A Comparison of Two Vertical Mixing Schemes in a Pacific Ocean General Circulation Model

The upper Pacific Ocean current and temperature have been simulated by a 3-dimensional ocean general circulation model (OGCM) with two different vertical mixing schemes. One corresponds to the modified Richardson number dependent scheme of Pacanowski and Philander (PP, 1981); the other is adapted from the newly developed K-Profile Parameterization (KPP) scheme (Large et al., 1994). Both schemes include the penetration of shortwave radiation. For the KPP scheme, an ocean boundary layer depth is predicted and the turbulent mixing within this boundary layer is parameterized using a nonlocal bulk Richardson number and the similarity theory of turbulence. Below the boundary layer, the vertical mixing is parameterized through the local gradient Richardson number and a background mixing coefficient similar to the PP scheme. The performances of both schemes in the Pacific OGCM are evaluated under the same model configuration and boundary conditions. Model and data comparisons are made for the mean state, annual cycle, and interannual-to-interdecadal variability. In the tropics, both the PP and KPP schemes produce reasonably realistic tropical thermal and current structures; however, the KPP is better than the PP in several important aspects. For example, the thermocline in the KPP scheme is tighter and the cold tongue in the eastern equatorial Pacific is less cold than that in the PP solution. The core of the equatorial Undercurrent from the KPP scheme is much closer to the observation. In the extra-tropics, however, the KPP scheme is significantly better than the PP in simulating the thermal and current structures ranging from the mean state to annual cycle and decadal-to-interdecadal variability.