Barrier Layers in E3SMv1 and Their Relationship with Atmosphere Model Biases

Barrier layers (BLs) occur in the ocean where salinity stratification causes the constant-density mixed layer to be shallower than the near-isothermal layer above the thermocline. BLs can affect ocean-atmosphere interaction by insulating the surface from colder water below the thermocline. They are a semi-permanent feature of some regions in the tropical oceans and occur seasonally in higher latitude oceans.

The representation of BLs in coupled Earth system models has not been widely explored. Here we assess the ability of the Department of Energy’s Energy Exascale Earth System Model version 1 (E3SMv1) to simulate BLs. The spatial distribution of tropical BLs in E3SMv1 shows good agreement with observations, although simulated BL thickness is generally too low. E3SMv1 correctly captures the seasonal nature of BLs in high latitudes but the simulated BLs are generally too thick and too widespread. Comparison between the fully coupled E3SMv1 and its ocean model forced by atmospheric reanalysis suggests that part of the BL bias in the tropics is caused by biases in the coupled model precipitation. Analysis of other coupled models reveals that such coupling of biases is not unique to E3SMv1. Further insight into BL biases are gained by examination of the ocean model tendency terms.