Surface Currents
These form from large circular patterns called gyres, which flow clockwise in the Northern Hemisphere, and anti-clockwise in the Southern Hemisphere because of the Coriolis effect. Wind patterns are a big factor influencing the formation of surface currents. Trade winds push water along the top of the ocean, and this aids the formation of surface currents. The Gulf Stream takes very warm water from the Gulf of Mexico and parts of the Caribbean Sea and transports it northward. During the winter, the Gulf Stream can have a great effect on storm systems along the East Coast of the United States. For example, nor’easters can strengthen and grow over the Gulf Stream and bring heavy snow or rain, strong winds, and damaging beach erosion to the East Coast. It used to be thought that the flow of the Gulf Stream alone helped keep Europe warm in winter, but it has been recently shown that the temperate climate of Great Britain comes from warm air moving along with the Gulf Stream over the Atlantic Ocean. This keeps temperatures along the western coast of Europe milder than continental areas farther east away from the coast. Surface currents can carry heat, which affects regional climates.
Deep ocean currents
These are generally influenced by the density of the ocean. Density depends on temperature and salinity of the water. Circulation reliant on density is also called thermohaline circulation. Warm water lies near the surface as it is less dense, whereas cooler water lies lower down in the water because it is more dense so ‘heavier’. These patterns can be because of fresh water runoff. At the ocean’s surface, fresh water from rivers or precipitation falling from the sky mixes with the ocean water and dilutes it. As a result, the water at the ocean surface is less salty than the water below it. It can also be due to ice forming at high latitudes. In the northern Atlantic and Pacific Oceans, when the air gets cold enough for ice to form on the surface of the ocean, the dissolved salt is excluded from the ice that forms, leading to cold and extra salty water forming at these latitudes as the fresh water becomes ice. This very dense water sinks to deep levels in the ocean and spreads out from there. The bottom topography of the ocean and the arrangement of the continents helps steer this deep salty water around the globe. From this, we can get underwater currents that can be thousands of feet below the surface of the ocean.
The process begins on the water surface near the North Atlantic Pole. Here, the water is chilled by arctic temperatures. It also gets saltier because when sea ice forms, the salt does not freeze and is left behind in the surrounding water. The cold water is now more dense, due to the added salts, and sinks toward the ocean bottom. Surface water moves in to replace the sinking water, thus creating a current. This deep water moves south, between the continents, past the equator, and down to the ends of Africa and South America. The current travels around the edge of Antarctica, where the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt gets “recharged.” As it moves around Antarctica, two sections split off the conveyor and turn northward. One section moves into the Indian Ocean, the other into the Pacific Ocean. These two sections that split off warm up and become less dense as they travel northward toward the equator, so that they rise to the surface (upwelling). They then loop back southward and westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins again.
It is much slower than wind influenced surface currents, and it is estimated that any cubic metre of water takes around 1000 years to complete the entire journey. It moves over 100 times the volume of water as the Amazon River.