Influences of meso-scale and submeso-scale variability on simulated subtropical mode water formation: comparing two different grid-resolutions

Abstract Coarse resolution climate models (1° horizontal grid spacing) and even high-resolution eddy-resolving ocean models (1/10° grid spacing) fail to accurately simulate eddy activity and suffer from deep mixed layer depth (MLD) biases in the Kuroshio Extension (KE) region. Higher-resolution models (1/30° grid spacing) have been successful in reproducing eddy seasonality and MLD consistent with altimetry and Argo float observations, but Subtropical Mode Water (STMW) variability has yet to be investigated in such high-resolution models. In a limited 1-year study, we compare the outputs of two high-resolution regional ocean models, one that resolves only mesoscale variability (1/10°), and one that permits the majority of the submesoscale variability (1/30°). We find that there is no significant difference in the annual STMW volume between the models despite a shallower MLD and narrower outcrop area during the late winter formation in the 1/30° simulation. We find that 1/30° anticyclonic eddies are stronger and extend deeper into the main thermocline than 1/10° eddies. They also carry up to 25% more STMW, have increased vertical tracer circulation, and transmit low potential vorticity STMW signal into the thermocline more efficiently through May, possibly explaining the maintained STMW volume. Submesoscale vertical buoyancy flux in the 1/30° simulation is shown to be consistent with increased potential vorticity and reduced STMW volume in case studies of KE frontal systems and select mesoscale eddies. The combined mesoscale and submesoscale effect on STMW formation and transport reveals the importance of resolving eddy processes in climate models.