General Ocean Circulation

Wind driven circulation• About 10% of the water is moved by

surface currents• Surface currents are primarily driven by

the wind and wind friction• Move fast relative to thermohaline

circulation (1 to 2 m/s)• Most water moved is above the pycnocline• Reflect global wind patterns and Coriolis

effect!

History

• Nansen first connected wind with currents (remember him? He froze his ship, the Fram into the ice and noticed how it drifted)

• Showed his measurements to Ekman who formulated a mathematical explanation of surface currents

• Moving water “piles up” in the direction the wind is blowing

• Water pressure increases where its piled up so tries to slide back along a pressure gradient

• Coriolis effect intervenes deflecting currents to the right of wind direction (in N hemisphere)

• Continents and land masses also deflect flow

Surface currents

Ocean gyres

• Circular flow around the periphery of an ocean basin

• This flow is often broken down into interconnected currents (e.g., North Atlantic gyre)

• Why doesn’t flow spiral toward center because of Coriolis force?

Ekman spiral• Wind flows over surface and creates drag on

water• Wind driven flow is deflected to right in N

hemisphere by Coriolis effect• Water flows at only about 3% of the speed of the

driving wind.• Current flows at 45o to the right of the wind

direction in the northern hemisphere• But, only the surface feels the wind• Each layer down only feels the layer above so is

deflected based on the layer above• Each layer down moves more slowly than the

layer above

•Wind creates a drag on surface waters and successive layersexert drag on each successive layer below.•Each layer is subject to Coriolis deflection

Ekman flow• Water doesn’t really spiral downward• At some depth water flow will be opposite surface

flow and at this depth friction dissipates horizontal flow

• Effects of surface wind felt to approximately 100m• The net motion of the water movement, after the

sum of the effects of the Ekman spiral is the Ekman transport or flow

• In theory, Ekman transport is 90o to the right of the wind in the N hemisphere

• In nature, it barely reaches 45o because of the interaction between the Coriolis effect and pressure gradient

Gyre circulation

• To deflect further than 45o, water would have to move uphill against a pressure gradient

• To deflect away from the pressure gradient would defy the Coriolis effect

• So water circulates clockwise around the gyre balanced between the pressure gradient in the center of the gyre and the Coriolis deflection

• Higher sea surface height at the center of gyres and maintained by wind energy

Water piles up in the direction of flow so piles up in middle of gyres due to Ekman transport.

Sea surface height

Hill is offset to the western side of basins because of western intensification

• Gyres in balance between pressure gradient and Coriolis effect

• Their currents are geostrophic currents• Because of wind patterns and positions of

continents, major gyres are largely independent of each other in each hemisphere.

• Six great surface current circuits in the world, one is technically not a geostrophic gyre

• The Antarctic circumpolar current (west wind drift) moves eastward around Antarctica driven by westerly winds and is never deflected by a continent

Geostrophic gyres/flow

More details next term

• Western boundary currents– Western intensification

• Eastern boundary currents• Transverse currents• Upwelling and downwelling• Langmuir circulation• Surface currents and climate• Differences in water masses among ocean

basins

Thermohaline circulation

• Vertical water movement• Driven by density differences (can be very small)

– Remember temperature and salinity diagrams and the properties of water

– Remember temperature and salinity profiles (with depth)

– Salty water is denser than fresh water– Cold water is denser than warm water

• Density gradients with latitude (due to temperature differences of surface waters)– Polar water has the most uniform density (weakest

pycnocline) so is least stable

Thermohaline circulation• Deep circulation is driven by density differences• Movement is very slow (0.1 m/s)• Three layer ocean

– surface mixed layer– Pycnocline– Deep water

• Deep water formed at 2 places – N Atlantic and Weddell Sea (Antarctica)

• Connection between surface and deep water– Diffusion (slow and along density gradients)– Mixing (e.g., storms)– Upwelling (polar, equatorial and coastal)

•S-curve tracks densitywith depth

•Points a and b on anIsopycnal so are the same density, despitedifferent temperaturesand salinities

•If the two water massesmix, will result in denser water!

Places where deep and surface water exchange

Idealized thermohaline circulation

Thermohaline circulation

• As for the atmosphere, there are convergence and divergence zones where water masses collide or diverge

• Global heat balance

• Deep circulation and basin exchange

• Studying currents

Deep circulation is like a conveyer belt thatmoves heat and water

Take home points

• The wind and density gradients are major drivers of ocean circulation

• Geostrophic flow – “earth turning” driven– Surface circulation

• Thermohaline circulation – density driven– Vertical movements and deep circulation