Currents are like rivers of water flowing within the ocean and can occur on the surface or deep within the ocean. They are influenced by factors including the wind, land, temperature and salinity gradients, and the rotation of the Earth (the Coriolis force). Ocean currents can travel for thousands of kilometers and are responsible for heat and salinity transfers between the tropics and high latitude regions. Currents have a major impact on the climate.
Large scale ocean currents tend to travel clockwise in the northern hemisphere and anticlockwise in the southern hemisphere due to the influence of the Coriolis force. The rotation of the Earth also produces boundary currents near coastlines. Eastern boundary currents tend to be shallow, wide, and slow. Conversely, western boundary currents to be deep, narrow, and fast. Western boundary currents are the main conveyor belts transporting warm tropical water to the mid latitudes. The most famous western boundary current is the Gulf Stream in the North Atlantic while the Kuroshio and Agulhas currents flow in the western north Pacific and Indian Oceans, respectively.
The Gulf Stream starts off the Florida coast, travels north along the eastern seaboard of the United states and then crosses the Atlantic towards Europe. The warm waters of the Gulf Stream help to moderate temperatures along its trajectory, most notably in north-western Europe, where winter in Ireland, the UK, and western Norway would be much more severe without the warming influence of the Gulf Stream. It is estimated that average winter temperatures in the UK would be 5 °C cooler without the Gulf Stream. The Gulf Stream is approximately 100 km wide, up to 1 km deep and can travel up to 5 kts. Because the Gulf Stream is transporting warm water relative to its surroundings, the Gulf Stream is associated with increased atmospheric instability and thunderstorms. Furthermore, when the wind and waves are against the direction of the current, waves will tend to back up and steepen, leading to uncomfortable and occasionally dangerous conditions. The Gulf Stream has also been associated with the strengthening of low pressure systems and hurricanes as they cross the strong atmospheric temperature gradients caused by the warm water flow of the current. Its path up the American coastline tends to meander quite markedly, so knowing its current position and speed is important when cruising around the Bahamas, Bermuda, and the US east coast.
The Kuroshio current is a western boundary current in the Pacific Ocean that travels north past Japan and the eastern seaboard of Asia. It is warm, deep, narrow, and fast, and influences weather in the western Pacific like the Gulf Stream influences Atlantic weather. The Kuroshio starts near the Philippines and merges with the easterly flowing North Pacific current north of Japan.
In the Indian Ocean, the Aghulas western boundary current travels poleward down the eastern seaboard of Africa. While it is less well known than its northern hemisphere counterparts, it is thought to be one of the largest western boundary currents, travelling faster and transporting more water than both the Gulf Stream and the Kuroshio. Ocean currents are known to have significant impacts on weather and sea state, and the waters of the Aghulas current are one of the most treacherous shipping channels in the world. The disappearances of several large merchant ships off the African coast have been attributed to rogue waves up to 30 m high caused by the confluence of the Aghulas current, prevailing strong south westerly winds, and large Southern Ocean swells.
At the equator, ocean currents move in an east to west direction at approximately 0.2–0.5 kts. In addition to the boundary currents, there are also many counter currents and eddies that form near coastlines in response to the general ocean circulation and as a result of local influences. Up to date information on worldwide currents is available on the windy.com website.
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