The Global Influence of Southern Ocean Shortwave Radiation Biases

Abstract:

One large long-standing bias in global climate models is excessive absorbed shortwave radiation (ASR) over the Southern Ocean. We evaluate this Southern Ocean ASR bias and its climate implications using experiments from one climate model (CESM-CAM5). We hypothesize that the excessive ASR bias in our model is due in part to insufficient supercooled liquid water in parameterized low-level shallow convective clouds. To test this hypothesis, we implement cloud phase diagnostics and compare the climate model with CALIPSO spaceborne lidar observations following Cesana et al. 2013. Guided by the satellite observation comparisons, we complete model experiments in which we increase the amount of supercooled liquid in the detrained condensate from the shallow convection scheme. Finally, we assess the mean state and climate feedback implications of increased supercooled liquid water in the modeled clouds. Our findings show that increasing supercooled liquid water in shallow convective clouds reduces Southern Ocean ASR biases, shifts tropical rain belts, and can alter global mean radiation balance sufficiently to cause runaway global cooling. In summary, Southern Ocean ASR has an appreciable influence on global radiation balance and is an important but challenging target for climate model bias reduction.