Southern Ocean Barrier Layer Assessment in CMIP6 Models with Argo: Historical Bias and Possible Reasons

The barrier layer (BL) influences air–sea heat exchange and plays a key role in regulating global climate change. Here, we provide a detailed assessment of the ability of models contributing to the Coupled Model Intercomparison Project phase 6 (CMIP6) to represent the BL in the Southern Ocean. Our findings indicate that, while the CMIP6 models generally reproduce the seasonal cycle and spatial distribution of the Southern Ocean BL, the simulation results for BL thickness (BLT) remain too thin in comparison to observations. The main reason for this bias is that the models simulate a mixed layer depth that is too deep, which is associated with excessive buoyancy loss. However, in the BL core region of the South Pacific, we find that the thin BLT bias is largely due to an underestimation of the isothermal layer depth, which is caused by the unrealistic subsurface temperature structure in the models. In addition, we find that the warm sea surface temperature (SST) bias in the Southern Ocean BL core regions is related to the thin BLT bias. This may be because the thin BLT is ineffective at inhibiting the entrainment of warm Subantarctic Mode Water into the mixed layer. Our results suggest that BLT biases in the Southern Ocean should be considered to reduce the Southern Ocean SST bias in climate models.