Relating extratropical atmospheric heat transport to cyclone life cycle characteristics and numbers in Southern Hemispheric winter

Abstract. Extratropical cyclones, which develop and propagate in regions known as storm tracks, account for the majority of poleward atmospheric heat transport (AHT) outside the tropics. This allows the intensity and position of the zonal mean storm track to be constrained by the hemispheric-wide radiative budget. Yet, the zonal mean nature of this approach masks the contribution of individual cyclones, which can locally constitute extreme eddy AHT. In this study, we adopt a cyclone-centered perspective to quantify in detail the relationship between poleward AHT and the life cycle characteristics of extratropical cyclones in Southern Hemispheric winter. Specifically, we combine objectively identified surface cyclone tracks derived from ERA5 data (1981–2021) with a moist static energy (MSE) framework that features an eddy-mean decomposition of the circulation. The eddy MSE flux maximizes during the cyclone intensification phase and is largest in the warm sector with a secondary maximum in the cold sector. Importantly, a considerable fraction of the flux in the warm-sector is located well equatorward of the cyclone center and thus outside the cyclonic region identified by the tracking algorithm. This leads to a latitudinal shift between maxima in cyclone frequency and eddy MSE fluxes. To bridge the gap between zonally integrated AHT and contributions from individual cyclones, local vertically integrated eddy MSE flux events are attributed to cyclones based on spatial overlap criteria. When integrating zonally and over the cyclone lifetime, the most intense cyclones (and not the ones intensifying most rapidly) generally exhibit the largest cyclone-attributed eddy MSE fluxes. Despite a disproportionate contribution to the cyclone-attributed eddy MSE fluxes by the most intense cyclones on the seasonal scale, the relationship between the seasonal number of intense cyclones and the poleward eddy MSE flux is sensitive to the choice of the eddy-mean decomposition. This result indicates that low wavenumber background flows mask the influence of cyclone characteristics in the vertical, zonal, and seasonal integral. Notably however, the relationship between the overall cyclone number and total AHT shows a peak at 50° S, which resembles the dominance of synoptic waves at this latitude in terms of AHT while the relevance of planetary waves increases poleward. Further research on the interplay between synoptic and planetary MSE fluxes in the vicinity of cyclones is needed to understand to which extent the cyclone number, which is projected to decrease under warming, could be constrained by the global energy budget.