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

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 degrees C, in comparison to 0.5 degrees 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. Plain Language Summary Nowadays, numerical ocean, weather, and climate forecasts play an important role in the daily life of human beings. An accurate prediction could help us prepare day-to-day activities orderly. However, the prediction ability has been much lower than expected. As an example, ocean models often simulate a warmer sea surface temperature and cooler subsurface (30-100 m deep) temperature in subtropical oceans, especially in summer, which can lead to big errors in the weather and climate forecasting. This situation was partly alleviated by distributing solar radiation in the upper ocean rather than only heating up the ocean surface. Although shortwave penetration makes some improvement on ocean model performance, it is still far from solving the common simulated temperature bias in the upper ocean. The simulated temperature is considerably improved by incorporating the mixing induced by nonbreaking surface waves into the new generation ocean model FESOM. It turns out that the nonbreaking wave is more capable in ameliorating the simulated upper-ocean temperature than the shortwave penetration.