A turbulent but well-behaved child!
The oceans and the atmosphere carry around the globe the energy and react on the dynamics of the climate!
The climate is not regular.
Depending on the seasons, the positions and the importance of depressions and anticyclones as well as the displacements of hot and cold air, time can become extreme and surprise us!
There is a relationship of union between Heaven and Earth, between earth and sea, between biosphere and hydrosphere!
Ocean circulation!
Water, water!
The ocean covers 71 % of the earth's surface, nearly 361 million square km, or nearly 1.322 billion cubic km!
The dynamics of the oceans are mainly driven by atmospheric circulation and the rotation of the Earth.
The pressure and especially the wind explain to a significant extent the existence and the direction of the surface currents.
The movements of the latter are also related to the density of the water, which varies according to temperature and salinity.
The Edman theory explains why currents describe cellular movements.
Phenomena of divergences and convergences are caused by the effect of the wind and the Coriolis deviation!
When the wind is anticyclonic(clockwise in the northern hemisphere), the water accumulates (convergence)in the center.
So to compensate for the elevation of the surface, the thermocline plunges deeply!
In the winds of depressions (counter-clockwise in the northern hemisphere) the phenomenon is reversed, which causes an upward movement of the sea water and a rise of the thermocline: Is the pumping of Ekman.
Frequently, upwelling near the coasts of Portugal, Mauritania, Peru ... take place to renew the water of the surface which is driven off by the winds!
We know different marine currents!
The first is the horizontal oceanic current, which is due to the winds like the Alizes, the Shrieking Fifties, and the rotation of the Earth
Among these currents are the Gulf Stream and the Labrador Current in the NorthAtlantic, the Kuroshio and many others in the Pacific.
The second circulation is the currents which plunge towards the depths to rise towards the surface of the oceans.
Differences in temperature (cold water is denser than hot water) and/or salinity(salt water is denser than fresh water) between different layers of the ocean, come into play in Their movements.
When horizontal circulation brings dense water over a lesser layer, the surface water then plunges into the depths and sets in motion a "vertical" circulation as is the case in North Atlantic!
Between the hot surface layer and a deeper, cold layer there is a "break" called the thermocline, which is between 10 and 800 meters deep.
The currents that take place in this superficial layer due to variations of the thermocline which slowly changes in depth with various processes, as for El Niño are called "thermocline circulation".
On the other hand, there is the thermohaline circulation, which is a very large scale circulation and which brews all the ocean basins.
In the North Atlantic, current flows into the Norwegian Sea and Labrador Sea where sea ice is formed.
The salinity of the waters is higher, the cold water is denser and plunges into the depths of the Atlantic Ocean along the North and South American coasts, crosses the South Atlantic to penetrate the Indian Ocean.
Then part of the water flows back to western Australia and the rest to the South Pacific.
In the northern Pacific, water re-emerges through the tropical areas where they warm up.
Ocean circulation in 11 steps!
Our story begins with the GulfStream and its extension.
The North Atlantic drift brings warm and salty water to the northeastern Atlantic by heating western Europe.
The water cools, mixes with cold water from the Arctic Ocean, and becomes so dense that it descends to the south and east of Greenland
If we go further, we see that this current is part of a larger system, linking the North Atlantic, the tropical Atlantic Ocean, the Southern Atlantic Ocean, the Indian Pacific oceans and the southern ocean.
Further more descent of dense water occurs near Antarctica.
If we look below the surface, there are two regions with large sinking slopes extending below the surface of the ocean.
These descents affect the oceans of most of the Earth to a depth of 1000 m and below.
The cold, dense, gradually heated water returns to the surface of the oceans!
This circulation in closed loop, is called Circulation of thermohaline.
In depth, the current is characterized by a circulation characterized by its slowness(often from 15 to 25cm/sec).
The water that goes up the North Pacific is the same as the one that came down from the North Atlantic some hundreds or thousands of years ago.
Oceans can be assimilated to the "long-term memory" of climate.
Water vapor remains in the atmosphere for an average of 10 years (before moving to another part of the biosphere), but the average "residence" time of water in the oceans would be about 3000 years.
The Gulf Stream
The Gulf Stream is a surface marine current.
It is a warm current that runs along the US coast from the Gulf of Mexico and travels north-east of the Atlantic Ocean,pushed by the prevailing south westerly winds, gradually cooling.
The Gulf Stream is among the strongest currents.
It moves warm water from the subtropical zones to the poles.
In autumn, the Gulf Stream shifts to the north, while in winter and spring it shifts to the south.
Compared to the current width (about 100-200 kilometers), the range of this variation (30 - 40 kilometers)is relatively small.
According to altimetry results from Geosat, the current carries a maximum amount of water in autumn and minimum in spring.
The Gulf Stream has marked seasonal variability.
The fluctuation is mostly confined to above 200-300 meters of water, it is a result of the warming and seasonal expansion of the surface waters.
But the variations of the transport of the deep waters seem to be almost opposite to the phase of the waters of the surface, and their magnitude is more significant.
Sea currents like the Gulf Stream distribute heat around the globe, as do the atmosphere
According to calculations, océan currents bring much of the heat released in winter.
But contrary to what many people think, the Gulf Stream has little influence on the thermal contrast between European and American winters.
On the other hand, marine currents play a more important role to the north, preventing the formation of sea ice along the Norwegian coasts.
Numerical simulations show that oceanic heat transport increases winter temperatures in eastern North America and western Europe by 2 to 3 degrees C, or 10 % of the warming generated by atmospheric movements.
Between sky and sea!
Like the atmosphere, the ocean plays an important role on the climate.
The water heats up less quickly than the air but it also cools less quickly than this one!
The ocean has a much longer "memory" than the atmosphere.
This memory is of the order of the season with regard to surface currents, and at least of the order of decade with regard to the great masses of water in the deep ocean!
The heat thus stored in the water of the tropical areas is restored to the atmosphereat the highest latitudes.
It is thus that the oceanic currents of surface and depth are generated which carry this heat from the equator to the poles.
This balances the excess solar radiation that equatorial regions receive!
The Atlantic probably carries more heat from the equator to the north than the Pacific.
It is estimated that the ocean contributes about 30 % to the transport of heat, from the equator to the poles, achieved by the climate system.
During this transport, the ocean and the atmosphere constantly exchange energy:
On the surface of the oceans, these exchanges constitute fluxes greater than those from the Sun in the form of visible light.
They are in the form of infrared radiation.
On the other hand, a very important exchange, in fact dominant, takes place in the form of latent heat, by evaporation and condensation in the atmosphere of the water of the oceans.
With the evaporation of water, which is more important in the tropics, salinityincreases, but with the input of fresh water from rains and rivers, it decreases!
A second essential term for the exchange of energy between the ocean and the atmosphere is due to the friction of the wind at the surface of the oceans.
In summer, the sunshine is more important, which warms the surface of the ocean.
The wind blows the water of the ocean and redistribute this heat on a layer a few tens of meters deep.
Then this layer stores this energy.
Then, when the winter arrives, the sun is less intense and the winds more violent.
Slowly the ocean cools down by releasing its heat into the atmosphere!
Thus, heat is stored at the surface of the oceans in summer.
After the release of this heat into the atmosphere, the waters become colder and denser, causing them to dip to the oceanfloor ... to be replaced by the deeper, warmer and less dense waters!
The contrast of winter temperatures depends on three phenomena:
The release in the atmosphere of the heat stored during the summer.
The transport of heat from the Gulf Stream to northern Europe.
The great atmospheric meanders which, partly shaped by the reliefs, extend over several thousand kilometers!
