Comparison of a coupled ocean-atmosphere model with and without oceanic eddy-induced advection. Part I: Ocean spinup and control integrations

The Gent and McWilliams (GM) parameterization for large-scare water transpo rt caused by mesoscale oceanic eddies is introduced into the oceanic compon ent of the Commonwealth Scientific and Industrial Research Organisation glo bal coupled ocean-atmosphere model. Parallel simulations with and without t he GM scheme are performed to examine the effect of this parameterization o n the model behavior for integrations lasting several centuries under condi tions of constant atmospheric CO2. The solution of the version with GM show s several significant improvements over that of the earlier version. First, the generally beneficial effects of the GM scheme found previously in stud ies of stand-alone ocean models, including more realistic deep water proper ties, increased stratification, reduced high-latitude convection, eliminati on of fictitious horizontal diffusive heat transport, and more realistic su rface fluxes in some regions, are an maintained during the coupled integrat ion. These improvements are especially pronounced in the high-latitude Sout hern Ocean. Second, the magnitude of flux adjustment is reduced in the GM v ersion, mainly because of smaller surface fluxes at high southern latitudes in the GM ocean spinup. Third, the GM version displays markedly reduced cl imate drift in comparison to the earlier version. Analysis in the present s tudy verifies previous indications that changes in the pattern of convectiv e heat flux are central to the drift in the earlier version, supporting the view that reduced convective behavior in the GM version contributes to the reduction in drift. Based on the satisfactory behavior of the GM model ver sion, the GM coupled integration is continued for a full 1000 yr. Key aspec ts of the model behavior during this longer period are also presented. Inte rannual variability of surface air temperature in the two model versions is briefly examined using some simple measures of magnitude. The variability differs between the two versions regionally, but is little changed on the g lobal scale. In particular, the magnitude of variability in the tropical Pa cific is little changed between the versions.