Some aspects of time variability of the Mediterranean Water off south Portugal

Mediterranean Water contributes significantly to the hydrological properties and circulation of the intermediate layers of the North Atlantic, yet the temporal variability of the Mediterranean Undercurrent is still not well understood, even at intra-annual time scales. The present study addresses the evolution of the temperature field in the layers of Mediterranean Water off south Portugal during one year (July 1993–July 1994), obtained with an almost weekly repetition of a high-resolution expendable bathythermograph (XBT) section in the vicinity of Portimão Canyon, during the AMUSE experiment. The information obtained with this set of data was complemented by velocity observations of the Mediterranean Undercurrent with RAFOS floats launched simultaneously with the XBT lines. The typical structure of the Mediterranean Undercurrent along the slope of the Gulf of Cadiz, consisting of a westward jet associated with the cores of the thermohaline structure, was observed in almost 60% of the 24 repetitions of the XBT section. The results show that this typical structure in the velocity was not always accompanied by a typical thermal structure. Anomalous behaviour of the Undercurrent – southward or southwestward flow or local recirculation – occurred only in winter and corresponded to some anomalies in the thermal field associated with the Mediterranean Water. The occurrence of warm features offshore of and separated from the main body of warm water found against the slope off south Portugal seemed to be connected with episodes of changes in direction and speed of the Undercurrent. Several scales of time variability within the Mediterranean Water layers were identified ranging from a week to a month. A possible seasonal trend in the temperature of these layers was apparent in the XBT data and seemed also to be found in time series obtained with moored current meters.