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Modelling the Mediterranean Seainterannual variability over the last 40 years:
focus on the Eastern Mediterranean Transient (EMT)
Jonathan BEUVIER, Météo-France/ENSTA
Florence SEVAULT, Météo-France
Marine HERRMANN, Météo-France
Karine BÉRANGER, ENSTA
Samuel SOMOT, Météo-France
2009 NEMO users meeting - Paris
2009/07/03 2009 NEMO users meeting Paris
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Introduction
Eastern Mediterranean Transient: switch of the main source of dense waters in the Eastern Mediterranean in the early 1990’s, from the Adriatic Sea to the Aegean Sea.
Need of modelling to understand the EMT (Roether et al. 2007).
Interests:– variabilities at different time-scales: requires long and stable simulations,
– good test for atmosphere and ocean models,
– improves knowledge of possible past or future EMT,
– need to be better simulated with realistic simulations (Samuel et al. 1999, Nittis et al. 2003, Bozec et al. 2006).
Questions: – are we able to reproduce the different phases of the EMT (winter deep
convection, filling, overflow and spreading)?
– what are the key processes that trigger the EMT?
2009/07/03 2009 NEMO users meeting Paris
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The NEMOMED8 configuration
Mediterranean version (Sevault et al. 2009), based on NEMO-v2 Resolution of 1/8° x 1/8°cos(lat) (9 to 12 km with square meshes) Grid tilted and stretched at Gibraltar (up to 6km resolution) Z-coordinate partial steps (43 vertical Z-levels) Atlantic buffer zone with 3D T-S damping (11°W to 7.5°W) Explicit river forcing for 33 rivers + Black Sea (simulated as a river)
Gulf of Lions
Adriatic Sea
Atlanticbuffer zone
Strait of Gibraltar
Sicily Strait
Ionian basin
Levantine basin
Aegean Sea
Otranto Strait
2009/07/03 2009 NEMO users meeting Paris
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Physics used in this study
Filtered free surface (with transfer of the evaporated water in the buffer zone).
TVD scheme for tracers.
Iso-neutral diffusion for tracers (laplacian operator).
Horizontal diffusion for momentum (bilaplacian operator).
Vertical diffusion based on TKE closure scheme.
EEN (energy and enstrophy conserving) scheme.
Feedback coefficient for SST damping: -40 W/m²/K.
No-slip condition for the lateral momentum boundary.
Non-linear bottom friction.
2009/07/03 2009 NEMO users meeting Paris
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Three 1961-2000 hindcast simulations 15-year spin-up. 40 years forced by ARPERA (dynamical downscaling
of ERA40, 50 km of resolution over the Med). SST relaxation (ERA40). No SSS relaxation: monthly water flux correction.
Climatological river runoff
(Vörösmarty et al. 1996) and
Black Sea input (Stanev et al. 2000)
Interannual river runoff (Ludwig et
al. 2009) and Black Sea input
(Stanev, personnal communication)
Climatological Atlantic buffer
zone (Reynaud et
al. 1998)
Interannual Atlantic
buffer zone (Daget et al. 2008)
NM8-atl-riv x x
NM8-riv x x
NM8-clim x x
Horizontal grid and relief of ARPERA(Herrmann & Somot 2008)
Climatological (---) and interannual(___) river and Black
Sea runoffs
Total
Black Sea
Nile PoRhone
Interannual Atlantic anomalies at 176m
T S
2009/07/03 2009 NEMO users meeting Paris
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Global validation of the simulations
Comparison with the interannual climatology of Rixen et al., 2005 (mean and standard deviation):
– T: good correlations (>0.7) despite global bias (+0.1°C), accurate surface layer, intermediate layer too warm (+0,2°C), trend in the bottom layer.
– S: well simulated in average but not enough variability, surface layer too fresh, intermediate layer too salty.
Total Med heat content Total Med salt content
Med heat content per layer Med salt content per layer
0-150m
150-600m
600m-bottom
NM8-atl-rivNM8-riv
NM8-clim
2009/07/03 2009 NEMO users meeting Paris
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Agean winter convection in NM8-atl-riv
Strong heat and water losses in winters 1991-92 and 1992-93, but not only.
Formation of dense and deep waters (> 29,2 kg/m3) during many successive winters in the 1970’s and the 1980’s.
Annual formation rate for σ>29.2kg/m3: 0,5 Sv in 1992 and 1,2 Sv in 1993.
In 1993, about 75% of the Aegean Sea filled by waters denser than 29,2 kg/m3.
Winter (NDJF) surface flux anomalies over the Aegean
Net surface heat flux anomalies (W/m²)
Net surface water flux anomalies (mm/day)
Monthly volume of Aegean dense waters (m3)
---- σ>29,2 kg/m3
___ σ>29,3 kg/m3
Qtot R+P-E
Annual formation rate (Sv)
---- σ>29,2 kg/m3
___ σ>29,3 kg/m3
2009/07/03 2009 NEMO users meeting Paris
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Filling and overflowing of Aegean deep waters
Increase of potential density above the Cretan Arc Straits sills.
Overflow of warm, salty and dense waters toward the Ionian and Levantine seas.
Potential density (kg/m3) on the Cretan Arc Straits sills
Antikithira534m Kassos
542m
Karpathos777m
Location of the Cretan Arc Straits and paths of the outflowing waters
2009/07/03 2009 NEMO users meeting Paris
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May 1993 May 1994
Salinity (colors, in psu) and depth (lines, in m) of the 29,165 kg/m3 isopycnal
Dispersion in the Eastern Mediterranean
Simulated EMT-waters warmer (0,3°C), saltier (0,05psu) and less dense (-0,03kg/m3) than the observed EMT-waters.
Simulated EMT-waters less dense than the bottom waters of the Eastern Mediterranean.=> they sink to a depth of 2200m (not to the bottom as observed).
2009/07/03 2009 NEMO users meeting Paris
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Impact of the interannual hydrological forcings
Main characteristics of the EMT not modified: same formation rates in 1992 and 1993, overflow, dispersion and sinking.
Affects mainly the chronology of the Aegean deep convection in the 1970’s and 1980’s.=> main motor of the EMT: atmospheric forcing.
Monthly volume of Aegean dense waters (m3, left) and associated annual formation rate (Sv, right)---- σ>29,2 kg/m3 ___ σ>29,3 kg/m3
NM8-atl-rivNM8-riv
NM8-clim
NM8-atl-rivNM8-riv
NM8-clim
2009/07/03 2009 NEMO users meeting Paris
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Conclusion
The EMT is well simulated:– realistic chronology (surface losses, winter deep convection),– good estimates of the dense water formation rates,– representation of an overflow and of a spreading.
The interannual hydrological forcings mostly impact the Aegean convection in the 1970’s and 1980’s.
Perspectives:– studies of the variability in other Mediterranean locations (e.g. the
convection in the Gulf of Lions, the cascading at the Otranto Strait, …)– EMT modelling:
• improvement of the initial conditions and of the spin-up,
• test of new physical parametrisations (horizontal diffusion, vertical mixing, …),
• use of higher resolution models (atmosphere and ocean).