Validation of Vertical Mixing in an Equatorial Ocean Model Using Large Eddy Simulations and Observations

Abstract

A nonlocal K-profile parameterization (KPP) of the upper-ocean boundary layer is tested for the equatorial regions. First, the short-term performance of a one-dimensional model with KPP is found to compare favorably to large eddy simulations (LES), including nonlocal countergradient heat flux. The comparison is clean because both the surface forcing and the large-scale flow are identical in the two models. The comparison is direct because the parameterized turbulent flux profiles are explicitly computed in LES. A similar comparison is less favorable when KPP is replaced by purely downgradient diffusion with Richardson-number-dependent viscosity and diffusivity because of the absence of intense convection after sunset. Sensitivity experiments are used to establish parameter values in the interior mixing of KPP. Second, the impact of the parameterization on annual means and the seasonal cycle in a general circulation model of the upper, equatorial Pacific Ocean is described. The results of GCM runs with and without KPP are compared to annual mean profiles of zonal velocity and temperature from the TOGA-TAO array. The two GCM solutions are closer to each other than to the observations, with biases in zonal velocity in the western Pacific and in subsurface temperature in the eastern Pacific. Such comparisons are never clean because neither the wind stress and the surface heat flux nor the forcing by the large-scale flow are known to sufficient accuracy. Finally, comparisons are made of the equatorial Pacific Ocean GCM results when different heat flux formulations are used. These include bulk forcing where prescribed air temperature and humidity are used, SST forcing where the use of such ocean-controlled parameters is avoided, and a fully coupled atmospheric general circulation model where there is no prescribed control over any surface fluxes. It is concluded, especially in the eastern Pacific, that the use of specified air temperature and humidity does not overly constrain the model sea surface temperature.