Improving the Upper-Ocean Temperature in an Ocean Climate Model (FESOM 1.4): Shortwave Penetration Versus Mixing Induced by Nonbreaking Surface Waves

Abstract

As the first mature global ocean general circulation model based on unstructured-mesh methods, the multiresolution Finite Element Sea ice-Ocean Model (FESOM) has shown great capability in reconstructing the ocean and sea ice in both standalone and coupled simulations at a relatively low computational cost. Parameterizations of some important processes, including the vertical mixing induced by surface waves, however, are still missing, contributing to temperature biases in the upper ocean. In this work we incorporate the vertical mixing induced by nonbreaking surface waves derived from a wave model into FESOM and compare its effect with that of shortwave penetration, another key process to vertically redistribute the heat in the upper ocean. Numerical experiments with and without the shortwave penetration scheme and the nonbreaking surface wave mixing reveal that both processes ameliorate the simulation of upper-ocean temperature in middle and low latitudes mainly on the summer hemisphere. The role of nonbreaking surface waves is more pronounced in decreasing the mean cold biases at 50 m (by 1.0 °C, in comparison to 0.5 °C achieved by applying shortwave penetration). We conclude that the incorporation of mixing induced by nonbreaking surface waves into FESOM is practically very helpful and suggest that it needs to be considered in other ocean climate models as well.