Pacific cold-tongue bias in CMIP5/6 linked to shifts in extratropical subduction zones

The Equatorial Pacific Cold Tongue (CT) bias is a systematic sea surface temperature (SST) bias, persisting throughout all generations of comprehensive climate models. Recent works suggested that extratropical SST biases contribute to the CT bias, mediated by the wind-driven ocean overturning circulation in the Pacific. However, as shown here, the northern and southern hemispheric "eastern exchange windows", which are the dominant extratropical sources for water upwelling in the Equatorial Pacific, are not characterized by cold SST biases. Here we explore these links using Lagrangian back trajectory analysis in 32 models participating in phases 5 and 6 of the coupled model intercomparison project (CMIP5/6), and four ocean reanalyses. Consistent with previous works, Equatorial Pacific upwelling is found to be sourced primarily from late-winter subduction in the extratropical eastern exchange windows. We also find that climatological dynamical fields dominate the probability distribution maps linking extratropical subduction with upwelling in the CT. Variations across CMIP5/6 models and between CMIP5/6 models and reanalyses consistently point to poleward shifts of the eastern exchange windows into colder extratropical waters as a likely key contributing factor to the CT bias, which, due to the strong meridional SST gradients in the extratropics, can drive CT biases regardless of extratropical SST biases. Consistent with previous works, cold biases in the northern extratropics, which partially overlap with the north-eastern exchange window, also contribute to the CT bias. Trajectory duration varies considerably across models and between models and reanalyses, but is not consistently related to the CT bias.