Water masses and their circulation off western Iberia during May 1993

Abstract

The distribution of the water masses and their circulation in the eastern North Atlantic region off the northern half of the west coast of the Iberian Peninsula are analysed here based on CTD measurements made in May 1993 during the “MORENA 1” cruise. The upper layers of the ocean, down to about 100m, showed a typical spring, non-upwelling situation with a very shallow seasonal thermocline overlying thicker remnants of the mixed layer from the previous winter. Below, Central Water extended between a subsurface salinity maximum near depths of 100–140m and a salinity minimum at about 500m. Using quantitative water mass analysis and geostrophic calculations, a poleward surface current reaching down to depths of 250m and transporting relatively warm and saline Eastern North Atlantic Central Water (ENACW) of subtropical origin was characterised, centred at a few tens of km offshore the shelf break, therefore indicating that the northward current identified previously in this region during winter (the Portugal Coastal Countercurrent) continues to be present through the spring season. A large-scale transition zone, the Galicia Front, was found in the northwest area of the study region, reaching from the surface layers down to the depths of the Mediterranean Water. To the north of this front, relatively fresh Central Water of Western Atlantic origin replaced the subtropical branch of ENACW. At the deeper Central Water levels, ENACW from subpolar North Atlantic origin was present through the whole study region. The influence of the upper and lower cores of Mediterranean Water was present, centred at levels of 800 and 1100m, as indicated by their characteristic thermohaline maxima. The deduced flow of Mediterranean Water was towards the north in the whole study region, in clear relation with the Portugal Slope Undercurrent, and showed some tendency to branch towards the west through the gap between the Vigo Seamount and the Galicia Bank. The Mediterranean upper core occupied a relatively thin layer, O (100m), and underwent a strong dilution of about 45% from the south to the north of the study region; the lower core spread over a larger depth range, of about 400–500m, and decayed only by about 20% through the region. Below the Mediterranean Water, the influence of North Atlantic Deep Water spread through the study region, and an intermediate layer of Labrador Sea Water was also present, particularly offshore in the northwest zone.