What Controls the Seasonality of Poleward Energy Transport by Intense Extratropical Cyclones?

Intense extratropical cyclones shape midlatitude weather extremes, but their role in the seasonal cycle of poleward energy transport remains poorly constrained. Here we develop a localized, Lagrangian‐mean moist static energy (MSE) framework that combines objective feature tracking with the Eulerian‐mean MSE budget. The framework attributes stormtrack intensity to individual cyclones, anticyclones, and the sources and sinks of their associated heat transport. It also separates changes in cyclone footprint from changes in per‐cyclone transport efficiency. Our results show that cyclones contribute more than anticyclones to total poleward MSE transport, although both are leading‐order contributions, while the most intense 6‐hourly transport events are dominated by intense cyclones. The poleward transport by weak cyclones broadly follows the seasonal cycle of the zonal‐mean stormtrack, but intense cyclones exhibit a much larger seasonal cycle in both hemispheres. In the Northern Hemisphere, this amplification is driven by zonal MSE advection associated with land‐ocean energy contrasts. In the Southern Hemisphere, surface heat fluxes damp the seasonality of weak cyclones but not that of intense cyclones, whose larger winter footprint offsets the opposite‐phased response of the local transport efficiency. These results show that intense cyclones are not merely stronger samples of the mean stormtrack. Their seasonality is instead controlled by localized energy pathways that are hidden in zonal‐mean diagnostics.