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Marine Herrmann (1) , Jonathan Beuvier (1,2) , Florence Sevault (1) , Samuel Somot (1)

Investigating the exceptionnal winter 2005 deep convection event in the Western Mediterranean sea using NEMOMED8. Marine Herrmann (1) , Jonathan Beuvier (1,2) , Florence Sevault (1) , Samuel Somot (1) (1) CNRM, Météo-France / CNRS, Toulouse, France (2) ENSTA, Palaiseau, France.

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Marine Herrmann (1) , Jonathan Beuvier (1,2) , Florence Sevault (1) , Samuel Somot (1)

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  1. Investigating the exceptionnalwinter 2005 deep convection eventin the Western Mediterranean sea using NEMOMED8 Marine Herrmann (1), Jonathan Beuvier (1,2) , Florence Sevault (1) , Samuel Somot (1) (1) CNRM, Météo-France / CNRS, Toulouse, France (2) ENSTA, Palaiseau, France

  2. The 2004-05 winter convection event in the NWMS Observed by several experimentators Lopez-Jurado et al., 2005; Salat et al., 2006; Font et al., 2007; Schroder et al., 2006,2008;Smith et al., 2008 NWMS Abrupt change of the WMDW characteristics 2004-2005: +0.05oC, +0.03 psu Trends during the last decades:0.001-0.005oC.yr-1, ~0.001 psu.yr-1(Béthoux and Gentili, 1999, Krahmann and Schott, 1998; Rixen et al., 2005) Exceptional extension “new” WMDW “old” WMDW MODIS Surface chlorophyll concentration April 2005, Schroeder et al., 2006

  3. The 2004-05 winter convection event in the NWMS MAW LIW WMDW What can explain this ? Atmospheric forcing ?2004-05 was one of the coldest and driest winter of the last 40 years (Lopez-Jurado et al., 2005) Impact of the EMT ?Propagation of a T,S anomaly from the Eastern Mediterranean basin that could have induced a warming and salting of the LIW (Schroeder et al., 2006) Climatological T, S profile in the NWMS Mertens and Schott, 1998 Our objectiveAssess the respective contributions of atmospheric forcing and oceanic conditions to the characteristics of the 2004-05 convection event

  4. Tools and methods Control simulation NEMOMED8 (presentation of Beuvier et al.)‏1/8o, 43 vertical levels Atmospheric forcing: ARPERA, 1960-2006: Downscaling of the ECMWF reanalysis and analysis (Herrmann and Somot, 2008) Climatological values for rivers, Atlantic and Black Sea (Vörösmarty et al., 1996; Reynaud et al., 1998; Stanev et al., 2000) LION Mean winter (DJFM) heat, water and buoyancy losses over the LION area between winters 1961-62 and 2005-06 Daily wind velocity and heat, water and buoyancy losses over the LION area between December 2004 and April 2005

  5. Tools and methods LION Sensitivity simulations investigating the impact of the pre-convection oceanic conditions 5 simulations, beginning in august 2004 with different initial conditions: CI65, CI70, CI75, CI80, CI86 investigating the impact of EMT on the oceanic conditions in 2004 1 simulation beginning in 1993 (just after the EMT) with the oceanic state of 1980 65 86 12/2006 75 1960 70 80 CTRL 08/2004 CI65 CI70 CI75 CI80 CI86 EMT period 08/1993 NEMT August thermal and saline content over the LION area between winters 1961 and 2006 in CTRL

  6. Results: validation of the CTRL simulation LION Propagation and effect of the EMT signal August 2004: T and S positive anomalies between 800 and 1500 m depth August 2004 August 1961-2000 Average august temperature, salinity and density profiles in LION

  7. Results: validation of the CTRL simulation Propagation and effect of the EMT signal T (oC) S (psu) T and S anomalies for the 800-1500 m layer during the period 1993-2004

  8. Results: validation of the CTRL simulation The winter 2004-05 convection event The strongest convection event during the 1960 – 2006 period Follows the meteorological chronology. Maximum on 08/03/2005, in agreement with the observations (Smith et al., 2008; MODIS) Annual maximum mixed layer depth over LION between 1961 and 2006 Maximum and mean MLD and DW volume over LION between December 2004 and April 2005 MODIS surface chlorophyll concentration

  9. Results: validation of the CTRL simulation The winter 2004-05 convection event T, S characteristics:12.89oC, 38.48 psu, 29.116 kg.m-3in strong agreement with the observations (Schroeder et al., 2006, 2008; Font et al., 2007; Smith et al., 2008) Extension of the convection area TS diagram in the LION area on 01/10/2004 (black) and 08/03/2005 (grey) MLD on March 8th MODIS surface chlorophyll concentration

  10. Results: impact of the oceanic conditions on the convection event “New” WMDW characteristics Impact on the DW characteristics DW produced in the CI.. simulations correspond to classical observed WMDW DW produced in the CTRL and NEMT simulations correspond to new observed WMDW The DW temperature and salinity are strongly related to the pre-convection thermal and heat content in the convection area “Old” WMDW characteristics TS characteristics of the densest water observed for each simulation during winter 2004-05 Relationship betweenthe T and S DW characteristicsandthe T and S content over LION in august

  11. Results: impact of the EMT on the 2005 DW characteristics Accumulation of heat and salt in both simulations: CTLR and NEMT Partly due to the absence of convection between 1993 and 1999 As a result, the 2005 DW characteristics are very close in both simulations CTRL NEMT Minor impact of the EMT on the 2005 DW characteristics

  12. Results: impact of the EMT on the 2005 convection intensity CTRL NEMT T (oC) S (psu) r (kg.m-3) IS (m2.s-2) The EMT signal induces a modification of the stratification of the water column As a result, the stratification is stronger in NEMT than in CTRL, and the volume of DW produced in the NEMT is twice smaller than in CTRL. Major impact of the EMT on the intensity of the 2005 convection event

  13. Conclusions and future work • Realistic simulation of the 2004-05 winter, reproduces correctly the convection characteristics (WMDW T, S, r, Intensity, Chronology) • Strong impact of the pre-convection thermal and saline contents in the convection area on the DW characteristics • Minor impact of the EMT on the increase of the thermal and saline content in the convection area between 1993 and 2004, and therefore on the DW T,S characteristics • Major impact of the EMT on the stratification in the convection area in 2004, and therefore on the volume of DW produced To be done: • Impact of ocean-atmosphere coupling during the convection: a coupled simulation is running • Impact of the mesoscale processes on the convection : implementing AGRIF in NEMOMED • Propagation of the “new” WMDW through Gibraltar ?

  14. The NEMOMED8 configuration Adriatic Sea Gulf of Lions Otranto Strait Strait of Gibraltar Sicily Strait Aegean Sea Atlanticbuffer zone Ionian basin Levantine basin • 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)

  15. 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.

  16. Downscaling of ERA40 using ARPEGE-Climate 50 km ERA40 125 km ERA40 orography ARPEGE-Climate orography Herrmann & Somot (2008) GRL

  17. Results : impact of the initial conditions - Impact on the volume ?

  18. Results: impact of the EMT on the 2005 convection intensity CTRL NEMT DCT 0 - 3000: ctrl: 163.1 8093: 205.0 DCT 3 - 176: ctrl: 36.7 8093: 57.4 DCT 208 - 796: ctrl: 42.6 8093: 80.6 DCT 901 - 1403: ctrl: 62.9 8093: 47.9 DCT 1549 - 2926: ctrl: 20.9 8093: 19.2 DCS 0 - 3000: ctrl: 49.9 8093: 21.5 DCS 3 - 176: ctrl: 17.1 8093: 2.5 DCS 208 - 796: ctrl: 13.0 8093: 5.6 DCS 901 - 1403: ctrl: 15.7 8093: 9.8 DCS 1549 - 2926: ctrl: 4.1 8093: 3.6

  19. Results: impact of the EMT on the 2005 DW characteristics CTRL, 08/2004 NEMT, 08/2004 1961-2000

  20. The 2004-05 winter convection event in the NWMS What can explain this ? Atmospheric forcing ?2004-05 was one of the coldest and driest winter of the last 40 years (Lopez-Jurado et al., 2005) Impact of the EMT ?Propagation of a T,S anomaly from the Eastern Mediterranean basin that could have induced a warming and salting of the LIW (Schroeder et al., 2006) MAW LIW WMDW Climatological T, S profile in the NWMS Mertens and Schott, 1998 Characteristics of the vein of maximum salinity in the Sicily Strait, Schroeder et al., 2006 Our objectiveAssess the respective contributions of atmospheric forcing and oceanic conditions to the characteristics of the 2004-05 convection event

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