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

Abstract

Outside the tropics, extratropical cyclones account for most of the poleward atmospheric heat transport, and extreme heat transport events are known to occur in their vicinity. Yet, it remains unclear how individual cyclones contribute to heat transport over the course of their lifetime and whether the seasonal heat transport – viewed from a zonally integrated standpoint – is determined by their number. This study adopts a cyclone-centered perspective to quantify in detail the relationship between poleward heat transport and the life cycle characteristics of extratropical cyclones in Southern Hemispheric winter. Specifically, objectively identified surface cyclone tracks derived from ERA5 data (1981–2021) are combined with a moist static energy (MSE) framework involving an eddy-mean decomposition of the meridional MSE flux. It is found that the local transient eddy MSE flux maximizes during the cyclone intensification phase and is largest in the warm sector with a secondary maximum in the cold sector. A considerable fraction of the flux in the warm sector is located well equatorward of the cyclone and thus outside the cyclonic region identified by the tracking algorithm. This leads to a latitudinal shift between maxima in cyclone frequency and transient eddy MSE fluxes. To bridge the gap between zonally integrated MSE flux and contributions from individual cyclones, local vertically integrated transient eddy MSE flux events are attributed to cyclones based on spatial overlap with the identified cyclone area. Poleward of 50° S, the cyclones that become most intense are the ones that exhibit the largest zonally integrated cyclone-attributed MSE flux while the strongly intensifying cyclones dominate equatorward thereof. Although both of these sets of cyclones contribute disproportionally to the cyclone-attributed transient eddy MSE fluxes, the relationship between their seasonal number and the seasonal mean poleward transient eddy MSE flux is sensitive to the choice of the eddy-mean decomposition method. This result indicates that low wavenumber background flows mask the influence of cyclone intensities and intensification rates in the vertical, zonal, and seasonal integral. Notably, at 50° S the relationship between the overall cyclone number and total MSE flux shows a peak. 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 may be constrained by the global energy imbalance.