1. Ocean Currents

2. Ocean Currents
The ocean has a complex circulation system, moving water, heat, salt and nutrients around the world.
Surface currents in the top 400m are driven mainly by wind.
Deeper currents are driven by changes in water density.
Both types of currents work with the atmosphere to help shape the Earth’s climate.
Ocean currents
Ocean circulation explained
Ocean currents 2

3. Mixing of ocean currents
Mixing is a key dynamic in the oceans, creating currents and exchanges between cold, deep waters and warmer surface waters. These processes redistribute heat from low to high latitudes, carry nutrients from deep waters to the surface, and shape the climates of coastal regions.
Mixing allows 3 main processes to occur in the ocean. What are they?

4. Causes of ocean mixing
Several types of forces cause ocean mixing. Waves and surface currents are caused mainly by winds. When winds "pile up" water in the upper ocean, they create an area of high pressure and water flows from high to low pressure zones. Ocean currents tend to follow Earth's major wind patterns, but with a difference: the Coriolis force deflects surface currents at an angle of about 45 degrees to the wind—to the right in the Northern Hemisphere, left in the Southern Hemisphere. This pattern is called Ekman transport, after Swedish oceanographer Vagn Ekman. Each layer of the ocean transfers momentum to the water beneath it, which moves further to the right (left), producing a spiral effect.
Coriolis force
Wind driven circulation (Ekman transport)
What are the main causes of ocean mixing?
Define Coriolis force and Ekman transport.

5. Ekman spirals contribute to ocean current patterns.

6. Ocean mixing at deeper levels
At deeper levels, ocean mixing is caused by differences in density between colder, saltier water and warmer, fresher water. Because the density of water increases as it becomes colder and saltier, it sinks at high latitudes and is replaced by warm water flowing northward from the tropics. (This pattern, called the Thermohaline Circulation, is a key mechanism that helps to regulate Earth's climate). Cold water typically flows below warmer water, but when winds blowing along coastlines deflect warm surface currents away from shore through Ekman transport, they allow cold, nutrient-rich water to rise to the surface. This coastal upwelling process occurs against western coastlines in the Atlantic, Pacific, and Indian oceans.
What causes ocean mixing at deeper levels?

7. Gyres
The combined effects of these forces create circular currents called gyres in the world's largest oceans, centered at about 25° to 30° north and south latitudes. Gyres rotate clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere, driven by easterly winds at low latitudes and westerly winds at high latitudes. Due to a combination of friction and planetary rotation, currents on the western boundaries of ocean gyres are narrower and flow faster than eastern boundary currents. Warm surface currents flow out of ocean gyres from the tropics to higher latitudes, and cold surface currents flow from colder latitudes toward the equator.
What are gyres?

8. Ocean currents flow in distinct patterns generated by wind, Earth's rotation, water heat content and salinity, and bottom topography.

9. Map of ocean gyres
Each of the 5 main oceans has an overall large-scale pattern of currents, called gyres, that circle around them.