Ocean Circulation. Si Hui Lee and Frances Wen. You can access ME at

Transcription

1 Ocean Circulation Si Hui Lee and Frances Wen You can access ME at

2 Earth - the blue planet - 71% area covered by the oceans - 3/4 of ocean area between m deep vs. 11% of land above 2000m altitude

3 What causes ocean circulation? Driven by energy and matter exchanges between the ocean and the atmosphere Evaporation, precipitation, plus heating and cooling change temperature and salinity of surface waters Density changes cause water to sink or rise in the ocean Wind transfers kinetic energy to ocean depths of a few hundred meters Winds give rise to horizontal currents and vertical water motions such as upwelling

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6 Vertical Structure of the Ocean Mixed Layer warm near uniform density stirred due to wind-driven surface currents Pycnocline water density increases rapidly with depth because of changes in temperature (thermocline) and/or salinity (halocline) colder and saltier water is denser stratification suppressing mixing between the mixed layer and deep layer Deep Layer cold density increases gradually with depth water moves slowly, mostly due to salinity differences (thermohaline mechanism)

7 Wind, Coriolis Force & Ekman Spiral topmost layer subject to wind stress layers below experience frictional drag o current about 2-3% of wind speed surface water moves in the same direction as the wind (at equator) with Coriolis force, surface water is deflected to the right of the wind direction in the Northern Hemisphere and to the left of the wind direction in the Southern Hemisphere by about 45 degrees Ekman modeled the ideal 3D current pattern caused by a steady wind Ekman layer is around m thick net water movement 90 degrees to the wind direction reality deviates from ideal conditions

8 Ocean Gyres large system of rotating ocean currents controlled by wind circulation, especially by the subtropical oceanic high-pressure cells and the westerlies

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10 Currents 30 degrees to equator: trade winds generate broad north and south equatorial currents water piles up near the equator on the western sides of the oceans some deflect poleward - increasing Coriolis deflection accumulated water flows down the hydraulic gradient eastwards as compensating narrow-surface equatorial counter-currents (11.5m/s) Poleward water tend to pile up against the continents around the western margins of the oceanic subtropical high pressure cells high temperature - cannot sink and continues poleward form relatively narrow currents of high velocity western boundary currents: Gulf Stream, Kuroshio current o 100km wide, 2m/s surface velocity eastern boundary currents: Canary and California currents o 1000km wide, <0.25m/s surface velocity

11 Gulf Stream a western boundary current heat flux of 1.2 x 1015 W 75% lost to the atmosphere 25% used to heat the Greenland-Norwegian seas area

12 Upwelling Coastal upwelling: occurs where Ekman effect moves surface waters away from the coast (mostly westward) surface waters are replaced by water that wells up from depths of m average rates of upwelling: 1-2m/day large arrow - dominant wind direction small arrows - current northern hemisphere case shown persistent offshore wind also implicated in upwelling Along the equator: divergent surface currents due to convergent trade winds

13 Thermohaline Circulation circulation driven by density gradients in the deep ocean due to salinity and temperature differences produced by surface processes cold saline water fed to deep ocean basins upwelling delivers deep water to surface upwelling occurs chiefly in narrow coastal locations subsidence takes place in broad ocean regions - northern North Atlantic and around parts of Antarctica North Atlantic: dry cool air causes evaporation and cooling of surface water from 10 to 2 degrees Celsius formation of sea ice expels brine dense water mass sinks and fuels a broad, slow, and diffuse southward-flowing density current at depths >1500m

14 Oceanic Conveyor Belt

15 Oceanic Conveyor Belt