A Hybrid Vertical Mixing Scheme and Its Application to Tropical Ocean Models

Abstract

A novel hybrid vertical mixing scheme, based jointly on the Kraus?Turner-type mixed layer model and Price's dynamic instability model, is introduced to aid in parameterization of vertical turbulent mixing in numerical ocean models. The scheme is computationally efficient and is capable of simulating the three major mechanisms of vertical turbulent mixing in the upper ocean, that is, wind stirring, shear instability, and convective overturning. The hybrid scheme is first tested in a one-dimensional model against the Kraus?Turner-type bulk mixed layer model and the Mellor?Yamada level 2.5 (MY2.5) turbulence closure model. As compared with those two models, the hybrid model behaves more reasonably in both idealized experiments and realistic simulations. The improved behavior of the hybrid model can be attributed to its more complete physics. For example, the MY2.5 model underpredicts mixed layer depth at high latitudes due to its lack of wind stirring and penetrative convection, while the Kraus?Turner bulk model produces rather shallow mixed layers in the equatorial region because of its lack of shear-produced mixing. The hybrid model reproduces the good results of the MY2.5 model toward the equator and the bulk model toward high latitudes, thereby taking the advantages of those two models while avoiding their shortcomings. The hybrid scheme is then implemented in a three-dimensional model of the tropical Pacific Ocean. This leads to an improved simulation of the large-scale equatorial circulation. Compared with the other two commonly used mixing schemes tested in this experiment, the hybrid scheme helps to produce more realistic velocity profiles in the eastern and central equatorial Pacific. This is mainly due to the improved parameterization of interior mixing related to the large shears of the Equatorial Undercurrent. Another feature in this model that is sensitive to the vertical mixing scheme is the equatorial instability waves; in the eastern Pacific Ocean these waves are most energetic when the hybrid scheme is used. The meridional heat flux associated with these waves can be locally important in the mixed layer heat budget.