DELIVERABLE D2.1 - Ifremer

1.5. General hydrography of the deep NE Atlantic1.5.1. Water column hydrography (Temperature and Salinity)Our interest in waters deeper than about 200m in the NE Atlantic straddles upperand deep circulations, and due to variability in the structure of the water column agiven depth maybe associated with the deep, intermediate or shallow circulationdepending on where it is in the region.Deep waters are separated from the surface by the permanent thermocline which isthe part of the water column where temperature changes strongly with depth. Abovethe main thermocline the water column changes seasonally. In the winter time strongheat loss due to wind and cold air temperature forms a thick mixed layer (Figure 12).In the Labrador Sea and Nordic Seas the main thermocline is weak and the wintermixed layer here can produce deep convection forming new deep water. In the mainNorth Atlantic the winter mixed layer can be deeper than 600 m in the RockallTrough, Iceland and Irminger Basins, and mixing deeper than 200 m extends into theBay of Biscay. The thick layers of water formed above the main thermocline bywinter-mixing contribute the volumetrically dominant water masses of the upperand intermediate North Atlantic circulation.Figure 12. Areas where the depth of winter mixing in the NE Atlantic (not including the Nordic Seas)was found to be greater than 200m by McCartney and Talley (1982).Most of what we know about the interior circulation of the ocean has been deducedby understanding water masses. The concept of water masses relies on the idea thatonce a water type is formed by interaction with the atmosphere in a given location itwill conserve some of its properties once it is no longer in contact with theatmosphere. The two properties that generally define water masses are its20