Rachid Benshila

Impact of the Mesoscale Dynamics on Ocean Deep Convection: The 2012–2013 Case Study in the Northwestern Mediterranean Sea

Winter 2012–2013 was a particularly intense and well‐observed Dense Water Formation (DWF) event in the Northwestern Mediterranean Sea. In this study, we investigate the impact of the mesoscale dynamics on DWF. We perform two perturbed initial state simulation ensembles from summer 2012 to 2013, respectively, mesoscale‐permitting and mesoscale‐resolving, with the AGRIF refinement tool in the Mediterranean configuration NEMOMED12. The mean impact of the mesoscale on DWF occurs mainly through the high‐resolution physics and not the high‐resolution bathymetry. This impact is shown to be modest: the mesoscale does not modify the chronology of the deep convective winter nor the volume of dense waters formed. It however impacts the location of the mixed patch by reducing its extent to the west of the North Balearic Front and by increasing it along the Northern Current, in better agreement with observations. The maximum mixed patch volume is significantly reduced from 5.7 ± 0.2 to 4.2 ± 0.6 × 1013 m3</sup<. Finally, the spring restratification volume is more realistic and enhanced from 1.4 ± 0.2 to 1.8 ± 0.2 × 1013 m3 by the mesoscale. We also address the mesoscale impact on the ocean intrinsic variability by performing perturbed initial state ensemble simulations. The mesoscale enhances the intrinsic variability of the deep convection geography, with most of the mixed patch area impacted by intrinsic variability. The DWF volume has a low intrinsic variability but it is increased by 2–3 times with the mesoscale. We relate it to a dramatic increase of the Gulf of Lions eddy kinetic energy from 5.0 ± 0.6 to 17.3 ± 1.5 cm2/s2, in remarkable agreement with observations.

On Eddy-Mixed Longshore Currents: Video Observation and 3D Modeling off Grand Popo Beach, Benin

ABSTRACT Marchesiello, P.; Almar; R.; Benshila; R.; Larnier, S.; Castelle, B., and McWilliams, J.C., 2016. Morphological Change near Artificial Reefs as a Beach Erosion Countermeasure. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). J. Coast. Res., Special Issue, No. 75, pp. 408–412. Coconut Creek (Florida), ISSN 0749-0208. The link between intrinsic shear-flow instability of longshore currents and their mean cross-shore profiles has long been suggested. Yet, recent investigations give increasing credit to extrinsic wave forcing mechanisms, downplaying the role of shear waves on the observed variability. Our results for Grand Popo beach, Benin, provide an original attempt to map mean longshore currents forced by an oblique swell. A 3D model investigation shows that their broad mean flow and the frequency band of eddy variability are consistent with shear instability, conversion of mean to eddy kinetic energy and eddy mixing of momentum. Their turbulent dynamics do not clearly fit in any classical 2D or 3D paradigms but show large transfer of energy across the wavenumber spectrum, to both larger and smaller scales.