Validation of vertical mixing in an equatorial ocean model using large eddy simulations and observations

A nonlocal K-profile parameterization (KPP) of the upper-ocean boundary lay er 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 counter gradient heat flux. The comparison is clean because both the surface forcing and the large-scale fl ow are identical in the two models. The comparison is direct because the pa rameterized turbulent flux profiles are explicitly computed in LES. A simil ar comparison is less favorable when KPP is replaced by purely downgradient diffusion with Richardson-number-dependent viscosity and diffusivity becau se of the absence of intense convection after sunset. Sensitivity experimen ts 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 Ocea n is described. The results of GCM runs with and without KPP are compared t o 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 observati ons, with biases in zonal velocity in the western Pacific and in subsurface temperature in the eastern Pacific. Such comparisons are never clean becau se 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 w hen different heat flux formulations are used. These include bulk forcing w here prescribed air temperature and humidity are used, SST forcing where th e use of such ocean-controlled parameters is avoided, and a fully coupled a tmospheric general circulation model where there is no prescribed control o ver any surface fluxes. It is concluded, especially in the eastern Pacific, that the use of specified air temperature and humidity does not overly con strain the model sea surface temperature.