ocean circulation deepsea

8/3/2019 Ocean Circulation Deepsea

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Earth Systems Science

OCEAN CIRCULATION II: DEEP OCEAN

Salinity

Thermohaline circulation

Ocean circulation and climate


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SALINITY

NOTE: Someof these will be

importantwhen studyingthe carboncycle


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SALINITY

1. Why is salinity important for ocean circulation?

2. From where does the salt in the ocean originate?

3. To where does the salt in the ocean go?

1. Salinity, in addition to temperature, determines the density of

the water, which is the driving force for deep oceancirculation.

2. Thesource of salts in the ocean is weathering. This is the

process whereby water flowing over and through the land

picks up dissolved minerals (i.e. salts). Those waterseventually flow out into the ocean.


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3. Salts do not evaporate along with the ocean water. So, when

ocean water evaporates, it leaves the salts behind. This wouldlead to ever-increasing salt concentrations if there were no

othersinks.

Sinks of salt in the ocean include:

In shallow seas evaporation leaves such high salt

concentrations that the water becomes saturated, and the

resultingprecipitate forms evaporite deposits.

Removal of salt by microorganisms to form shells, some ofwhich are eventually deposited on the ocean floor.

Chemical reactions with newly formed sea floor

Removal by wind from evaporated sea spray

SALINITY


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Over long time periods, and averaged over the globe, the amount

of salt in the oceans is in dynamic equilibrium.

Regionally, there can be differences in salt concentrations due to

variations in the salt concentration of river runoff, and the

amount of precipitation, runoff, and outflow. For example,

where evaporation exceeds precipitation, and there is little

runoff, salinity usually increases.

SALINITY


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THERMOHALINE CIRCULATION

Deep ocean circulation.

Driven by density variations which are due to temperatureand salinity variations.

Slower than surface circulation, but important over long

time scales (100s to millions of yrs)


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SURFACE ZONE

Top 50100 meters

Exchanges energy with the atmosphere through latent heat,

radiation, and kinetic energy

Generally well-mixed by the wind, so is often referred to as

themixed layer

PYCNOCLINE

~1 km below the mixed layer

Region of sharp increase in density, very stable, prevents

mixing between mixed layer and deep oceanDEEP OCEAN

1-5 km deep

Stable density gradient


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Observed temperature distributions

Western Atlantic

Central Pacific

no gradientno gradient

no gradient


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Observed salinity distributionsno gradientweak gradient

no gradient

Western Atlantic

Central Pacific


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DEEP WATER FORMATION

Dense water formed in the Weddell Sea (Antarctic Bottom

Water, AABW) and in the North Atlantic Ocean near

Greenland (North Atlantic Deep Water, NADW) sinks

This sinking water initiates a slow moving deep ocean

circulation

Average residence time in the deep ocean is hundreds of

years

Why is this water dense?

Ice formation: rejection of salt

Evaporation from Gulf Stream


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DEEP WATER FLOW AT 4000 m DEPTH


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THE THERMOHALINE CONVEYOR BELT


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http://seawifs.gsfc.nasa.gov/SEAWIFS/IMAGES/

SEAWIFS Mean Chlorophyl September 97 - August 2000

Center of gyres downwelling few sources of nutrients little biological activity

Areas with nutrient input from rivers or from upwelling more biological activity


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MEDITERRANEAN OUTFLOW AT 1000m

Temperature (C)

Salinity 0/00


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Remember this from chapter 4


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ocean circulation deepsea

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