The seasonality of mesoscale motion in the northern current of the western mediterranean - several years of evidence

The Liguro-Provenco-Catalan or Northern Current, which contours the continental slope of the north-western Mediterranean Sea, has been described as a permanent flow affected by important variability at different scales. Many observations indicate a higher flux in winter than in summer, while the occurrence of mesoscale events appears to be a common characteristic. The opportunity of examining several years of current meter data at a location close to the inner border of the Northern Current has definitively demonstrated the existence of a seasonality in the mesoscale variability. Although the records do not concern the core of this current, the consistency of the several characteristics we have determined with results found by other authors in the Ligurian Sea allows us to consider them as representative of its general trends. Three current meters moored at 15, 50 and 100 m depth from an oil drilling ring near the shelf break off the Ebro delta, with a considerable coverage from mid-1987 to mid-1992, provide a clear image of the current behaviour. The general flow is along-slope and presents occasional disruptions for periods of several days, described here as mesoscale events. Most of the motion we have recorded displays a marked barotropic character. The intensity of the current is relatively low, increasing suddenly in autumn at the three levels monitored, in a manner that appears not to be connected to the local wind regime. One of the most relevant pieces of information contained in the whole current data set concerns the repeated presence of clockwise rotations in a near-inertial frequency, even at 100 m. The principal objective of the present study was to investigate any seasonal signal in the appearance and characteristics of the mesoscale motion. The main observed feature in a low-passed time series below the thermocline, namely a rapid and strong autumn increase in mesoscale activity, is shown to be consistently present, with very small inter-annual temporal variability. This maximum of mesoscale activity is followed by a rapid decrease in winter and then by a continuous decline until the end of the summer. A complete characterization of the mesoscale motion in the Northern Current, including its origin and interaction with the main flow, requires extensive experimental and numerical work. Our results have demonstrated that this motion displays a clear seasonal cycle and provided evidence that the recorded mesoscale activity is linked far more to the shelf/slope front evolution than to local wind variability.