Enhanced Upper Ocean Warming Projected by the Eddy-Resolving Community Earth System Model

Ocean warming is a key factor impacting future changes in climate. Here we investigate vertical structure changes in globally averaged ocean heat content (OHC) in high- (HR) and low-resolution (LR) future climate simulations with the Community Earth System Model (CESM). Compared with observation-based estimates, the simulated OHC anomalies in the upper 700 and 2,000 m during 1960-2020 are more realistic in CESM-HR than -LR. Under RCP8.5 scenario, the net surface heat into the ocean is very similar in CESM-HR and -LR. However, CESM-HR has a larger increase in OHC in the upper 250 m compared to CESM-LR, but a smaller increase below 250 m. This difference can be traced to differences in eddy-induced vertical heat transport between CESM-HR and -LR in the historical period. Moreover, our results suggest that with the same heat input, upper-ocean warming is likely to be underestimated by most non-eddy-resolving climate models. With the rise in anthropogenic emissions, the ocean has absorbed over 90% of greenhouse gas-related heat, resulting in well-known ocean warming. This warming is the main cause of severe deoxygenation, coral bleaching, and sea-level rise, among others. Furthermore, the upper ocean experiences more warming than the deep ocean, which leads to strengthened ocean stratification. On a global scale, the heat into the ocean is vertically redistributed by a downward mean-flow-induced heat transport, upward eddy-induced heat transport, and vertical turbulent mixing. In this study, we compare vertical structures of ocean heat uptake in response to anthropogenic forcing between high (similar to 10 km) and low horizontal resolution (similar to 100 km) future climate simulations. We find that, with similar heat input at the ocean surface, the high-resolution simulations exhibit more warming in the upper 250 m of the ocean than their low-resolution counterparts. This difference is caused by the different representations of vertical heat transport by mesoscale ocean eddies in high- and low-resolution models. Simulated upper-ocean ocean heat content anomalies are more realistic in high-resolution simulations than in low-resolution counterpartsWith similar ocean surface heating, high-resolution simulations project stronger upper-ocean warming than low-resolution simulationsFuture changes in vertical heat transport depend on the representation of the mean ocean states at present day