Abstract
AbstractWe document the configuration and emergent simulation features from the Geophysical Fluid Dynamics Laboratory (GFDL) OM4.0 ocean/sea ice model. OM4 serves as the ocean/sea ice component for the GFDL climate and Earth system models. It is also used for climate science research and is contributing to the Coupled Model Intercomparison Project version 6 Ocean Model Intercomparison Project. The ocean component of OM4 uses version 6 of the Modular Ocean Model and the sea ice component uses version 2 of the Sea Ice Simulator, which have identical horizontal grid layouts (Arakawa C‐grid). We follow the Coordinated Ocean‐sea ice Reference Experiments protocol to assess simulation quality across a broad suite of climate‐relevant features. We present results from two versions differing by horizontal grid spacing and physical parameterizations: OM4p5 has nominal 0.5° spacing and includes mesoscale eddy parameterizations and OM4p25 has nominal 0.25° spacing with no mesoscale eddy parameterization. Modular Ocean Model version 6 makes use of a vertical Lagrangian‐remap algorithm that enables general vertical coordinates. We show that use of a hybrid depth‐isopycnal coordinate reduces the middepth ocean warming drift commonly found in pure z* vertical coordinate ocean models. To test the need for the mesoscale eddy parameterization used in OM4p5, we examine the results from a simulation that removes the eddy parameterization. The water mass structure and model drift are physically degraded relative to OM4p5, thus supporting the key role for a mesoscale closure at this resolution.
Highlights
We present results from two versions differing by horizontal grid spacing and physical parameterizations: OM4p5 has nominal 0.5° spacing and includes mesoscale eddy parameterizations and OM4p25 has nominal 0.25° spacing with no mesoscale eddy parameterization
We describe in this paper the result of employing all three strategies, making use here of a new ocean model dynamical core, improvements to physical parameterizations, and refinements in the horizontal and vertical grid spacing
The two baseline models are the OM4p25 0.25° configuration, which has no parameterization of mesoscale eddies in the tracer equation, and the OM4p5 0.5° configuration, in which unresolved mesoscales are parameterized
Summary
Numerical models are an essential part of the arsenal of science tools used to mechanistically understand how the ocean works within the earth climate system. Three strategies for improving simulations from ocean‐sea ice climate models include (i) enhanced formulations of the dynamical core, such as treatment of the vertical coordinate, (ii) improved parameterization of unresolved physical processes, such as mesoscale eddies and boundary layers, and (iii) refined numerical resolution to better capture both resolved and unresolved physical processes. We describe in this paper the result of employing all three strategies, making use here of a new ocean model dynamical core, improvements to physical parameterizations, and refinements in the horizontal and vertical grid spacing. Given the importance of boundary features for the ocean circulation, even modest resolution enhancements benefit the simulations This perspective provides the central motivation for refining horizontal grid spacing into a regime that some refer to as the ”gray zone” given that it only partially resolves transient mesoscale eddies.
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