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Ocean currents


Graph extracted from Dinter (2001): Biogeography of the OSPAR Maritime Area. German Federal agency for Nature Conservation.

The physical setting of the mid-Atlantic Ocean: Hydrodynamic modelling and satellite observations of regional watermasses and frontal zone features.


Physics shape ecosystems. Oceanic animals are not distributed evenly or at random. Their distribution patterns are influenced by a number of factors, both abiotic and biotic, and the more important structuring factor at the regional scale is the character and distribution of water characteristics. Frontal zones are highly productive border areas, often separating animal communities of markedly differing composition.

MAR-ECO area influenced by major Atlantic features. The mid-Atlantic Ridge (MAR) influences regional surface and deep-water circulation, and the major flows and watermass patterns are well known. The circulation of the North Atlantic Ocean is characterised by two large gyres: the subpolar and subtropical gyres.

As the Gulf Stream approaches the Great Banks, some water continues east and crosses the MAR in the Azores Current and the remainder forms the North Atlantic Current (NAC) that continues as a well-defined boundary current along the eastern slope of the Grand Banks.

At about 51°N the NAC moves to the east. As the waters flow eastward the NAC looses its structure as a well-defined jet, and the water is transported eastward in the Sub Polar Front (SPF), which is the boundary between the warm water in the subtropical gyre and the cooler and less saline water in the subpolar gyre to the north.
 
The eastward transport of warm water is split in several branches and at the MAR these branches are found between 45°N and the Charlie Gibbs Fracture Zone (CGFZ, ~52°N). East of the MAR the SPF makes a sharp turn toward the north and eventually feeds the Irminger Current and the inflow of warm water to the Nordic Seas.


Main pathways of intermediate and deep-water masses in the North Atlantic.
Subsurface drifters launched in the SPF to the west of the MAR, were typically (but not all!) funneled across the MAR at the CGFZ (Bower et al. 2002). Also surface drifters indicate that very little surface water is transported across the ridge between the CGZF and the Azores Current (AC) that crossed the MAR to the south of the Azores.

The CGFZ is also a main passage way for low salinity intermediate depth mode waters from the Labrador Sea, Labrador Sea Water (LSW), into the eastern North Atlantic. Below the LSW, Iceland-Scotland Overflow Water (ISOW) originating in the Nordic Seas flows toward the west in the CGFZ.

The eastward transport of warm water is split in several branches and at the MAR these branches are found between 45°N and the Charlie Gibbs Fracture Zone (CGFZ, ~52°N). East of the MAR the SPF makes a sharp turn toward the north and eventually feeds the Irminger Current and the inflow of warm water to the Nordic Seas.
Subsurface drifters launched in the SPF to the west of the MAR, were typically (but not all!) funneled across the MAR at the CGFZ (Bower et al. 2002). Also surface drifters indicate that very little surface water is transported across the ridge between the CGZF and the Azores Current (AC) that crossed the MAR to the south of the Azores.

The CGFZ is also a main passage way for low salinity intermediate depth mode waters from the Labrador Sea, Labrador Sea Water (LSW), into the eastern North Atlantic. Below the LSW, Iceland-Scotland Overflow Water (ISOW) originating in the Nordic Seas flows toward the west in the CGFZ.


Every March, one of the ocean's grandest biological events begins just north of the Sargasso Sea and Bermuda. As days in the northern Hemisphere begin to lengthen, phytoplankton, nourished by the abundant nutrients in the surface waters that the winter storms have supplied, explode in numbers producing a massive greening of the North Atlantic Ocean which is often referred to as the Spring Bloom. Sometimes, when ocean current systems interact, surface water masses move toward each other and sink along the meeting line -- concentrating phytoplankton and the nutrients that they need for growth. These zones of enhanced productivity are often visible in satellite ocean colour data.*these images are from the SeaWiFS Project (Images are from the SeaWiFS Project)
The value of pre-cruise modelling and satellite images. The MAR influence local and regional circulation patterns, which in turn are likely to affect the distribution of pelagic organisms.

Pre-cruise knowledge of patterns, e.g. position and character of frontal zones and flow patterns, is crucial for designing sampling programmes to test central ecological hypotheses.

A major feature strongly influencing distribution patterns, the SPF is clearly visible in sea surface temperatures (SST) satellite images.

Vertically structured circulation models can be used to simulate zooplankton advection.

How and who

Oceanographers and modellers of the Norwegian Institute of Marine Research and the Nansen Centre….. can provide the necessary model outputs and satellite images, and supply outputs to project participants from other partner countries involved in MAR-ECO. This work is essential to all cruises and all partners should have access to high-quality data. However, the work requires new costly model runs and analyses using powerful computers.


This image from the SeaWiFS Project shows where there is more or less plant life on our planet. On land, the dark greens show where there is abundant vegetation and tans show relatively sparse plant cover. In the oceans, red, yellow, and green pixels show dense phytoplankton blooms, those regions of the ocean that are the most productive over time, while blues and purples show where there is very little of the microscopic marine plants called phytoplankton.

For more information, please contact Henrik Søiland , Institute of Marine Research, Bergen.

More information

About North Atlantic (and ocean) circulation:
article by Henrik Søiland, "Large scale circulation and water masses in the North Atlantic"

On physical oceanography in the North-Atlantic:

NASA Goddard Space Flight Center SeaWiFS Project

ICES The Annual Ocean Climate Status Summary 2000/2001

Climate Variability and Predictability CLIVAR

WOCE Atlantic Atlas horizontal Maps

MAST-II EUROFLOAT-project

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