The mixed-layer depth in the Ocean Model Intercomparison Project (OMIP): impact of resolving mesoscale eddies

• Published in: Geoscientific Model Development
• Main Authors: Treguier, Anne Marie, de Boyer Montégut, Clement, Bozec, Alexandra, Chassignet, Eric P., Fox-Kemper, Baylor, McC. Hogg, Andy, Iovino, Doroteaciro, Kiss, Andrew E., Le Sommer, Julien, Li, Yiwen, Lin, Pengfei, Lique, Camille, Liu, Hailong, Serazin, Guillaume, Sidorenko, Dmitry, Wang, Qiang, Xu, Xiaobio, Yeager, Steve
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2023
• Subjects: article; Verlagsveröffentlichung; Southern Ocean; Sea ice
• Online Access: https://doi.org/10.5194/gmd-16-3849-2023; https://noa.gwlb.de/receive/cop_mods_00067706; https://noa.gwlb.de/servlets/MCRFileNodeServlet/cop_derivate_00066153/gmd-16-3849-2023.pdf; https://gmd.copernicus.org/articles/16/3849/2023/gmd-16-3849-2023.pdf

The ocean mixed layer is the interface between the ocean interior and the atmosphere or sea ice and plays a key role in climate variability. It is thus critical that numerical models used in climate studies are capable of a good representation of the mixed layer, especially its depth. Here we evaluate the mixed-layer depth (MLD) in six pairs of non-eddying (1∘ grid spacing) and eddy-rich (up to 1/16∘) models from the Ocean Model Intercomparison Project (OMIP), forced by a common atmospheric state. For model evaluation, we use an updated MLD dataset computed from observations using the OMIP protocol (a constant density threshold). In winter, low-resolution models exhibit large biases in the deep-water formation regions. These biases are reduced in eddy-rich models but not uniformly across models and regions. The improvement is most noticeable in the mode-water formation regions of the Northern Hemisphere. Results in the Southern Ocean are more contrasted, with biases of either sign remaining at high resolution. In eddy-rich models, mesoscale eddies control the spatial variability in MLD in winter. Contrary to a hypothesis that the deepening of the mixed layer in anticyclones would make the MLD larger globally, eddy-rich models tend to have a shallower mixed layer at most latitudes than coarser models do. In addition, our study highlights the sensitivity of the MLD computation to the choice of a reference level and the spatio-temporal sampling, which motivates new recommendations for MLD computation in future model intercomparison projects.