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 !