Modelling deep-water formation in the north-west Mediterranean Sea with a new air–sea coupled model: sensitivity to turbulent flux parameterizations

Abstract

Abstract. In the north-western Mediterranean, the strong, dry, cold winds, the Tramontane and Mistral, produce intense heat and moisture exchange at the interface between the ocean and the atmosphere leading to the formation of deep dense waters, a process that occurs only in certain regions of the world. The purpose of this study is to demonstrate the ability of a new coupled ocean-atmosphere modelling system based on MESONH-SURFEX-SYMPHONIE to simulate a deep-water formation event in real conditions. The study focuses on summer 2012 to spring 2013, a favourable period that is well documented by previous studies and for which many observations are available. Model results are assessed through detailed comparisons with different observation data sets, including measurements from buoys, moorings and floats. The good overall agreement between observations and model results shows that the new coupled system satisfactorily simulates the formation of deep dense water and can be used with confidence to study ocean-atmosphere coupling in the north-western Mediterranean. In addition, to evaluate the uncertainty associated with the representation of turbulent fluxes in strong wind conditions, several simulations were carried out based on different parameterizations of the flux bulk formulas. The results point out that the choice of turbulent flux parameterization strongly influences the simulation of the deep-water convection and can modify the volume of the newly formed deep water by a factor of 2.

A New Coupled Ocean‐Waves‐Atmosphere Model Designed for Tropical Storm Studies: Example of Tropical Cyclone Bejisa (2013–2014) in the South‐West Indian Ocean

“…This is mainly attributed to the Charnock parameter and roughness length being larger in the OWA simulation than the OA one (Figures 10c and 10f). However, divergences in turbulent flux parameterizations at high wind speeds imply a need to revisit them (Lebeaupin Brossier et al, 2008;Seyfried et al, 2017) and meaning that this sensitivity of turbulent fluxes to oceanic waves is not generalizable. Further studies with longer simulations and statistical diagnoses (as in Lengaigne et al, 2018;Samson et al, 2014) are necessary to address the physical origin of these differences.…”

Preface to the Special Section: Dense Water Formations in the Northwestern Mediterranean: From the Physical Forcings to the Biogeochemical Consequences

“…Uncertainties on heat and water fluxes, in particular related to uncertainties in the parameterization of turbulent fluxes under strong wind conditions, remain an important problem for ocean modeling in the Mediterranean, as shown by the sensitivity study by Seyfried et al () on the modeling of the convection during winter 2012–2013. Unfortunately, no direct observation of the turbulent fluxes was available during the strong wind events of DeWEX/HyMeX SOP2.…”

Dynamics of North Balearic Front during an autumn Tramontane and Mistral storm: air–sea coupling processes and stratification budget diagnostic

“…To perform the simulation, we used the Meso-NH -SURFEX -Symphonie system (Meso-NH: Lafore et al (1997) (Voldoire et al, 2017). This coupled system was described and validated in Seyfried et al (2017).…”

“…Flux calculations are carried out on the atmospheric model grid (after bilinear interpolation of the oceanic fields) and are then bilinearly interpolated on the grid of the ocean model. The atmospheric model covers the whole western Mediterranean basin (same spatial coverage as in Seyfried et al (2017)).…”

“…1). These winds produce strong air-sea exchanges (Flamant, 2003;Hauser et al, 2003) which lead to important diabatic buoyancy losses for the sea and which also impact the NBF position in autumn (Seyfried et al, 2017). Furthermore, the Tramontane and Mistral have up-or down-front winds components over the NBF and the Northern Current.…”