The Mixed Layer Depth in the Ocean Model Intercomparison Project (OMIP): Impact of Resolving Mesoscale Eddies

• Main Authors: Treguier, Anne Marie, Boyer Montégut, Clement, Bozec, Alexandra, Chassignet, Eric P., Fox-Kemper, Baylor, Hogg, Andy McC., Iovino, Doroteacino, Kiss, Andrew E., Sommer, Julien, Li, Yiwen, Lin, Pengfei, Lique, Camille, Liu, Hailong, Serazin, Guillaume, Sidorenko, Dmitry, Wang, Qiang, Xu, Xiaobio, Yeager, Steve
• Format: Text
• Language: English
• Published: 2023
• Subjects: Southern Ocean; Sea ice
• Online Access: https://doi.org/10.5194/egusphere-2023-310; https://egusphere.copernicus.org/preprints/2023/egusphere-2023-310/

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 <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="27pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="22a3600bd5eb5cb9a3249b3a22652254"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-16-3849-2023-ie00001.svg" width="27pt" height="14pt" src="gmd-16-3849-2023-ie00001.png"/></svg:svg> ∘ ) 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.