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Abstract The impact of resolving western boundary currents and mesoscale eddies on a carbon circulation simulation for the North Pacific Ocean is investigated to evaluate the merits of using high‐resolution ocean biogeochemical models for climate projections. Simulations by a 100‐km resolution global ocean biogeochemical model with and without embedding a 10‐km resolution model in the North Pacific Ocean are compared. The major improvement in the high‐resolution simulation is the representation of the Kuroshio, its extension current, and the recirculation gyres formed to its south and north, resulting in a proper representation of the North Pacific subtropical mode water (STMW) and an increase in storage of the anthropogenic CO2 (Canth) in STMW by about two‐thirds. The larger storage rate in STMW is caused by supply of a larger amount of warm surface water containing rich Canth to the formation region by the intensified Kuroshio. A huge buoyancy loss from this warm water results in the increased formation of STMW that occupies a vast area in the western subtropical gyre. The surface uptake of Canth in the formation region of STMW is slightly increased but is largely comparable to that of the low‐resolution model. Moreover, there is no structural difference in Canth uptake in other parts of the subtropical region. Thus, the improvement of Canth distribution can be understood as a redistribution of water mass in the subtropical gyre by the improved circulation. The present assessment motivates the use of a high horizontal resolution ocean model in next‐generation projection experiments with carbon cycles.

We present a new framework for global ocean–sea-ice model simulations based on phase 2 of the Ocean Model Intercomparison Project (OMIP-2), making use of the surface dataset based on the Japanese 55-year atmospheric reanalysis for driving ocean–sea-ice models (JRA55-do). We motivate the use of OMIP-2 over the framework for the first phase of OMIP (OMIP-1), previously referred to as the Coordinated Ocean–ice Reference Experiments (COREs), via the evaluation of OMIP-1 and OMIP-2 simulations from 11 state-of-the-science global ocean–sea-ice models. In the present evaluation, multi-model ensemble means and spreads are calculated separately for the OMIP-1 and OMIP-2 simulations and overall performance is assessed considering metrics commonly used by ocean modelers. Both OMIP-1 and OMIP-2 multi-model ensemble ranges capture observations in more than 80 % of the time and region for most metrics, with the multi-model ensemble spread greatly exceeding the difference between the means of the two datasets. Many features, including some climatologically relevant ocean circulation indices, are very similar between OMIP-1 and OMIP-2 simulations, and yet we could also identify key qualitative improvements in transitioning from OMIP-1 to OMIP-2. For example, the sea surface temperatures of the OMIP-2 simulations reproduce the observed global warming during the 1980s and 1990s, as well as the warming slowdown in the 2000s and the more recent accelerated warming, which were absent in OMIP-1, noting that the last feature is part of the design of OMIP-2 because OMIP-1 forcing stopped in 2009. A negative bias in the sea-ice concentration in summer of both hemispheres in OMIP-1 is significantly reduced in OMIP-2. The overall reproducibility of both seasonal and interannual variations in sea surface temperature and sea surface height (dynamic sea level) is improved in OMIP-2. These improvements represent a new capability of the OMIP-2 framework for evaluating process-level responses using simulation results. Regarding the sensitivity of individual models to the change in forcing, the models show well-ordered responses for the metrics that are directly forced, while they show less organized responses for those that require complex model adjustments. Many of the remaining common model biases may be attributed either to errors in representing important processes in ocean–sea-ice models, some of which are expected to be reduced by using finer horizontal and/or vertical resolutions, or to shared biases and limitations in the atmospheric forcing. In particular, further efforts are warranted to resolve remaining issues in OMIP-2 such as the warm bias in the upper layer, the mismatch between the observed and simulated variability of heat content and thermosteric sea level before 1990s, and the erroneous representation of deep and bottom water formations and circulations. We suggest that such problems can be resolved through collaboration between those developing models (including parameterizations) and forcing datasets. Overall, the present assessment justifies our recommendation that future model development and analysis studies use the OMIP-2 framework.