Observations of the Mediterranean Undercurrent and eddies in the Gulf of Cadiz during 2001

Abstract

The North Atlantic is strongly influenced by the presence of a high salinity and temperature water mass – the Mediterranean Water (MW) – lying at intermediate levels. A detailed field survey in the region around the Portimão Canyon, which constitutes the first large canyon that the Mediterranean Undercurrent encounters downstream of the Strait of Gibraltar, has been repeated in three different seasonal conditions (winter, spring and summer) of the year 2001, in the frame of Project MEDTOP. This paper is based on the observations undertaken during MEDTOP and focus on relevant aspects of the thermohaline and velocity fields associated with the Mediterranean Undercurrent and the associated eddies, bringing further evidence of hypotheses presented in previous work on the dynamical behaviour of the Mediterranean Outflow and its intra-annual variability, and describing new observations of meddy formation and subsequent evolution in the centre of the Gulf of Cadiz. The analysis of the CTD data at the MW levels has shown the presence of four MW cores: the Upper and Lower Cores (centred at about 800 and 1200 m), the Shallow Core (between 400 and 600 m) and, in the region close to the Portimão Canyon, a core with higher density than the classical Lower Core at depths between 1300 and 1600 m. The analysis of the RAFOS floats trajectories has shown the Upper and Lower Cores flowing independently along the continental slope, the Upper Core reaching systematically higher velocities than the Lower Core. Within the meddies, there was evidence of a weak vertical coherence of the Cores in the early stages of the eddy life, and a tendency for acquiring a solid-body rotation as they evolve away from their source. The RAFOS floats evidenced the formation of MW eddies at three sites off the Portuguese coast: in Portimão Canyon, off Cape St. Vincent and in the Estremadura Promontory. The period of eddy generation could be quantified within the range 45–60 days. The comparison between the winter and summer cruises has confirmed some aspects of the Mediterranean outflow which had been identified in previous data sets, namely the higher temperature and salinity values, the broader lateral extension and the higher velocities of the outflow in winter relative to summer values.