Revisiting Cloud Radiative Heating and the Southern Annular Mode

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. Plain Language Summary Atmospheric circulation controls cloud formation, and clouds change atmospheric radiative and latent heating, thereby affecting the circulations in which they form. These two‐way interactions affect regional climate by a potentially large but uncertain amount. Here we use satellite data to investigate relationships between cloud radiative heating and poleward shifts of the extratropical jet stream in the Southern Hemisphere. We show that poleward jet shifts cause bottom‐heavy radiative heating anomalies throughout the extratropics. These heating anomalies result from an increase in the low‐cloud fraction equatorward of the mean jet‐stream latitude and a poleward shift of clouds in extratropical cyclones. The bottom‐heavy heating structure occurs because low‐level clouds strongly emit radiation. These observed relationships advance our understanding of cloud‐circulation interactions in the extratropics. Key Points Poleward shifts of the Southern Hemisphere extratropical jet stream change atmospheric cloud radiative heating through two key mechanisms The fraction of low clouds that are exposed to space increases equatorward of the mean jet latitude, enhancing local radiative cooling Clouds in extratropical cyclones shift poleward, causing radiative heating anomalies throughout the troposphere