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

Abstract

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. 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.

Multiscale Observations of Deep Convection in the Northwestern Mediterranean Sea During Winter 2012–2013 Using Multiple Platforms

“…As pointed out by Leger et al (), "L'Hévéder et al () and Somot et al (), numerical simulations are very sensitive to the initial conditions with regards to winter convection and numerical outputs, including operational products like MERCATOR PSY2V4R4 (Estournel et al, ), have generally serious difficulties to describe well the intermediate and deep layers, because stratification is influenced by initial conditions derived from smoothed climatologies encompassing decades of observations. Waldman et al () and Estournel et al () showed it is possible to correct the initialization and forcing of their model and to significantly improve the realism of the simulations using the DEWEX data set both for initial conditions correction in Summer and later validation.…”

“…This data set was then used for validation purposes to assess the realism of numerical simulations in particular in terms of timing and geography of the phenomena as well as in terms of quantitative estimates of the deep water formation rate (Waldman et al, ) and in terms of mesoscale and submesoscale processes (Damien et al, ; Waldman et al, ) by performing similar diagnostics in the observations and the simulations, and sensitivity studies. They were thus able to reach a better understanding of deep convection processes from autumn to winter together with quantitative estimates.…”

“…The methodologies proposed for estimating the deep convection rate are complementary. In particular, Waldman et al () have shown from a modeling study that the Mixed Patch volume computed as the volume of MLD > 1,000 m (or from a cold signature (<13°C) of intermediate waters (400–600m)) reached a lower value by 1.5 × 10 4 km 3 (0.4 Sv averaged over a year) than the dense water formation rate computed with the volume of waters denser than 29.11 kg m −3 in their run. Both estimates have different physical origins, the former resulting exclusively from the intense vertical mixing during the deep water formation events and the latter also resulting from lateral advection and mixing with surrounding waters.…”

“…Waldman et al () have studied the impact of oceanic intrinsic variability on deep water formation with eddy resolving and permitting simulations. By comparing ensemble results they conclude mesoscale could have a significant impact on deep water formation.…”

On the Chaotic Variability of Deep Convection in the Mediterranean Sea

“…However, the turbulent mesoscale dynamics, primarily stemming from baroclinic instability, is intrinsic to the ocean itself and can modulate deep convection through the restratification process. The question of how important this IOV is to the total variability of deep convection activity in the NWMed, which has been addressed at the daily time scale by Waldman, Somot, et al () and Waldman, Herrmann, et al (), remains open at the interannual and climatic time scales, which are the focus of the present work.…”