Synoptic perspective on the conversion and maintenance of local available potential energy in extratropical cyclones

Abstract

The global atmospheric circulation is maintained by the conversion of available potential energy (APE) into kinetic energy. At midlatitudes, this conversion occurs to a large extent in extratropical cyclones through baroclinic instability. Although kinetic energy is easily defined locally, APE is typically defined as a global integral. Therefore, local APE conversion is not well understood. Here, we investigate local APE conversion within the North Atlantic storm track using ERA5 reanalysis data. We utilize a recently introduced formulation of APE, which is exact and defined locally for individual air parcels. First, we explore APE conversion during a period of rapid cyclogenesis, which we then extend to a climatology of extratropical cyclones. Our results indicate that the synoptic upper-level flow determines the distribution of high APE values, which are primarily located in the high-latitude upper troposphere. We show that APE is converted locally into kinetic energy by descending air parcels within the ageostrophic circulation, for example, induced by a jet streak upstream of an extratropical cyclone. The local APE originates not only from advection from the polar, upper-tropospheric APE reservoir, but also from local generation by vertical motion. In fact, the net baroclinic conversion of APE to kinetic energy is the result of much larger positive and negative local contributions. Thus, the global Lorenz energy cycle is more complex on synoptic scales. In addition, we show that surface heat fluxes resulting from air-sea interactions and latent heat release act as diabatic sinks for APE. However, the effect of surface heat fluxes is small compared to the conversion of APE to kinetic energy, as little APE is located in the mid-latitude lower troposphere. In summary, the study shows that the local APE perspective allows the energetics of North Atlantic extratropical cyclones to be better understood in terms of local APE advection as well as adiabatic (ascent and descent) and diabatic effects.