VARIABILITY OF THE NORTHERN CURRENT OFF MARSEILLES, WESTERN MEDITERRANEAN-SEA, FROM FEBRUARY TO JUNE 1992

A hydrological transect was performed on seven occasions from February to June 1992 in the coastal zone off Marseilles, a region of the western Mediterranean Sea where such data were not yet available. This lack of information is surprising, especially if one considers the large amount of data already collected both further seawards and in neighbouring regions such as the Ligurian and the Catalan Seas. The time interval between the transects (one week to one month) does not permit a distinction between mesoscale and seasonal variations, but the fine space interval between the casts (3-4 km) and the length of the transect (similar to 45 km) provide significant information on the characteristics of the Northern Current (NC) when it penetrates into the Gulf of Lions, and on those of the Modified Atlantic Water (MAW), the Winter Intermediate Water (WIW), and the Levantine Intermediate Water (LIW). Large mesoscale meanders of the NC have been evidenced; within the space of one week, its core can be found at a distance of more than 35 km or less than 20 km, while the LIW vein can be shifted by more than 10 km seawards from its mean location close to the slope. This also accounts for the fact that the NC was, in general, relatively deep (5 cm/s at 400-500 m), with surface speeds up to 30-50 cm/s continuously decreasing with depth. Its transport displayed a specific variation: low (similar to 0.7 Sv) in late February-early March, it increased up to 1.75 Sv in April-May and decreased to similar to 0.5 Sv in June. Although significant, such a seasonal variation differs from that encountered in the Ligurian Sea (the maximum occurs earlier in the winter), but is not inconsistent with what is predicted from numerical models with regard to the rim current associated with the formation of the Western Mediterranean Deep Water (WMDW). A mechanism for the formation of WIW is proposed which can occur more or less everywhere in the western Mediterranean Sea, throughout the general circuit of MAW. It is clear that MAW, cooled and homogenized in winter during gusts of northwesterly winds, is rapidly overlaid, when the wind ceases, by warmer MAW that is preserved from such strong interactions in neighbouring places; this cool MAW will subsequently be recognized as WIW and characterized by a temperature minimum with respect to both MAW above and LIW below. It is confirmed that the general circuit of LIW in the whole sea is cyclonic along the continental slope, and it is shown that the variations of both its distribution and hydrological characteristics there are clearly dependent on the formation of WMDW. At that time, LIW spreads seawards along the isopycnals while its temperature and salinity maxima are low due to intense mixing; it then re-structures itself as a vein flowing westwards along the slope while its maximum values continuously increase, due to lower and lower mixing.