Ocean Circulation

Si Hui Lee and Frances Wen

You can access ME athttp://tinyurl.com/oceancirculation

Earth - the blue planet- 71% area covered by the oceans

- 3/4 of ocean area between 3000-6000m deepvs. 11% of land above 2000m altitude

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

Vertical Structure of the OceanMixed Layer • warm• near uniform density• stirred due to wind-driven

surface currentsPycnocline• 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)

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 100-150m thick• net water movement 90 degrees to the

wind direction• reality deviates from ideal conditions

Ocean Gyres

large system of rotating ocean currentscontrolled by wind circulation, especially by the subtropical oceanic

high-pressure cells and the westerlies

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 (1-1.5m/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

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

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

100-300m• 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

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

Oceanic Conveyor Belt

Oceanic Conveyor Belt