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 co ntinental slope of the north-western Mediterranean Sea, has been descr ibed as a permanent flow affected by important variability at differen t scales. Many observations indicate a higher flux in winter than in s ummer, 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 Cu rrent has definitively demonstrated the existence of a seasonality in the mesoscale variability. Although the records do not concern the cor e of this current, the consistency of the several characteristics we h ave determined with results found by other authors in the Ligurian Sea allows us to consider them as representative of its general trends. T hree current meters moored at 15, 50 and 100 m depth from an oil drill ing 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 curre nt behaviour. The general flow is along-slope and presents occasional disruptions for periods of several days, described here as mesoscale e vents. Most of the motion we have recorded displays a marked barotropi c character. The intensity of the current is relatively low, increasin g suddenly in autumn at the three levels monitored, in a manner that a ppears not to be connected to the local wind regime. One of the most r elevant pieces of information contained in the whole current data set concerns the repeated presence of clockwise rotations in a near-inerti al frequency, even at 100 m. The principal objective of the present st udy was to investigate any seasonal signal in the appearance and chara cteristics of the mesoscale motion. The main observed feature in a low -passed time series below the thermocline, namely a rapid and strong a utumn increase in mesoscale activity, is shown to be consistently pres ent, with very small inter-annual temporal variability. This maximum o f mesoscale activity is followed by a rapid decrease in winter and the n by a continuous decline until the end of the summer. A complete char acterization of the mesoscale motion in the Northern Current, includin g its origin and interaction with the main flow, requires extensive ex perimental 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 fro nt evolution than to local wind variability.