In this study, we use an ensemble seasonal hindcast approach to investigate the development of the equatorial Pacific cold SST bias and its characteristic annual cycle in the Community Earth System Model version 1 (CESM1). Six-month long hindcasts with CESM version 1.1.2 covering the period 2001-2005 are performed, with initial conditions derived from ERA-Interim for the atmosphere and from NCAR-DART for the ocean. In observations, the eastern equatorial Pacific SSTs exhibit a warm phase during boreal winter to spring and a cold phase during boreal summer to fall. The CESM1 climatology shows a cold bias during both warm and cold phases. Our results reveal that in the hindcasts, the cold bias during the cold phase develops quickly, whereas the cold bias during the warm phase takes longer than six months of leadtime to emerge. An upper ocean heat budget analysis of the cold phase cold bias in the hindcasts shows that the anomalous cooling comes from a too strong vertical advection. The vertical advection bias is associated with boreal summer easterly wind stress anomalies, which emerge as early as the first two months of leadtime and precede the onset of the cold SST bias. Using an ocean-only hindcast with enhanced zonal wind over the eastern equatorial Pacific, we demonstrate that the cold phase cold bias develops as a response to too strong zonal winds. We further explore the mechanisms behind the wind bias and test its sensitivity to the representation of moist convection.

[This study is funded by the Regional and Global Climate Modeling and Atmospheric System Research Programs of the U.S. Department of Energy as part of the Cloud-Associated Parameterizations Testbed. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-755457]