ocean: thermohaline circulation i - github pages
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Ocean: Thermohaline circulation I
ATM2106
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Two processes driving ocean circulation
• Stresses from the wind at the ocean’s surface
• Convection induced by the loss of buoyancy
• Due to cooling and/or salt input
• Localized regions in polar latitudes
• Causing surface waters to sink to depth
• Global responses to the sinking
• Thermohaline circulation
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Thermohaline circulation
• Setting up properties of the abyssal ocean • Very long timescale and weak current • Hard to observe directly • Tracer distributions (e.g. oxygen) reflect the circulation
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Ocean convection is most prevalent in…
• The coldest regions where the interior stratification is small.
• High latitudes in winter where surface density increases by
• direct cooling, reducing temperature
• brine rejection in sea-ice formation, increasing salinity
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Buoyancy loss
• Temperature and salinity changes at the surface result in the density change.
• Either cooling or sea-ice formation is associated with buoyancy loss.
• What processes control the surface temperature and salinity in general?
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Surface temperature change
• cw is the heat capacity of water.
• Q is the turbulent vertical flux of heat.
• At the surface, Q=Qnet, which is net heat flux.
• When Qnet is positive (upward, out of the ocean into the atmosphere), T decreases.
DT
Dt= � 1
⇢refcw
@Q
@z
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Surface heat flux
• Shortwave flux (QSW) heats up the surface and also down to a depth of 100-200 m.
• Longwave flux (QLW) cools the ocean’s surface following the black-body law.
Qnet = QSW +QLW +QS +QL
Shortwave Longwave Sensible heat Latent heat
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Surface heat flux
• Sensible heat flux is due to turbulent heat exchange that depends on the wind speed and air-sea temperature difference.
Qnet = QSW +QLW +QS +QL
Shortwave Longwave Sensible heat Latent heat
QS = ⇢aircpcSu10(SST � Tair)
Density of the air at the surface
Specific heat of the air
Coefficients for the heat transfer
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Surface heat flux
• Latent heat flux is introduced from evaporation.
• It depends on the wind speed and relative humidity.
Qnet = QSW +QLW +QS +QL
Shortwave Longwave Sensible heat Latent heat
Density of the air at the surface
Transfer coefficients for water vapor
Latent heat of evaporation
QL = ⇢airLecLu10(q⇤(SST )� qair)
Specific humidity
Specific humidity at saturation
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Surface heat flux
• Latent heat flux is introduced from evaporation.
Qnet = QSW +QLW +QS +QL
Shortwave Longwave Sensible heat Latent heat
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Surface heat flux
• High wind and dry air lead to more evaporation
• It is always positive.
Qnet = QSW +QLW +QS +QL
Shortwave Longwave Sensible heat Latent heat
Density of the air at the surface
Transfer coefficients for water vapor
Latent heat of evaporation
QL = ⇢airLecLu10(q⇤(SST )� qair)
Specific humidity
Specific humidity at saturation
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Major source of cooling
Adjust the total heat flux so that global heat flux becomes zero.
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Heat into the atmosphereHeat into the ocean
Net upward shortwave and longwave heat flux
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Heat into the atmosphereHeat into the ocean
Net upward sensible heat flux
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Heat into the atmosphereHeat into the ocean
Net upward latent heat flux
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Heat into the atmosphereHeat into the ocean
Warm water + Cold air from the land in winter
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Surface salinity change
• is the turbulent vertical flux of freshwater.
• At the surface, (Evaporation - Precipitation, including river runoff and ice formation)
DS
Dt= S
@E@z
E
E = Esurface = E � P
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Sinking branch of Hadley cell
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Rising branch of Hadley cell
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Green : net evaporationBrown : net precipitation
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The zonally-averaged buoyancy forcing
Biggest buoyancy loss
Deepest convection
* Brine rejection is not included here
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Phases of deep convection
Schematic diagram of the three phases of open-ocean deep convection: (a) preconditioning, (b) deep convection and mixing (c) sinking and spreading. Buoyancy flux through the sea surface is represented by curly arrows, and the underlying stratification/outcrops are shown by continuous lines. The volume of fluid mixed by convection is shaded.
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Sites of deep convection
• There is no direct relationship between air-sea buoyancy forcing and the pattern of deep mixed layer depth.
• Underlying stratification plays important role in preconditioning of the ocean for deep convection.
• Labrador and Greenland Seas in the north Atlantic have convection deeper than 1 km.
• No deep convection in the North Pacific (because it is fresh.)
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Buoyancy frequency normalized by a reference value of the Coriolis parameter. Note that N/f < 20 in regions where deep mixed layers are common Sites of deep-reaching convection are
marked in the Labrador Sea, the Greenland Sea, the Western Mediterranean and the Weddell Sea.
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The observed thermohaline circulations
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Observations of CFCs at a depth of 2 km in ocean
• Chloroflourocarbons (CFCs) from industrial and household use can be used to track the ocean circulation patterns.
• The concentration of CFCs reveals the flow in the ocean interior along the western boundaries.