The Relationship between Precipitation and Extra-Tropical Cyclone Intensity in Different Idealised Climates

Extra-tropical cyclones (ETCs) are the main cause of precipitation in the mid-latitudes and there is substantial evidence that ETC-related precipitation will increase in the future. However, little is known about how this increased precipitation may impact the dynamical strength of ETCs and thus how the linear relationship between ETC maximum vorticity and ETC precipitation may change. Furthermore, it remains unknown whether the impact of increased precipitation differs for different types of ETCs. To address these knowledge gaps, we perform aqua-planet simulations with the state-of-the-art global numerical weather prediction model, OpenIFS, which is a version of ECMWF’s operational forecast model. Three 10-year long simulations are performed which differ only in terms of their sea surface temperature (SST) distributions and are (1) a control simulation, (2) a uniform warming experiment, and (3) a polar amplification experiment. For each experiment, the large-scale circulation is examined. Uniform warming causes the jet stream to move polewards whereas polar amplification results in an equatorward shift. Both uniform warming and polar amplification lead a decrease in the Eady growth rate, but this is caused by an increase in static stability in the uniform warming case and a decrease in the meridional temperature gradient in the polar amplification case. ETCs are objectively tracked, and their maximum vorticity and related precipitation computed. In all experiments, ETCs with stronger maximum vorticity are associated with more precipitation but the slope of the linear regression between maximum ETC vorticity and ETC precipitation is larger in the uniform warming and polar amplification simulations than in the control simulation. We hypothesise that if an increase in precipitation in warmer climates were to feed back, via diabatic heating and potential vorticity anomalies, onto the dynamical intensity of the ETCs, precipitation and vorticity would increase at similar rates, and hence the slope of the linear regression line between precipitation and vorticity would remain similar. Our results indicate either that there is no feedback or that the increase in vorticity due to diabatic heating is masked by the decrease in the Eady growth rate. Lastly, k-means clustering is applied to the ETC precipitation field to group the ETCs into clusters with similar precipitation structures. Four distinct and physically realistic types of ETCs, which are present in all experiments, are identified, meaning that the average precipitation patterns associated with ETCs are unlikely to change in the future. Only two of the four ETC types identified have strong linear correlations between maximum vorticity and precipitation. The strongest dependency between ETC maximum vorticity and precipitation occurs for ETCs that have the most precipitation associated with the warm front, which suggests that the position of the precipitation and diabatic heating may be critical to whether there is a feedback onto the intensity of extra-tropical cyclones.