Aspects of the seasonal and mesoscale variabilities of the Northern Current in the western Mediterranean Sea inferred from the PROLIG-2 and PROS-6 experiments

This paper presents a study of the Northern Current, which is a major component of the circulation in the western Mediterranean Sea. Our analysis is based on a monthly CTD survey (PROS-6 experiment), and on 12 current-meter time series collected at three depths on four moorings (PROLIG-2 experiment) from May to December 1985. These data support the fact that the mesoscale activity of the Northern Current smoothly decreases in spring-summer and then rapidly increases in early autumn. From the first period to the second, the fluctuations become more isotropic, their time and space scales become smaller and they have a more reduced vertical extent. The mesoscale phenomena are characterized by two main fluctuation bands: at all depths, a 10–20-d band only appears in the direction of the mean flow while a 3–6 d band mainly appears in a perpendicular direction, being polarized anticlockwise at some distance from the coast. The phase speeds are generally equal to or lower than ∼ 10 km/d in both bands. These values compare rather well with those deduced from previous analyses of hydrological data and baroclinic instability models. We hypothesize that the fluctuations at 10–20 d are pulses of the mean flow while those at 3–6 d are current meanders that are more intense in autumn than in spring-summer.