Oceanographic Context

Brief summaries of the hydrology of the studied regions are given below.

The North Atlantic and Adjacent Seas

The North Atlantic and adjacent seas include the North Atlantic Ocean from ca. 40ºN to 80ºN, and the Gulf of St. Lawrence, Hudson Bay, Labrador Sea, Baffin Bay, North Sea, Barents Sea, and the Greenland, Iceland, and Norwegian Seas (Fig. 1; Rochon et al., 1999). The major surface current in the North Atlantic Ocean is the Gulf Stream that flows northward along the eastern coast of the United States. After diverging from the continental platform, it continues as the north-eastward flowing North Atlantic Drift (NAD). The NAD transports relatively warm water into the Nordic seas. Low pressure centres, which originate from the east coast of North America, travel approximately along the same route. These cells remain, however, peripheral to a primary semi-permanent low-pressure system centred near Iceland. The relatively warm waters within these regions cause high atmospheric temperatures compared to those of the surrounding areas (Gathman, 1986). The NAD is characterised by sea-surface temperature (SST) and sea-surface salinity (SSS) gradients from the south to the north, from 6 to 10ºC winter SST, 12 to 22ºC summer SST, and 33 to 35 summer SSS. At about 50ºN part of the NAD waters branch eastward, after which a component flows to the south, to the region off Portugal and a component continues its way south-eastward, toward the Bay of Biscay. In the same region, another part of the NAD turns westward, south of Iceland, and separates into two components as well. The first of the two component turns northward and becomes the Irminger Current, which flows around Iceland in a clockwise direction and mixes north of Iceland with cold waters of the southward flowing East Icelandic Current. The second branch turns southward to mix with the Arctic waters of the East Greenland Current and continues as the West Greenland Current after passing the southern tip of Greenland, moving northward along the west coast of Greenland. A part of this current turns back towards the south in the area of Davis Strait and a part continues to flow northward alongside Greenland into Baffin Bay. Waters of the West Greenland Current are characterised by summer SST and SSS of about 3-8 ºC, and 34-31, respectively.

The area between Iceland and the Faeroes is characterised by a strong temperature and salinity gradient, which is called the Iceland Gap Front (Johannessen, 1986). In this area, the remaining part of the NAD flows north-east along the Norwegian coast and becomes the Norwegian Atlantic Current (NAC). Part of this current enters the North Sea through the Shetland-Faeroe Channel. Within the North Sea, the NAC flows in a anticlockwise direction. North of Denmark, the NAC joins the Baltic Sea outflow waters Sea and becomes the Norwegian Coastal Current flowing northward along Norway, more or less parallel to the NAC. At latitude 72ºN, the NAC separates into three branches: the North Cape Current, the West Spitsbergen Current and a further westward flowing branch that enters the Greenland Sea. The North Cap Current turns east and enters the Barents Sea where it flows along the coast of northern Norway, Finland and Russia. The main branch of the NAC, the West Spitsbergen Current continues its way northward. The branch which enters the Greenland Sea mixes with Arctic waters that are flowing southward. SST varies between 2 and 5ºC, and SSS between 31 and 34 and in addition a variable amount of ice cover, varying between 0 to 10 months a year, is found in this region also characterised by the presence of the Polar Ocean Front.

Arctic waters enter the North Atlantic via the southward flowing East Greenland Current. The East Greenland Current is characterised by summer SST and SSS of ca. 0 to 6ºC and 30-32 respectively. Sea ice covers the east Greenland coast for 3 to 11 months each year, with the exception of the NE polynya (north-east of Greenland), which sometimes remains ice free. In Baffin Bay another Arctic current enters the region: the Baffin Land Current, which is characterised by summer SST between 2 to 4ºC, SSS of ca. 31-32) and covered by sea ice for more than 6 months per year. In contrast to the surrounding waters, the North Water Polynya off the north-west coast of Greenland, remains ice free for several months a year. The Baffin Land Current mixes with the return flow of the West Greenland Current in the Davis Strait area and also with the outflow from Hudson Bay further to the south. This current forms then the Labrador Current with SSS of ca. 32-33. The Labrador Current flows southward and is covered by ice ca. 3 to 4 months per year. A minor branch enters the Gulf of St. Lawrence through the Belle Isle Strait. The major branch continues its southward movement and flows around the Newfoundland Grand Banks into the Gulf of St. Lawrence. A relatively small proportion of the Labrador Current mixes with water from the NAD, whereas the major part sinks and becomes part of the southward flowing intermediate and bottom waters.

Hudson Bay

Cold, relatively saline waters (ca. 31-32) enter Hudson Bay from the north-east whereas river runoff enters the southern part of the Bay; this produces an estuarine circulation and results in low salinity surface water being exported out of the Bay and saline water entering the bay at depth. Sea ice can cover the sea surface for up to 8 months per year. The surface circulation is counterclockwise and exits the bay through the Hudson Strait. Mean summer SST varies between 3.5 and 6ºC, and SSS ranges from ca. 25 nearshore, to ca. 30 in the centre of the Bay.

Gulf of St. Lawrence

The surface circulation in the Gulf of St. Lawrence is estuarine and thus dominated by the fresh water inputs from the St. Lawrence River. Cold Arctic waters enter the Gulf and results in mean summer SST and SSS of 11ºC and 30, respectively. In the south-western part of the Gulf of St. Lawrence, summer SST might be as high as 17ºC. The Gulf is covered by sea ice ca. 3 to 4 months per year. Surface water exits the Gulf to the south-east and turns westward along the Scotian Shelf, forming the Nova Scotia Current.

Pechora Sea

The Pechora Sea is located between Novaya Zemlya and northern Russia. Surface waters in this region are influenced by the North Cape Current, which flows eastward along the Finnish and Russian coasts, and by the Arctic waters of the Kara Sea Current, which flow southward along the eastern coast of Novaya Zemlya. Both of these currents mix to form the Novaya Zemlya Current that flows northward along the western coast of Novaya Zemlya. Summer SST and SSS are about 5-6ºC and 32-34, respectively, and the sea-ice cover varies between 3 and 6 months per year.

The North Tropical Atlantic (Canary Islands)

The most prominent oceanic surface current of the North Canary Basin, which is located off north-west Africa between 28° and 32°N, is the southward flowing Canary Current (CC) that is driven by the steady Trade Wind field or NE-trades (Mittelstaedt, 1991; Fig. 1). The intensity and position of the NE-trades are influenced by the pressure difference between the subtropical high-pressure cell located above the North Atlantic Ocean and the low-pressure cells situated above the African continent (Rognon and Coudé-Gaussen, 1996). The position of the NE-trades changes in relation to the seasonal movement of the Inter-tropical Convergence Zone between the tropics of Cancer and Capricorn (Wooster et al., 1976; Fiuza et al., 1982). During boreal summers, the high-pressure cell is centred over the Azores and the trade-wind field extends its influence up to 40°N, off the Portuguese coast. During boreal winters, the high-pressure cell is expanded southwards and the trade-wind influence only reaches to about 25°N. The NE-trades blow sub-parallel to the African coast, causing an offshore movement of the surface-water masses that results in Ekman pumping and coastal upwelling. During boreal summers, intense coastal upwelling is observed along the Portuguese and Canary margins, whereas during boreal winters upwelling is intensified south of the Canaries, according to the seasonal movement of the NE-trades (Fiuza et al., 1982; Nykjaer and Van Camp, 1994). Short episodes of winter upwelling can exceptionally occur in the Canary region during this season. The area between 20° and 3°N is characterised by permanent coastal upwelling with seasonal changes in intensity proportional to the strengthening of the NE-trades (Haynes et al., 1993; Fig. 1). In addition to the control by NE-trade wind dynamics, upwelling intensity is strongly dependent on physical factors such as coastal geometry and/or sea-floor topography, especially near the capes of the North Canary Basin. At the transition between the active upwelling region and the open ocean, a sharp density front can be observed, that results in the surfacing of pycnoclines. Along this front, intense downwelling occurs and small eddies can be observed at the ocean surface. Filaments of cold nutrient-rich water can thus migrate into the open oligotrophic ocean as far as 30 km offshore (Brink and Cowles, 1991).

Equatorial Atlantic Ocean, Guinea Basin and Mexico Bay

Relatively warm South Atlantic surface waters, that are reinforced by the Benguela Oceanic Current move through the Equatorial Atlantic as the South Equatorial Current (SEC; Fig. 1, 2). In the Equatorial Atlantic the SEC consists of two branches; the northern branch is a fast current located at 2-4ºS whereas the southern broad branch is around 10-11ºS. These two branches are separated by the South Equatorial Counter Current. The northern boundary of the SEC is formed by the North Equatorial Counter Current, with the contact between these two currents leading to convergence and downwelling of surface waters, thereby depressing the thermocline and resulting in warm, oligotrophic and generally stratified water masses. Within the thermocline, water is transported equatorwards, where it supports the eastward flowing Equatorial Under Current (EUC). The EUC is a fast flowing undercurrent which runs through the entire Equatorial Atlantic, extending between 5°N and 5°S before surfacing in the eastern part of the Gulf of Guinea. A northern branch of the EUC becomes the Guinea Current, whereas a southern offshoot forms the southward flowing Angola Current. In the eastern Atlantic, the contact between the EUC and SEC forms the equatorial divergence zone, centred on the equator between 25°W to 5°E (Voituriez et al., 1982; Voituriez, 1983), where cooler water-masses upwell from the thermocline, and results in high nutrient concentrations and enhanced productivity in surface waters (Fig. 3).

Different types of surface waters can be distinguished in the Gulf of Guinea on the basis of the temperature and salinity gradients (Berrit, 1966). Low salinity (< 33 in summer) and warm waters (> 24°C in winter) are observed in the vicinity of the Niger Delta, the Bay of Biafra, Congo Delta, and off the Guinea-Liberia region, due to intense river discharge. Saline surface waters (ca. 35) of a southern origin are evident off Cape Three Points (off Ghana). Seasonal fluctuation of surface temperatures above and below 24°C leads to distinguish a "cold water" mass (August) and a "warm water" mass (February). Seasonal coastal upwelling (between July and September) is controlled by atmospheric and oceanic circulation patterns (Herbland et al., 1983; Collin, 1991; Fig. 3). Thermal anomalies, or thermal domes, are observed off Guinea and Angola and probably linked to the flow pattern of the equatorial counter-currents (Voituriez, 1981). The degree of such doming is at its maximum during boreal summer for the Guinea Dome and during boreal winter for the Angola Dome.

In the western Equatorial Atlantic the SEC branches to form in the oligotrophic waters of the southward flowing Brazil Current (BrC) and the northward flowing North Brazil Current (NBC). Fresh, nutrient-rich Amazon waters enter the Equatorial Atlantic by flowing northward along the coast of northern Brazil.

 

The equator in the Atlantic Ocean (photo Donath)

South Atlantic

A summary of the most important oceanic features of the South Atlantic Ocean based on studies of Peterson and Stramma (1991), Berger and Wefer (1996), Fu (1996), Lutjeharms (1996), Reid, (1996), Shannon and Nelson (1996) is given below (Fig. 1). The present-day surface oceanic circulation of the eastern South Atlantic is coupled to the surface atmospheric circulation, characterised by the south-east trade-winds (SE trade-winds). The strength of these trade-winds is controlled by the position of the Inter Tropical Convergence Zone (ITCZ) and the pressure difference between the high-pressure cell above the South Atlantic, which has a more or less permanent position at about 30°S, and a low-pressure area situated above the African continent. During boreal summers, SE trades are at their maximum intensity, due to strengthening of the subtropical anticyclonic gyre, and the ITCZ migrates to 15°N (Leroux, 1983, 1992). The SE trade-winds are deflected to the east on passing the equator and become the SW-monsoon. During boreal winters, when the ITCZ reaches 5°N, dry north-easterly trade-winds (the Harmattan) dominate the surface flow pattern over much of Africa (Pye, 1987).

The southern boundary of the South Atlantic gyre, characterised by oligotrophic waters, is formed by the Subantarctic Convergence Zone, where the eastward flowing south Atlantic gyre contacts the Antarctic Circumpolar Current (ACC). In the south-east South Atlantic Ocean, the relatively cool south Atlantic gyre and Antarctic Circumpolar Current waters contact warm and saline water, that was transported out of the south-western Indian Ocean by the Agulhas Current. As a result of this contact, a major part of the south-westward flowing Agulhas Current waters is retroflected offshore from the south-western tip of south Africa to form the Agulhas Return Current. The terminal region of the Agulhas Current exhibits extreme mesoscale variability, and a range of warm rings and eddies are formed there resulting in highly variable nutrient concentrations in surface waters. The region is also characterised by relatively warm sea-surface temperatures (ca. 20°C), maintained by a continuous inflow of warm Agulhas water. Some of the shedded warm-saline rings drift into the less saline South Atlantic Ocean at an estimated frequency of ca. 4 to 8 rings per year. Depending on their energy, it is estimated that these rings may have a life span of 5 to 10 years and can be transported far into the South Atlantic Ocean by the north-westward flowing South Equatorial Current (SEC). Waters off the northern landward border of the Agulhas Current can rapidly be transported into the South Atlantic Ocean by the shelf edge current, as well as along the edges of Agulhas rings in form of filaments.

The oceanic circulation along the south-western African coast is dominated by the northward flowing Benguela Current (BC). Between ca. 35°S and ca. 17°S, cells of permanent upwelling of cold thermocline waters occur, inducing high-nutrient concentrations in surface waters (Fig. 3). High-nutrient concentrations in surface waters can also be observed at several fronts running parallel to the coast as a result of an unstable mixed layer. Land-derived nutrients are brought into the system by the Orange River and Cunene River discharges. The BC contains two branches, a north-westward flowing branch that leaves the coast at about 20°S (the Benguela Oceanic Current) and a northward branch that moves up to 14°S-16°S where it meets the warm southward-flowing Angola Current at the Angola-Benguela Front region (the Benguela Coastal Current). The northern extent of the Benguela upwelling system coincides approximately with the termination of upwelled water at 15°S. During one year, the average latitudinal shift of the Angola Benguela Front is only two to three degrees latitude. However, the front may also exhibit short-period (days to weeks) fluctuations to the north (up to 10ºS) and south (up to 20ºS).

Arabian Sea

The oceanic circulation of the Arabian Sea is strongly influenced by the semi-annual reversal of wind patterns; the south-west (SW) monsoon in the boreal summer and north-east (NE) monsoon in the boreal winter. In summer, differential heating of the continental and oceanic regions leads to low-atmospheric pressure above the Asian Plateau and high-atmospheric pressure over the relatively cool southern Indian Ocean. This results in the development of a strong low-level jet stream, the Findlater Jet (e.g. Smith et al., 1991; Fig. 4). The response of the ocean to these south-westerly winds includes the development of the south-western Somali Boundary Current, with a north-north-eastward current speed of about 150 cm/s (Bruce, 1973, 1974, 1979; Schott et al., 1990; Brock et al., 1991). This current speed decreases to about 10 cm/s at 100m depth. The Somali Current can contain one or several clockwise-rotating eddies that transport low-salinity water northward from the equator (Schott et al., 1990). Development of this current results in upwelling along the Somali and Arabian coasts, and brings relatively cold, nutrient-rich waters, to the surface. In the Arabian Sea region north of the Findlater Jet, a large area of open oceanic upwelling develops (e.g. Currie et al., 1971; Bauer et al., 1991; Brock et al., 1991). To the south, enhanced deepening of the mixed-layer associated with Ekman pumping can be observed (Bauer et al., 1991). Both upwelling and processes associated with Ekman pumping result in increased nutrient availability and consequently enhanced bioproduction in the surface waters of the Arabian Sea (e.g. Currie et al., 1971; Bauer et al., 1991; Brock et al., 1991; Fig. 5).

During the NE monsoon both wind and surface circulation patterns are reversed. However, ocean current speeds are not as high as during the SW monsoon, and over a year a net northward flow of 20-30 cm/s can be observed (Schott et al., 1990). Cold winds originating from the north-east induce increased mixing and cooling of waters near the Gulf of Oman and on the Pakistan shelf, resulting in high bioproduction in these regions (e.g. Qasim, 1982; Smith et al., 1991).

Arabian Sea subsurface waters are composed of Red Sea and Gulf of Oman outflow waters, that spread slowly southward at a depth of ± 600 m to 800 m and eventually mix with other water masses (Wyrtki, 1971; Shapiro and Meschanov, 1991). Bottom waters enter the Somali Basin from the south (Wyrtki, 1971; Barton and Hill, 1989; Johnson et al., 1991a, b). Part of this water probably leaves the basin in the North by passing the Owen Fracture Zone, to become part of a south-westward flow along the Carlsberg Ridge.

The Southern Ocean

The surface-oceanic circulation in the Southern Ocean is unique in many aspects, having a latitudinal zonation due to the absence of obstacles around much of the Antarctic continent; the Drake Passage is the only region where water masses are constricted (Fig. 5). Pioneering work completed by Deacon (1937) with subsequent research by Gordon et al. (1977), Deacon (1984), Gordon and Owens (1987) and summarised by Knox (1994), documents the complexity of the Southern Ocean-Atmosphere-Ice system. Two main surface currents were identified by Deacon (1937); the East Wind Drift (or Antarctic Coastal Current, ACoC) and the West Wind Drift (or Antarctic Circumpolar Current, ACC), with both currents driven by prevailing katabatic winds. The ACoC (flowing westwards) follows the coastline of the Antarctic continent, whereas the ACC (flowing eastwards) displays significant variations with longitude, in relation to bottom topography. This current subsequently joins the great current gyres of the South Atlantic, South Indian and South Pacific Oceans. A number of fronts are recognised in the Southern Ocean and divide it into several regions. The Antarctic Divergence (AD), which forms the boundary between the AcoC and the ACC, and the Antarctic Convergence (or Polar Front), are the southern and northern limits of the cold, nutrient-rich, Antarctic surface waters. The Polar Front, which covers an area of about 2º to 4º latitude, is the southern boundary of the warmer, nutrient-poor, Sub-Antarctic surface waters, and is positioned at about 55ºS and 60ºS in the Scotia Sea. This front is characterised by steep gradients in temperature and salinity and abrupt changes in phytoplankton and zooplankton composition of the surface waters. The Subtropical Convergence Zone (STC) borders the Sub-Antarctic surface waters to the north. The Southern Ocean is an important area of upwelling and high primary production, linked to the strong and persistent Antarctic Circumpolar Current. The occurrence of eddies is quite common and are particularly important since they contribute to the meridional transport of heat. Sea-ice cover occurs seasonally, extending as far as ca. 56ºS in the South Atlantic during September. The presence of polynyas, high-productivity spots in polar environments, has also been observed by satellite imagery (Knox, 1994). Deep-water formation also occurs at the shelf of the Antarctic continent, mainly in the Weddell Sea, and plays an important role in the global thermohaline circulation.

 

Surface circulation in the Southern Ocean. AAC: Antarctic Circumpolar Current, AcoC: Antarctic Costal Current. Fronts are outlined with dotted lines: STC: Subtropical Convergence Zone; AC: Antarctic Convergence; AD: Antarctic Divergence. Samples sites are represented with grey dots.

The West Pacific Ocean (Japan)

The surface oceanic circulation in the subtropical west Pacific is characterised by the Kuroshio Current (KC) that forms the western part of the anticyclonic oceanic gyre, also named the subtropical gyre (e.g. Masuzawa, 1972; Tchernia, 1980; Tomczak and Godfrey, 1994). The KC begins around 15ºN as a swift and narrow segment of the western boundary of the subtropical gyre (Fig. 6). This current flows northwards close to the east coast of Taiwan, as the Taiwan Current, and then into the East China Sea through the shallow strait between Taiwan and the Ryukyu islands. In the East China Sea, the KC splits south-west of Kyushu into two branches. A major part passes through the Tokara Strait and flows along the east-coast of the Japanese islands. A minor branch flows north as the Tsushima Current (TC) west of Kyushu, through the Korea Strait and into the Sea of Japan.

The East China Sea and the Yellow Sea are part of the broad continental shelf that reaches from the Chinese mainland to Taiwan and stretches as far south as Vietnam. In the East China Sea, two water masses are recognised, one advected by the KC, bringing warm and saline waters before becoming the Yellow Sea Warm Current. The second water mass is controlled by river runoff from monsoonal rainfall in summer, mainly from the Changjan River. The China Coastal Current is strengthened by this runoff and transports low salinity water southward from the northern Yellow Sea. A narrow coastal current along the west coast of Korea transports also low salinity waters from the Bohai Sea south-westward. Sea-surface temperatures in this area ranges from 2 to 20ºC during winter monsoon and between 24 and 28ºC during summer monsoon.

The Sea of Japan consists of an isolated deep basin, where exchange with the surrounding seas takes place through mostly narrow passages with sill depths of less than 100 meters. Waters of the warm Tsushima Current (TC) and the cold southward flowing currents meet in this sea. In the south of the Sea of Japan, the TC separates into two branches around the Tsushima Islands (in the Korea Strait). The western branch, that transports summer heat, flows north-westward along the Korean coast up to 37-38ºN. The eastern branch is weak all year and closely follows the Japanese coast. In the north of the Sea of Japan, the most westerly component of the TC enters the Pacific as the Tsugaru Warm Current and meets the cold Oyashio Current at about 42ºN. Cold waters enter the Sea of Japan from the Sea of Okhotsk. In the Sea of Japan, a strong latitudinal gradient in sea-surface temperatures is observed from north to south, with temperatures ranging from 0 to 12ºC during winter and from 18 to 26ºC during summer. Salinity is above 34 in the south and well below 34 in the north.

Florida Bay and Mississippi Sounds

The study area includes the coastal areas offshore of eastern Mississippi and westernmost Alabama, U.S.A. The area is adjacent to Biloxi Bay, the Pascagoula River, Mobile Bay, and numerous bayous. Samples were taken between the coast and the barrier islands (Mississippi Sound) and seaward of the barrier islands (Gulf of Mexico) and represent modern sedimentation.

Climate in the Florida Bay and Mississippi Sounds is generally tropical to subtropical, with summer atmospheric temperatures average 28ºC and winter atmospheric temperatures are 10-11ºC (National Climatic Center, 1983; National Oceanographic and Atmospheric Administration, 1985). Summer and winter sea-surface temperatures average 30ºC and 12-15ºC respectively. Surface salinity is highly variable, both landward and seaward of the barrier islands. Landward, it varies from 1 in the spring to 30 in the automm. Even 25 km offshore, direct measurements of surface salinity in September were less than 30. The bayous, rivers, and bays draining the adjacent land area bring in considerable sediment and freshwater. Hurricanes and tropical storms are a major influence in the maintenance of the barrier islands. Historic records (Eleuterius and Beaugez, 1979) indicate one hurricane strikes the area every four years, and one tropical storm every three years. Minor currents between barrier islands may transport cysts short distances.