Sensitivity of deep ocean biases to horizontal resolution in prototype CMIP6 simulations with AWI-CM1.0

• Published in: Geoscientific Model Development
• Main Authors: T. Rackow, D. V. Sein, T. Semmler, S. Danilov, N. V. Koldunov, D. Sidorenko, Q. Wang, T. Jung
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2019
• Subjects: Geology; QE1-996.5; Southern Ocean; Alfred Wegener Institute; North Atlantic; Sea ice
• Online Access: https://doi.org/10.5194/gmd-12-2635-2019; https://doaj.org/article/d8e60ed88e034999b45772518f151f10

Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) show substantial biases in the deep ocean that are larger than the level of natural variability and the response to enhanced greenhouse gas concentrations. Here, we analyze the influence of horizontal resolution in a hierarchy of five multi-resolution simulations with the AWI Climate Model (AWI-CM), the climate model used at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, which employs a sea ice–ocean model component formulated on unstructured meshes. The ocean grid sizes considered range from a nominal resolution of <math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>∼</mo><mn mathvariant="normal">1</mn><msup><mi/><mo>∘</mo></msup></mrow></math> <svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="398feeb91a53908b63a9a04779046c3c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-12-2635-2019-ie00001.svg" width="23pt" height="11pt" src="gmd-12-2635-2019-ie00001.png"/></svg:svg> (CMIP5 type) up to locally eddy resolving. We show that increasing ocean resolution locally to resolve ocean eddies leads to reductions in deep ocean biases, although these improvements are not strictly monotonic for the five different ocean grids. A detailed diagnosis of the simulations allows to identify the origins of the biases. We find that two key regions at the surface are responsible for the development of the deep bias in the Atlantic Ocean: the northeastern North Atlantic and the region adjacent to the Strait of Gibraltar. Furthermore, the Southern Ocean density structure is equally improved with locally explicitly resolved eddies compared to parameterized eddies. Part of the bias reduction can be traced back towards improved surface biases over outcropping regions, which are in ...