Warm bias of sea surface temperature in Eastern boundary current regions—a study of effects of horizontal resolution in CESM

Abstract

Warm bias of modeled sea surface temperature (SST) in the eastern boundary upwelling systems (EBUS) is a ubiquitous feature in coupled climate models. This paper investigates the causes underlying this warm bias, with a focus on the effect of horizontal resolution in the atmospheric component of coupled models, by using Community Earth System Model (CESM) as an example. By breaking down the energy budget of the upper ocean, we conclude that surface net heat flux and Ekman upwelling process exert a considerable influence (over 80%) on upper ocean temperature of EBUS in CESM. Besides, the problem of underestimation of stratocumulus cloud is not present near the coast, and hence not responsible for this warm bias in CESM. On the contrary, downward shortwave radiation bias is overcompensated by longwave radiation and latent flux bias on the open ocean. Therefore, the insufficient ocean dynamic upwelling is the dominantly cause for SST warm bias. Finer horizontal resolution atmosphere component of CESM enables better representation of low-level coastal jet structure, with stronger and closer alongshore wind stress and curl leading to realistic representation of upwelling process and horizontal water mass transportation. Furthermore, low-level coastal jet is shown to be sensitive to coastal mountain topography, especially in South East Pacific region, through both thermodynamic and dynamic atmospheric processes and oceanic response. This article provides further proof of improving coupled climate models in reducing the SST biases in EBUS regions.