Gyres & Major Currents

Five main gyres:

1. North Pacific Ocean
2. South Pacific Ocean
3. North Atlantic Ocean
4. South Atlantic Ocean
5. South Indian Ocean
   (what happened to the North Indian Ocean Gyre? Blame India - it juts down into the North Indian Ocean, causing the currents to go around it. The seasonal monsoonal winds also disrupt the current flow.)

Four main currents exist within each gyre

• A Western Boundary Current
• An Eastern Boundary Current
• Two Transverse Currents

Western boundary currents Narrow, fast and deep Move warm water poleward Well defined boundaries Can create features like eddies Little to no coastal upwelling – nutrient poor

Eastern boundary currents

• Slow, shallow & broad
• Carry cold water to the Equator
• Poorly defined boundaries
• Tend not to form eddies
• Upwelling common – nutrient rich

Transverse Boundary Currents

• Currents which flow from east to west and west to east
• Link eastern and western boundary currents

Other Currents

• Countercurrents flow on the surface in the opposite direction from the main currents.
• Undercurrents are countercurrents that flow beneath the surface current.

Effects of Surface Currents on Climate

Warm water currents bring warm, humid air to higher latitudes. Cold water currents bring cool, arid air to lower latitudes. Both help to moderate global temperatures. Together with pressure systems these currents determine where certain climates are or not. For example, high pressure systems (H) dry out and heat up the air; couple this with a cold water current and the dry air it brings with it and you have the conditions necessary for a desert. On the other hand, low pressure systems (L) are associated with humid conditions; couple this with a warm water current which brings humid air with it, place both in the equator, and you have the conditions necessary for a tropical rainforest.

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