The Role of Synoptic Eddies in Driving Extreme Weather in the Midlatitudes Climate

I investigate the role of synoptic-scale eddies in driving the extreme weather of the midlatitudes, with particular attention to how diabatic processes---surface heat fluxes, latent heat release in extratropical cyclones, and latent heating from recurving tropical cyclones---modulate the eddies that ultimately determine weather and climate impacts. Three studies are presented, each focused on a different facet of the synoptic-eddy population. First, I develop a localized, Lagrangian-mean moist static energy (MSE) framework that combines objective cyclone and anticyclone tracking with a storm-centered MSE budget. Applied to reanalysis, the framework decomposes storm-track energy transport into feature footprint and per-feature transport efficiency, and uses this decomposition to explain why intense extratropical cyclones exhibit a substantially larger seasonal cycle than weak cyclones in both hemispheres. In the Northern Hemisphere, this enhanced seasonality is amplified by zonal MSE advection tied to land--ocean energy contrasts; in the Southern Hemisphere, surface heat fluxes damp weak cyclones but amplify intense cyclones through an offsetting interplay of footprint and transport efficiency. I then address how eddies change under the influence of latent heating, modulating atmospheric blocking statistics in a moist two-layer quasi-geostrophic model. Over two hundred long integrations span an extensive moisture--jet parameter sweep. Blocking frequency is a non-monotonic function of latent heating at fixed jet speed, the result of a competition between latent heating's positive influence on eddy injection of wave activity (the dynamic effect) and its direct condensational damping of upper-layer wave activity. The zonal-mean wave-activity budget of the model agrees with that of reanalysis when averaged over the full troposphere. Finally, I show that latent heating can be a net damping term in a large-scale wave-activity budget while still acting as a large local wave source with important downstream weather impacts. I apply the column-mean local wave activity (LWA) budget, together with the time-dependent jet carrying capacity, to one of the most extreme latent-heat-releasing eddies: recurving tropical cyclones undergoing extratropical transition. The analysis is applied to reanalysis and to two ten-year, 15-km ensembles of the Model for Prediction Across Scales-Atmosphere (MPAS-A) representing present-day and end-of-century climates. Latent heating is identified as the dominant non-advective LWA source along the recurving track in both basins, in both reanalyses, and in both MPAS-A ensembles. The basin asymmetry between the western North Pacific and the North Atlantic is controlled by the jet carrying capacity (the North Atlantic jet has 54% more ``headroom''), whereas within the western North Pacific the discriminator between large- and small-downstream-Rossby-wave-packet storms is the latent-heating LWA source itself (a factor of 2.7 between the two pools, against essentially identical carrying capacities). In the future MPAS-A ensemble, the storm-track latent-heating source rises by only 7% while the downstream-waveguide carrying capacity rises by 62%, so the dominant climate-change signal is an enhanced jet capacity rather than an enhanced source. Together, the three studies show that synoptic eddies are best understood as energetic intermediaries between large-scale, diabatically modulated background flows and the localized extreme weather they sustain, and that climate-change projections of midlatitude weather extremes should track both changes in the large-scale flow and changes in the behavior of individual eddies.