An ensemble seasonal hindcast approach is used to investigate the development of the equatorial Pacific cold sea surface temperature (SST) bias and its characteristic annual cycle in the Community Earth System Model version 1 (CESM1). 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.