The hydrography of the mid-latitude Northeast Atlantic Ocean: II: The intermediate water masses

Abstract

The intermediate water masses in the eastern Atlantic Ocean between 31°N and 53°N were studied by analysis of the distributions of potential temperature, salinity, dissolved nutrients and oxygen. Sub-surface salinity minima are encountered everywhere in the area. At the northern and southern boundary they are connected with the presence of Sub-Arctic Intermediate Water and Antarctic Intermediate Water, respectively, but towards the European ocean margin the sub-surface salinity minima shift to shallower density levels. The sub-surface salinity minima observed west of the Iberian Peninsula represent a water mass formed by winter convection in the Porcupine Sea Bight and the northern Bay of Biscay. These minima gain salt by diapycnal mixing with the underlying Mediterranean Sea Outflow water and with the overlying permanent thermocline. The core of Antarctic Intermediate Water appears to contribute to the formation of Mediterranean Sea Outflow Water since it becomes entrained into the overflow near Gibraltar. This entrainment gives rise to an enhanced concentration of the nutrients in the Mediterranean water in the North Atlantic. The deep salinity minimum, due to the presence of Labrador Sea Water, is restricted mainly to the Porcupine Abyssal Plain. In the Bay of Biscay this water type is strongly modified by enhanced diapycnal mixing near the continental slope. At all intermediate levels the continental slope in the Bay of Biscay seems to be a focal point for water mass modification by diapycnal mixing. Below the core of the Mediterranean Sea Outflow Water the Labrador Sea Water is also strongly modified. Its salinity is strongly enhanced by diapycnal mixing with the overlying core of Mediterranean Sea Outflow Water. An analysis of the oxygen and nutrient data indicates that the large spatial concentration differences at the level of the Labrador Sea Water are caused mainly by ageing of the water. The youngest water is observed at 52°N, and, especially in the Bay of Biscay and off south-west Portugal, the water at levels of about 1700 dbar are strongly enriched in nutrients and depleted in oxygen.