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Ocean Stratification and Circulation
Martin VisbeckDEES, Lamont-Doherty Earth Observatory
General Atmosphere Ocean CirculationThe surface energy balance
Top of atmosphere
Air-sea interface
seafloor
Imbalance of energy flux at the top can be balanced by:
Atmospheric Heat Transport
Oceanic Heat Transport
Sea Surface Temperature
The link between ocean and climate depends on exchange of energy (mainly heat and radiation) and materials (water, gases) across the sea surface. T atmosphere 'sees', influences and responds to the sea surface temperature (SST), by way of sea-air heat flux. SST generally cools with increasing latitude, but important deviations from a pure latitudinal dependence occurs. These are generally due to the movement of sea water in both the horizontal and vertical directions. Temperature and density of ocean water are related inversely: warm water means low density, cold water means denser sea water.
Ocean Salinity
As the range of salt concentration in the ocean varies from about 3.2 to 3.8%, oceanographers, who refer to salt content as 'salinity', express salt concentration as parts per thousand; 34.9 ppt is the average salinity.
Ocean Salinity
The more saline, the denser the sea water
Density of sea water is a function of temperature and salinity, both play an important role
Stratification
Waters warmer than 10°C dominate the sea surface but do not extend much below 500 m in the ocean; the warm waters provide just a veneer of warmth over a cold ocean. The sharp drop off in temperature with depth is called the thermocline. Deeper cold waters derive their properties at the sea surface during winter at high latitude.
Ocean simulation in tank
Cool waterlightlight
Ocean Propertie
s Temperature
Salinity Diagramsat the surface
Ocean Propertie
s Temperature
Salinity at 150 meter water depth
Ocean Propertie
s Temperature
Salinity at 500 meter water depth
Ocean Propertie
s Temperature
Salinity at 1000 meter water depth
Ocean Propertie
s Temperature
Salinity at 3000 meter water depth
How does the Ocean move heat?
Top of atmosphere
Atmosphere
Ocean
Effect of Atmospheric Forcing on Ocean
Ocean Circulation
Ocean circulation is produced by:
1) the wind stress acting on the sea surface and
2) by buoyancy (heat and freshwater) fluxes between the ocean and atmosphere.
The former induces the wind driven ocean circulation, the latter the thermohaline circulation.
Ocean Circulation
The wind driven flow is by far the more energetic and for the most part resides in the upper kilometer.
The sluggish thermohaline circulation forces ocean overturning reaching in some regions to the sea floor, resulting in the formation of the major water masses of the global ocean:
North Atlantic Deep Water and
Antarctic Bottom Water.
Wind Driven Ocean Circulation
The wind driven circulation, which by far is the more energetic, though confined mostly to the upper kilometer of the ocean and generally moves water in the horizontal plane.
Wind Driven Ocean Circulation
How, does the wind induce an ocean circulation?
The wind exerts a force or stress on the ocean surface. This stress is proportional to the square of the wind speed. This produces ocean waves and ocean currents. The wind makes the surface layer of the ocean move ….
Wind Driven Ocean Circulation
The wind makes the surface layer of the ocean move, though not in the way that intuition might dictate - its not in the direction of the wind stress, but rather at an angle to it.
This is because of the Coriolis Force.
Eventually a balance is achieved between the wind stress and the Coriolis Force. The surface Ekman layer extends to about 100 to 300 meters depth, is a boundary layer feature in which the direct stress of the wind are felt.
Wind Driven Ocean Circulation
The transport within the Ekman layer is 90° towards the right of the wind in the northern hemisphere, to the left in the southern hemisphere.
The Ekman transport is proportional to the wind stress, which is proportional to the square of the wind speed. Typically a surface current is around 2 or 3% of the wind speed.
Wind Driven Ocean Circulation
One clear effect of the Ekman transport can be seen in the eastern side of the subtropical ocean where cold subsurface water is pumped up to the sea surface from a depth of perhaps 200 meters as the sea surface water is forced offshore by the Ekman transport. These regions are rich in nutrients and support important fisheries. The cold SST of these regions also induce a specific climate, called Cape Verde climate, one of a very stable atmosphere, cool, fog, few storms.
Large Scale Wind Driven Ocean Circulation
It is this movement that produces the wind driven circulation of the ocean.
How?
Well the wind field changes in its strength and direction from place to place. This causes Ekman transport to either pile up water (convergence) in some places or remove it (divergence) in other regions. As surface water is less dense than deeper water this has the effect of heaping buoyant water in the convergence regions and removing it from the divergence regions.
Large Scale Wind Driven Ocean Circulation
The hills and valleys of the sea surface produced by the convergence and divergent causes a sea level relief (difference from lowest to highest sea level, neglecting tides and waves) of around 2 meters.
Large Scale Wind Driven Ocean Circulation
The wind produces convergences and divergences of surface water, which causes hills and depressions in sea level, which produce a horizontal gradient of pressure, or a pressure head reaching down to perhaps 1000 meters.
Convergent hills have an outward directed pressure gradient, depression an inward directed pressure gradient.
Large Scale Wind Driven Ocean Circulation
As the pressure gradients make the water move from high pressure to low pressure, the Coriolis Force starts its action, and eventually a balance is achieved in these two forces, the horizontal pressure gradient equals the magnitude of the Coriolis Force, but is directed in the opposite direction.
This balance is called the geostrophic balance, and a current in such a balance is called a geostrophic current. Ocean currents are very close to being in geostrophic balance
Large Scale Wind Driven Ocean Circulation
As the pressure gradients make the water move from high pressure to low pressure, the Coriolis Force starts its action, and eventually a balance is achieved in these two forces, the horizontal pressure gradient equals the magnitude of the Coriolis Force, but is directed in the opposite direction.
This balance is called the geostrophic balance, and a current in such a balance is called a geostrophic current. Ocean currents are very close to being in geostrophic balance
Large Scale Wind Driven Ocean Circulation
How does the Ocean move heat?
Top of atmosphere
Atmosphere
Ocean
Buoyancy Driven Ocean Circulation
The buoyancy forces are capable of inducing overturning that reach from the sea surface to the sea floor.
Buoyancy fluxes are those fluxes between air and water that alter the density of the sea water.
Cooling of the ocean and evaporation makes the ocean (colder, saltier) denser, removing buoyancy.
Heating and excess precipitation has the opposite effect, they add buoyancy to the ocean.
Overturning
North Atlantic Deep Water
Antarctic Bottom Water
Intermediate Water
Overturning
North Atlantic Deep Water
Antarctic Bottom Water
Intermediate Water
Oceanic Heat Transport
Overturning
North Atlantic Deep Water
Antarctic Bottom Water
Intermediate Water
Intermediate Water
At the base of the thermocline is the low salinity Antarctic Intermediate water derived from the Antarctic Circumpolar Current.
The Oceans Role in Climate
The sum of the wind driven and buoyancy driven ocean current transport large amounts of heat and fresh water over large distances.
Can you rationalise the signs?
Atmospheric Heat Transportsimulation in tank
Cool waterlightlight
Atmospheric Heat Transportsimulation in tank
Cool waterlightlight
Ocean Heat Transportsimulation in tank
Cool waterlightlight
Atmosphere-Ocean Heat Transportsimulation in tank
Cool waterlightlight
Atmosphere
Ocean
Why is the ocean heat transport so much smaller than the one in the atmosphere?
Some Final Remarks
The atmosphere carries: • 75 percent of the heat transport in the Northern Hemisphere and• 90 percent in the Southern Hemisphere
• It would take a million 1,000-megawatt electric power stations - the largest power plants now used - to produce a quantity of energy equivalent to the heat the atmosphere carries on average from the tropics to polar regions.
