The dynamic-thermodynamic sea ice module in the Bergen Climate Model Helge Drange and Mats Bentsen Nansen Environmental and Remote Sensing Center Bjerknes Centre for Climate Research

Zonal mean change in surface-T in 19 CMIP-models ! Reasons for model differences Different states of natural high-latitude climate variability modes Variations in sea ice extent in CTRL integration Actual winter sea ice extent in transient integration

Simulated change in surface temperature at 2xCO 2 in 19 climate GCMs Räisänen (2001), J. Climate, 14,

Miami Isopycnic Coordinate Ocean Model (MICOM; Bleck et al., 1992) - a mixture of versions 2.6 to 2.8 is used Dynamic-thermodynamic sea ice modules included Reference pressure at the surface 24 model layers with potential density ranging from to Stretched grids with focus in the North Atlantic-Arctic region (Bentsen et al., 1999) Daily atmospheric forcing Daily forcing from NCEP/NCAR reanalysis (Kalnay et al., 1996) Period 1948 to present Many integrations conducted: - min 80, 40 and 20 km resolution, differing by different initial conditions only MICOM-configuration

BCM-configuration, ARPEGE+MICOM The atmospheric grid (red dots) has a resolution of T63 (2.8° by 2.8°), L31 The ocean grid (blue dots) has a resolution of 0.8° by 2.4° at the Equator, gradually transforming to approximate square grid cells towards the poles (Mercator projection), L24 (MICOM v dyn/thermodyn sea ice)

Old 300-yr BCM-CTRL: Sea ice thickness Winter: Too thin but realistic extent in Arctic Summer: Too thin and too small extent MarchSep

Simulated change in sea ice extent in old version of the Bergen Climate Model Purple Control run White Doubled CO 2 March September

Sea ice module Old: Treatment of heat fluxes in atmosphere coupled/uncoupled mode were different New: Heat fluxes split between solar and non-solar components, with temperature-dependent tendency term for non-solar component New: Improved conservation of heat and fresh water New: WENO advection scheme New: Bug fixes

Sea ice module in general Viscous-plastic rheology based on Hibler III (1979), based on the implementation of Harder (1996) 1 snow layer, 1 ice layer, linear temp profile in each layer Salinity-dependent freezing temperature Each grid cells accept ice and open water Metric terms included

Sea ice thermodynamics

Short wave radiation

Albedo formulations

Exchange of heat between water and ice Ifthe is ice layer is melted (frozen) from below.

Ex: Computation of sea ice temperature

Simulated Arctic sea ice in the old version of NERSC-MICOM

Sea ice extent anomalies, obs and simulated Lisæter et al.

Sea ice thickness (m), obs and simulated

Lisæter et al.

Sea ice thickness (m), obs and simulated Lisæter et al.

Simulated Arctic sea ice in the first version of NERSC-MICOM Fairly realistic anomalies in sea ice thickness and extent Too thin sea ice, in general (not shown)

Comparison: Old and new sea ice module

Sea ice thickness, monthly NCEP/NACR forcing New Old March New Old SeptemberBoth versions produce realistic sea ice extent Thickness distribution more realistic in new version

Sea ice concentration, monthly NCEP/NACR forcing New Old March New Old SeptemberBoth versions produce realistic maximum sea ice extent Concentration distribution more realistic in new version

Example from new BCM-integration (IPCC CTRL)

MarchSeptember New BCM control integration

Summary New version of the sea ice module has been implemented and tested; improved numerics + conservation of heat and fresh water + bug fixes Improved sea ice thickness and concentration distributions Currently used for the new IPCC-simulations

300-yr CTRL: SST, SSS, Arctic and Antarctic sea ice extent