01 December 2016

Global Climate Impacts of Fixing the Southern Ocean Shortwave Radiation bias in the Community Earth System Model (CESM)

Science

This study examines the mechanisms responsible for changes in mid-latitude clouds and cloud-radiative effects that occur in conjunction with poleward shifts in the jet streams over the North Atlantic, North Pacific, and Southern Oceans. When the mid-latitude jet shifts poleward, extratropical cyclones and their associated upward vertical velocity anomalies closely follow. As a result, a poleward jet shift contributes to a poleward shift in high clouds and their associated longwave CRE. However, when the jet shifts poleward, downward vertical velocity anomalies increase equatorward of the jet, contributing to an enhancement of the boundary layer inversion strength (EIS) and an increase in low cloud amount there. 

Impact

Because shortwave CRE depends on the reflection of solar radiation by clouds in all layers, the shortwave cooling effects of mid-latitude clouds increase with both upward vertical velocity anomalies and positive EIS anomalies. Over mid-latitude oceans where a poleward jet shift contributes to positive EIS anomalies but downward vertical velocity anomalies, the two effects cancel, and net observed changes in shortwave CRE are small.  

Summary

Climate models capture the observed anomalies associated with jet shifts. However, there is large inter-model spread in the shortwave CRE anomalies, with some models showing a large shortwave cloud-radiative warming over mid-latitude oceans with a poleward jet shift. In these models, mid-latitude shortwave CRE is sensitive to vertical velocity perturbations, but the observed sensitivity to EIS perturbations is underestimated. Consequently, these models might incorrectly estimate future mid-latitude cloud feedbacks in regions where appreciable changes in both vertical velocity and EIS are projected.

Contact
Brian Medeiros
National Center for Atmospheric Research (NCAR)