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 45 o 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 layers exert 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 90 o to the right of the wind in the N hemisphere In nature, it barely reaches 45 o because of the interaction between the Coriolis effect and pressure gradient

Gyre circulation To deflect further than 45 o, 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 density with depth Points a and b on an Isopycnal so are the same density, despite different temperatures and salinities If the two water masses mix, 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 that moves 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