  The International Ocean Model Benchmarking (IOMB) software package is a new
community resource used here to evaluate surface and upper ocean variables from
CMIP5 and CMIP6 Earth System Models (ESMs) Our analysis reveals general
improvement in the multi-model mean of CMIP6 compared to CMIP5 for most of the
variables we examined including surface nutrients, temperature, and salinity.
We find that both CMIP5 and CMIP6 ocean models underestimate anthropogenic
carbon dioxide uptake after the 1970s. For the period of 1994 to 2007, the
multi-model mean from CMIP6 yields a mean cumulative carbon uptake of 27.2
+-2.2 Pg C, which is about 15% lower than the 32.0+-5.7 Pg C estimate derived
from two sets of observations. 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{\deg}S). For the few models that provided
simulations of chlorofluorocarbon (CFC), we demonstrate that regions with
negative anthropogenic DIC biases coincide with regions that have a negative
bias in CFC concentrations. This relationship suggests that underestimates of
anthropogenic carbon storage in some models originates, in part, from weak
transport between the surface and interior ocean. To examine the robustness of
this attribution across the full suite of CMIP5 and CMIP6 models, we examined
the vertical temperature gradient between 200 and 1000m as a metric for
stratification and exchange between the surface and deeper waters. On a global
scale across different models and different MIPs we find a linear relationship
between the bias of vertical temperature gradients and the bias in
anthropogenic carbon uptake, consistent with the hypothesis that model biases
in the ocean carbon sink are related to biases in surface-to-interior
transport.
