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
The 2004-05 winter convection event in the NWMS
Exceptional extension
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)
NWMSObserved by several experimentatorsLopez-Jurado et al., 2005; Salat et al., 2006; Font et al., 2007; Schroder et al., 2006,2008;Smith et al., 2008
MODIS Surface chlorophyll concentration
April 2005, Schroeder et al., 2006
“old” WMDW“new” 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)
The 2004-05 winter convection event in the NWMS
Our objectiveAssess the respective contributions of atmospheric forcing and oceanic
conditions to the characteristics of the 2004-05 convection event
Climatological T, S profile in the NWMS Mertens and Schott, 1998
MAW
LIW
WMDW
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)
Tools and methods
Daily wind velocity and heat, water and buoyancy losses over the LION area
between December 2004 and April 2005
LION
Mean winter (DJFM) heat, water and buoyancy losses over the LION area
between winters 1961-62 and 2005-06
Sensitivity simulations
investigating the impact of the pre-convection oceanic conditions5 simulations, beginning in august 2004 with different initial conditions:
CI65, CI70, CI75, CI80, CI86
investigating the impact of EMT on the oceanic conditions in 20041 simulation beginning in 1993 (just after the EMT) with the oceanic state of 1980
Tools and methods
August thermal and saline content over the LION areabetween winters 1961 and 2006 in CTRL
1960 12/2006
08/2004
08/1993
CTRL
CI65CI70CI75CI80CI86
NEMT
7065 7580
86
LION
EMT period
August 2004: T and S positive anomalies between 800 and 1500 m depth
Results: validation of the CTRL simulation
Average august temperature, salinity and density profiles in LION
August 2004August 1961-2000
LIONPropagation and effect of the EMT signal
Results: validation of the CTRL simulation
T and S anomalies for the 800-1500 m layer during the period 1993-2004
T (oC) S (psu)
Propagation and effect of the EMT signal
Results: validation of the CTRL simulation
The strongest convection event during the 1960 – 2006 period
The winter 2004-05 convection event
Annual maximum mixed layer depth over LION between 1961 and 2006
Follows the meteorological chronology. Maximum on 08/03/2005, in agreement with the observations (Smith et al., 2008; MODIS)
Maximum and mean MLD and DW volume over LION between December 2004 and April 2005 MODIS surface chlorophyll concentration
Results: validation of the CTRL simulation
T, S characteristics:12.89oC, 38.48 psu, 29.116 kg.m-3
in strong agreement with the observations (Schroeder et al., 2006, 2008; Font et al., 2007; Smith et al., 2008)
Extension of the convection area
The winter 2004-05 convection event
MLD on March 8th
TS diagram in the LION area on 01/10/2004 (black) and 08/03/2005 (grey)
MODIS surface chlorophyll concentration
Results: impact of the oceanic conditions on the convection event
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
Impact on the DW characteristics
TS characteristics of the densest water observed for each simulation during winter 2004-05
“Old” WMDW characteristics
“New” WMDW characteristics
Relationship between
the T and S DW characteristics
andthe T and S content over LION in august
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
CTRLNEMT
Minor impact of the EMT on the 2005 DW characteristics
Results: impact of the EMT on the 2005 convection intensity
CTRLNEMT
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
T (oC) S (psu) (kg.m-3) IS (m2.s-2)
Realistic simulation of the 2004-05 winter, reproduces correctly the convection characteristics (WMDW T, S, , 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 ?
Conclusions and future work
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
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.
ERA40125 km
Herrmann & Somot (2008) GRL
Downscaling of ERA40 using
ARPEGE-Climate50 km
ERA40 orography ARPEGE-Climate orography
Results : impact of the initial conditions
- Impact on the volume ?
DCT 0 - 3000: ctrl: 163.1 8093: 205.0DCT 3 - 176: ctrl: 36.7 8093: 57.4DCT 208 - 796: ctrl: 42.6 8093: 80.6DCT 901 - 1403: ctrl: 62.9 8093: 47.9DCT 1549 - 2926: ctrl: 20.9 8093: 19.2DCS 0 - 3000: ctrl: 49.9 8093: 21.5DCS 3 - 176: ctrl: 17.1 8093: 2.5DCS 208 - 796: ctrl: 13.0 8093: 5.6DCS 901 - 1403: ctrl: 15.7 8093: 9.8DCS 1549 - 2926: ctrl: 4.1 8093: 3.6
Results: impact of the EMT on the 2005 convection intensity
CTRLNEMT
Results: impact of the EMT on the 2005 DW characteristics
CTRL, 08/2004NEMT, 08/2004
1961-2000
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)
The 2004-05 winter convection event in the NWMS
Our objectiveAssess the respective contributions of atmospheric forcing and oceanic
conditions to the characteristics of the 2004-05 convection event
Characteristics of the vein of maximum salinity in the Sicily Strait, Schroeder et al., 2006
Climatological T, S profile in the NWMS Mertens and Schott, 1998
MAW
LIW
WMDW
