Linking midlatitude transient eddy moist static energy transport and extratropical cyclones

Abstract

Earth's equator-to-pole net radiation gradient is counteracted by poleward atmospheric energy transport. In the extratropics, the largest contribution to this poleward flux can be attributed to variability on the timescale of weather systems. Even though the radiative imbalance has been argued not to strongly differ in a warmer climate, the partitioning of heat flux into moist and dry components is expected to change due to a moister atmosphere. On the synoptic scale, an increase in moisture and associated latent heat release enhances the intensification of cyclones, prolongs cyclone lifetimes, and also strengthens downstream anticyclones. Conversely, latent heating locally alters static stability and thereby affects projected trends in baroclinicity, which in turn vary across height due to different trends in temperature. Given that these drivers of cyclones and thereby storm tracks are subject to change and the resulting interplay is complex, isolating the influence of changes in latent heating on cyclone number and storm track intensity is not straight-forward. By combining the global moist static energy (MSE) budget perspective with cyclone numbers and other feature-based characteristics such as intensity and intensification, we aim to better understand the role of latent heat transport and release on midlatitude storm tracks. In particular, we ask: How are changes in zonal and time mean poleward transient eddy MSE flux and its divergence related to changes in cyclone number and intensities? We start investigating the linkage between MSE fluxes and surface cyclones in reanalysis data by calculating cyclone composites. These analyses reveal that in general, poleward flux in the vicinity of low-pressure systems reaches its maximum during the intensification phase and drops after cyclones reaching mature stage. Furthermore, MSE flux peaks slightly equatorward and downstream of the cyclone center. In the mean picture, this signal can be related to warm-sector flux along the cold front, also indicating that the footprint of cold-sector flux is not as dominant. Further separating dry and moist flux components is expected to reveal additional insight into how heat transport is distributed across cyclones. These diagnostics can readily be applied to climate model data and idealized aquaplanet simulations, which we make use of to reduce the complexity and single out the effect of individual drivers of storm track changes.