Effect of penetrating momentum flux over the surface boundary/mixed layer in a z-coordinate OGCM of the Tropical Pacific

A simple scheme is proposed for penetrating atmospheric momentum flux over the ocean surface boundary layer or mixed layer (BL/ML) and is tested in the z-coordinate NOAA/Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM 3) for improving its performance. Analogous to the treatment in layered ocean models, wind stress is applied, as a body force, to the entire BL/ML whose depth is calculated from a nonlocal K-profile parameterization scheme. The penetrating scheme presents an explicit and effective way to distribute a priori momentum flux throughout the BL/ML that has varying depth in space and time, instead of just over the uppermost model level with fixed thickness. This additional procedure introduces an explicit mechanism that directly relates wind stress to the BL/ML formulation, which in turn controls current and thermal structure in the upper ocean and the interaction with the underlying thermocline. Two penetrating runs, one over the BL and the other over the ML, have similar results that differ systematically from those with the penetration over fixed depths (control run). It is demonstrated that, with coherent and systematic improvements, this penetrating scheme can have significant effects on simulated equatorial ocean currents and thermal structure not only in the surface layer, but also in the thermocline. Besides more reasonable ML depth simulation in the equatorial central basin, there is substantial reduction in the mean offset of simulated isotherm depths and warm bias in the thermocline, due to downward shift of the maximum upwelling zone in the equatorial central Pacific. Consistent with observations, the penetrating scheme realistically reproduces the springtime reversal of the South Equatorial Current and the corresponding surface warming in the central equatorial Pacific, with accompanying surfacing of the Equatorial Undercurrent Current in March-May.