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 Great differences near the coast MAR-ERA meridional wind stress in N/m 2 (Petrelli et al. 2006, submited) Wind speed close Terra Nova Bay MAR-ERA zonal wind stress in N/m 2

Meridional wind stress above the first ocean point (katabatic winds) MAR ERA -0,400,2 0,4 -0,2 month Katabatic mask in tauyKatabatic mask in taux ,4

Increasing the wind stress on the first sea points Used the simulation MAR ( ) to increase ERA wind stress on each coast point. Before correction After correction Correlation ≈ 0,85 ERA/MAR before ≈ 0,55 ERA/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 marked  No 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 area Obs KATA9P SKATA 16 X km 2 2

Polynya in the model ICE Polynya KATA9P Definition of polynya in model Criterion (Marsland et al. 2004) : Ice fraction < 0,7 Ice 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 polynya KATA9P SKATA 0,55 0,45 0,04 0,12 Ice produced in polynya/Antarctica Ice area of polynya/Antarctica 3 X 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

Out flow In flow 200 Mass flux in polynya Depth Year 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. Salinity Density - 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 FEBMAR SEPAUGJUL JUNMAYAPR NOV DECOCT

Summary Wind stress correction factor : tau MAR / tau ERA 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 !

ObservationsG70 Summer sea ice in ORAC025 G70 G70 Obs X 10 3 km 2 Time correlation : 0,49 (significant at 95%) Ice area in G70 : -85% (obs.)

ObservationsG70 Obs Winter sea ice in ORAC025 G X 10 3 km 2 Time correlation : 0,29 (not significant at 95%) Ice area in G70 : +5% (obs.)

Sea ice and polynyas in ORCA025 KATA9P Criterion (Marsland et al. 2004) : Ice fraction < 0,7 Ice production > 1 m month -1 ICE Polynya

Tauy in ORCA025 G70 Taux in ORCA025 G70 Wind stress in polynya  Same magnitude than simulation KATA9P for taux and tauy

Ice in polynya and in Antarctic on G70 Mean ice fraction : 0,5% 1,5% of total area produces 10% of total sea ice volume (in Marsland et al. 2004, Mertz Glacier polynya (less than 0,1% of total sea ice extend) produces 1,3% of total sea ice volume) 0,02 0,1 0,5 2 X Ice produced in polynya/Antarctica Ice area of polynya/Antarctica Polynya area Polynya ice free

Water exported in polynya ORCA025~ 1,8 Sv SKATA : 2,5 Sv KATA9P : 4 Sv Strong effect of SSS restoring ? ( Levitus ) Depth in m Out flow In flow