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

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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

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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

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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).