Search for: All records

Abstract. Recent work has shown that variability in the subtropical jet's (STJ) latitude, ϕSTJ, is not coupled to that of the Hadley cell (HC) edge, ϕHC, but the robustness of this disconnect has not been examined in detail. Here, we use meteorological reanalysis products, comprehensive climate models, and an idealized atmospheric model to determine the necessary processes for a disconnect between ϕHC and ϕSTJ in the Northern Hemisphere's December–January–February season. We find that a decoupling can occur in a dry general circulation model, indicating that large-scale dynamical processes are sufficient to reproduce the metrics' relationship. It is therefore not reliant on explicit variability in the zonal structure, convection, or radiation. Rather, the disconnect requires a sufficiently realistic climatological basic state. Further, we confirm that the robust disconnect between ϕSTJ and ϕHC across the model hierarchy reveals their differing sensitivities to midlatitude eddy momentum fluxes; ϕHC is consistently coupled to the latitude of maximum eddy momentum flux, but ϕSTJ is not. 

Abstract Atmospheric rivers (ARs), narrow intense moisture transport, account for much of the poleward moisture transport in midlatitudes. While studies have characterized AR features and the associated hydrological impacts in a warming climate in observations and comprehensive climate models, the fundamental dynamics for changes in AR statistics (e.g., frequency, length, width) are not well understood. Here we investigate AR response to global warming with a combination of idealized and comprehensive climate models. To that end, we developed an idealized atmospheric GCM with Earth-like global circulation and hydrological cycle, in which water vapor and clouds are modeled as passive tracers with simple cloud microphysics and precipitation processes. Despite the simplicity of model physics, it reasonably reproduces observed dynamical structures for individual ARs, statistical characteristics of ARs, and spatial distributions of AR climatology. Under climate warming, the idealized model produces robust AR changes similar to CESM large ensemble simulations under RCP8.5, including AR size expansion, intensified landfall moisture transport, and an increased AR frequency, corroborating previously reported AR changes under global warming by climate models. In addition, the latitude of AR frequency maximum shifts poleward with climate warming. Further analysis suggests the thermodynamic effect (i.e., an increase in water vapor) dominates the AR statistics and frequency changes while both the dynamic and thermodynamic effects contribute to the AR poleward shift. These results demonstrate that AR changes in a warming climate can be understood as passive water vapor and cloud tracers regulated by large-scale atmospheric circulation, whereas convection and latent heat feedback are of secondary importance.