Under the action of the North-West/West gales (Mistral) blowing off the calanques of Marseille and the Toulon area, an oceanic circulation of an impulsive nature is set up, characterised by the presence of coastal upwellings. These upwellings induce a strong cooling of the surface waters in summer, and are generally accompanied by an enrichment favouring primary production from a biogeochemical point of view. Due to the high intensity of the phenomenon and the contrasting response of the coastal ecosystem according to the different upwelling episodes during the year, the calanques area is of particular interest. The upwellings are particularly intense in that area and have an impact on biogeochemistry, on the cold water corals that live in the Cassidaigne canyon, as well as on the gorgonians. They will also have an impact on the transfer of matter and contaminants from the Marseilles metropolis (discharged in particular via the outlet of the Cortiou wastewater treatment plant).

Numerous modelling studies of upwelling dynamics have been carried out to date. Although the upwelling mechanism is correctly resolved at a first order by ocean models, because it is directly forced by strong winds, and although the occurrence, location and spatial extension are realistic, a recurrent flaw in the model results concerns the relaxation of the upwellings which is too slow, sometimes accompanied by an underestimation of their intensity. Intermittent upwellings are therefore an ideal object of study to understand whether this erroneous representation can be improved by working on the ocean/atmosphere coupling, the representation of hydrodynamics, and/or the biogeochemical processes involved (AMICO-NEXT project, "Action towards Operational Integrated Coastal Modelling").

Within the framework of this campaign, it was planned to explore the response of the water column to the upwelling process and its relaxation in two contrasting areas, Cassidaigne and Toulon (between the islands), using innovative sensors. Low-cost thermistor lines developed in the framework of the JERICO-NEXT european project were installed in the upwelling areas on the shelf break for one month in late summer. Their use, combined with the realisation of hydrological (TSG, CTD and MVP) and currentometric (hull ADCP) profiles along radials during the campaign, aimed at improving our understanding of the upwelling process and its possible interactions with other processes at play in the study area (North Current, local circulation in the canyon,...).

These data are also valuable in the key stage of model validation in the Toulon to Cassis area, particularly in the Cassidaigne canyon area (Lénaïg Brun phD thesis, 2021-2024).

The contribution of the campaign therefore concerned several aspects:

• Evaluating and validating the technological development of the MASTODON-2D low-cost thermistor chains carried out within the framework of the JERICO-next project, and testing their implementation on deep sites on the shelf (depths greater than 100 metres),
• To acquire hydrological (and if possible currentological) data on the water column at the edge of the shelf next to the MIO BOMBYX mooring (area of the islands off Toulon) over a period of 1 month, as well as in the Cassidaigne canyon area, so as to broaden our knowledge of the temporal variability of the physical processes in these zones,
• To carry out complementary measurements along radials at the end of the deployment, to further characterise intermittent upwellings at the edge of the shelf at the end of the summer and (sub)mesoscale processes associated with the presence of the North Current.

In order to optimise deployments, the campaign also made it possible to deploy platforms for the study of the air-sea interface (Koursk catamaran and Ocarina trimaran), within the framework of the LEFE TURBORADAR project (Improvement of turbulent flow parametrisations of the oceanic surface layer from HF radar data, in situ and laboratory measurements, PI F. Fraunié, MIO)