There is only one ocean basin which connects all the worlds oceans. But all this water is always on the move through currents. The most well known current is the rise and fall of the tides, the other two currents are the wind-driven surface currents and the deep water thermohaline current. But these currents also feed an immense ''river'' within the ocean. This ''river'' moves more than a 100 times the volume of water that flows through the Amazon river. In this blog post I am going to dive into these currents and river that flows within the ocean.
The rise and fall of the tides.
The currents caused by the tides are most noticeable and strongest near the shore, in bays and estuaries along the coasts. Tidal currents are predictable and are affected by the gravitational pull of the moon. The highest tides in the world occur in Bay of Fundy, Canada, with a difference between high and low tide of 16.3meters! The lowest tides are around 1m and occur in the Mediterranean sea and the Caribbean sea. But these tides are not always the same in height. Tides are stronger after a full moon and a new moon, these tides are called spring tides. When the moon is half full, 1st and 3rd quarter, the tides are weaker and these tides are called neap tides.
Wind-driven currents
The friction of wind over the surface of the water cause the top 10% of the water layer to move with the wind. Water as deep as 400m is affected by the friction of the wind. When you look at the patterns of the surface currents, these currents form loops called gyres. In the Northern Hemisphere these gyres rotate clockwise and in the Southern Hemisphere anti-clockwise. This happens because of the rotation of the earth. Without the earth’s rotation these currents would move up and down between the equator and the poles. The turning of the earth takes the wind moving towards the pole in an easterly direction and the wind moving towards the equator in a westerly direction in the Northern hemisphere. In the southern hemisphere this happens in opposite direction. This causes the main stream winds and thus the surface currents to move in a loop and form gyres, this is called the Coriolis effect. There are five main gyres: North-Atlantic gyre, South-Atlantic gyre, the Indian gyre, the North-Pacific gyre and the South-Pacific gyre.
These currents / gyres distribute the warmer waters from the equator towards the poles and colder water from the poles to the equator. A well-known current is the gulf stream which brings warm water from the Caribbean past the east coast of America across the North Atlantic towards Europe.
Thermohaline current
The top 10% of the water is moved by the wind-driven current. The other 90% is moved though the thermohaline current. This current happens in the depths of the ocean and is density driven. The difference in density happens through difference in temperature (thermo) and salinity (haline). The warmer waters from the equator that move towards the poles gets colder the closer they get to the poles. But the colder the water gets the more salty it becomes. This is because the ice crystals that form trap the water while leaving the salt behind, thus the salinity level rises. This cold and salty water has a higher density and sinks through the water column and warmer water rises to the surface, this vertical circulation is called the thermohaline circulation.
The combination of the wind driven surface currents and the thermohaline circulation forms the largest current of them all: the global conveyor belt.
The global conveyor belt.
As the name suggest this current or conveyor belt crosses all the oceans. This current ‘’starts’’ in the Norwegian sea, where the warmer waters of the gulf stream warms the atmosphere in the north. The loss of heat to the atmosphere cools the waters and this water sinks below. But because the warm waters keep coming to the north the cold waters are pushed to the south again, this is the start of the conveyor belt. The water moves slowly towards the equator and on to Antarctica where the current gets recharged, by the cooling of the waters once again. As the currents moves along Antarctica, the conveyor belt splits in two section. One section moves northward into the Indian ocean and the second moves along the Antarctic coast and north into the Pacific ocean. These two sections become less dense as they travel towards the equator which causes the belt to rise to the surface (upwelling). This upwelling of colder water brings along nutrients from the depths which are important for marine life. These cool nutrient rich waters feed and support the growth of algae and seaweed. At the surface the waters become warm again and loop back westward and stream back into the Atlantic ocean towards the Caribbean and to Europe where the cycle starts over again.
Now you might think that the conveyor belt moves quite fast, however it is estimated that 1 cubic meter of water takes about a 1000 years to complete the entire conveyor belt! Whereas waters being transported through wind-driven currents can travel tens to hundreds of centimetres per second. Which makes the global conveyor belt a massive but slow moving river flowing through the worlds oceans.
In a future post I am going to look into the effect of the warm/cold water distribution of the ocean circulation on our climate.
Sources:
Invitation to Oceanography, Paul R. Pinet. Sixth edition. Chapter 6 Wind and ocean circulation.
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