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73-648: The closure of the Central American Seaway had tremendous effects on oceanic circulation and the biogeography of the adjacent seas, isolating many species and triggering speciation and diversification of tropical and sub-tropical marine fauna. The inflow of nutrient-rich water of deep Pacific origin into the Caribbean was blocked and so local species had to adapt to an environment of lower productivity. It had an even larger impact on terrestrial life. The seaway had isolated South America for much of

146-543: A volume flow rate of 1,000,000 m (35,000,000 cu ft) per second. There are two main types of currents, surface currents and deep water currents. Generally surface currents are driven by wind systems and deep water currents are driven by differences in water density due to variations in water temperature and salinity . Surface oceanic currents are driven by wind currents, the large scale prevailing winds drive major persistent ocean currents, and seasonal or occasional winds drive currents of similar persistence to

219-616: A . The increased availability of nutrients in upwelling regions results in high levels of primary production and thus fishery production. Approximately 25% of the total global marine fish catches come from five upwellings, which occupy only 5% of the total ocean area. Upwellings that are driven by coastal currents or diverging open ocean have the greatest impact on nutrient-enriched waters and global fishery yields. The three main drivers that work together to cause upwelling are wind , Coriolis effect , and Ekman transport . They operate differently for different types of upwelling, but

292-399: A decisive role in influencing the climates of regions through which they flow. Ocean currents are important in the study of marine debris . Upwellings and cold ocean water currents flowing from polar and sub-polar regions bring in nutrients that support plankton growth, which are crucial prey items for several key species in marine ecosystems . Ocean currents are also important in

365-413: A decrease in productivity as the waters are no longer receiving nutrient-rich water. Without these nutrients, the rest of the trophic pyramid cannot be sustained, and the rich upwelling ecosystem will collapse. Coastal upwelling has a major influence over the affected region's local climate. This effect is magnified if the ocean current is already cool. As the cold, nutrient-rich water moves upwards and

438-414: A decreasing population, especially in species that do not breed often under normal circumstances or become reproductively mature late in life. Another problem is that the decrease in the population of a species due to fisheries can lead to a decrease in genetic diversity, resulting in a decrease in biodiversity of a species. If the species diversity is decreased significantly, this could cause problems for

511-400: A delay in the reconstruction of this upwelling community. The possibility of such an ecosystem collapse is the very danger of fisheries in upwelling regions. Fisheries may target a variety of different species, and therefore they are a direct threat to many species in the ecosystem, however they pose the highest threat to the intermediate pelagic fish . Since these fish form the crux of

584-430: A high number of commercial fishers and fisheries. On one hand, this is another benefit of the upwelling process as it serves as a viable source of food and income for so many people and nations besides marine animals. However, just as in any ecosystem, the consequences of over-fishing from a population could be detrimental to that population and the ecosystem as a whole. In upwelling ecosystems, every species present plays

657-663: A matter for debate: The closing of the seaway allowed a major migration of land mammals between North and South America, known as the Great American Interchange. That allowed species of mammals such as cats, canids, horses, elephants, and camels to migrate from North America to South America, and porcupines , ground sloths , glyptodonts and terror birds made the reverse migration. There is much controversy about glacial and interglacial climates in South America. Research shows that vegetation in most of

730-569: A much colder northern Europe and greater sea-level rise along the U.S. East Coast." In addition to water surface temperatures, the wind systems are a crucial determinant of ocean currents. Wind wave systems influence oceanic heat exchange, the condition of the sea surface, and can alter ocean currents. In the North Atlantic, equatorial Pacific, and Southern Ocean, increased wind speeds as well as significant wave heights have been attributed to climate change and natural processes combined. In

803-837: A north and south split. The split in this system occurs at Point Conception , California due to weak upwelling in the South and strong upwelling in the north. The Canary Current is an eastern boundary current of the North Atlantic Gyre and is also separated due to the presence of the Canary Islands . Finally, the Humboldt Current or the Peru Current flows west along the coast of South America from Peru to Chile and extends up to 1,000 kilometers offshore. These four eastern boundary currents comprise

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876-505: A rate of about 5–10 meters per day, but the rate and proximity of upwelling to the coast can be changed due to the strength and distance of the wind. Deep waters are rich in nutrients, including nitrate , phosphate and silicic acid , themselves the result of decomposition of sinking organic matter (dead/detrital plankton) from surface waters. When brought to the surface, these nutrients are utilized by phytoplankton , along with dissolved CO 2 ( carbon dioxide ) and light energy from

949-453: A result, influence the biological composition of oceans. Due to the patchiness of the natural ecological world, dispersal is a species survival mechanism for various organisms. With strengthened boundary currents moving toward the poles, it is expected that some marine species will be redirected to the poles and greater depths. The strengthening or weakening of typical dispersal pathways by increased temperatures are expected to not only impact

1022-545: A significant flow of water northwards. This is actually a type of coastal upwelling. Since there are no continents in a band of open latitudes between South America and the tip of the Antarctic Peninsula, some of this water is drawn up from great depths. In many numerical models and observational syntheses, the Southern Ocean upwelling represents the primary means by which deep dense water is brought to

1095-418: A significant role in influencing climate, and shifts in climate in turn impact ocean currents. Over the last century, reconstructed sea surface temperature data reveal that western boundary currents are heating at double the rate of the global average. These observations indicate that the western boundary currents are likely intensifying due to this change in temperature, and may continue to grow stronger in

1168-436: A vital role in the functioning of that ecosystem. If one species is significantly depleted, that will have an effect throughout the rest of the trophic levels. For example, if a popular prey species is targeted by fisheries, fishermen may collect hundreds of thousands of individuals of this species just by casting their nets into the upwelling waters. As these fish are depleted, the food source for those who preyed on these fish

1241-557: A wasp-waist richness pattern. In this type of pattern, the high and low trophic levels are well-represented by high species diversity. However, the intermediate trophic level is only represented by one or two species. This trophic layer, which consists of small, pelagic fish usually makes up about only three to four percent of the species diversity of all fish species present. The lower trophic layers are very well-represented with about 500 species of copepods , 2500 species of gastropods , and 2500 species of crustaceans on average. At

1314-425: Is also known as the ocean's conveyor belt. Where significant vertical movement of ocean currents is observed, this is known as upwelling and downwelling . The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content , factors which together determine the density of seawater. The thermohaline circulation is a part of the large-scale ocean circulation that

1387-430: Is depleted. Therefore, the predators of the targeted fish will begin to die off, and there will not be as many of them to feed the predators above them. This system continues throughout the entire food chain , resulting in a possible collapse of the ecosystem. It is possible that the ecosystem may be restored over time, but not all species can recover from events such as these. Even if the species can adapt, there may be

1460-537: Is driven by global density gradients created by surface heat and freshwater fluxes . Wind -driven surface currents (such as the Gulf Stream ) travel polewards from the equatorial Atlantic Ocean , cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water ). This dense water then flows into the ocean basins . While the bulk of it upwells in the Southern Ocean ,

1533-600: Is the Great American Interchange of vertebrates between North and South America which required a continuous land bridge across the two areas for the organisms to travel along with a climate that was very different from the climate today. Lastly is the development of differences in marine assemblages and their isotopic signatures in the Caribbean from those in the Pacific. The Central American Seaway

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1606-543: Is the eastern boundary of the South Atlantic subtropical gyre and can be divided into a northern and southern sub-system with upwelling occurring in both areas. The subsystems are divided by an area of permanent upwelling off of Luderitz , which is the strongest upwelling zone in the world. The California Current System (CCS) is an eastern boundary current of the North Pacific that is also characterized by

1679-493: The Amazon basin has changed very little since glacial times, but it is believed there was more savanna present during that period. A closed seaway would have led to a very different North Atlantic Ocean circulation, but it impacted the surrounding atmospheric temperatures, which in turn affected the glacial cycle. The emergence of the isthmus caused a reflection of the westward-flowing North Equatorial Current northward and enhanced

1752-482: The Atlantic meridional overturning circulation (AMOC) is in danger of collapsing due to climate change, which would have extreme impacts on the climate of northern Europe and more widely, although this topic is controversial and remains an active area of research. The "State of the cryosphere" report, dedicates significant space to AMOC, saying it may be enroute to collapse because of ice melt and water warming. In

1825-820: The Canary Current (off Northwest Africa ), the Benguela Current (off southern Africa ), the California Current (off California and Oregon ), the Humboldt Current (off Peru and Chile ), and the Somali Current (off Somalia and Oman ). All of these currents support major fisheries. The four major eastern boundary currents in which coastal upwelling primarily occurs are the Canary Current, Benguela Current, California Current, and Humboldt Current. The Benguela Current

1898-681: The Cenozoic , which allowed the evolution of a wholly-unique diverse mammalian fauna there. When it closed, a faunal exchange with North America ensued and led to the extinction of many of the native South American forms. The evidence for when the Central American landmass emerged and the closing of the Central American Seaway can be divided into three categories. The first is the direct geologic observation of crustal thickening and submarine deposits in Central America. The second

1971-465: The Coriolis effect , breaking waves , cabbeling , and temperature and salinity differences. Depth contours , shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents move both horizontally, on scales that can span entire oceans, as well as vertically, with vertical currents ( upwelling and downwelling ) playing an important role in

2044-506: The East Australian Current , global warming has also been accredited to increased wind stress curl , which intensifies these currents, and may even indirectly increase sea levels, due to the additional warming created by stronger currents. As ocean circulation changes due to climate, typical distribution patterns are also changing. The dispersal patterns of marine organisms depend on oceanographic conditions, which as

2117-542: The El Nino-Southern Oscillation (ENSO) event. The Peruvian upwelling system is particularly vulnerable to ENSO events, and can cause extreme interannual variability in productivity. Changes in bathymetry can affect the strength of an upwelling. For example, a submarine ridge that extends out from the coast will produce more favorable upwelling conditions than neighboring regions. Upwelling typically begins at such ridges and remains strongest at

2190-543: The Humboldt Current . The largest ocean current is the Antarctic Circumpolar Current (ACC), a wind-driven current which flows clockwise uninterrupted around Antarctica. The ACC connects all the ocean basins together, and also provides a link between the atmosphere and the deep ocean due to the way water upwells and downwells on either side of it. Ocean currents are patterns of water movement that influence climate zones and weather patterns around

2263-409: The ocean surface. It replaces the warmer and usually nutrient-depleted surface water . The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton . The biomass of phytoplankton and the presence of cool water in those regions allow upwelling zones to be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll

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2336-437: The seasons ; this is most notable in equatorial currents. Deep ocean basins generally have a non-symmetric surface current, in that the eastern equator-ward flowing branch is broad and diffuse whereas the pole-ward flowing western boundary current is relatively narrow. Large scale currents are driven by gradients in water density , which in turn depend on variations in temperature and salinity. This thermohaline circulation

2409-410: The sun , to produce organic compounds , through the process of photosynthesis . Upwelling regions therefore result in very high levels of primary production (the amount of carbon fixed by phytoplankton ) in comparison to other areas of the ocean. They account for about 50% of global marine productivity. High primary production propagates up the food chain because phytoplankton are at the base of

2482-551: The apex and near-apex trophic levels, there are usually about 100 species of marine mammals and about 50 species of marine birds. The vital intermediate trophic species however are small pelagic fish that usually feed on phytoplankton . In most upwelling systems, these species are either anchovies or sardines, and usually only one is present, although two or three species may be present occasionally. These fish are an important food source for predators, such as large pelagic fish, marine mammals, and marine birds. Although they are not at

2555-405: The base of the trophic pyramid, they are the vital species that connect the entire marine ecosystem and keep the productivity of upwelling zones so high A major threat to both this crucial intermediate trophic level and the entire upwelling trophic ecosystem is the problem of commercial fishing . Since upwelling regions are the most productive and species rich areas in the world, they attract

2628-503: The catch. Besides directly causing the collapse of the ecosystem due to their absence, this can create problems in the ecosystem through a variety of other methods as well. The animals higher in the trophic levels may not completely starve to death and die off, but the decreased food supply could still hurt the populations. If animals do not get enough food, it will decrease their reproductive viability meaning that they will not breed as often or as successfully as usual. This can lead to

2701-459: The circulation has a large impact on the climate of the Earth. The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. On occasion, it is imprecisely used to refer to the meridional overturning circulation , (MOC). Since the 2000s an international program called Argo has been mapping the temperature and salinity structure of

2774-554: The cost and emissions of shipping vessels. Ocean currents can also impact the fishing industry , examples of this include the Tsugaru , Oyashio and Kuroshio currents all of which influence the western North Pacific temperature, which has been shown to be a habitat predictor for the Skipjack tuna . It has also been shown that it is not just local currents that can affect a country's economy, but neighboring currents can influence

2847-597: The direction of the wind, and the friction between that layer and the layer beneath it causes the successive layers to move in the same direction. This results in a spiral of water moving down the water column. Then, it is the Coriolis forces that dictate which way the water will move; in the Northern hemisphere, the water is transported to the right of the direction of the wind. In the Southern Hemisphere,

2920-570: The dispersal and distribution of many organisms, inclusing those with pelagic egg or larval stages. An example is the life-cycle of the European Eel . Terrestrial species, for example tortoises and lizards, can be carried on floating debris by currents to colonise new terrestrial areas and islands . The continued rise of atmospheric temperatures is anticipated to have various effects on the strength of surface ocean currents, wind-driven circulation and dispersal patterns. Ocean currents play

2993-400: The entire trophic process of upwelling ecosystems, they are highly represented throughout the ecosystem (even if there is only one species present). Unfortunately, these fish tend to be the most popular targets of fisheries as about 64 percent of their entire catch consists of pelagic fish. Among those, the six main species that usually form the intermediate trophic layer represent over half of

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3066-514: The equator, upwelling still occurs just north and south of the equator. This results in a divergence, with denser, nutrient-rich water being upwelled from below, and results in the remarkable fact that the equatorial region in the Pacific can be detected from space as a broad line of high phytoplankton concentration. Large-scale upwelling is also found in the Southern Ocean . Here, strong westerly (eastward) winds blow around Antarctica , driving

3139-410: The general effects are the same. In the overall process of upwelling, winds blow across the sea surface at a particular direction, which causes a wind-water interaction. As a result of the wind, the water has transported a net of 90 degrees from the direction of the wind due to Coriolis forces and Ekman transport. Ekman transport causes the surface layer of water to move at about a 45-degree angle from

3212-618: The majority of coastal upwelling zones in the oceans. Upwelling at the equator is associated with the Intertropical Convergence Zone (ITCZ) which actually moves, and consequently, is often located just north or south of the equator. Easterly (westward) trade winds blow from the Northeast and Southeast and converge along the equator blowing West to form the ITCZ. Although there are no Coriolis forces present along

3285-471: The moisture supply to Arctic latitudes, which contributed to both Arctic continental glaciation and sea ice formation. This eventually led, with the aid of the orbitally-paced extension of Gelasian ice sheets, to the Quaternary ice age . Ocean current An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind,

3358-407: The moon in the form of tides , and by the effects of variations in water density. Ocean dynamics define and describe the motion of water within the oceans. Ocean temperature and motion fields can be separated into three distinct layers: mixed (surface) layer, upper ocean (above the thermocline), and deep ocean. Ocean currents are measured in units of sverdrup (Sv) , where 1 Sv is equivalent to

3431-456: The movement of nutrients and gases, such as carbon dioxide, between the surface and the deep ocean. Ocean currents flow for great distances and together they create the global conveyor belt , which plays a dominant role in determining the climate of many of Earth's regions. More specifically, ocean currents influence the temperature of the regions through which they travel. For example, warm currents traveling along more temperate coasts increase

3504-462: The near future. There is evidence that surface warming due to anthropogenic climate change has accelerated upper ocean currents in 77% of the global ocean. Specifically, increased vertical stratification due to surface warming intensifies upper ocean currents, while changes in horizontal density gradients caused by differential warming across different ocean regions results in the acceleration of surface zonal currents . There are suggestions that

3577-435: The northward-flowing Gulf Stream . The Pacific coast of South America would have cooled as the input of warm water from the Caribbean was cut off. That trend is thought to have caused the extinction of the marine sloths of the area. The closure of the seaway led to an increased poleward salt and heat transport, which strengthened the North Atlantic thermohaline circulation 2.95–2.82 million years ago. That in turn increased

3650-433: The ocean circulation suggest that broad-scale upwelling occurs in the tropics, as pressure driven flows converge water toward the low latitudes where it is diffusively warmed from above. The required diffusion coefficients, however, appear to be larger than are observed in the real ocean. Nonetheless, some diffusive upwelling does probably occur. Upwelling intensity depends on wind strength and seasonal variability, as well as

3723-476: The ocean interior, upwelling associated with eddies, topographically-associated upwelling, and broad-diffusive upwelling in the ocean interior. Coastal upwelling is the best known type of upwelling, and the most closely related to human activities as it supports some of the most productive fisheries in the world. Coastal upwelling will occur if the wind direction is parallel to the coastline and generates wind-driven currents. The wind-driven currents are diverted to

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3796-515: The ocean with a fleet of automated platforms that float with the ocean currents. The information gathered will help explain the role the oceans play in the earth's climate. Ocean currents affect temperatures throughout the world. For example, the ocean current that brings warm water up the north Atlantic to northwest Europe also cumulatively and slowly blocks ice from forming along the seashores, which would also block ships from entering and exiting inland waterways and seaports, hence ocean currents play

3869-495: The oceanic food chain. The food chain follows the course of: Coastal upwelling exists year-round in some regions, known as major coastal upwelling systems , and only in certain months of the year in other regions, known as seasonal coastal upwelling systems . Many of these upwelling systems are associated with relatively high carbon productivity and hence are classified as Large Marine Ecosystems . Worldwide, there are five major coastal currents associated with upwelling areas:

3942-408: The oldest waters (with a transit time of around 1000 years) upwell in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of

4015-444: The process of upwelling. However, during El Niño events, trade winds are weaker, causing decreased upwelling in the equatorial regions as the divergence of water north and south of the equator is not as strong or as prevalent. The coastal upwelling zones diminish as well since they are wind driven systems, and the wind is no longer a very strong driving force in these areas. As a result, global upwelling drastically decreases, causing

4088-434: The ridge even after developing in other locations. The most productive and fertile ocean areas, upwelling regions are important sources of marine productivity. They attract hundreds of species throughout the trophic levels; these systems' diversity has been a focal point for marine research . While studying the trophic levels and patterns typical of upwelling regions, researchers have discovered that upwelling systems exhibit

4161-571: The right of the winds in the Northern Hemisphere and to the left in the Southern Hemisphere due to the Coriolis effect . The result is a net movement of surface water at right angles to the direction of the wind, known as the Ekman transport (See also Ekman Spiral ). When Ekman transport is occurring away from the coast, surface waters moving away are replaced by deeper, colder, and denser water. Normally, this upwelling process occurs at

4234-582: The same latitude North America's weather was colder. A good example of this is the Agulhas Current (down along eastern Africa), which long prevented sailors from reaching India. In recent times, around-the-world sailing competitors make good use of surface currents to build and maintain speed. Ocean currents can also be used for marine power generation , with areas of Japan, Florida and Hawaii being considered for test projects. The utilization of currents today can still impact global trade, it can reduce

4307-450: The same time, the Antarctic Circumpolar Current (ACC) is also slowing down and is expected to lose 20% of it power by the year 2050, "with widespread impacts on ocean circulation and climate". UNESCO mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for

4380-565: The sea surface temperature gets cooler, the air immediately above it also cools down and is likely to condensate, forming sea fog and stratus clouds . This also inhibits the formation of higher altitude clouds, showers and thunderstorms and results in rainfall over the ocean leaving the land dry. In year-round upwelling systems (like that of the western coasts of Southern Africa and South America), temperatures are generally cooler and precipitation scarce. Seasonal upwelling systems are often paired with seasonal downwelling systems (like that of

4453-441: The species in an environment that is so variable and quick-changing; they may not be able to adapt, which could result in a collapse of the population or ecosystem. Another threat to the productivity and ecosystems of upwelling regions is El Niño-Southern Oscillation (ENSO) system, or more specifically El Niño events. During the normal period and La Niña events, the easterly trade winds are still strong, which continues to drive

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4526-509: The surface. In some regions of Antarctica, wind-driven upwelling near the coast pulls relatively warm Circumpolar deep water onto the continental shelf, where it can enhance ice shelf melt and influence ice sheet stability. Shallower, wind-driven upwelling is also found in off the west coasts of North and South America, northwest and southwest Africa, and southwest and south Australia , all associated with oceanic subtropical high pressure circulations (see coastal upwelling above). Some models of

4599-413: The survival of native marine species due to inability to replenish their meta populations but also may increase the prevalence of invasive species . In Japanese corals and macroalgae, the unusual dispersal pattern of organisms toward the poles may destabilize native species. Knowledge of surface ocean currents is essential in reducing costs of shipping, since traveling with them reduces fuel costs. In

4672-452: The temperature contrast is greatly seasonably variable, creating periods of strong upwelling in the spring and summer, to weak or no upwelling in the winter. For example, off the coast of Oregon, there are four or five strong upwelling events separated by periods of little to no upwelling during the six-month season of upwelling. In contrast, tropical latitudes have a more constant temperature contrast, creating constant upwelling throughout

4745-510: The temperature of the area by warming the sea breezes that blow over them. Perhaps the most striking example is the Gulf Stream , which, together with its extension the North Atlantic Drift , makes northwest Europe much more temperate for its high latitude than other areas at the same latitude. Another example is Lima, Peru , whose cooler subtropical climate contrasts with that of its surrounding tropical latitudes because of

4818-428: The vertical structure of the water , variations in the bottom bathymetry , and instabilities in the currents . In some areas, upwelling is a seasonal event leading to periodic bursts of productivity similar to spring blooms in coastal waters. Wind-induced upwelling is generated by temperature differences between the warm, light air above the land and the cooler denser air over the sea. In temperate latitudes ,

4891-787: The viability of local fishing industries. Currents of the Arctic Ocean Currents of the Atlantic Ocean Currents of the Indian Ocean Currents of the Pacific Ocean Currents of the Southern Ocean Oceanic gyres Upwelling Upwelling is an oceanographic phenomenon that involves wind -driven motion of dense, cooler, and usually nutrient-rich water from deep water towards

4964-417: The water is transported to the left of the wind. If this net movement of water is divergent, then upwelling of deep water occurs to replace the water that was lost. The major upwellings in the ocean are associated with the divergence of currents that bring deeper, colder, nutrient rich waters to the surface. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in

5037-527: The wind powered sailing-ship era, knowledge of wind patterns and ocean currents was even more essential. Using ocean currents to help their ships into harbor and using currents such as the gulf stream to get back home. The lack of understanding of ocean currents during that time period is hypothesized to be one of the contributing factors to exploration failure. The Gulf Stream and the Canary current keep western European countries warmer and less variable, while at

5110-404: The winds that drive them, and the Coriolis effect plays a major role in their development. The Ekman spiral velocity distribution results in the currents flowing at an angle to the driving winds, and they develop typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere . In addition, the areas of surface ocean currents move somewhat with

5183-401: The world. They are primarily driven by winds and by seawater density, although many other factors influence them – including the shape and configuration of the ocean basin they flow through. The two basic types of currents – surface and deep-water currents – help define the character and flow of ocean waters across the planet. Ocean currents are driven by the wind, by the gravitational pull of

5256-406: The year. The Peruvian upwelling, for instance, occurs throughout most of the year, resulting in one of the world's largest marine fisheries for sardines and anchovies . In anomalous years when the trade winds weaken or reverse, the water that is upwelled is much warmer and low in nutrients, resulting in a sharp reduction in the biomass and phytoplankton productivity. This event is known as

5329-559: Was closed by the elevation of the Central American Isthmus which is proposed to have occurred three and a half to five million years ago. The closing of the Central American Seaway is also supported by the evolution of taxa on different sides of the Central American Isthmus along with the different histories of the oceans on either side of the isthmus. The first closure and the final closure remain

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