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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 semipermanent 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, E3SMv1s BL thickness biases most affect entrainment here. Tropical BLT biases appear driven by atmosphere biases, mainly through the effect of precipitation minus evaporation on mixed layer depth. At higher latitudes BL thickness biases are dominated by thermocline depth errors related to ocean circulation and vertical mixing.

Plain Language Summary Most regions of the Earth's oceans exhibit a thermocline, separating relatively warm surface water from colder water below. In some regions, salinity varies sharply within the warm layer, displaying a fresh layer at the surface and a salty warm layer, termed a barrier layer, between the surface layer and the thermocline. Here we assess barrier layers in three Earth system models, focusing on the Energy Exascale Earth System Model. We show the following: Earth system models can capture barrier layers, albeit with errors in thickness; barrier layers affect exchange of water and heat between the surface and the thermocline in the tropics, but not at midlatitudes; and barrier layer model errors are not purely due to the ocean model component but are caused by several model components (ocean, atmosphere, land, and river runoff) and interactions between them.