general ocean circulation. wind driven circulation about 10% of the water is moved by surface...
TRANSCRIPT
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