Robust Effects of Ocean Heat Uptake on Radiative Feedback and Subtropical Cloud Cover: A Study Using Radiative Kernels

Abstract:

Transient climate change depends on both radiative forcing and ocean heat uptake. A substantial fraction of the inter-model spread in transient warming under future emission scenarios can be attributed to differences in “efficacy” of ocean heat uptake (suppression of surface warming per unit energy flux into the deep oceans relative to CO2 forcing). Previous studies have suggested that this efficacy depends strongly on the spatial pattern of ocean heat uptake. Rose et al (2014) studied this dependence in an ensemble of aquaplanet simulations with prescribed ocean heat uptake, and found large differences in model responses to high-versus low latitude uptake. In this study we use radiative kernel analysis to accurately partition these responses into feedbacks associated with temperature, water vapor and clouds. We find large and robust differences in both clear-sky longwave feedbacks and shortwave cloud feedbacks, with high-latitude uptake exciting substantially more positive feedback (higher efficacy) than low-latitude uptake. These robust clear-sky longwave feedbacks are particularly associated with lapse rate feedbacks, implying differences in large-scale circulation patterns associated with ocean heat uptake. A particularly surprising result is the robustness across several independent GCMs of the differences in subtropical low cloud feedback (positive under high-latitude uptake, strongly negative under tropical uptake). We trace these robust differences to thermodynamic constraints associated with lower-tropospheric stability and boundary layer moisture. Our results imply that global cloud feedback under global warming may be partly modulated by the spatial pattern of ocean heat uptake.