Southern Hemisphere Winter Storm Tracks Respond Differently to Low and High CO2 Forcings

Abstract

In the Southern Hemisphere, Earth system models project an intensification of winter storm tracks by the end of the 21st century. Previous studies using idealized models showed that storm track intensity saturates with increasing temperatures, suggesting that the intensification of the winter storm tracks might not continue further with increasing greenhouse gases. Here, we examine the response of mid-latitude winter storm tracks in the Southern Hemisphere to increasing CO2 from two to eight times preindustrial concentrations in more realistic Earth System Models. We find that at high CO2 levels (beyond 4×CO2), winter storm tracks no longer exhibit an intensification across the extratropics. Instead, they shift poleward, weakening the storm tracks at lower mid-latitudes and strengthening at higher mid-latitudes. By analyzing the eddy kinetic energy (EKE) budget, the non-linear storm track response to an increase in CO2 levels in the lower mid-latitudes is found to stem from a scale-dependent conversion of eddy available potential energy to EKE. Specifically, in the lower mid-latitudes, this energy conversion acts to oppositely change the EKE of long and short scales at low CO2 levels, but, at high CO2 levels, it mostly reduces the EKE of shorter scales, resulting in a poleward shift of the storms. Furthermore, we identify a “tug of war” between the upper and lower temperature changes as the primary driver of the non-linear scale-dependent EKE response in the lower mid-latitudes. Our results suggest that in the highest emission scenarios beyond the 21st century, the storm tracks’ response may differ in magnitude and latitudinal distribution from projected changes by 2100.