Evaluation of the CMIP models with the International Ocean Model Benchmark tool: Rates of contemporary ocean carbon uptake linked with vertical temperature gradients and transport to the ocean interior

The International Ocean Model Benchmarking (IOMB) software package is a new community resource, used here to evaluate integrated anthropogenic carbon uptake and surface and upper ocean biogeochemical variables from the Coupled Model Intercomparison Project (the CMIP5 and the CMIP6) Earth System Models (ESMs). IOMB generates novel plots and tables for comparing multiple model output against numerous observational datasets. Our analysis reveals some improvement in the multi-model mean from the CMIP5 to CMIP6 for most examined variables. We find that both the CMIP5 and the CMIP6 ocean models underestimate anthropogenic CO2 uptake after the 1970s. From 1994 to 2007, the multi-model mean from CMIP6 yields a cumulative anthropogenic carbon uptake of 27.2 ± 2.2 Pg C, about 15% lower than the 32.0 ± 5.7 Pg C from observation-based estimates. Negative biases in the change in anthropogenic carbon inventory exist in the northern North Atlantic and at mid-latitudes in the southern hemisphere (30-60°S). With chlorofluorocarbon (CFC) available for a few models, we demonstrate that negative anthropogenic DIC biases coincide with negative biases in CFC concentrations. This suggests the underestimation of anthropogenic carbon storage originates, in part, from weak exchange between the surface and interior ocean. To examine the robustness of this attribution across the full suite of CMIP5 and CMIP6 models, we calculate the vertical temperature gradient between 200 and 1000m as a metric for global stratification and exchange between the surface and deeper waters. We find a linear relationship between the bias of vertical temperature gradients and the bias in global anthropogenic carbon uptake, consistent with the hypothesis that model biases in uptake are related to low biases in surface-to-interior exchange by physical processes. Our analysis shows that a low rate of CO2 uptake is a common feature of most of the CMIP5 and the CMIP6 models. The weak ocean CO2 uptake implies some overestimation of the climate warming trend, in model projections forced by CO2 emissions, rather than prescribed atmospheric CO2 concentrations.