Understanding the Varied Influence of Midlatitude Jet Position on Clouds and Cloud-Radiative Effects in Observations and Global Climate Models

Over the 21st century, the midlatitude jet streams and storm tracks are projected to shift poleward with increasing greenhouse gases in the atmosphere, and it is likely that these shifts in the large-scale circulation will have direct linkages with cloud feedbacks at midlatitudes. In this study, by examining interannual variability in ERA-Interim reanalysis data and ISCCP and CERES satellite observations, we aim to provide a better understanding of the dynamical mechanisms responsible for the changes in midlatitude clouds and their radiative effects that occur in conjunction with meridional shifts in the jet streams over the Southern Ocean, North Atlantic Ocean, and North Pacific Ocean. When the midlatitude 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-topped storm track clouds and their associated longwave cloud-radiative effects (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. Because shortwave CRE depends on the reflection of solar radiation by clouds in all layers, the shortwave cooling effects of midlatitude clouds increase with both upward vertical velocity anomalies and positive EIS anomalies. Over midlatitude 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. Global climate models generally capture the observed anomalies associated with midlatitude jet shifts. However, there is large inter-model spread in the shortwave CRE anomalies, with a subset of models showing a large shortwave cloud-radiative warming over mid-latitude oceans with a poleward jet shift. In these models, midlatitude shortwave CRE is sensitive to vertical velocity perturbations, but the observed sensitivity to EIS perturbations is underestimated. Consequently, these models might incorrectly estimate future midlatitude cloud feedbacks in regions where appreciable changes in both vertical velocity and EIS are anticipated to occur over the 21st century.