Diapycnal mixing in a joint mixed-layer/isopycnic coordinate numerical model of the wind- and thermohaline-driven ocean general circulation

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Meteorology and Physical Oceanography

First Committee Member

Rainer Bleck - Committee Chair


A joint mixed-layer/isopycnic coordinate numerical model of the wind- and thermohaline- driven ocean general circulation is developed based on the work of Bleck et al. (1989). In the present model the complete set of primitive equations for conservation of momentum, mass, heat and salt are solved. Diapycnal and isopycnal mixing of temperature and salinity, and the diapycnal velocity associated with them, are included. The model is forced by a Newtonian type of surface heat flux with seasonal variations and an E-P type of surface salt flux varying with latitude only. Vertical stratification of the model ocean is determined by two kinds of diapycnal mass fluxes, first the flux across the bottom of the mixed layer due to mixed layer entrainment/detrainment, and second the interior diapycnal advective flux associated with diapycnal and isopycnal mixing of temperature and salinity. The distinguishing feature of the model is that three-dimensional distributions of both types of mass fluxes are easily diagnosed, thus allowing detailed water mass budget calculations for each layer.Based on a physical definition of neutral surfaces, a mathematical definition of dianeutral velocity is derived which is independent of turbulence closure schemes for interior mixing. Also derived is an Eulerian type of mathematical definition for neutral surfaces, which simplifies the use of neutral surface coordinates in data analysis and in numerical models, compared to McDougall's mathematical definition.Numerical algorithms are developed to solve the numerical implementation difficulties in computing vertical scalar gradients and diapycnal advection velocities in isopycnic coordinates. The algorithms diagnose diapycnal velocity objectively and, at the same time, maintain isopycnic coordinates. In addition, the algorithms are free of ad hoc parameters. The algorithms are verified in one-dimensional isopycnal models of diapycnal diffusion. The verification results show that the algorithms are successful in simulating various one-dimensional diapycnal diffusion processes.A model sensitivity study is aimed at establishing a basic understanding of the role of vertical diffusivity in determining thermocline structure and meridional heat transport in ocean general circulation models (OGCMs). Contrary to F. Bryan's results for the GFDL model sensitivity study, meridional heat transport in the present model is found to be rather insensitive to the vertical diffusivity. A simple analysis shows that sensitivity of meridional heat transport in OGCMs to vertical diffusivity depends on the way the model surface heat flux is parameterized. For OGCMs using a Newtonian cooling type of surface heat flux, high sensitivity of the model meridional heat transport to vertical diffusivity can be avoided by choosing an appropriate reference temperature and vertical e-folding damping rate, and therefore the vertical diffusivity can be tuned to obtain a realistic thermocline.


Geophysics; Physical Oceanography

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