Topic 14Density Driven Currents

GEOL 2503Introduction to Oceanography

THERMOHALINE CIRCULATION

• VERTICAL circulation driven by DENSITY• Density of sea water mainly controlled by

temperature and salinity• Temperature is more important• Warm water has low density, and oceans are

heated from the top. So how does water sink?

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IMPORTANCE OF VERTICAL CIRCULATION

DOWNWELLINGCarries oxygen to the

deep ocean

UPWELLINGCarries carbon

dioxide and nutrients to the

surface

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Driven by surface processes

• Recall atmospheric circulation– These zones control evaporation and

precipitation patterns• Latitude controls surface temperature

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Recall patterns of rising and falling air 5

Precipitation and Evaporation Patterns

• Average ocean salinity is 35 ‰• Tropics—rising air—heavy rain

– Oceans: lower salinity (34.5 ‰)– Land: tropical rain forests

• 30° N+S—falling air—evaporation– Oceans: higher salinity (36.7 ‰)– Land: deserts

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Precipitation and Evaporation Patterns

• 50-60° N+S—rising air again—heavy rain again– Oceans: cool and lower salinity (34.0 ‰)– Land: northern hemisphere forests

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Another Important Factor

• Polar latitudes—seasonal influences– Winter—sea ice forms—salinity increases– Summer—sea ice melts—salinity decreases

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Ocean surface salinity 9

Ocean Surface WaterDensity Summary

• Temperature and Salinity are the major controls on density of ocean surface water

• As salinity increases, density increases• As salinity decreases, density decreases• As temperature increases, density decreases• As temperature decreases, density increases

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Controls on Salinity

• Increased by:– evaporation– sea ice formation

• Decreased by:– precipitation– sea ice melting– inflow of river water

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Many combinations of salinities and temperatures produce the same density 12

Surface Processes

• Less-dense water stays at surface– Warmer, less saline

• Denser water tends to sink– Colder, more saline

• The result is a density-layered ocean

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Changes with Depth

• Well-mixed surface layer to about 100 m• Increasing density to about 1,000 m• Constant density below 1,000 m

• Region between 100-1,000 m is called a pycnocline

• ‘Pycno-’ refers to density

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Same for Temperature, Salinity

• Thermocline—zone with a rapid change in temperature with depth

• Halocline—zone with a rapid change in salinity with depth

• Pycnocline—zone with a rapid change in density with depth

• All occur between about 100-1,000 m

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There is no pycnocline (or halocline or thermocline) in high latitudes because of rapidly sinking water

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Density-Driven Circulation

• Vertical circulation in the oceans is driven by density

• Density is mostly controlled by surface changes in temperature and salinity

• Because of this, vertical circulation is called thermohaline circulation

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Upwelling and Downwelling

• Vertical motions of water up or down:• Sinking water—downwelling• Rising water—upwelling

• Continuity of flow is basic concept

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Another way to drive upwelling and downwelling

• Push surface water together or apart• Has nothing to do with density• Processes called surface convergences and

divergences

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http://maritime.haifa.ac.il/departm/lessons/ocean/

Convergence zones—surface water pushed together by winds and surface currents

Divergence zones—surface water pushed apart by winds and surface currents

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Ocean surface convergence and divergence driven by global wind patterns

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Remember the El Niño Story

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Antarctic Convergence is the northern limit of the Southern Ocean

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Antarctic Convergence

• Also known as the Polar Front• It’s a line encircling Antarctica where cold,

northward-flowing Antarctic waters sink beneath the relatively warmer waters of the sub-Antarctic.

• It is a dynamic boundary, meaning it's precise location may shift, but is generally located between 48º S and 61º S latitude

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Coastal Upwelling/

Downwelling

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All driven by differences in density 30

Water Masses

• Water with similar temperature and density characteristics

• Acquire characteristics at the surface• Thermohaline circulation moves masses

throughout the ocean depths• Named by:

– Where from– Where found

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The Mediterranean Seafloor

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Mediterranean outflow water

Map view

Side view

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Southern Ocean Water Masses and Circulation

• Schematic depth-latitude diagram showing the major circulation and water masses of the Southern Ocean.

• The following water masses are highlighted: – (1) Antarctic Bottom Water flowing along the abyssal

ocean, – (2) Circumpolar Deep Water upwelling into the

Antarctic Divergence Zone, – (3) Antarctic Intermediate Water in the temperature

range 4-6°C, and – (4) Subantarctic Mode Water in the upper ocean north

of the Subantarctic Front (SAF).

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T-S Curves

• Temperature and Salinity (T-S) plotted on graphs

• Remember, water masses have characteristic patterns of temperature and salinity

• So T-S curves delineate water masses

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Deep water forms only in a few places

surface deep

sinking

sinking

sinking

1-Weddell Sea 2-Ross Sea 3-Labrador Sea 4-Norwegian Sea

3 4

1 2

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2

3

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1-Weddell Sea 3-Labrador Sea2-Ross Sea 4-Norwegian Sea 43

Could it happen?

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A. thermocline

B. mixed layer

C. layer with the highest density

D. curve for low latitudes

E. curve for high latitudes

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A. thermocline and pycnoclineB. warmest waterC. seasonally warm waterD. Antarctic Bottom Water

E. Antarctic Intermediate WaterF. North Atlantic Deep WaterG. upwelling

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Polynya (polynia)

• An area of open water surrounded by sea ice• Now used as geographical term for areas of sea in

Arctic or Antarctic regions which remain unfrozen for much of the year.

• The term derives from the Russian word for a hole in the ice

• Adopted in the 19th century by polar explorers to describe navigable portions of the sea.

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http://www2.fsg.ulaval.ca/giroq/now//dundas.jpg 48

Whales often use polynias for access to air.

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Seawater: Temperature and Density

Seawater: Ice Formation