Explaining and predicting the Southern Hemisphere eddy-driven jet

Abstract

The summertime eddy-driven jet (EDJ) in the Southern Hemisphere is a critical mediator between regional climate and large-scale phenomena, guiding synoptic systems that shape weather patterns. Uncertainties in global climate models (GCMs)-particularly in projecting changes in remote drivers like tropical warming, stratospheric polar vortex strengthening, and asymmetric tropical Pacific warming-hinder predictions of EDJ trends and associated regional outcomes. In this study, we develop a causal framework that combines observations, reanalysis datasets, and storylines estimated from the Coupled Model Intercomparison Project (CMIP) projections to attribute past EDJ changes and predict plausible future trajectories. Our findings indicate that tropical warming has evolved along the low end of plausible CMIP trajectories, while the stratospheric polar vortex shows robust strengthening, both strongly influencing observed EDJ trends. Our results suggest that 50% of the observed EDJ latitude shift can be directly attributed to global warming (GW), and the remaining 50% to remote drivers whose attribution to GW remains uncertain. Importantly, GCMs appear to accurately estimate the observed latitudinal shifts but underestimate the observed strengthening of the EDJ, while the proposed storylines are able to capture the observed trend. By integrating causal inference with climate storylines, our approach narrows the divide between attribution and prediction, offering a physically grounded method to estimate plausible pathways of future climate change.