A synoptic perspective on Northern Hemisphere available potential energy collapses

Abstract

The atmospheric circulation is maintained against dissipation by the conversion of available potential energy (APE) into kinetic energy (KE). Most of this conversion is performed by baroclinic waves in the midlatitudes. Episodes of intense hemispheric depletion of the Northern Hemisphere (NH) APE content have been shown to occur repeatedly throughout the NH winters. Despite these episodes suggesting an intense baroclinic conversion of APE to KE, a number of unknowns remain with regard to the synoptic dynamics of such collapses, their link to the intensification of extratropical cyclones and their implications for storm track variability. In this study, we conduct a case study of an APE collapse event and develop a methodology to identify instances of boreal winter APE collapse from time series of volume-integrated NH APE over the 42 winters within the period 1979- 2021. Subsequently, a climatological study of the 113 identified NH APE collapse events is undertaken, with the objective of determining both where APE depletion occurs during APE collapses and the synoptic environment that favours this depletion. Using a novel local APE framework, we demonstrate that the planetary-scale depletion of APE during NH APE collapses is governed by the southward excursion of high-APE air from the polar APE reservoir and subsequent synoptic-scale APE conversion in the baroclinic region. Three regions are identified as hotspots for APE depletion during collapses: the Western North Pacific, the Western North Atlantic, and the Eastern North Pacific. The depletion of APE in these regions occurs as a result of the adiabatic descent of cold, high-APE air parcels on the western flank of deep upper-level troughs. Subsequently, the regional depletion hotspots are associated with the presence of deep upper-level troughs, increased cyclogenesis in the downstream storm track, and are responsible for a burst in NH KE release. We demonstrate a 15% increase in the local frequency of explosive cyclones in the storm track downstream of the Western Pacific depletion hotspot for Western Pacific collapse events. In conclusion, our findings illustrate the importance of APE collapses as indicators of periods of intense baroclinic activity. Additionally, we present evidence that synoptic-scale processes can exert a significant influence on planetary-scale energetics by tapping into the polar APE reservoir.