
Water near ocean surface moves to the right of wind direction in Northern Hemisphere
Water near ocean surface moves to the left of wind direction in Southern Hemisphere

6. Coriolis Effect Coriolis Effect Causes currents to move
Clockwise in the Northern Hemisphere
Counterclockwise in the Southern Hemisphere
Coriolis Effect

7. Surface Currents Affect the uppermost 10% of world ocean
Some rapid & riverlike, some slow and diffuse
Largest organized into gyres

8. The circular patterns of surface water currents are called gyres
The circular patterns of surface water currents are called gyres. The characteristics of a current depend upon where the water is coming from in the gyre.

12. Western boundary currents are located on the western side of oceans.
Eastern boundary currents are located on the eastern side of the OCEAN - not the continent.
East Coast of United States
Gulf Stream
in red
Western boundary currents are located on the western side of oceans.
Florida

13. Comparison of Boundary Currents
Western
Eastern
currents bring warm water from the equator
1. currents bring cold water from the poles
2. high salinity;
low oxygen content
2. low salinity; high oxygen content
3. nutrient poor, infrequent upwelling
3. nutrient rich due to upwelling
4. lower biomass
4. high biomass

14. Surface Currents

15. Effect of Surface Currents on Climate
Surface currents distribute heat worldwide
Warm water flows to higher latitudes transfers heat to the air and cools
Moves back to low latitudes
Absorbs heat
Repeats

16. Vertical Movement Upwelling – upward movement of water
Can be caused by wind blowing parallel to shore or offshore (Coastal Upwelling)
Brings nutrients

19. Vertical Movement
Downwelling – downward movement of water – caused by:
Increase in surface water density
Wind blowing parallel to shore or onshore (Coastal Downwelling)
Brings dissolved gases and nutrients to deeper ocean

21. EL NIÑO

22. NORMAL
Normally, trade winds blow warm surface water toward the western Pacific.
This causes the thermocline to rise near the surface in the eastern Pacific.

23. Notice the warm water is in a pool in the western Pacific Ocean.
Cold, nutrient-rich water wells up from the thermocline to the surface waters off the coast of Peru - good for fishing.
This image shows normal sea surface temperature in the Equatorial Pacific Ocean in January 1997.
Notice the warm water is in a pool in the western Pacific Ocean.
NOAA

25. EL NIÑO El Niño describes abnormal oceanic and atmospheric conditions.
When the Trade Winds falter, warm water piles up in the western Pacific about 1 & ½ feet higher than the average ocean surface
First noticed off the coast of Peru around Christmas time, the condition was called El Niño referring to the Christ child.

26. El Niño’s weaker trade winds blow warm water eastward, across the equatorial region of the Pacific.
NOAA
This depresses the thermocline across the Pacific, and stops upwelling.

28. Because warm water evaporates faster, the air fills with more water vapor and it produces more rain.
El Niño occurs primarily in the tropical Pacific Ocean but it affects global weather.

29. LA NIÑA
NOAA
When too much westerly wind blows surface water near the Americas, it has an effect on the weather that is often opposite to El Niño, called La Niña.

30. Comparison of El Niño and La Niña
warmer ocean temperatures
colder ocean temperatures
occurs every 3-7 years
frequency unknown
wetter than normal summers
drier than normal summers
weak trade winds
strong trade winds
decreases hurricanes forming in N. Atlantic

31. 1997 was the warmest year of the century and the strongest El Niño in 40 years. Oddly, five of the century’s warmest years have been in the last decade.
NASA

32. DENSITY DRIVEN CURRENTS
AKA THERMOHALINE CURRENTS

33. Thermohaline currents found on the ocean floor.
“Thermo” refers to temperature and “haline” refers to salinity.
NASA
Water slows down below the pycnocline (area defining water masses of different densities) to 1-2 meters/day.
Cold water sinks at the poles and crawls toward the rising warmer water at the equator.
NASA

34. Density Structure of the Ocean
Salt water weighs 2-3% more than pure water
Cold, salty water is more dense than warm, less salty water
Much of the ocean can be divided into three density zones:
surface, pycnocline & deep

35. Density Zones Surface or Mixed Zone
Temperature, salinity & density constant due to currents and waves
In contact with atmosphere
Exposed to sunlight
Least dense water
2% all of ocean water
~150 meters deep

36. Density Zones Pycnocline aka Thermocline aka Halocline
Density increases with depth, temperature decreases, salinity increases
Isolates surface water from denser layer below
18% of all ocean water

37. Density Zones Deep Zone Below pycnocline Depth below 1000 m
Little change with depth
Very cold (1-3C) and stable
80% of all ocean water

38. Density stratification in the ocean – surface layer does not mix with bottom layer

39. Density Stratification & Water Movement
Even deepest water originates at surface
Dense water forms:
near poles (as water freezes)
in enclosed areas (where water evaporates)

40. Density Stratification & Water Movement
Water masses below pycnocline do not mix – no energy – too cold

41. Thermohaline Circulation
90% of ocean water below surface zone gravity driven
Dense water sinks, less dense water rises
Density is a function of temperature and salinity – movement of water due to density differences called thermohaline circulation
Can take hundreds of years to circulate

42. Water masses form at the surface
Densest & deepest form where water becomes very cold and salty (at the poles)

44. Circulation of the Atlantic Ocean

46. The Global Heat Connection
Because they transfer large quantities of heat, ocean currents affect world weather and climate
The transport of tropical water to the polar regions is part of the global conveyor belt for heat transfer

47. Ocean Flow The slow, steady flow of water
distributes dissolved gases and solids
mixes nutrients
transports juvenile marine organisms among ocean basins