Changes in the Local Eddy Energetics of Extratropical Storm Tracks

Traveling atmospheric disturbances arrange into storm tracks that determine midlatitude weather, but changes in temperature gradients drive competing influences on storm track activity over time. We use local eddy energetics on the ERA5 reanalysis during 1980–2020 to determine local trends in the conversion and transfer of eddy kinetic energy (EKE) for synoptic-scale eddies. EKE increases most rapidly in the Southern Hemisphere throughout the troposphere and over the entire year. In the Northern Hemisphere, EKE is increasing over the North Atlantic and North Pacific at pressures below 300 hPa but only during boreal winter and spring; EKE is decreasing during boreal summer and fall. Most EKE changes correspond with trends in baroclinic energy conversion upstream of storm tracks. Barotropic energy conversion of EKE to the mean-flow becomes more intense downstream of the largest EKE, with conversion to long-period transient eddies playing a secondary role. The increased baroclinic conversion in the Southern Hemisphere appears related to upper-level tropical temperature increases. In the Northern Hemisphere, increased baroclinic energy conversion is enabled by a combination of increased vertical heat fluxes and a region of temperature increases within 30-60°N, most prominent during boreal winter.