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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. - PowerPoint PPT Presentation
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High resolution modelling of dense water formation in the Northwestern Mediterranean: benefits from an improved initial
stratification in summerC. 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 2012-2013 as many data are available
D’Ortenzio, pers. comm.
The numerical domain
3D ModelS26 (Marsaleix et al., 2008, 2009, 2011, 2012)1 km horizontal resolution
ForcingOGCM : MERCATOR operational modelMeteo: 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 + 139 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 convectionDiagnostic : stratification index profile
dzzzzFz
))((0
111
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
Obse
rvati
on
without initial state correction with initial state correction without meteo correction
Latent heat flux X 1.25 Wind X 1.13
correlation: 0.32 correlation: 0.5
correlation: 0.59 correlation: 0.64
Density at 2000 m
Map of the stratification index at 1000 m
model V x1.13Observation
(logarithmic scale)
2
1
0
-1
15000 km3
46000 km3
53000 km3
Difference of volume by density class15 March 2013 – 1 September 2012
Conclusion
Observations in summer are unvaluable to improve the model initial statethe 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 km3
46000 km3
53000 km3
Difference of volume by density class15 March 2013 – 1 September 2012
Mean buoyancy flux over autumn winter kg/m2/day
-0.727
-0.783
-0.863
X 3 X 1.08
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