Subtropical Eastern North Pacific SST Bias in Earth System Models

This study systematically evaluates the warm sea surface temperature (SST) bias in the Subtropical Eastern North Pacific, a problem plaguing most Coupled Model Intercomparison Project Phase 6 models, using the Energy Exascale Earth System Model version 1 (E3SM). In the model at its standard resolution (1 degrees atmosphere, 30-60 km ocean), the SST bias, exceeding several degrees, is mainly concentrated along the coast between 25 degrees N and 40 degrees N. In the high-resolution (0.25 degrees atmosphere, 18-6 km ocean) version of the model, the nearshore SST bias improves considerably with a better representation of coastal upwelling. However, the offshore SST bias, approximately centered at 125 degrees W and 25 degrees N, is relatively stronger in the high-resolution version. To better understand the offshore warm bias in the model, a mixed-layer heat budget analysis is performed. While errors in surface radiative fluxes occur at both resolutions, positive biases in horizontal heat advection also play a role in the SST bias at high-resolution. Analysis of HighResMIP models indicates that the shift in the location of the prominent SST bias from nearshore to offshore with an increase in model spatial resolution, is not native to E3SM alone.