Why the Southern jet stream is shifting - and what it means

A fast belt of westerly winds that helps shape summer weather across South America, southern Africa, Australia, and Oceania is now sliding toward the South Pole and blowing harder.

New research shows that about half of this shift in the Southern Hemisphere’s eddy‑driven jet (EDJ) comes straight from global warming, while the other half traces to a cluster of interacting climate drivers.

By sorting those influences and testing models against real‑world data, scientists have sharpened near‑term forecasts for the next decade.

The work was led by Julia Mindlin and colleagues at the Institute for Meteorology at Leipzig University, within the university’s Climate Causality working group, headed by Professor Marlene Kretschmer.

“The aim of our research is to better understand climate risks and reduce the uncertainties of regional predictions and projections when it comes to extreme weather and climate events,” Kretschmer said.

The researchers focused on the summertime EDJ, which steers storms and controls heat and rainfall across the Southern Hemisphere.

Tracking decades of wind shifts

Prompted by recent changes seen in Southern Hemisphere wind patterns, the researchers began by examining historical climate observations back to 1950.

They confirmed two clear trends: summer EDJ wind speeds have increased, and the jet’s core latitude has crept steadily poleward.

To untangle why, they turned to a statistical framework known as causal inference, which helps isolate the influence of individual drivers – even when those drivers co‑vary in the climate system.

They then linked the causal results to a “storyline” approach – a way of tracing climate changes as chains of cause and effect. Storylines help explain why different climate models can yield different futures and give researchers a structured way to explore uncertainty.

Forces behind the jet stream shift

Combining the two approaches produced a clean attribution. Roughly 50 percent of the observed poleward shift in the Southern Hemisphere summer jet is directly attributable to global warming.

The remaining half reflects the combined push of several other climate‑related changes: warming of the upper tropical atmosphere, strengthening winds in the stratosphere, and warming in the tropical Pacific.

Some of these influences are themselves partly driven by human‑caused climate change; others are harder to pin down. The same mix of factors also explains the measured acceleration of the jet, with global warming contributing to the rise in wind speeds.

“The findings show how complex the jet stream’s reaction to climate change is, especially in terms of how rapidly the winds are strengthening,” Mindlin said.

Forecasting the next decade

Because climate models often disagree – especially over short horizons – the team next asked which models best mirrored the real atmosphere.

The researchers compared simulations to observed EDJ behavior to see which captured core features such as latitude, variability, and sensitivity to the identified drivers.

By giving more weight to models that get those pieces right, they narrowed the plausible range of jet changes expected over the coming ten years. That tighter range is critical for planners in agriculture, water management, wildfire preparedness, energy, and infrastructure.

“In the past, research primarily focused on long-term climate developments. Recently, however, short-term developments have taken center stage, as they are increasingly relevant for decision-makers. The methods we suggest can be used to improve climate predictions for the next ten years,” Mindlin said.

What a shifting jet stream means

When the EDJ edges poleward, the storm tracks it helps steer often shift with it. Regions on the jet’s equatorward side can see fewer frontal systems, bringing a tendency toward drier summers and greater drought risk.

Farther south, where the jet now spends more time, stronger winds can intensify storms, enhance ocean mixing, and reshape sea ice and marine ecosystems.

These downstream changes matter for crop yields in southern Australia and reservoir inflows in Chile and Argentina. They also affect fire weather in South Africa and coastal infrastructure exposed to more frequent wind-driven extremes.

Causal tools reveal the truth

Many large‑scale climate drivers occur together: greenhouse warming, tropical Pacific sea surface shifts, and stratospheric circulation changes often overlap in time. Traditional correlations blur their separate roles.

Causal inference lets researchers ask counterfactual questions – what would the jet have done without the tropical Pacific warming, for instance? – and assign shares of responsibility.

Coupling those answers to storylines makes the science more usable, because decision‑makers can follow the chain from driver to local impact.

Matching models to history

No single climate model is perfect, but each gets some features right. By comparing existing models to observed EDJ behavior, Mindlin’s team effectively filtered out poor performers and leaned on those that reproduce the jet’s history.

These findings will help water agencies, grid operators, and insurers work with narrower, more actionable climate windows for the coming decade.

The team is extending this framework to related wind belts over the Pacific and the coupled Atlantic‑Indian sector. The researchers want to see how shifts in these jet streams link to regional heatwaves and multi‑year droughts – events that strain societies and ecosystems.

As causal‑storyline methods mature, they could become standard practice for translating global climate signals into the local risk information that communities need.

The study is published in the journal Proceedings of the National Academy of Sciences.

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