Ocean Currents: Surface and Deep Ocean

• Ocean currents are flowing masses of ocean water
• Wind-driven (surface) currents move water horizontally and are set in motion by wind
• Deep ocean currents move vertically and horizontally, being density-driven

Measuring Currents

• Direct methods include using floating drifters and current meters
• Indirect methods include pressure gradients, radar altimeters, and Doppler flow meters
• Chemical tracers, characteristic temperature/salinity, and Argo floats are used to measure deep ocean currents

Surface Currents

• Impact the top 10% of ocean water
• Friction exists between wind and the ocean surface, with only 2% of wind energy transferred to the ocean surface
• Surface currents move slower than corresponding winds
• Surface currents occur above the pycnocline

Observation by Fridtjof Nansen

• Observed that floating ice does not move in the same direction as the wind
• Instead it moves at a 20- to 40-degree angle to the right of the wind

Ekman Spiral

• Developed by V. Walfrid Ekman, in 1905
• Explains the balance between friction and the Coriolis effect
• Describes the direction and flow of waters at different depths near the ocean surface

Ekman Transport

• Average movement of surface waters is:
  • 90° to the right of the wind in the Northern Hemisphere
  • 90° to the left of the wind in the Southern Hemisphere

Wind Belts and Surface Currents

• Wind belts set water in motion
• The distribution of continents disrupts flow
• Flow is influenced by gravity, friction, and the Coriolis effect

Subtropical Gyres

• These are large, circular loops of moving water
• Five are centered around 30° latitude
• Bounded by four currents, including:
  • Northern or Southern Boundary currents
  • Eastern Boundary current
  • Equatorial current
  • Western Boundary currents

Geostrophic Flow

• Coriolis effect causes convergence, piling up water in the gyre's middle
• Gravity pulls water downhill
• Forces nearly balance, creating rotational flow
• Ideal geostrophic flow follows a path around the hill of water in equilibrium
• Actual flow is slightly downslope due to friction

Western Intensification

• Coriolis effect increases toward the poles
• Eastward-flowing high latitude water turns more strongly toward the equator

Comparing Boundary currents

• Western Boundary:

  • Fast
  • Narrow and Deep
  • Large transport volume
  • Move warm water from equator toward poles
  • Examples include the Gulf Stream, Kuroshio Current, East Australian Current, Brazil Current, and Agulhas Current

• Eastern Boundary

  • Slow
  • Wide and Shallow
  • Small transport volume
  • Move cold water from poles toward the equator
  • Examples include the Canary, Benguela, California, Humboldt, and West Australian Currents

Effects of Gyres on Climate

• Wind belts (2/3) and ocean currents (1/3) transfer heat from the tropics to the poles
• Temperatures and humidity levels on land are affected