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P. Mathiot , B. Barnier, J.M. Molines, T. Penduff : LEGI - CNRS

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

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P. Mathiot , B. Barnier, J.M. Molines, T. Penduff : LEGI - CNRS

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  1. Correction of katabatic winds in ORCA05 and ORCA025 P. Mathiot, B. Barnier, J.M. Molines, T. Penduff : LEGI - CNRS H. Gallée : LGGE - CNRS

  2. 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

  3. 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

  4. Wind speed close Terra Nova Bay Differences between MAR and ERA wind stress around Antarctica (Petrelli et al. 2006, submited) Great differences near the coast MAR-ERA zonal wind stress in N/m2 MAR-ERA meridional wind stress in N/m2 7 6 1 2 5 4 3

  5. Meridional wind stress above the first ocean point (katabatic winds) MAR month ERA month 1 2 3 4 5 6 7 -0,4 -0,2 0 0,2 0,4 Katabatic mask in taux Katabatic mask in tauy 1 0 6 0,4

  6. Increasing the wind stress on the first sea points Before correction Used the simulation MAR (1980-1989) to increase ERA wind stress on each coast point. After correction Correlation ≈ 0,85 ERA/MAR before ≈ 0,55 ERA/MAR after ≈ 0,94

  7. 2 simulations : • The first one with correction (KATA9P) • The second one without correction (SKATA) Between the two simulations : ONLY changed the wind stress near the coast Tauy(KATA9P-SKATA) Taux(KATA9P-SKATA)

  8. Seasonal cycle General trend after 10 years of integration : X10+6 km 2 Ice area Underestimation of sea ice in summer as in winter in SKATA and KATA9P 16 Obs KATA9P SKATA KATA9P - SKATA sea ice concentration 2 • Coastal polynyas more marked • No significant change in ice area : Coastal polynya

  9. Definition of polynya in model Polynya in the model Polynya ICE Criterion (Marsland et al. 2004) : Ice fraction < 0,7 Ice production > 1 m month-1 KATA9P All values in polynya after this, are mean values over all polynya (it is not a study of a single polynya)

  10. Wind stress in polynya Tauy in KATA9P Tauy in SKATA Taux in KATA9P Taux in SKATA • Wind stress doubles with correction in polynya

  11. Ice area and ice produced in polynya KATA9P SKATA X10+11 KATA9P SKATA 3 Polynya area 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) Polynya ice free 0,12 Ice produced in polynya/Antarctica Ice area of polynya/Antarctica 0,04 Ice fraction in polynya 0,55 0,45

  12. Mass flux in polynya Creation of an overturning circulation Mass flux in polynya ICE Polynya 200 m Out flow In flow Year Water exported out of polynya KATA9P : 4 Sv SKATA : 2,5 Sv 200 Depth Year

  13. Vertical profile in polynya • Less stratified with katabatic correction in October - SKATA - KATA9P FEB MAR 27,9 Temp. • Colder with katabatic correction in October APR MAY JUN OCT JUL AUG SEP • Saltier with katabatic correction in October Salinity 27,9 Density OCT OCT DEC NOV 28

  14. 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)

  15. Thank you !

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