Abstract
Abstract. The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth system models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) vs. when integrated within fully coupled Earth system models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled to ocean circulation models, initialized with observational data or output from a model spin-up, and forced by repeating the 1948–2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF6) and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin-up, preferably for 2000 years or more, and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols are founded on those from previous phases of the Ocean Carbon-Cycle Model Intercomparison Project. They have been merged and updated to reflect improvements concerning gas exchange, carbonate chemistry, and new data for initial conditions and atmospheric gas histories. Code is provided to facilitate their implementation.
Highlights
Centralized efforts to compare numerical models with one another and with data commonly lead to model improvements and accelerated development
Ocean Model Intercomparison Project (OMIP) focuses on comparison of global ocean models that couple circulation, sea ice, and optional biogeochemistry, which together make up the ocean components of the Earth system models (ESMs) used within CMIP6
As described by Griffies et al (2016), the OMIP-Physics simulations consist of forcing physical model systems with the interannually varying atmospheric data reanalysis known as the Coordinated Ocean-ice Reference Experiments (CORE-II) available over 1948–2009 (Large and Yeager, 2009)
Summary
Centralized efforts to compare numerical models with one another and with data commonly lead to model improvements and accelerated development. To help assess simulated circulation fields, OCMIP included another passive tracer, radiocarbon, focusing on both its natural and anthropogenic components. OMIP focuses on evaluating and comparing preselected “combined” ocean models (circulationice-biogeochemistry) largely defined already by individual groups planning to participate in CMIP6 Those combined ocean models will be evaluated when forced by reanalysis data as well as when coupled within the CMIP6 ESMs. OMIP-BGC model groups will use common physical forcing for ocean-only models and common formulations for carbonate chemistry, gas exchange, gas solubilities, and Schmidt numbers. That foundation includes (1) the OMIP-BGC protocols for groups that will include inert chemical tracers and biogeochemistry in OMIP’s two forced global ocean model simulations, which couple circulation, sea ice, and biogeochemistry, and (2) the complete list of ocean biogeochemical diagnostics for OMIP, and for CMIP6 (Eyring et al, 2016) and any ocean-related MIPs under its umbrella, e.g., C4MIP (Jones et al, 2016). OMIP aims to provide new insight into the ocean’s contribution to internal variability while quantifying the relative importance of the contribution of internal variability to the overall uncertainty of model projections
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