Sensitivity of a global coupled ocean-sea ice model to the parameterization of vertical mixing

Three numerical experiments have been carried out with a global coupled ice -ocean model to investigate its sensitivity to the treatment of vertical mi xing in the upper ocean. In the first experiment, a widely used fixed profi le of vertical diffusivity and viscosity is imposed, with large values in t he upper 50 m to crudely represent wind-driven mixing. In the second experi ment, the eddy coefficients are functions of the Richardson number, and, in the third case, a relatively sophisticated parameterization, based on the turbulence closure scheme of Mellor and Yamada version 2.5, is introduced. We monitor the way the different mixing schemes affect the simulated ocean ventilation, water mass properties, and sea ice distributions. CFC uptake i s also diagnosed in the model experiments. The simulation of the mixed laye r depth is improved in the experiment which includes the sophisticated turb ulence closure scheme. This results in a good representation of the upper o cean thermohaline structure and in heat exchange with the atmosphere within the range of current estimates. However, the en or in heat flux in the exp eriment with simple fixed vertical mixing coefficients can be as high as 50 W m(-2) in zonal mean during summer. Using CFC tracers allows us to demons trate that the ventilation of the deep ocean is not significantly influence d by the paramertization of vertical mixing in the upper ocean. The only ex ception is the Southern Ocean. There, the ventilation is tao strong in all three experiments. However, modifications of the vertical diffusivity and, surprisingly, the vertical viscosity significantly affect the stability of the water column in this region through their influence on upper ocean sali nity, resulting in a more realistic Southern Ocean circulation. The turbule nce scheme also results in an improved simulation of Antarctic sea ice cove rage . This is due to to a better simulation of the mixed layer depth and t hus of heat exchanges between ice and ocean. The large-scale mean summer ic e-ocean heat flux can vary by more than 15% between the three experiments. Because of this influence of vertical mixing on Southern Ocean ventilation, sea ice extent, and ocean-atmosphere heat fluxes, we recommend that global climate models adopt a sufficiently realistic representation of vertical m ixing in the ocean.