The datasets presented here aim at the examination of global ocean temperature and circulation response to atmospheric CO2 drawdown (from 400 to 200 ppmv) and Antarctic ice-sheet expansion (43 m of sea-level equivalent) across the middle Miocene climate transition.The data provided are climatological means obtained from the CCSM3 experiments named MMCO400, MMG200, MMCO200, MMG400, and PI. MMCO400 and MMG200 represent the periods before and after the middle Miocene climate transition, respectively. The design of MMCO200 is identical to that of MMCO400 except for the atmospheric CO2 concentration, which is set to 200 ppmv instead of 400 ppmv. The design of MMG400 is identical to that of MMG200, except for CO2, which is set to 400 ppmv instead of 200 ppmv. PI is a pre-industrial control run. See the description in the methods section of the associated manuscript for more details. All data are presented in NetCDF format. Datasets D1 to D5 (D6 to D10) contain CCSM3 ocean/ice (atmosphere) model data. Datasets D11 to 15 contain offline calculations from the CCSM3 ocean model runs. Dataset D1: ds01.nc contains CCSM3 ocean/ice model output data from the PI experiment, including potential temperature (TEMP), salinity (SALT), barotropic streamfunction (BSF), and sea-ice thickness. Dataset D2: ds02.nc contains CCSM3 ocean/ice model output data from the MMCO400 experiment, including potential temperature (TEMP), salinity (SALT), barotropic streamfunction (BSF), sea-ice thickness, boreal winter mixed layer depth (HMXL_JFM), potential density (PD), and salt, melt, precipitation, and evaporation fluxes (SALT_F, MELT_F, PREC_F, EVAP_F) from the coupler. Dataset D3: as ds02.nc but for the MMG200 experiment. Dataset D4: as ds02.nc (except for sea-ice thickness) but for the MMCO200 experiment. Dataset D5: as ds02.nc (except for sea-ice thickness) but for the MMG400 experiment. Dataset D6: ds06.nc contains CCSM3 atmosphere model output data from the PI experiment, including 2-meter surface air temperature (TREFHT), and wind components (U and V fields respectively). Dataset D7: ds07.nc contains CCSM3 atmosphere model output data from the MMCO400 experiment, including 2-meter surface air temperature (TREFHT), upward longwave flux at the top of the atmosphere (FLUT), surface sensible heat flux (SHFLX), and wind components (U and V fields respectively). Dataset D8: as ds07.nc but for the MMG200 experiment. Dataset D9: as ds07.nc but for MMCO200 experiment. Dataset D10: as ds07.nc but for the MMG400 experiment. Dataset D11: ds11.nc contains the ocean meridional overturning circulation streamfunction (Eulerian component) (MOC field) and ocean global meridional heat transport (transport field) calculated from the PI experiment model output data through NCAR CCSM3 offline calculation tools. Dataset D12: as ds11.nc but for the MMCO400 experiment. Dataset D13: as ds11.nc but for the MMG200 experiment. Dataset D14: as ds.11.nc but for the MMCO200 experiment. Dataset D15: as ds.11.nc but for the MMG400 experiment.