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Ocean Current Ocean current Ocean current is the general horizontal movement of a body of ocean water, generated by various factors, such as earth's rotation, wind, temperature, salinity, tides etc. These movements are occurring on permanent, semi- permanent or seasonal basis. Knowledge of ocean currents is essential in reducing costs of shipping, as efficient use of ocean current reduces fuel costs. Ocean currents are also important for marine lives, as well as these are required for maritime study. Ocean currents are measured in Sverdrup with the symbol Sv, where 1 Sv is equivalent to a volume flow rate of 106 cubic meters per second (0.001 km³/s, or about 264 million U.S. gallons per second). On the other hand, current direction is called set and speed is called drift. Causes of ocean current are a complex method and not yet fully understood. Many factors are involved and in most cases more than one factor is contributing to form any particular current. Among the many factors, main generating factors of ocean current are wind force and gradient force. Current caused by wind force: Wind has a tendency to drag the uppermost layer of ocean water in the direction, towards it is blowing. As well as wind piles up the ocean water in the wind blowing direction, which also causes to move the ocean. Lower layers of water also move due to friction with upper layer, though with increasing depth, the speed of the wind-induced current becomes progressively less. As soon as any motion is started, then the Coriolis force (effect of earth’s rotation) also starts working and this Coriolis force causes the water to move to the right in the northern hemisphere and to the left in the southern hemisphere. Theoretically the resultant effect between wind force and Coriolis force is 45º to the right of the wind direction for NH (left for SH), but practically this effect varies between 20º to 45º. Many experts tries to find out the ratio between the wind speed and surface current speed, but it is not yet decided due to so many complicacy related with earth’s curvature, friction amount, viscosity & density of water, type of costal areas, period of wind blow etc. Just for an idea it is said that if wind blows for 10 Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 1 For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury hours over the ocean, then can cause an ocean current, with rate of 2% of the wind speed. Ekman's Theory of wind affect on surface current: The first reasonable theory, regarding wind affects on surface currents was introduced by Swedish oceanographer and mathematician Valfrid Ekman, in 1890. Ekman considered an infinitely wide and deep ocean of constant density, divided into an infinite number of horizontal layers, where the top horizontal layer of ocean is affected by two forces, one is wind and the other is friction by below layer. Similarly the second layer is also affected by friction at top and bottom, and so on. As the same, the Coriolis force is also affecting each layer, as they are moving. Due to resultant of wind force, friction and the Coriolis force, Ekman concluded that the currents rate decreased exponentially with depth and current deviations to the wind direction increased with depth, where surface current moves at a 45º angle to the wind direction, forming a spiral (which is known as Ekman spiral). Such Ekman spirals are rare, as the oceans are neither infinitely wide, nor of constant density, as Ekman assumed. Ekman layer a relatively thin surface layer of ocean, which is 10 to 200m thick. Due to the low viscosity of water, wind stress is not directly communicated to the below of this layer. Within the Ekman layer, the currents actually form a spiral Ekman transport is the motion of Ekman layer. Because of the deflection by the Coriolis force, the Ekman transport is not in the direction of the wind, but to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The amount of water flowing in this layer depends on the wind and the Coriolis force. The large scale convergence or divergence of Ekman transport serves to drive the general ocean circulation Ekman pumping is the convergence of Ekman transports. Current caused by gradient: Water has a tendency to move due to pressure gradient in water. This pressure gradient develops, where water surface creates differences, caused by variations in heat and salt. The density of the ocean varies both horizontally and vertically. Scientists can use this density structure of the ocean, to calculate the pressure field and hence the pressure gradient. Current generated by this force is modified by the rotation of the earth. Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 2 For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury In practice, the distribution of density is determined by measurements of salinity and temperature. From this the horizontal pressure gradient, as well as surface currents can be obtained. Currents at Bay of Bengal in February are an interesting example of gradient current. In this month, a temperature gradient arises between the cold water in the north and warm water in the south. Due to this, a density difference is created, which makes a current flow towards north, then deflection to east, which is against the wind flow, caused by north-east monsoon. Other then above two factors; some other factors, such as depth of water, underwater topography including shape of the basin in which the current is running, extent and location of land, evaporation from ocean water etc are involved with ocean current formations. The other factor is centrifugal force, as currents may move in a curved path rather than in a straight line. In cyclonic motion the centrifugal force augments the Coriolis force. In anti-cyclonic motion the centrifugal force augments the pressure-gradient force. Types of Ocean Currents as per ocean depth: 1. Surface Currents are the movements of ocean waters, at the upper 400 meters of the ocean, with about 10% of ocean water, generally moves clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere Surface ocean currents are generally wind driven, as due to the low thickness of water, wind stress is not directly communicated to the ocean interior. The amount of water flowing in this layer depends upon the wind and the Coriolis force, as Coriolis force acts 90° to the right of wind effect in the Northern Hemisphere and 90° to the left of wind effect, in the Southern Hemisphere. 2. Deep Water Currents are the movements of deeper ocean water, around the ocean, with about 90% of ocean water. Deep Ocean currents are driven by density and other factors, as density of water varies with salinity, temperature and pressure. This type of ocean circulation is also called Thermohaline circulation (THC), where ‘thermo’ means temperature and ‘haline’ means salt content. The thermohaline circulation is sometimes called as the “ocean conveyor belt”, the “great ocean conveyor”, or the “global conveyor belt”. Some definitions Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 3 For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury Periodic current changes speed or direction cyclically at regular intervals, such as a tidal current. This current is produced by the tidal influence of moon and sun or by any other oscillatory forcing activities. Seasonal current changes speed or direction due to seasonal winds. Coastal current flows roughly parallel to a coast, outside the surf zone. Longshore current flows parallel to a shore, inside the surf zone. It moves along the beach, eroding it from some areas and depositing it in other areas. This process is called longshore drift. This way, it is able to move entire islands in the direction of the current. It can carry people too, so can be very dangerous for people, swimming in the ocean Offshore current is a current away from the shore. Inshore current is a current close to the shore. Warm ocean currents are the corridors of warm water at ocean, moving from the tropics towards pole. Since this current starts from equatorial areas, so it will have an affect of easterly trade winds, which blow to west. For that reason, warm currents are normally available along the western shore of large oceans. Gulf Stream, Norwegian current, Brazilian current, Alaska current, Kuroshio current, East Australian current, Mozambique current, Agulhas current etc are the examples of warm current. Warm ocean currents Cold ocean currents Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 4 For the officers, who are appearing Deck Officer Class 1, 2 & 3 examinations. For more details, please study reference book “Meteorology”, written by Capt. Rashidul Hasan Chowdhury Cold ocean currents are the corridors of cold water at ocean, moving from higher latitudes toward the equator. Since this current starts from higher latitudes areas, so it will have an affect of westerlis winds, which blow to east. For that reason, cold currents are normally available along the eastern shore of large oceans. Labrador current, Falkland current, Benguela current, California current, Humboldt current, west Australian current, Oyashio current, Baffin land current etc are the examples of cold current.
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