Abstract: Extratropical forcing exerts a strong influence on the tropical Pacific climate, with the Southern Ocean emerging as a key player. Based on analyses of a set of more than 50 CMIP5&6 models , this study found that the intermodel spread in future equatorial Pacific climate could be largely traced to the background Southern Ocean (SO) salinity that dominates the spread in ocean stratification relative to ocean temperature. Higher salinity leads to deeper SO pycnocline and therefore deepened warm water ventilation under warming scenarios, which further mitigates surface warming. The SO salinity impact on local surface warming affects equatorial East Pacific surface warming via their teleconnection demonstrated in previous studies. Based on the link between SO salinity and equatorial Pacific warming, we develop a new emergent constraint on the sea surface temperature (SST) zonal gradient in the equatorial Pacific and Walker Circulation strength. The typical fresh SO biases in CMIP models imply the overestimated weakening of SST zonal gradient and Walker Circulation under climate warming by models. The link between SO salinity and equatorial Pacific is weaker in CMIP5 relative to CMIP6, primarily driven by that CMIP5 models typically underestimate shortwave cloud feedback in western South America and therefore the teleconnection strength between SO and Southeast Pacific. Reducing SO salinity biases in climate models provides a potentially useful pathway for improved simulation and projection in air-sea interactions and Pacific climate.

Southern Ocean Salinity; Equatorial Pacific SST Zonal Gradient; Walker Circulation;Cloud Feedback;CMIP Models