Constraints on Southern Ocean Shortwave Cloud Feedback from the
Hydrological Cycle
Abstract
Shifts in Southern Ocean (SO, $40 - 85^{o}S$) shortwave cloud
feedback ($SW_{FB}$) 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 $SW_{FB}$, we use a simplified
physical model to connect SO $SW_{FB}$ 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 $SW_{FB}$ in the SO. Observations of SW radiation
fluxes, LWP, and CCFs from reanalysis are used to constrain the SO
$SW_{FB}$. The response of SO LWP to warming is constrained to
$2.76\ -\ 4.19$ $g\
m^{-2}\ K^{-1}$, relative to a GCM prior of
$0.64\ -\ 9.33$ $g\
m^{-2}\ K^{-1}$. The susceptibility of albedo
to LWP is constrained to be $0.43\ -\
0.90$ $ (kg\ m^{-2})^{-1}$, relative to
$0.30\ -\ 3.91$ $(kg\
m^{-2})^{-1}$. The overall constraint on the contribution of
SO to global mean $SW_{FB}$ is $-0.168\
-\ 0.051$ $W\
m^{-2}\ K^{-1}$, relative to
$-0.277\ -\ 0.270$ $ W m^{-2}
K^{-1}$. In summary, observations suggest SO $SW_{FB}$ is
less likely to be as extremely positive as predicted by some CMIP6 GCMs,
but more likely to range from moderate negative to weakly positive.