1. The Role of Surface Freshwater Flux Boundary Conditions in Arctic Ocean/Sea-Ice Models EGU General Assembly, Nice, April 2004 Matthias Prange and Rüdiger Gerdes Research Center Ocean Margins, University of Bremen and Alfred Wegener Institute, Bremerhaven, Germany

2. Wind Stress Density Gradients ≈ Salinity Gradients What drives the circulation in the Arctic Ocean?

3. In spite of its importance, little attention as been paid to freshwater forcing in Arctic ocean/sea-ice models. Only few models omit salinity restoring to climatological data. In these models, surface freshwater fluxes are conventionally treated as virtual salt fluxes; i.e., (P-E+R) is neglected in the kinematic S.B.C.

4. Present work: We use the regional ocean/sea-ice model COSMOS with open surface (i.e., surface freshwater forcing is naturally implemented) We compare a COSMOS control run with two modified model versions, in which a material surface is used along with virtual salt fluxes

5. Ocean w0w0w0w0 w 0 = M t  + (E-P-R) MtMtMtMt (E-P-R) z = 0 z =  (Non-material) open surface:

6. Ocean model MOM 2 Open surface FCT advective scheme 19 levels Ocean model MOM 2 Open surface FCT advective scheme 19 levels Coupling Heat, salt and momentum fluxes following Hibler & Bryan (1987) Coupling Heat, salt and momentum fluxes following Hibler & Bryan (1987) Dynam. ice model (Hibler/Harder) Viscous-plastic rheology Modified upstream scheme Snow layer Dynam. ice model (Hibler/Harder) Viscous-plastic rheology Modified upstream scheme Snow layer Model grid Arakawa B Rotated grid 1° (c. 100 km) Model grid Arakawa B Rotated grid 1° (c. 100 km) Atmosph. forcing Typical year (based on ECMWF 1979-1993) with daily winds (Roeske 2001) Atmosph. forcing Typical year (based on ECMWF 1979-1993) with daily winds (Roeske 2001) Model Setup: Bering Strait inflow Monthly varying, mean values: 0.8 Sv, 32.5 psu Bering Strait inflow Monthly varying, mean values: 0.8 Sv, 32.5 psu

7. + 700 km 3 /yr ungauged (diffuse) runoff Implemented Arctic river runoff:

8. Model domain:

9. Model experiments: Experiment S1: Neglecting surface freshwater volume fluxes corresponds to using the virtual salt flux F S = (  z 1 ) -1 (-P+E-R) S 1 Experiment S1: Neglecting surface freshwater volume fluxes corresponds to using the virtual salt flux F S = (  z 1 ) -1 (-P+E-R) S 1 Experiment SREF: Use a constant reference salinity S ref = 35 psu in the salt flux boundary condition, i.e. F S = (  z 1 ) -1 (-P+E-R) S ref Experiment SREF: Use a constant reference salinity S ref = 35 psu in the salt flux boundary condition, i.e. F S = (  z 1 ) -1 (-P+E-R) S ref For each experiment the model is integrated 30 years, starting from the same spin-up run. Experiment CTRL: Control run with open surface Experiment CTRL: Control run with open surface

10. Mean salinity (0-80 m) Experiment CTRL:

11. Mean velocity (0-80 m) [cm/s] Experiment CTRL:

12. Mean salinity difference (0-80 m) Experiment S1 minus CTRL:

13. Mean velocity difference (0-80 m) [cm/s] Experiment CTRL minus S1:

14. Mean sea ice thickness difference[m]

15. Upstream: d  i  dt = Q (  i-1 –  i ) Solution for  i (t=0) = 0  0 = Q = 1 Experiment CTRL versus S1:

16. d    dt = Q (  0 –  1 ) Solution for  i (t=0) = 0  0 = Q = 1 Experiment CTRL versus S1:

17. Mean salinity difference (0-80 m) Experiment SREF minus CTRL:

18. Mean velocity difference (0-80 m) [cm/s] Experiment CTRL minus SREF:

19. Arctic Ocean mean salinity: S1SREFCTRL

20. P-E River water Bering Str. Ice Water Ice Water Ice Water Barents S. Fram Str. C. Archipel. km 3 /yr Reference: 35 psu Output Input Freshwater balance of the Arctic Ocean:

21. Conclusions: Neglecting the volume input of surface freshwater fluxes leads to significant salinity increases in the upper Arctic Ocean. Introducing a constant reference salinity S ref = 35 psu in the salt flux b.c. results in hydrographic fields which are much more similar to those from the control run with open surface. The Canadian Archipelago is an important sink in the Arctic Ocean freshwater balance: 20% of the Arctic Ocean freshwater input is exported by ocean currents through the archipelago. Neglecting the volume input of surface freshwater fluxes leads to significant salinity increases in the upper Arctic Ocean. Introducing a constant reference salinity S ref = 35 psu in the salt flux b.c. results in hydrographic fields which are much more similar to those from the control run with open surface. The Canadian Archipelago is an important sink in the Arctic Ocean freshwater balance: 20% of the Arctic Ocean freshwater input is exported by ocean currents through the archipelago.