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Correction of katabatic winds in ORCA05 and ORCA025. P. Mathiot , B. Barnier, J.M. Molines, T. Penduff : LEGI - CNRS. H. Gallée : LGGE - CNRS. Outlines. Motivation : ERA40 is weak Correction of katabatic winds in ERA40 Description of the correction of katabatic winds used - PowerPoint PPT Presentation
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P. Mathiot, B. Barnier, J.M. Molines, T. Penduff : LEGI - CNRS
Correction of katabatic winds in ORCA05 and ORCA025
H. Gallée : LGGE - CNRS
Outlines
Motivation : ERA40 is weak
Correction of katabatic winds in ERA40 Description of the correction of katabatic winds used Effect on sea ice Effect on polynyas and water mass formation
Summary
Motivation
Re-analysis have weak katabatic winds
To improve this when driving an interannual global ocean/sea ice simulation, one way is to make a correction of ERA40 katabatic winds
Our Work : test this way by running twin experiments - with wind correction- without wind correction
Differences between MAR and ERA wind
stress around Antarctica
1
2
34
5
67
Great differences near the coast
MAR-ERA meridional wind stress in N/m2
(Petrelli et al. 2006, submited)
Wind speed close Terra Nova Bay
MAR-ERA zonal wind stress in N/m2
Meridional wind stress above the first ocean point (katabatic winds)
1 2 3 4 5 6 7
MAR
ERA
-0,4 0 0,2
0,4
-0,2
mon
thm
onth
Katabatic mask in tauyKatabatic mask in taux
601
0,4
Increasing the wind stress on the first sea points
Used the simulation MAR (1980-1989) to increase ERA wind stress on each coast point.
Before correction
After correction
Correlation ≈ 0,85ERA/MAR before ≈ 0,55ERA/MAR after ≈ 0,94
Between the two simulations : ONLY changed the wind stress near the coast
2 simulations : The first one with correction (KATA9P)The second one without correction (SKATA)
Taux (KATA9P-SKATA)Tauy (KATA9P-SKATA)
General trend after 10 years of integration :
Coastal polynyas more markedNo significant change in ice area
Seasonal cycle
Underestimation of sea ice in summer as in winter in SKATA and KATA9P
: Coastal polynya
KATA9P - SKATA sea ice concentration
Ice areaObs
KATA9PSKATA
16
X10+6 km 2
2
Polynya in the model
ICEPolynya
KATA9P
Definition of polynya in model
Criterion (Marsland et al. 2004) : Ice fraction < 0,7Ice production > 1 m month-1
All values in polynya after this, are mean values over all polynya (it is not a study of a single polynya)
Wind stress in polynya
Tauy in KATA9P
Taux in KATA9P
Tauy in SKATA
Taux in SKATA
Wind stress doubles with correction in polynya
Ice area and ice produced in polynyaKATA9P
KATA9P
SKATA
SKATA
0,55
0,45
0,04
0,12
Ice produced in polynya/AntarcticaIce area of polynya/Antarctica
3
X10+11
Polynya area
Polynya ice free 3% of total area produces 13% of total sea ice volume (in Marsland et al. 2004, Mertz Glacier polynya (less than
0,1% of total area) produces 1,3% of total sea ice volume)
Ice fraction in polynya
Ou
t fl
ow
In f
low
200
Mass flux in polynya
Depth
Yea
r
Mass flux in polynya
ICE
200 m
Polynya
Creation of an overturning circulation
SKATA : 2,5 Sv
KATA9P : 4 Sv
Water exported out of polynya
Year
Temp.
SalinityDensity
- SKATA- KATA9P
27,9
Vertical profile in polynya
Less stratified with katabatic correction in October
Colder with katabatic correction in October
Saltier with katabatic correction in October
27,9
28
FEB MAR
SEPAUGJUL
JUNMAYAPR
NOV DECOCT
OCT
OCT
Summary
• Wind stress correction factor : tauMAR / tauERA
• Increase area of polynya
• Increase of sea ice produced in polynya
• Increase export of bottom water in polynya (4 Sv against 2,5 Sv)
Thank you !