Revisiting Cloud Radiative Heating and the Southern Annular Mode

Abstract

Cloud-circulation interactions have a potentially large but uncertain influence on regional climate. Here we use satellite observations to investigate relationships between atmospheric cloud radiative heating and hemispheric-scale shifts in the Southern Hemisphere extratropical jet stream, as represented by the Southern Annular Mode. In contrast to a previous study, we find that poleward jet shifts cause bottom-heavy heating anomalies. The heating anomalies arise from two distinct mechanisms: First, poleward jet shifts promote anomalous large-scale subsidence equatorward of the mean jet latitude. This increases the fraction of low clouds that are exposed to space, thereby enhancing lower-tropospheric radiative cooling. Second, deep and multi-layer clouds in extratropical cyclones shift poleward with the jet, causing radiative heating anomalies throughout the troposphere. The bottom-heavy structure of the heating anomalies occurs because low clouds strongly emit radiation. These results establish new observational benchmarks for understanding extratropical cloud-circulation interactions.