I- Oceanic currents
At first sight, the oceans seem quite still when we exclude tides and waves. Don’t be fooled, they are tremendously moving. Indeed, powerful and ubiquitous currents move the Oceans. These latter are produced wind friction, gravity and water density variation in different parts of the ocean. They are present on the ocean’s surface and in its depths, flowing both locally and globally. Interestingly, they are similar to winds in the atmosphere in that they possess a general circulation pattern and transfer significant amounts of heat from Earth’s equatorial areas to the poles. Moreoverthey influence one another (by heat transfer, evaporation for example). Thus it is not surprising that both ocean current and atmospheric circulation influence our climate, our local ecosystem as well as the seafood that we eat.
How does this general circulation pattern works?
The general ocean circulation derives its energy at the sea surface from two sources that define two circulation types: (1) wind-driven circulation forced by wind stress on the sea surface, inducing a momentum exchange, and (2) thermohaline circulation driven by the variations in water density imposed at the sea surface by exchange of ocean heat and water with the atmosphere, inducing a buoyancy exchange. Since the momentum and the buoyancy exchange are both depending of the wind speed, these two circulation types are not fully independent. The wind-driven circulation impacts mostly the sea surface circulation (up to 100m) while the thermohaline circulation extends to the seafloor. Thus although the wind-driven circulation is the most vigorous, the thermohaline one sets a larger amount of water in motion.
For example, if a wind blow on a relatively warm sea surface it will set in motion this water mass (a water body of same buoyancy characteristics) but also induce its evaporation. By reducing the seawater temperature and freshwater content, evaporation induces a decrease of its buoyancy. When the buoyancy of that sea surface water mass is under that of underlying water masses, they flow in a phenomenon called downwelling. Besides, the motion of the surface layer by the wind has also produced a suction effect, which tend to pull deep waters to the surface in a phenomenon called upwelling.
If the two circulation types are now clear in your mind, we will further discuss the sea surface circulation and most notably, the gyres! A gyre is a large swirling current system. There are five gyres on Earth and you probably heard about some of them because of the waste that we found there. Indeed, because of their strong swirling current, they are accumulation points for any sort of floating garbage and explain why the North Pacific gyre has been recently recalled the great pacific garbage patch! Looking at the figure you have maybe noted that the gyre circulation (which is driven by the wind) depends on their location (clockwise in the north and counterclockwise in the south). Its called Coriolis force and it is due to Earth’s rotation.
Explanation of the Coriolis force
Compared that at the Equator, the speed of rotation of the Earth decreases when approaching a pole (because the distance to travel in a day is shorter). Thus, if you throw a ball in the direction of a Pole from the equator the ball will gain a higher rotation speed than the Earth and will be therefore diverted to the East. The resulting direction will be to the right and to the left in the northern and southern hemisphere respectively.
The general circulation of the oceans defines the average movement of seawater, which, like the atmosphere, follows a specific pattern. Superimposed on this pattern are oscillations of tides and waves, which are not considered part of the general circulation. There also are meanders and eddies that represent temporal variations of the general circulation. In the following video you will observe the general circulation with all the temporal variations (eddies and meanders) along two years! Try to spot the five gyres!