Spin-up strategy for ocean biogechemistry in a high resolution Earth System Model

<p class="western" align="left">Increasing computational power enables Earth System <span lang="en-US">M</span>odels (ESMs) to be run at higher resolutions than on conventional grids with spacings of O(100km). The new generation of ESMs running at resolutions of O(5-10km) are able to resolve phenomena such as mesoscale eddies in the ocean and convective storms in the atmosphere. Resolving these features can be a step towards reducing uncertainties in carbon cycle modeling as they directly affect the ocean uptake of anthropogenic carbon (Harrison et al. 2018). However, the spin-up of ocean biogeochemistry in globally high resolution ESMs remains computationally challenging. Traditionally, the spin-up duration of ocean biogeochemical components (e.g., in CMIP5/CMIP6 models) ranges from one hundred to several thousand years to avoid model drifts in the euphotic, mesopelagic and even deeper ocean that has an overturning time of O(1000 years). <span lang="en-US">T</span>his long spin-up time is, however, not yet computationally affordable in high resolution ESMs despite recent advances in improving their <span lang="en-US">scalability</span> (Linardakis et al. 2022). Therefore, different spin-up strategies are required and need to be explored.</p> <p class="western" align="left">We here present our strategy to run the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al. 2013, Jungclaus et al. 202<span lang="en-US">2</span>) in the high resolution ICON-Sapphire ESM (Hohenegger et al. 2022) configuration. We discuss the steps we take from tuning and spin-up of HAMOCC in a cascade of resolutions and configurations: initially in a 40km ocean only setup, subsequently in a 10km ocean-only and eventually a 5km ESM setup. Furthermore, we examine the possibility of replacing interpolated results (used as initialization for the next higher resolution) with available observations (e.g., nutrients, alkalinity, dissolved inorganic carbon) and its consequence on biogeochemical drifts of key tendencies such as CO₂ surface fluxes. Finally, we discuss the scientific questions that can be addressed using this spin-up strategy and its limitations.</p> <p class="western" align="left">&#160;</p> <p class="western" align="left">References:</p> <p class="western" align="left">Harrison et al. 2018: https://doi.org/10.1002/2017GB005751</p> <p class="western" align="left">Hohenegger et al. 2022: https://doi.org/10.5194/gmd-2022-171</p> <p class="western" align="left">Ilyina, T., et al. 2013: https://doi.org/10.1029/2012MS000178.</p> <p class="western" align="left">Jungclaus et al. 202<span lang="en-US">2</span>: https://doi.org/10.1029/2021MS002813</p> <p class="western" align="left">Linardakis et al. 2022: https://doi.org/10.5194/gmd-2022-214</p>