What are ocean currents?
The horizontal movement of water near the surface is caused due to either terrestrial reasons or astronomical/tidal forces. The movement caused due to terrestrial reasons is called current, whereas the movement caused by gravitational pull by celestial bodies is called tidal stream. In British Admiralty charts, the current element is included in the tidal stream data and hence when using this data, additional application of current is not necessary.
Surface currents prevail over about 10% of all the water in the ocean. Surface currents are generally restricted to the upper 400 m of the ocean. The movement of deep water in the ocean basins is by density driven forces and gravity. The density difference is caused due difference in temperatures and salinity. The thermohaline circulation plays an important role in supplying heat to the polar regions. The thermohaline circulation governs the rate at which deep waters are exposed to the surface, thus, controlling Carbon Dioxide in the atmosphere.
Ocean currents are organized flows, that persist over some geographical region and over some time period such that water is mass-transported from one part of the ocean to another part of the ocean. Ocean currents are measured in Sverdrup (Sv), where 1Sv is equivalent to a volume flow rate of 1,000,000 m3 per second.
OSCAR data is used extensively in climate studies. OSCAR currents are routinely used to evaluate surface currents in Global Circulation Models.
What do the currents do for us?
Currents bring stability in climate. They reduce fluctuations in temperature of an area. Many of the Earth’s regions have considerable influence. Gulf Stream makes Northwest Europe warmer than other areas of same latitude. It also prevents formation of ice in Northwest European areas.
The Hawaiian Islands, have a climate that is cooler than the similar latitudes for the same reasons.
Currents are a significant component of the global Bio-Geo-Chemical and Hydrological Cycles. They move plankton, fish, etc. over large distances. Knowledge of ocean currents helps saving on fuel, time and thus, money. We know from the history that the Agulhas Current in the Southwest Indian Ocean dissuaded the Portuguese sailors from reaching eastern shores of land.
A large quantity of heat is transported vertically in oceans, which can probably be capitalised in future. Deep ocean water is the name for cold, salty water found deep below the surface of Earth’s oceans. Deep Ocean Water has a very low temperature, with a maximum of 1.6 °C and a salinity of about 3.5%. Only about 5% of the sunlight reaches a depth of 100 meters. At 200 meters, only about 1% of light at the surface reaches. The top 200 meters is the euphotic zone.
Depths between 200 meters and 1000 meters are referred to as the ‘dysphotic’ or twilight zone, where visibility is close to zero. Below 1000 meters, ocean depths are in the aphotic zone, where no sunlight ever reaches. The ocean water is pumped vertically for research and other activities. Currents help in the dispersal of pollutants. Salts, Oxygen and various minerals are also transferred maintaining the balance. With ocean thermal energy conversion, the temperature difference can be turned into electricity.
Primary current producing forces
1. Wind Drift
Wind is one of the primary forces that drive ocean currents. Winds influence surface fluid by friction, imparting it speed and momentum. Winds can blow for long ‘fetches’ or distances and for prolonged periods of time. Friction between the air and the surface of water causes drift current. Thus, a wind blowing for about 10 hours can produce a surface current in the water at about 2% of the wind velocity. So, a steady wind blowing in a certain direction at 40 kn for about 10 hours will produce a surface water current of about 0.8 kn. Initially water moves with winds and then the Earth’s rotation, deflects it to the right/left in the Northern/Southern hemispheres respectively. The original current producing force and the Coriolis force cause equilibrium and provide direction to the surface liquid, thus creating a wind drift current.
2. Evaporation
Wind moving over the water also produces a great deal of evaporation and cooling, leading to a decrease in temperature, called evaporative cooling. Evaporation removes only water molecules, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass.
3. Cooling
In polar regions the water at surface is continuously getting cooled due the cold winds and the contact with ice bergs. Colder sea water being denser sinks, creating vertical movement of water. Thus, thermohaline are deep ocean density-driven ocean basin currents, with original cause as cooling. When water is cooled, at polar latitudes, by cold air, it gets denser and sinks. The vertical sinking motion causes horizontal water motion as surface waters replaces the sinking water. The sinking of water in the Nordic and Greenlans Seas and around Antarctica makes the oceanic conveyor belt.
These currents are called submarine rivers. Deep downwelling occurs in the North polar regions as well as the South poles. A bulk of water body from deep ocean conveyor belt upwells in the Southern Ocean.
4. Heating
Heat expands water and thus less dense water mass floats over denser ones (this is known as ‘stable stratification’). Solar energy provides a bulk of the energy which drives the circulation of water in the oceans. The heating of water can cause a portion of sea to bulge out and eventually create an oceanic gyre. The uneven distribution of solar energy across the globe produces an uneven heating of water in oceans. The sea level at the equator is about 8 cm higher than at temperate latitudes.
As per one of the theories there are five major permanent gyres in the world’s oceans. Their locations are dictated by the temperature of the water and geography i.e. the ocean-land boundary.
The central bulge is caused by heat induced expansion of water. These bulges can be huge – hundreds of thousands of square kilometers in size over the expanse of the open oceans. Under the force of gravity, water flows down from the top of the bulges to the bottom. However, because of the Coriolis effect, water does not take a straight path. Instead, it curves Eastwards or Westwards depending upon its flow direction. Gyres are usually bounded by shallow waters of continental shelf. These gyres are responsible for much of the world’s surface currents. Debris floating in the ocean also tends to converge in certain zones because of these currents.
5. Coriolis force
Because of the Earth’s rotation, any movement away from the equator (in both the Northern and Southern hemispheres) is deflected eastwards, while movements towards the equator are deflected westwards. This effect is very pronounced in movements that happen within a fluid medium (atmosphere and oceans) and over long distances. The effect is minimum at the equator and increases towards the pole. The large bodies of air or liquid thereby turn to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Secondary Factors Influencing Currents
1. Ekman’s Transport
Fridtjof Nanson, a Norwegian scientist, noted for the first time that icebergs in the North Atlantic moved to the right of the wind. It was his student Walfrid Ekman, however, who demonstrated that the Earth’s rotation caused this effect and in particular, that the Coriolis force was responsible for the net movement of water, forced by large-scale winds. The movement is to the right of the wind in the Northern Hemisphere. Reverse would occur in the Southern Hemisphere. The surface water in the Northern Hemisphere is pushed to the right, so it flows in a direction about 45° to the right of downwind direction. In deeper layers, the direction is deflected even further to the right and in the deepest layers, at about 100-150 meters, which is about the maximum depth affected by the wind, current flows almost opposite to the downwind direction of the wind. However, if all these vectors are summed up, the net direction of flow is about 90° to the right of the wind direction.
Upwelling in Northern Hemisphere: Northerly winds cause wind drift current to south and Ekman transport moves surface waters away from the coast to east, surface waters are replaced by water that comes up from below in the process known as upwelling.
Downwelling in Northern Hemisphere: Southerly winds cause wind drift current to north and Ekman transport moves surface waters towards the coast to east. The water piles up and sinks in the process known as downwelling.
In Southern hemisphere the Ekman’s transportation is to the left of the wind flow. A very well known upwelling occurs due to the South Easterly winds along the West coast of South America and West Africa.
2. Deflection due to obstructions
Depth contours, shoreline configurations and interaction with other currents influence the original current’s direction and strength.
Why the currents are stronger on western shores?
In the late 1940s, Henry Stommel showed that this variation of the Coriolis force was responsible for the observed fact that Western boundary currents, such as the Gulf Stream and the Kuroishio, are much narrower and faster than Eastern boundary currents, such as the California Current and the Canary Current. The oceanic bulge in say North Atlantic is more to the West of the basin rather than in centre. The clockwise movement of current has narrow space available for Northward movement and wide space for Southward movement. If the same amount of water should go in the two directions, the Northward currents on the West side should be stronger than Southward currents on the East side.
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