STABILITY DEPENDENT PARAMETERISATIONS OF VERTICAL MIXING IN OCEAN GENERAL-CIRCULATION MODELS

Parameterisations of mixing induced through shear instability, interna l wave breaking, and double diffusion are investigated in simulations of ocean climate using a global ocean general circulation model (OGCM) . Focus is placed on the sensitivity of the large scale circulation wa ter mass formation and transport of heat as measures of the model's ab ility to represent current climate. The model resolution is typical of OGCMs being coupled to atmospheric. GCMs in climate models and the pa rameterisations investigated are all computationally inexpensive enoug h to allow for integrations on long time scales. Under the assumption of constant vertical eddy coefficients (the control case), the model c limatology displays acceptable values of North Atlantic Deep Water for mation, Antarctic Circumpolar Current (ACC) transport, and Indonesian through-flow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that various c irculation and water mass properties are sensitive to the choice of pa rameterisation of vertical mixing and that determining a scheme which works satisfactorily over all regions (tropical, mid-latitude, and pol ar) of the domain is not straightforward. Parameterisations of interna l wave breaking or upper ocean shear instability lead to some improvem ents in the model water mass formation. ACC and poleward heat transpor t when compared to the control case whereas parameterisations of doubl e diffusive processes did not. Based on these and other results, vario us recommendations are made for mixing parameterisations in ocean clim ate models.