Mesoscale circulation of the North gyre
Due to the wind and heat forcing from the atmosphere as well as to the inflow of Atlantic water (AW), the circulation in the North Western Mediterranean sub-basin displays a permanent Northern Current (NC) flowing westward along slope from the Gulf of Genova further than the Balearic sub-basin. In the south, the North Balearic Front circulates part of this water from the Balearic Islands back to Corsica. This gyre is included into the basin scale circulation via the Corsica and Balearic channels as well as through the coastal turbulent flow west of Sardinia and Corsica. In the centre of this gyre, mainly off the Gulf of Lions, deep convection occurs in winter and plays a major role in shaping the Mediterranean thermohaline circulation as well as the primary productivity in a large part of the Northwestern basin.
This area has been widely investigated since the sixties with hydrological surveys, moorings equipped with current meters and even basin scale acoustic tomography arrays. Major results have been achieved about the description of the general circulation, including its seasonal and interannual variability, the role of mesoscales and submesoscales like the recently discovered small coherent vortices able to spread water across large distances, as well as a better understanding of fundamental dynamical processes concerning open ocean convection. Convection in the MEDOC area has been studied in the past because it is a suitable model for understanding the role of open ocean convection at the global ocean scale. In the central part of the Ligurian sub-basin, the DYFAMED is one of the world longest biogeochemical observation time series but despite these long-lasting observational efforts, a permanent monitoring at the sub-basin scale at the right temporal and spatial scales is still a challenge.
Such a monitoring is needed for two main reasons. First, fixed moorings and regular ship CTD surveys at large scale are not able to provide any information about fluxes of physical, chemical and biological water properties : fluxes at the boundaries and internal redistributions in the basin thus remain unknown making difficult to assess any kinds of budgets. Second, operational oceanography today relies on sea surface height remote sensing and some scattered temperature and salinity profile provided by the ARGO floats or ships. Because of its relatively large scale, this information does not provide the adequate constraints to oceanic models through data assimilation. The mesoscale and submesoscale variability of the NC is of dramatic importance for coastal circulations, and cannot be monitored. It thus precludes any kind of reliable understanding of small scale coastal systems. MOOSE is the opportunity to start providing insight of the role of these small scales motions on the evolution of the ecosystem, from the surface to the bottom and from the short to the long term.
The backbone of MOOSE is based on five permanent deep offshore moorings giving temporal reference values : one at the centre of the convection area, the other in the Central Ligurian Zone (DYFAMED area) in order to establish the large scale gradient and also to maintain the continuity of this existing 20 year-long time series, the last one in the NC in relation with the ANTARES site. Two additional mooring are placed on the continental margin or on the shelf provide fruitful complementary information, in particular for the monitoring of deep ocean exchange with the shelf and the margin.
Profiling floats and (sub)surface drifters are perfectly suited for operational oceanography needs, and can be released very easily by commercial crews (low cost). They provide information on general circulation features and (sub)mesoscale related activity : since such devices drift with the flow, they cannot yield information on current transects. Moreover, the risk of stranding in coastal/shallow areas is higher.
Complementary strategy and data sets will come first from gliders. Due to their operating range, gliders are perfectly suited to survey with high frequency so called “endurance lines” between the coast and these moorings where profilers cannot sample. Other glider lines would extend the hydrological survey at the scale of the whole gyre and will monitor the exchanges through the North Balearic front south of this region. Gliders are also suitable for adaptive sampling of mesoscale where/when such structures are supposed to play a major role (blocking of coastal currents by eddies, …).
Second, coastal HF radars can provide the high resolution (3 km) synoptic (100 km) view of surface currents from the shore up to the central basin at hourly time scales, which is essential for the understanding and monitoring the NC variability and associated transport and off shore/coastal exchanges. Moreover, the conjoint use of these two types of real-time autonomous measuring instrument should bring a new dimension in terms of adaptability of sampling strategies and coverage of complex 3D structures due to hydrodynamic instabilities of the NC and its interactions with coastal eddies.
Last, periodic hydrological ship borne surveys at the gyre scale and across the boundaries of this region provide the missing data to better constraint budgets and for cross-calibration of all these systems. This should include the monitoring in narrow passages based on moorings like in the Corsican Channel.