A Comparison of Vertical Eddy Mixing Parameterizations for Equatorial Ocean Models

Abstract

The vertical eddy mixing formulations employed in the K-theory model of Pacanowski and Philander and in second-moment closure models are compared for an equatorial Pacific Ocean simulation. The Pacanowski and Philander model is found to be mainly driven by changes in the stratification rather than shear-generated instabilities, and the position and width of the mixing transition zone between high and low mixing values is found to be sensitive to the parameters of the model. In the second-moment closure models the master length scale limit effectively determines the threshold of the mixing zone, while the inclusion of storage, advection, and diffusion terms in the turbulent kinetic energy equation affects both the position and extent of the transition zone. Viscous mixing is more intense than diffusive mixing in the Pacanowski and Philander scheme, but in the second-moment closure models the reverse tends to be true. As expected, there is no simple functional relationship between the gradient Richardson number and the intensity of mixing in the second-moment closure schemes.