Maofeng Liu , Brian Soden, Gabriel Vecchi, Haozhe He, Chenggong Wang

The uncertainty in equilibrium climate sensitivity (ECS) has remained persistently unchanged for the past four decades1–4, with cloud feedback3,5–11 as a primary source of the uncertainty. Here we show that a key component of this uncertainty is rooted in the impact of base-state Southern Ocean salinity on cloud feedback. Sea surface salinity in the sinking zone of the Southern Ocean (45o-60oS) statistically explains half of the inter-model variance in shortwave cloud feedback from a set of 40 Coupled Model Intercomparison Project Phase 6 climate models. Models with greater salinity in this region sequester more heat in the deep ocean12, reducing the surface warming in the Southern Ocean. This acts to increase lower tropospheric stability13 which, combined with reduced surface warming, induce a more negative shortwave cloud feedback14,15, both locally and over remote tropical and subtropical oceans. This remote impact16–19 is related to enhanced northward advection of Southern Ocean surface waters associated with the strengthening of the southeasterly trade winds, especially in the Southeastern Pacific, transporting the surface warming differences to subtropical oceans. Using observed surface salinity as an emergent constraint argues against models with a strongly positive cloud feedback and high ECS due to their fresh bias in the Southern Ocean20. Our results highlight the potential of improved simulation of cloud feedback through dynamical constraint of climate models with salinity observations.