D’Ortenzio & Ribera d’Alcalà, 2009

1. 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 Importance of convection and dense water formation in the western Med basin - thermohaline circulation - ecosystem productivity - ventilation of deep layers Bergamasco & Malanotte-Rizzoli, 2010 D’Ortenzio & Ribera d’Alcalà, 2009 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

2. Time series from the « Lion » mooring line (de Madron et al., 2013; Houpert) does not provide the volume of dense water D’Ortenzio, pers. comm. Numerical modelling should be able to provide detailed information on newly-formed waters if accurate initial conditions and meteorological forcing are used Objective : to derive a method based on observations and modelling to characterize the interannual variability of dense water formation The method is tested on winter 2012-2013 as many data are available

3. 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.

4. During convection in 2013, DEWEX cruise of the MERMEX programme 75 CTD + 139 ARGO profiles used to check the simulation of convection Data available Each summer : cruise of the MOOSE monitoring programme 2012 : 88 CTD + 52 ARGO profiles numerical domain used to improve the initial state of the model

5. Initial conditions 1 August 2012

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

7. 6 months later Assessment of the simulation based on the CTD available during convection r 0 Diagnostic : stratification index profile z1 z2 simulationwithout initial state correction simulation withinitial state correction Anomaly of the stratification index (simulation-observation) averaged over all the CTD

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

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

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

11. Difference of volume by density class 15 March 2013 – 1 September 2012 15000 km3 46000 km3 53000 km3

12. Conclusion The method combining observations and modelling seems promising to monitor dense water formation in the northwest basin Observations in summer are unvaluable to improve the model initial state the simulation keeps the benefits of the improved initial state until winter 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

13. Mean buoyancy flux over autumn winter kg/m2/day Difference of volume by density class 15 March 2013 – 1 September 2012 15000 km3 -0.727 X 1.08 X 3 46000 km3 -0.783 53000 km3 -0.863