Modifications of the MICOM version used in the Bergen Climate Model

1. 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. Outline • BCM components and configuration • Conservation • Physical parameterizations • Some results

3. 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. 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. Conservation of layer mass MICOM conserves total mass, but not mass within each layer! Red dotted lines: After layer advection Blue lines: After bottom pressure restoration

6. Method 1 to reduce the bottom pressure error

7. 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: Let . Then typically

8. 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. 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. Mean salinity (upper figure) and temperature (lower figure) in a simulation with BCM

11. Turbulent fluxes The ocean surface velocity is now included in the computation of turbulent fluxes.

12. 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. 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. Conceptual presentation of convective adjustment Old scheme New scheme

16. 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. Monthly mean SST and sea-ice cover from transient BCM simulation March 1940 September 1940

18. Monthly mean SST and sea-ice cover from transient BCM simulation March 1940 September 1940

19. Monthly mean SSS from transient BCM simulation March 1940 September 1940

20. Monthly mean barotropic stream function from transient BCM simulation March 1940 September 1940