What are the challenges simulating historical ocean deoxygenation?

   This study presents an analysis of the historical upper ocean (0-700m) dissolved oxygen (O₂) and heat content changes from a suite of  the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemistry simulations. The simulations include both forced ocean-only models (the Ocean Model Intercomparison Project, OMIP1 and OMIP2) and coupled historical simulations from Earth System Models (ESMs)  (CMIP6 Historical). Simulated changes in the O₂ inventory and ocean heat content (OHC) over the past five decades spatially and temporally diverge across the models. The multi-model mean and spread of the upper ocean global O₂ inventory trend for each of the simulations is 0.03 ± 0.39 × 10¹⁴ [mol/decade] for OMIP1,  -0.37 ± 0.15 × 10¹⁴ [mol/decade] for OMIP2, and -1.06 ± 0.68 × 10¹⁴ [mol/decade] for CMIP6 Historical simulations, respectively. The latest observational trend based on the World Ocean Database 2018 is -0.98 × 10¹⁴ [mol/decade],  in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends.  A comparison between OMIP1 and OMIP2 simulations suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across simulations in O₂ inventory and OHC changes. An additional comparison between OMIP and CMIP6 Historical simulations indicates that differences in background mean states due to differences in spin-up strategy and equilibrium states result in substantial differences in the climate change response of O₂. In this presentation, we will discuss gaps and gaps and uncertainties in both ocean biogeochemistry simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O₂ and changes in associated ocean biogeochemical cycles. This presentation is based on the recently published work (Takano et al., 2023).

Reference

Takano Y., Ilyina T., Tjiputra J., Eddebbar Y.A., Berthet S., Bopp L., Buitenhuis E., Butenschön M., Christian J.R., John  J.P., Gröger M., Hayashida H., Hieronymus J., Koenigk T., Krasting J.P., Long M.C., Lovato T., Nakano H., Palmieri J., Schwinger J., Séférian R., Suntharalingam P., Tatebe H., Tsujino H., Urakawa S., Watanabe M., and Yool A.: Simulations of ocean deoxygenation in the historical era: insights from forced and coupled models, Front. Mar. Sci., 10:1139917, doi: 10.3389/fmars.2023.1139917.