1. Nansen Environmental and Remote Sensing Center Modifications of the MICOM version used in the Bergen Climate Model Mats Bentsen and Helge Drange Nansen Environmental and Remote Sensing Center Bjerknes Centre for Climate Research Nansen-Zhu International Research Centre, Beijng

2. Nansen Environmental and Remote Sensing Center Outline BCM components and configuration Conservation Physical parameterizations Some results

3. Nansen Environmental and Remote Sensing Center BCM components and configuration MICOM Based on version 2.8 Horizontal resolution of ~1.5° with enhanced meridional resolution near the equator and the northern hemisphere grid singularity located over Siberia. 35 model layers Reference pressure at the surface Dynamic-thermodynamic sea ice modules included ARPEGE Spectral model. Hybrid vertical coordinate (terrain following/pressure). Can use semi-Lagrangian time integration. Horizontal resolution: T63 (~2.8°) 31 model layers.

4. Nansen Environmental and Remote Sensing Center Main motivations for MICOM improvements Get rid of flux adjustments or at least reduce the magnitude of the adjustments. Improve the Southern Ocean sea-ice extent and volume. Improve Arctic sea-ice volume and summer extent. Reduce model drift. Add features required by IPPC.

5. Nansen Environmental and Remote Sensing Center MICOM conserves total mass, but not mass within each layer! Red dotted lines: After layer advection Blue lines: After bottom pressure restoration Conservation of layer mass

6. Nansen Environmental and Remote Sensing Center Method 1 to reduce the bottom pressure error

7. Nansen Environmental and Remote Sensing Center Method 2 to reduce the bottom pressure error Layer mass before advection: Layer mass after advection and with of correction to restore the bottom pressure: Apply correction to layer thickness so layer mass is conserved:. Then typicallyLet

8. Nansen Environmental and Remote Sensing Center Advection/diffusion of temperature and salinity in an isopycnic layer In the continuous case: Isopycnic advection does not lead to density change Isopycnic diffusion increases density (cabbeling)

9. Nansen Environmental and Remote Sensing Center Density changes due to lateral diffusion is compensated by diapycnal fluxes. Layer density can deviate from the target density, but the latter is used in computing the pressure gradient. The remaining density deviation from target density is kept small by relaxing the water mass towards a water mass with correct density.

10. Nansen Environmental and Remote Sensing Center Mean salinity (upper figure) and temperature (lower figure) in a simulation with BCM

11. Nansen Environmental and Remote Sensing Center Turbulent fluxes The ocean surface velocity is now included in the computation of turbulent fluxes.

12. Nansen Environmental and Remote Sensing Center Left/right figure show snapshots of SST (C°) without/with ocean surface velocities included in the computation of turbulent fluxes. Time series of SST in the region [5S,5N] and [120W,90W]. Green curve is climatology while blue/red curve is without/with ocean surface velocitiy dependent turbulent fluxes.

13. Nansen Environmental and Remote Sensing Center Left figure is monthly mean SST with region and coloring as previous figure. Right figure show eastward surface velocity in the same region. Coloring as before.

14. Nansen Environmental and Remote Sensing Center Conceptual presentation of convective adjustment Old schemeNew scheme

15. Nansen Environmental and Remote Sensing Center

16. Nansen Environmental and Remote Sensing Center Diapycnal mixing Old code: Background mixing (diffusivity ~1/N) New code: Background mixing (diffusivity ~1/N) Shear instability mixing (Ri dependent, Large et. al 1994) Gravity current entrainment (Ri dependent, Turner 1986) Cabbeling (due to lateral mixing) Momentum mixing (Pr=10) Absorption of short-wave radiation below mixed layer

17. Nansen Environmental and Remote Sensing Center Monthly mean SST and sea-ice cover from transient BCM simulation March 1940 September 1940

18. Nansen Environmental and Remote Sensing Center Monthly mean SST and sea-ice cover from transient BCM simulation September 1940 March 1940

19. Nansen Environmental and Remote Sensing Center Monthly mean SSS from transient BCM simulation September 1940 March 1940

20. Nansen Environmental and Remote Sensing Center Monthly mean barotropic stream function from transient BCM simulation March 1940 September 1940