Dependence of cloud feedback on Southern Ocean salinity

The uncertainty in equilibrium climate sensitivity (ECS) directly projected by climate models has remained persistently unchanged for the past four decades, with cloud feedback as a primary source of the uncertainty. We propose 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 heat into the deeper ocean, reducing the surface warming in the Southern Ocean. This acts to increase lower tropospheric stability which, combined with reduced surface warming, induces a more negative shortwave cloud feedback, both locally and over remote tropical and subtropical oceans. This remote impact is related to enhanced northward advection of Southern Ocean surface waters associated with climatological southeasterly trade winds and its strengthening, 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 strongly positive cloud feedback and high ECS due to their fresh bias in the Southern Ocean. A series of climate model experiments are conducted to further understand the link between salinity and cloud feedback. Our results highlight the potential of improved simulation of cloud feedback through dynamical constraints of climate models with salinity observations.