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

Léo Seyfried,Patrick Marsaleix,Evelyne Richard,Claude Estournel

doi:10.5194/os-13-1093-2017

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.

Highlights

The north-west Mediterranean Sea (NWMS) is one of the few regions in the world where the deep open-ocean convection process is regularly observed (Marshall and Schott, 1999)
The succession of strong wind events in autumn leads to a progressive decrease of the sea surface temperature (Fig. 4d), which reaches its minimum value of 12.9 ◦C in early winter and remains nearly constant during the convective period
This study focused on assessing the ability of a regional ocean–atmosphere coupled system, based on the SYMPHONIE, SURFEX and MESONH models, to correctly represent ocean convection and deep-water formation in the NWMS

Summary

Introduction

The north-west Mediterranean Sea (NWMS) is one of the few regions in the world where the deep open-ocean convection process is regularly observed (Marshall and Schott, 1999). The strong, dry, cold winds, the Tramontane (northwesterly) and the Mistral (northerly), play a major role in this process. These winds induce intense exchanges between the atmosphere and the sea (Flamant, 2003; Hauser et al, 2003), with a marked loss of surface buoyancy (Schott and Leaman, 1991). In some years, vertical mixing induced by strong winds leads to the formation of a vertically homogeneous water mass (convective phase) identifiable by its temperature and salinity properties and generally referred to as new western Mediterranean deep water (nWMDW). The mixed zone undergoes a re-stratification while the nWMDW is spread throughout the basin by the general circulation (Schott et al, 1996) and submesoscale eddies (Testor and Gascard, 2006) (re-stratification/spreading phase)

Objectives

Results

Conclusion