What are the Causes of Equatorial Pacific Cold Sea Surface Temperature Bias?
Scientists at Lawrence Livermore National Laboratory, along with collaborators from the National Center for Atmospheric Research, identify possible causes for the large cold SST bias seen in many global climate model (GCM) or earth system model (ESM) simulations over the equatorial Pacific. A realistic representation of the mean state and annual cycle of equatorial Pacific SSTs is key to an accurate simulation of ENSO.
Diagnosing causes of systematic SST errors in long-term, fully coupled GCM or ESM simulations is challenging because of the non-linear feedback processes. In this study, we diagnose the causes of SST bias over the equatorial Pacific by applying an initialized coupled hindcast approach to study the initial drift of SST from the seasonal hindcasts and attribute the causes of model bias. We demonstrate the usefulness of the coupled hindcast approach, which can better isolate the possible causes of model errors without performing model simulations for hundreds of years.
In observations, eastern equatorial Pacific SSTs exhibit a warm phase during boreal spring and a cold phase during late boreal summer to fall. The CESM1 climatology shows a cold bias during both warm and cold phases. In our hindcasts, the cold bias during the cold phase develops in less than six months, whereas the cold bias during the warm phase takes longer to emerge. The fast-developing cold phase cold bias is associated with too strong vertical advection and easterly wind stress over the eastern equatorial region. The antecedent boreal summer easterly wind anomalies also appear in atmosphere-only simulations, indicating that the errors are intrinsic to the atmosphere component. For the slower- developing warm phase cold bias, we find that the too cold SSTs over the equatorial region are associated with a slowly-evolving upward displacement of subsurface ocean zonal currents and isotherms that can be traced to the ocean component.