2004: The Community Climate System Model, version 2

The fourth version of the Community Climate System Model (CCSM4) was recently completed and re-leased to the climate community. This paper describes developments to all CCSM components, and docu-ments fully coupled preindustrial control runs compared to the previous version, CCSM3. Using the standard atmosphere and land resolution of 18 results in the sea surface temperature biases in the major upwelling regions being comparable to the 1.48-resolution CCSM3. Two changes to the deep convection scheme in the atmosphere component result in CCSM4 producing El Niño–Southern Oscillation variability with a much more realistic frequency distribution than in CCSM3, although the amplitude is too large compared to ob-servations. These changes also improve the Madden–Julian oscillation and the frequency distribution of tropical precipitation. A new overflow parameterization in the ocean component leads to an improved sim-ulation of theGulf Stream path and the NorthAtlantic Oceanmeridional overturning circulation. Changes to the CCSM4 land component lead to a much improved annual cycle of water storage, especially in the tropics. The CCSM4 sea ice component uses much more realistic albedos than CCSM3, and for several reasons the Arctic sea ice concentration is improved in CCSM4. An ensemble of twentieth-century simulations produces a goodmatch to the observed SeptemberArctic sea ice extent from 1979 to 2005. The CCSM4 ensemblemea