Ocean Circulation
= One of the most important ocean processes
= currents => 2 major types
1. Surface currents
2. Thermohaline currents
• Forces
• Primary versus Secondary forces
• Primary = force of the wind, thermal expansion and contraction, density differences between layers
• Secondary = coriolis, gravity, friction, shape of ocean basin.

Surface Currents - rivers in the ocean
• Wind (westerlies and trades) exerts stress on the surface of the water.
• The wind blows across the water - friction and drag
• inefficient transfer! - but density and inertia

Eckman Transport
• Eckman Spiral - friction in layers of water
• 45 degree deflectionat surface to the right in northern hemisphere
• effective to 100 m
• net transport 90¾ to right of prevailing wind direction.
• combined effects of wind, coriolis, and Eckman = hill in the west central parts of gyres

Gyres
• because of position of continents, coriolis, and friction effects within the ocean .... Surface Currents = large circles = Gyres
• => Gyres circle to right in Northern Hemisphere and to the left in the Southern Hemisphere
Geostrophic Gyres
• Most of the major surface currents are geostrophic = in balance with equatorial to polar pressure gradient (thermal expansion) and the coriolis effect.

Surface currents w/i the 6 great currents or gyres
• Western boundary currents (warm currents)
• narrow and intense
• 2 m/s, depth of 450 m, width of 70 km
• carry warm water towards higher latitudes
• ex: Gulf Stream (55 X 106 m3/sec)

Western boundary currents - features
• Eddies => looping meanders split off.
– Cold core = counterclockwise
– Warm core = clockwise
• Western Intensification
– pressure gradient and rotation
– trade winds on both sides of the equator converge on the west side of ocean basins

Eastern Boundary currents
• cold currents
• wide and diffuse
• 2 km/hr and very shallow
• carry cold water towards the equator
• ex: Canary Current (16 X 106 m3/sec)

Transverse (Zonal) currents
• high latitude eastern flowing (westerlies)
• low latitude western flowing (trades)
• ex: North Equatorial

Other Surfaces Currents
• high latitude currents
• Greenland and Labrador - iceberg freewayUpwelling and Downwelling

• horizontal movement of water results in vertical transport of water => wind-induced vertical circulation
• Equatorial Upwelling
• low latitude zonal deflected slightly towards poles
• warm surface waters replaced by cold bottom waters.Upwelling and Downwelling

• Coastal Upwelling (seasonal)
– wind blowing parallel to shore causes warm surface water to move away from shore. Ekman transport.
– warm water replaced by cold nutrient rich water
• Downwelling (seasonal)
– wind blowing parallel to shore causes water to move towards shore.

Thermohaline Circulation
• global ocean circulation
• slow moving (2-10 cm/s) deep-water currents driven by differences in density. Cold hyper saline water sinks and buoys up less dense water.
• Water Masses
• 1. Surface water (200 m)
• 2. Central water (bottom of thermocline)
• 3. Intermediate water (1500 m)
• 4. Deep water (4000 m)
• 5. Bottom water (contact with sea floor)
• water mass reflect surface conditions where they formed.

Thermohaline Circulation
• Deep Water => Formation and Downwelling
• cold dense brines generated at the poles and move along the sea floor towards the equator.
• Patterns
• sinking dense water offset by an equal volume of water rising elsewhere..Global Currents
• Surface and Deep-Water currents = connected
• Distribution of heat, salt, and dissolved gasses
• Very complicated system (oversimplified)
• Could help explain climate change

Great Ocean Conveyor Belt
• Climate change => past and future
• Will require more study
• Global warming - could spark the next ice age

Long-Term Climate Change
• ICE AGES = Ice House conditions
– 5 times in earth history
• Causes
– Plate Tectonics
– CO2
Climate Change in an Ice House World
• Variations in solar radiation
• Milankovitch Orbital Perturbations