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, semipermanent 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. 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

1

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. 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

2

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. 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

3

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. 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

4

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. Upwelling Upwelling is an oceanographic phenomenon, where wind force pushes dense, cooler deep water towards the ocean surface and also replaces warmer surface water. There are various types of upwelling; some of them are described as below. Coastal upwelling is the best known type of upwelling of ocean current. Here wind blows away from coastline for a considerable length of time, which causes outflow of surface water from the coast. So the deep water of about 150 meters comes up, to replace surface water. Due to easterly trade wind’s blow, along the eastern shores of ocean, in low latitudes, causes upwelling at these areas. Some examples are Canary & Benguela currents (east Atlantic), Californian & Peru Currents (east Pacific) etc. Some regions of coastal upwelling include coast of Peru, Chile, California, western South Africa, eastern New Zealand, southeastern Brazil etc. Equatorial upwelling is a related phenomenon available near equator or can tell more accurately, it occurs within Intertropical Convergence Zone (ITCZ, located at north or south of the equator). In this zone, easterly (westward) winds are blowing over Pacific and Atlantic areas, causing this equatorial upwelling at these areas. This phenomenon upwells denser, nutrient-rich water from below, for this reason, equatorial area in the Pacific can be detected from space, as a broad line of high phytoplankton concentration. Southern Ocean upwelling is available in the Southern Ocean. Here strong westerly (eastward) winds are blowing around Antarctica, which drives the water significantly to northwards. Since there are no lands between South America and the tip of the Antarctic Peninsula, so some water is drawn up from great depths, causes this Southern Ocean upwelling. Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

5

Tropical cyclone upwelling occurs when a tropical cyclone transits an area, usually at speeds of less than 5 mph (8 km/h). This way cooler water comes up from lower layers of the ocean. This phenomenon causes the cyclone to be weakened. Artificial upwelling is produced artificially, to pump ocean water to the surface from below. The nutrients brought to the surface by this upwelling process, increase biological production. Non-oceanic upwelling occurs not at oceans, mainly available at other types of fluid environments, such as the lakes, the magma in Earth's mantle or the plasma within a star. This occurs due to convection.

Charting of currents Ocean currents may be shown in a variety of ways, for various requirements. The changes of rate and drift of currents, in some areas are very small and in some areas are very large. For that reason, charting of currents becomes necessary for the mariners, at ocean voyages. There 3 forms of current display are commonly used. 1. Vector mean chart: It represents the overall movements of ocean water over a considerable period. It is good for averages readings of rate and drift, over a prolonged period, but does not give the most likely current on any particular occasion. It gives the direction by an arrow pointing towards the direction, in which the current is moving. It also gives the rate in knots and number of observations, upon which the mean chart is based. Normally upper figure indicates rate and lower figure indicates the number of observations.

Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

6

2. Predominant current chart: It represents an approximation to most frequent or most likely current’s rate and drift. Here percentage of ‘legend for constancy’ is indicated by thickness of arrows, where constancy is defined as the percentage of the number of observations, within the selected sector to the total number of observations. So for high constancy areas, there is little difference between the vector mean current and predominant current. Here arrow indicates the mean current’s set and figure beside arrow indicates the mean speed in knots.

Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

7

3. Current rose chart: It represents the relative frequency, with which the current sets in each direction and also the relative frequency of various categories of speed within each direction. Here most commonly either 8points or 16-points compass is used, so that individual sectors cover 45º or 22.5º. Each arrow extending from the centre of rose is subdivided, for that reason the relative frequency of observations within various speed categories can be indicated. Although this chart provides more information, but to be safe side it is better use both Predominant current chart and Current rose chart.

Some currents Agulhas Current is the narrow, swift and strong current of South-West Indian Ocean. It originates from the East Madagascar Current & the Mozambique Current; then flows down the east coast of Africa. Some waters from this current also re-curve to south-eastwards and enter the northern part of the Southern ocean current. Alaska Current is a northward warm-water current along the coast of British Columbia and the Alaska Panhandle. It forms a part of the counterclockwise gyre, in the Gulf of Alaska. In the northern Gulf of Alaska, the Alaska Current continues into the Alaskan Stream, which starts near Kodiak Island and flows southwestward along the Alaska Peninsula. Angola current is a temporary ocean surface current. It is an extension of the Guinea Current, flowing near western Africa's coast. It is created by weaker upwelling effect, at west coast of Africa. Antarctic Circumpolar Current (ACC) or West Wind Drift is an ocean current, which flows from west to east around Antarctica. It connects the Atlantic, Pacific Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

8

and Indian Ocean basins and also serves as a principal lane of exchange between these basins. It is strongly constrained by landform and bathymetric features. The current consists of a number of fronts, i.e. the northern boundary, which is defined by the Subtropical Front, a boundary between warm, salty subtropical waters and fresher, cooler sub polar waters. Baffin Island Current (or Baffin Current) is an ocean current, along Baffin Island, in the Arctic Ocean. It stats from West Greenland Current and runs southward, at western side of Baffin Bay. Its speed is approximately 17 km per day. Benguela Current is the northward flowing, relatively cool ocean current. It originates forms the eastern portion of the South Atlantic Ocean gyre, which is driven by the prevailing South Easterly Trade winds. The water of this current is mainly derived by the upwelling of water off the south-west coast of Africa. It flows north-westerly along the south-west coasts of Africa. It is 2-300 km wide and widens further as it flows north and northwest. Brazil Current is a warm ocean water current, flowing southward, from the equator along the Brazilian south coast to the mouth of the Río de la Plata. It originates by diversion of a portion of the Atlantic South Equatorial Current. It is considerably shallower and weaker. Canary current is an ocean current, located by Northern Africa, flowing toward the south-west up to about Senegal, then it turns west. It is named as per Canary Islands. It is responsible for the generally southward movement of ocean circulation and takes actions for the improvement of coastal fisheries in western Morocco. The water of this current is cold. Cape Horn Current is a cold water current and flowing west-to-east around Cape Horn. It is caused by the strengthening of the West Wind Drift. East Madagascar Current is an oceanic flow near Madagascar. It flows southward, on the east side of Madagascar, to the southern limit at Cape Saint Marie. Equatorial Counter Current is a significant ocean current in the Pacific and Indian Oceans, which flows west-to-east, at approximately five degrees north. It results from balancing the westward flow of water, in each ocean, by the North and South Equatorial currents. Falkland Current is a cold water current and flowing northward along the Atlantic coast of Patagonia, up to the mouth of the Río de la Plata. This current originates from the movement of water from the West Wind Drift. Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

9

Florida Current is a thermal ocean current and flowing from the Gulf of Mexico to the Atlantic Ocean. This current originates from the movement of water, pushed from the Atlantic into the Caribbean Sea, by the rotation of the Earth. It is sometimes named as the Gulf Stream, however it joins the Gulf Stream off the east coast of Florida. Gulf Stream is a warm Atlantic ocean current, originates in the Gulf of Mexico, runs through the Strait of Florida and follows the eastern coastlines of the United States and Newfoundland, before crossing the Atlantic Ocean. At about 30°W, 40°N, it divides in two parts, with the northern stream crossing to northern Europe and the southern stream to off West Africa. It influences the climate of the east coast of North America (from Florida to Newfoundland) and the west coast of Europe. It is a strong ocean current, which weakens at the east side of Grand Banks of Newfoundland. It is 100 kilometers wide and 800 meters to 1,200 meters deep. The current velocity is fastest near the surface, with the maximum speed of about 2.5 meters per second. Humboldt Current is a cold, low-salinity ocean current, which flows northwestward along the west coast of South America, from the southern tip of Chile to northern Peru. The presence of the Humboldt Current and its associated wind shear prevents the formation of tropical cyclones. It also has a considerable cooling influence on the climate of Chile.

Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

10

Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

11

Kuroshio Current (or Japan Current, Kuro Siwo, or Black Stream) is a strong western boundary current in the western North Pacific Ocean. It starts off the east coast of Taiwan and flows northeastward and then it merges with the easterly drift of the North Pacific Current. It is similar to the Gulf Stream in the Atlantic Ocean, transporting warm, tropical water northward towards the polar region. The warm waters of the Kuroshio Current maintain the coral reefs of Japan. It is also responsible for the mild weather, experienced around Alaska's southern coast. Labrador Current is a North Atlantic Ocean cold current, flowing from the Arctic Ocean towards south, along the coast of Labrador and Newfoundland. It is a continuation of the West Greenland Current and the Baffin Island Current. It meets the warm Gulf Stream at the Grand Banks southeast of Newfoundland and again north of the Outer Banks of North Carolina. In spring and early summer, it transports icebergs from the glaciers of Greenland to the trans-Atlantic shipping lanes. The waters of the Labrador Current have a cooling effect on the Canadian Atlantic provinces and the coastal New England. Leeuwin Current is a warm ocean current, flows southwards near the western coast of Australia. Its strength varies through out the year, which is weakest during November to March and apparently stronger during March to November. Due to this current, the continental shelf waters of Western Australia are warmer in winter and cooler in summer than the corresponding regions off the other continents. Madagascar Current is an ocean current, flows northwards near the western coast of Madagascar. From southern Africa to India voyage, this current is helpful for the sailors. This current is not strong and wide, as its opposing Agulhas current. Mediterranean Current is a gradient current, caused due to differences in level and density between Mediterranean and Atlantic, as Mediterranean Sea, being land locked, experiences severe evaporation. The main circulation of this current is a counterclockwise. Here the water is coming in through the Strait of Gibraltar, flows eastward along the north coast of Africa and then branches off. It is more powerful in summer than in winter, because there is more evaporation during the summer.

Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

12

Mozambique Current is a strong, steady Indian Ocean current, which flows southward along the African east coast in the Mozambique Channel, between Mozambique and the island of Madagascar. North Atlantic Current is a powerful warm ocean current that continues the Gulf Stream to northeast. At west of Ireland, it splits in two braches, one is Canary Current, which goes south and the other continues to north along the coast of northwestern Europe. It is driven by the global thermohaline circulation (THC). North Atlantic Drift is an ocean current, which continues from the North Atlantic Current. North Equatorial Current is a significant Pacific and Atlantic Ocean current that flows east to west between about 10° north and 20° north. In both oceans, it is separated from the equatorial circulation by the Equatorial Countercurrent (North Equatorial Countercurrent), which flows eastward. Norwegian Current (Norwegian Coastal Current) flows north-easterly along the Atlantic coast of Norway. It is a colder (winter temperatures is typically between 2 - 5 °C) and less salty current, but considerably warmer and more salty than the Arctic Sea. South Equatorial Current is a significant Pacific, Atlantic, and Indian Ocean current, which flows east to west between the equator and about 20º south. In the Pacific and Atlantic Oceans, it also extends across the equator to about 5º north. On the equator, the South Equatorial Current is driven directly by the trade winds, which blow from east to west. Directly above the equator, the winds are reversed twice a year due to the monsoons and so here current can be westward also. Currents at Bay of Bengal Since this area is an “Area of Monsoon”, so the monsoons have an effect on currents. In the more open part of Bay, at S of about 10ºN latitude, the current’s respond to the monsoon is more observed. At this area, during December to March, current’s general flow is westerly, due to NE monsoon and during May to September, current’s general flow is easterly, due to SW monsoon. Here Coriolis force also works on currents. Due to this, instate of completely monsoon’s wind direction, current flow is inclined by about 45º to the right. In restricted part of Bay, at N of about 10ºN latitude, the currents respond to the monsoon is less observed. Here coastal boundaries and seasonal temperature Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

13

changes also have an effect on currents. So due to so many effects, such as costal shape, areas, Coriolis Effect, pressure gradient, temperature etc currents circulations become clockwise at NE monsoon and anti-clockwise at SW monsoon. At Bay of Bengal, Tropical revolving storms (TRS) also have some effects on currents, which become stronger at some times. It is estimated that a wind with 40 knots speed is bowing for 48 hrs at same direction, can increase the current at a considerable amount, which can be achieved with slow moving TRS. Since the wind effects on currant depend on duration and wind flow with steady direction, which normally can not be offered by a TRS, as very occasionally wind moves in a steady direction for a longer duration with TRS. Except in the vicinity of Sri Lanka, currents normally get stronger and occasionally attain 2 or 3 knots.

Vector-mean currents during January at Bay of Bengal (courtesy: Bay of Bengal pilot)

Vector-mean currents during July at Bay of Bengal (courtesy: Bay of Bengal pilot) Published by GSP Training Centre (GTC), to distributive with FREE OF CHARGE. 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

14