WATER MASSES AND THEIR CIRCULATION OFF WESTERN IBERIA DURING MAY 1993

The distribution of the water masses and their circulation in the east ern North Atlantic region off the northern half of the west coast of t he Iberian Peninsula are analysed here based on CTD measurements made in May 1993 during the ''MORENA 1'' cruise. The upper layers of the oc ean, down to about 100 m, showed a typical spring, non-upwelling situa tion with a very shallow seasonal thermocline overlying thicker remnan ts of the mixed layer from the previous winter. Below, Central Water e xtended between a subsurface salinity maximum near depths of 100-140 m and a salinity minimum at about 500 m. Using quantitative water mass analysis and geostrophic calculations, a poleward surface current reac hing down to depths of 250 m and transporting relatively warm and sali ne Eastern North Atlantic Central Water (ENACW) of subtropical origin was characterised, centred at a few tens of km offshore the shelf brea k, therefore indicating that the northward current identified previous ly in this region during winter (the Portugal Coastal Countercurrent) continues to be present through the spring season. A large-scale trans ition zone, the Galicia Front, was found in the northwest area of the study region, reaching from the surface layers down to the depths of t he Mediterranean Water. To the north of this front, relatively fresh C entral Water of Western Atlantic origin replaced the subtropical branc h of ENACW. At the deeper Central Water levels, ENACW from subpolar No rth Atlantic origin was present through the whole study region. The in fluence of the upper and lower cores of Mediterranean Water was presen t, centred at levels of 800 and 1100 m, as indicated by their characte ristic thermohaline maxima. The deduced flow of Mediterranean Water wa s towards the north in the whole study region, in clear relation with the Portugal Slope Undercurrent, and showed some tendency to branch to wards the west through the gap between the Vigo Seamount and the Galic ia Bank. The Mediterranean upper core occupied a relatively thin layer , O (100 m), and underwent a strong dilution of about 45% from the sou th to the north of the study region; the lower core spread over a larg er depth range, of about 400-500 m, and decayed only by about 20% thro ugh the region. Below the Mediterranean Water, the influence of North Atlantic Deep Water spread through the study region, and an intermedia te layer of Labrador Sea Water was also present, particularly offshore in the northwest zone. (C) 1998 Elsevier Science Ltd. All rights rese rved.