1st PART





The ocean covers 71% of the surface of the Earth (about 361 millions km2) representing 1,322 billion km3 of water !

Ocean dynamics is mainly driven by atmospheric circulation and the rotation of the Earth. Pressure and wind especially explain to a considerable extent the existence and current direction of the surface. The movements of these latters are also related to the density of water, which varies depending on temperature and salinity.

The Ekman theory helps explain why currents describe cell movements. Phenomena of divergences and convergences are caused by the effect of wind and the Coriolis deflection.
- When we have a wind of anticyclonic origin (in the sense of turning clockwise in the northern hemisphere), the water accumulates (convergence) in the center. Then to compensate for the elevation of the surface, thermocline immerses in the depth.
- When we have winds depressions (rotating in the opposite direction clockwise in the northern hemisphere) the phenomenon is reversed, causing a rising of sea water and a rise in the thermocline movement: it's the Ekman pumping.

Frequently upwelling occur near the coasts of Portugal, Mauritania, Peru ... to renew the water surface which is driven off by the winds.


Different oceanic currents are known :

  • The first is the horizontal oceanic current, which is due to the winds like the Trade Winds, the Furious Fifties... and the rotation of the Earth. Among these currents there is the Gulf Stream and the Labrador Current in the North Atlantic, the Kuroshlo and others in the Pacific as seen on the map of ocean circulation on the surface.

The oceanic currents

Click here to enlarge map ocean surface circulation

  • The second circulation are currents that plunge to depths up to the surface of the oceans. The differences of temperatures (cold water is denser than hot water) and / or salinity (salt water is denser than fresh water) between the different layers of the ocean, come into play in their movements. When the horizontal circulation leads the dense water over a layer which is less, then the water surface plunges into the depths sets in rotation a "vertical" movement as is the case in the North Atlantic :

    • Between the warm surface layer and a deeper cold layer there is a "break" called the thermocline between 10 and 800 meters of depth. Currents that take place in the surface layer due to changes in the thermocline (which slowly changes of depth with various processes, such as El Niño) are called the"thermocline circulation".


- Sinking of the thermocline in the gyrating loop (hot sphere)
- less variable temperature in the cold sphere (thermohaline circulation)

    • We have the thermohaline circulation which is a very large-scale circulation and brews all ocean basins. In the North Atlantic, the current arrives in the Norwegian Sea and Labrador where sea ice forms. The rate of salinity of the water is higher, cold water, more dense, plunges into the depths of the Atlantic Ocean along the US North and South coasts through the South Atlantic into the Indian Ocean. Then some of the water go back to the western Australia and the rest in the South Pacific. In the North Pacific waters resurface across the tropic areas where they heat up.


Thermohaline circulation

Click to see an animation of the trajectory of a
particle in Ocean courtesy of B. Blanke, LPO, Brest


Click here to see an animation that explains the
ocean circulation in 11 patterns described below

  1. The Gulf Stream (and its extension, the North Atlantic drift) provide hot and salted water at northeast Atlantic, warming the Western Europe.
  2. The water cools, mixes with cold water coming from the Arctic Ocean, and becomes so dense it goes down the south and east of Greenland.
  3. If we go further, we see that this current is a part of a larger system, linking the North Atlantic...
  4. ... The tropical Atlantic...
  5. ... The South Atlantic...
  6. ... Indian and Pacific Oceans and...
  7. ... The Southern Ocean. More dense water occur near Antarctica.
  8. If we look beneath the surface, we see two areas with significant slopes (Sinking) extending below the surface of the ocean....
  9. ... the oceans of most of all the Earth to a depth of 1000 m and below...
  10. The cold and dense water gradually heated returns to the surface of oceans.
  11. This closed loop circulation is called the thermohaline circulation.


In depth, current knows circulation characterized by its slowness (often 15 to 25 cm / Second). The water that goes up the North Pacific is the same than this one that went down in the North Atlantic there are hundreds or thousands of years.

The oceans can be compared to the "long-term memory" of climate. Water vapor remains in the atmosphere for 10 years on average (before moving to another part of the biosphere), but the average length of "residence" of water in the oceans would be about 3.000 years .

The age of deep ocean waters around 3000 m. This age is calculated from
the decrease of concentration of the 14C (According Duplessy 1996)

Patricia Régnier helped me correct mistakes, please you to visit her blog
I’m not english speaker, some improprieties can appear to english masters.
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