Flow variability in the Gulf of Lions during the MATER HFF experiment (March–May 1997)

Hydrological and current meter data were gathered during the High Frequency Flux Experiment that took place in the Gulf of Lions from March to May 1997, within a 20×40-km experimental box over the shelf edge and the continental slope offshore of Marseille. The data set has been supplemented and jointly analysed with sea surface temperature images to characterise flow variability with a particular regard on the mesoscale effects associated to the regional circulation. The current meter observations showed a southwestward flow characteristic of the Northern Current. It is strongest near the shelf break, constrained to flow along topography near the bottom but much less polarised over the upper and mid-slope. Mesoscale variability shows up as fluctuations around 3.5- and 7-day periods at 250- and 650-m depth, and around 7-day periods at 1230-m depth. The Northern Current, as observed on satellite images, forms a 30-km-wide stream with meanders displaying length scales longer than 60 km. These meanders are observed to embrace smaller structures, which are responsible for the intense mesoscale activity recorded in current meter data. Both the hydrological observations and current meter data down to 650-m depth match the superficial structures. Two major flow patterns are observed during the experiment: (i) the core of the Northern current flows south of the experimental site during March and early April, with transitory eddies moving over the experimental site; (ii) following the traverse of a westward-propagating large meander on mid-April, the Northern Current remains over the experimental site. We provide evidence that the synoptic current variability observed in the upper layer may be related to baroclinic instability. A distinct near-bottom current variability is also documented on the mid-slope. Bottom trapped topographic waves are proposed as the mechanism which produces the along-slope deep flow variability.