An Improved Parameterization of Wind‐Driven Turbulent Vertical Mixing Based on an Eddy‐Resolving Climate Model. (23rd October 2021) - Yuan, Man
Abstract: Turbulent vertical mixing in the stratified ocean interior has a huge impact on global ocean circulations and the climate system. Although parameterizations of vertical mixing furnished by internal tides have been built into state‐of‐the‐art coupled global climate models (CGCMs), efforts in parameterizing wind‐driven vertical mixing in CGCMs are still limited. In this study, we apply a modified finescale parameterization (MFP) to an eddy‐resolving Community Earth System Model (CESM) to represent the wind's contribution to vertical mixing in the stratified ocean interior. The spatial pattern of the MFP‐parameterized wind‐driven vertical mixing in the thermocline agrees with the observation derived from the finestructure measurements of Argo floats, reproducing the enhanced values in the Kuroshio, Gulf Stream extensions, and the Southern Ocean where the winds inject great amount of energy into the internal wave field. The MFP also captures the observed seasonal variation of wind‐driven vertical mixing in the thermocline of these regions that exhibits enhancement and weakening in winter and summer, respectively. Application of the MFP to a non‐eddy‐resolving CESM fails to reproduce the observed wind‐driven vertical mixing. Specifically, the magnitude of parameterized wind‐driven vertical mixing in the thermocline of Kuroshio, Gulf Stream extensions, and the Southern Ocean is systemically smaller than those in the observation and eddy‐resolving CESM; so is the case for the amplitude of seasonal cycle. The results highlight the benefit of eddy‐resolving CESM compared to its standard‐resolution counterpart in parameterizing the wind‐driven vertical mixing and provide insight into developing parameterizations for wind‐driven vertical mixing in eddy‐resolving CGCMs. Plain Language Summary: Vertical mixing of fluids with different properties caused by small‐scale turbulence plays an important role in the global ocean circulations and climate system. Yet it is far from being resolved by state‐of‐the‐art climate models and thus needs to be parameterized. Parameterizing the vertical mixing driven by wind‐generated internal waves turns out to be challenging mainly due to the strong interaction of wind‐generated internal waves with ocean mesoscale eddies that are swirling, time‐dependent circulations about 100 km. In this study, we apply a modified finescale parameterization (MFP) to an eddy‐resolving (ocean model's grid size of ∼0.1°) Community Earth System Model (CESM) to parameterize the wind‐driven vertical mixing in the stratified ocean interior. The MFP‐parameterized wind‐driven vertical mixing in the thermocline agrees well with the observations, reproducing enhanced values in the regions where winds input large amount of energy into the internal wave field as well as an evident seasonal cycle. However, applying the MFP to a non‐eddy‐resolving (∼1°) CESM fails to reproduce the spatial and temporal variations of wind‐driven vertical mixing as in the observations. The results suggest that the eddy‐resolving climate models provide a new pathway for circumventing the conundrum for parameterizing the wind‐driven vertical mixing. Key Points: A modified finescale parameterization (MFP) scheme is applied to an eddy‐resolving Community Earth System Model (CESM) The parameterized wind‐driven vertical mixing in the CESM is validated against observations Eddy‐resolving CESM with MFP shows good skills in representing the wind‐driven vertical mixing in the thermocline