DYNAS

DYNAS: Dynamics and turbulence in the Sicily channel

User group PI: Pascale Bouruet-Aubertot, Université Pierre et Marie Curie, Laboratoire d’Océanographie et du Climat, France

Hosting infrastructure: CNR-ISMAR Sicily Channel Observatory, Italy

Main Objectives

The Sicily channel which connects the Western and Eastern Mediterranean basins is a key region for observational tracking of variations in thermohaline circulation through water mass changes – intermittent events and long term variations. The exchanges between the two basins have a strong variability that results from the complex flow pattern. The circulation is characterized by meso and submesoscale processes with high variability.  At smaller temporal and spatial scales, the Sicily Channel presents a strong internal wave activity as a result of a permanent energy source arising from the barotropic semi-diurnal tide as well as from an indirect excitation the near-inertial frequency band induced by the atmospheric forcing. The anomalous existence of internal tides in the Mediterranean Sea has been evidenced in observational studies (Gasparini et al, 2004) and is the subject of high resolution numerical simulations (Abdennadher and Boukthir, 2016). Nevertheless the turbulence induced has been only characterized recently with the first in-situ microstructure measurements (Ferron et al, 2017) revealing that the Sicily channel is a hotspot for turbulence. At the mooring sites where most measurements were performed, different mechanisms lead to turbulent events induced by shear instability in the region of maximum mean shear flow and internal wavebreaking. Noticeable modulation in dissipation rate with the semi-diurnal cycle has been evidenced (Vladoiu et al, 2017).

Our purpose is to characterize mass, heat and salt transports from the observations of the two moorings making the CNR-ISMAR Sicily Channel Observatory over a year. To this aim we will add temperature and conductivity sensors along the mooring lines as well as a high frequency acoustic Doppler profiler to follow the bottom flow. The second purpose is to identify the different flow spectral components and their contribution to the transport. There is indeed in addition to the seasonal mesoscale vortex structure, a large set of waves at specific frequencies (diurnal, inertial, semi-diurnal). Eventually the idea is to get insights on the impact of turbulence in local water mass transformation with indirect estimates from the mooring measurements. The ultimate ambitious goal is to get a vision on scale interactions with the quantification of energy transfers (bi-spectral analysis) and how they modulate the transport in the Sicily channel. Finally, additional turbulence measurements will be performed along the Eastern Deep Mediterranean Water outflow through the channel in order to better quantify the relationship between turbulence and the complex topography of this channel.

References

Abdennadher, J., Boukthir, M. 2016. Numerical study of the spatial distribution of the M internal tides in the strait of Sicily. Geophys. And Astrophys. Fluid Dyn.,doi:10.1080/03091929.2015.1135915

Ferron, B., P. Bouruet Aubertot, Y. Cuypers, K. Schroeder, and M. Borghini (2017), How important are diapycnal mixing and geothermal heating for the deep circulation of the Western Mediterranean?, Geophys. Res. Lett., 44, doi:10.1002/2017GL074169.

Gasparini, G.P., Smeed, D.A., Alderson, S., Sparnocchia, S., Vetrano, A. and Mazzola, S., 2004. Tidal and subtidal currents in the Strait of Sicily. J. Geophys. Res., 109(C2).

Vladoiu, A., Bouruet-Aubertot, P., Cuypers, Y., Ferron B. Schroeder, K., Borghini, M., Leizour, S., Ben Ismail, S., Bryden, H., 2017. Turbulence in the Sicily Channel from microstructure measurements,  submitted to Deep Sea Res.