Ocean barrier layers (BLs) separate the mixed layer from the top of the thermocline, and are able to insulate the mixed layer from entrainment of cold thermocline water. Here, we provide the first global BL assessment in E3SMv1 and two other earth system models. Compared to observations, models reproduce the global distributions as semi‐permanent features in some tropical regions and seasonal features elsewhere. However, model BLs are generally too thin in tropical regions and too thick in higher latitudes. BLs' ability to insulate the ocean surface from entrainment of cold thermocline water is most apparent in the tropics. Thus, E3SMv1's BL thickness (BLT) biases most affect entrainment here. Tropical BLT biases appear driven by atmosphere biases, mainly through the effect of precipitation minus evaporation (P‐E) on mixed layer depth (MLD). At higher latitudes BLT biases are dominated by thermocline depth errors related to ocean circulation and vertical mixing.