High resolution modelling of dense water formation in the Northwestern Mediterranean: benefits from an improved initial stratification in summer C. Estournel, P. Testor, P. Damien, L. Mortier, P. Marsaleix, J.M. Lellouche, C. Ulses, F. Kessouri, P. Raimbault, L. Coppola D’Ortenzio & Ribera d’Alcalà, 2009Bergamasco & Malanotte-Rizzoli, 2010 Importance of convection and dense water formation in the western Med basin - thermohaline circulation - ecosystem productivity - ventilation of deep layers Dense water formation is suspected to be highly sensitive to climate change Monitoring DWF at interannual scale (characteristics of newly-formed waters) is a priority

Time series from the « Lion » mooring line (de Madron et al., 2013; Houpert) does not provide the volume of dense water Objective : to derive a method based on observations and modelling to characterize the interannual variability of dense water formation Numerical modelling should be able to provide detailed information on newly-formed waters if accurate initial conditions and meteorological forcing are used The method is tested on winter as many data are available D’Ortenzio, pers. comm.

The numerical domain 3D Model S26 (Marsaleix et al., 2008, 2009, 2011, 2012 ) 1 km horizontal resolution Forcing OGCM : MERCATOR operational model Meteo: 3 hrs ECMWF forecasts. Turbulent fluxes calculated by Bulk F.

Data available Each summer : cruise of the MOOSE monitoring programme numerical domain 2012 : 88 CTD + 52 ARGO profiles used to improve the initial state of the model During convection in 2013, DEWEX cruise of the MERMEX programme used to check the simulation of convection 75 CTD ARGO profiles

Initial conditions 1 August 2012

Anomalies at 1500 m Interpolation of anomalies at 1500 m The initial state (and boundary conditions) is corrected from these anomalies

z1 z2  0 Assessment of the simulation based on the CTD available during convection Diagnostic : stratification index profile simulation without initial state correction simulation with initial state correction Anomaly of the stratification index (simulation-observation) averaged over all the CTD 6 months later

Anomaly of the stratification index (simulation-observation) Sensitivity study to meteorological fluxes without initial state correction

Model Observation without initial state correctionwith initial state correction without meteo correction Latent heat flux X 1.25 Wind X 1.13 correlation: 0.32correlation: 0.5 correlation: 0.59correlation: 0.64 Density at 2000 m

Map of the stratification index at 1000 m model V x1.13 Observation (logarithmic scale) 2 1 0

15000 km km km 3 Difference of volume by density class 15 March 2013 – 1 September 2012

Conclusion Observations in summer are unvaluable to improve the model initial state the simulation keeps the benefits of the improved initial state until winter The method combining observations and modelling seems promising to monitor dense water formation in the northwest basin After correction of the initial state, a stratification bias remains. Increasing wind (13%) or directly heat fluxes (LHF x 1.25) allowed to reduce the bias and to be very close to observations The priority is now to apply the method on other years to estimate the robustness of these results

15000 km km km 3 Difference of volume by density class 15 March 2013 – 1 September 2012 Mean buoyancy flux over autumn winter kg/m 2 /day X 3 X 1.08