Constraints on Southern Ocean Shortwave Cloud Feedback From the Hydrological Cycle

Shifts in Southern Ocean (SO, 40-85 degrees S) shortwave cloud feedback (SWFB) toward more positive values are the dominant contributor to higher effective climate sensitivity (ECS) in Coupled Model Intercomparison Project Phase 6 (CMIP6) models. To provide an observational constraint on the SO SWFB, we use a simplified physical model to connect SO SWFB with the response of column-integrated liquid water mass (LWP) to warming and the susceptibility of albedo to LWP in 50 CMIP5 and CMIP6 GCMs. In turn, we predict the responses of SO LWP using a cloud-controlling factor (CCF) model. The combination of the CCF model and radiative susceptibility explains about 50% of the variance in the GCM-simulated SWFB in the SO. Observations of SW radiation fluxes, LWP, and CCFs from reanalysis are used to constrain the SO SWFB. Observations suggest a SO LWP increase in response to warming and albedo susceptibility to LWP that is on the lower end relative to GCMs. The overall constraint on the contribution of SO to global mean SWFB is -0.168 to +0.051 W m-2 K-1, relative to -0.277 to +0.270 Wm-2 K-1. In summary, observations suggest SO SWFB is less likely to be as extremely positive as predicted by some CMIP6 GCMs, but more likely to range from moderately negative to weakly positive. Previous studies suggest that SO clouds reflect more sunlight in response to global warming and more strongly cool the planet - a negative shortwave cloud feedback (SWFB). The SO SWFB in the latest generation of global climate models (GCMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) has shifted toward more positive values, leading to the larger predicted temperature responses to greenhouse gas increases in these GCMs. In this study, we examine if this more positive SWFB is consistent with observations. We connect the effect of SO clouds on reflected sunlight with the predicted response of cloud liquid content to global warming. The linkage between cloud liquid water content and large-scale meteorology is applied to predict this cloud liquid response. Satellite observations of reflected sunlight, cloud liquid, and observations of large-scale meteorology are applied to constrain the SO SWFB for 50 CMIP5 and CMIP6 GCMs. The results suggest that SO cloud liquid will increase with warming around the average of predictions of 50 GCMs. Satellite records suggest that SO SWFB is moderately negative to weakly positive, instead of extremely positive or negative as suggested by some GCMs. Southern Ocean liquid water path increased over the past two decades due to enhanced moisture convergence Enhanced moisture convergence contributes to a negative cloud feedback in the Southern Ocean Across global climate models, the sensitivity of upwelling shortwave to cloud opposes the sensitivity of cloud to moisture convergence