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Jet streams are often referred to as the “motor” of global weather: High-altitude wind currents steer areas of high and low pressure, playing a crucial role in shaping our weather. However, how these atmospheric flows are affected by climate change remains uncertain. Now, a team of climate scientists from Leipzig University, working with other research institutions, has developed a new method that enables a deeper understanding of what is referred to as the Eddy-Driven Jet in the Southern Hemisphere. This advance paves the way for more accurate predictions in the coming years about how this wind belt may respond to climate change.
In South America, southern Africa, Australia and Oceania, summer weather is heavily influenced by the Southern Hemisphere’s Eddy-Driven Jet (EDJ), a wind belt that flows between 1.3–1.5 kilometres above the ground. Current climate models have difficulty predicting its behaviour, however. The interactions among the various factors that can lead to changes in the air current are too complex.
Prompted by changes in wind patterns over the Southern Hemisphere in recent decades, Dr Julia Mindlin and her colleagues from the Institute for Meteorology at Leipzig University launched their research project. They analysed measurement data and showed that the wind speeds in the EDJ have increased while the wind belt has continually moved further toward the South Pole. They applied innovative methods to investigate the causes of these changes.
Research team combines statistical methods
They first analysed historical climate data to determine how the summer Eddy-Driven Jet in the Southern Hemisphere has changed since 1950. To understand what was driving the changes in the jet stream, the team used a statistical framework called causal inference. This method helps to disentangle the distinct influence that individual climate drivers have even if they are closely linked. Using this approach, the researchers were able to separate the various effects and quantify the specific role of each factor.
They linked these findings with a “storyline” approach. Climate storylines depict changes as a causal chain of events and are an important tool in climate research for explaining uncertainties in climate projections that arise from differing model simulations.
Causes of changes in the jet stream untangled
By combining these calculation methods, the researchers found that 50 percent of the observed shift in the Southern Hemisphere’s jet stream is directly attributable to global warming. The other half is caused by a combination of other climate-related changes. These include the warming of the upper tropical atmosphere, the strengthening of the winds in the stratosphere (that is, the second layer of the Earth’s atmosphere) and the warming of the tropical Pacific. While some of these factors are also influenced by anthropogenic climate change, others are more difficult to attribute.
The researchers also found that the combination of these factors is responsible for the observed acceleration of the jet stream. In addition, global warming plays a role in the jet stream’s wind speeds. The team is also looking into how these factors influence the jet stream in other parts of the world, for example over the Pacific and the Atlantic-Indian Ocean. “The findings show how complex the jet stream’s reaction to climate change is, especially in terms of how rapidly the winds are strengthening,” Julia Mindlin explains.
To better predict how the jet stream could behave in the near future, the researchers drew on numerical climate models. These models do not always agree with one another, however, especially when it comes to near-term changes. As a solution to this problem, the team compared the models’ predictions to the actual observations to check how well the models capture important parts of the climate puzzle. By focusing on the aspects that the models accurately capture, the researchers were able to narrow down the range of possible future scenarios. This results in more reliable predictions for the changes in the jet stream in the next ten years.
Better climate predictions for the next ten years
Julia Mindlin explains: “In the past, research primarily focused on long-term climate developments. Recently, however, short-term developments have taken centre 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.”
This research is a result of Leipzig University’s Climate Causality working group. It conducts research into changes in the climate system and is headed by Junior-Professor Dr Marlene Kretschmer, who also worked on the study. “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,” says Kretschmer. Researchers from the University of Reading (UK), the Forschungszentrum Jülich and the University of Buenos Aires (Argentina) also participated in the current publication. In the future, the working group aims to continue using the combination of causal methods and storylines to better understand phenomena such as regional droughts and heatwaves.
Dr Julia Mindlin
Institute for Meteorology
julia.mindlin@uni-leipzig.de
Jun-Prof Dr Marlene Kretschmer
Institute for Meteorology
marlene.kretschmer@uni-leipzig.de
Original title of the publication in PNAS: “Explaining and predicting the Southern Hemisphere eddy-driven jet”, https://www.pnas.org/doi/10.1073/pnas.2500697122
The researchers were able to show which factors influence the jet stream in the southern hemisphere. ...
Quelle: Graphic: Leipzig University
Copyright: Office for University Communications
Graphic „The jet stream – Four factors that impact the weather motor" (German/English), PDF
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The researchers were able to show which factors influence the jet stream in the southern hemisphere. ...
Quelle: Graphic: Leipzig University
Copyright: Office for University Communications
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