Impact of Increased Horizontal Resolution of an Ocean Model on Carbon Circulation in the North Pacific Ocean

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. Accurately representing absorption and accumulation of anthropogenic CO2 (Canth) by the ocean in Earth system models is important to reliably project future climate change. This study investigates the impact of realistically expressing the Kuroshio, which is not expressed adequately in the majority of current Earth system models, on a carbon circulation simulation for the North Pacific Ocean. Two simulations differing in the horizontal grid spacing, about 100 and 10 km, are compared. The intensified Kuroshio in the fine resolution model results in an improved oceanic structure for the upper 500-m depths in the western North Pacific Ocean, and more Canth is accumulated there, which is closer to observations. Budget analysis suggests that source for the increased Canth accumulation is not local surface flux but a supply of Canth-rich surface water from low latitudes by the intensified Kuroshio. Such improvements would give more fidelity to projections of regional subsurface water acidification rate. Overall, the use of a fine horizontal resolution ocean model can be recommended in next-generation projection experiments with carbon cycles for the North Pacific Ocean. Storage of anthropogenic carbon in the western North Pacific Ocean is improved by using a 10-km horizontal resolution ocean modelImprovement in the formation and distribution of the North Pacific subtropical mode water is a keyUse of 10-km class high horizontal resolution ocean models is recommended as part of Earth system models with carbon cycles