Sailors who sailed the globe for centuries learned quickly where the fiercest storms lurked: the southern hemisphere. “The waves rose like mountains and threatened to crush the ship at every turn,” noted a traveler who crossed the tip of South America in 1849.
Decades later, satellite data allowed scientists to quantify what sailors sensed. The southern hemisphere proved stormier than the northern by about 24 percent. Yet the reason behind this imbalance remained a mystery.
Today a study led by climatologist Tiffany Shaw from the University of Chicago has provided the first solid explanation. Shaw and her colleagues identify two main drivers: the global pattern of ocean circulation and the northern hemisphere’s vast mountain range.
Additionally, the research shows that this asymmetry has grown since the onset of the satellite era in the 1980s. The rise aligns with climate change projections produced by physics based models. The findings appear in the Proceedings of the National Academy of Sciences as a peer reviewed report.
The story of the two hemispheres
For many years the southern hemisphere remained less understood because most weather observations were ground based. The emphasis on land areas limited the view, while the southern hemisphere hosts far more ocean than land.
With satellite based global observation beginning in the 1980s, researchers could measure how large the difference actually is. The southern half of the globe shows a stronger jet stream and more intense weather events overall.
As ideas circulated, no definitive explanation existed for this asymmetry. Shaw along with collaborators Osamu Miyawaki and Aaron Donohoe from the University of Washington pursued a rigorous next step. They integrated observations, theories, and physics based simulations to test competing ideas about what drives the difference.
Shaw explained that Earth cannot be put in a lab jar, so the team turned to physics backed climate models and conducted experiments to test hypotheses.
Using a numerical climate model that mirrors real world observations, the researchers systematically varied one factor at a time to measure its effect on storms.
Mountains affect
The first factor tested was topography. Wide mountain systems interrupt airflow and reduce storm formation, and the Northern Hemisphere is endowed with more high terrain.
When scientists effectively leveled all the mountains on the planet, roughly half of the storm disparity between hemispheres disappeared.
The other half of the difference traced back to ocean circulation. The global conveyor belt moves water around the planet, sinking at the poles, traveling along the seafloor, rising near Antarctica, and seeping back toward the equator. This energy transport creates a persistent hemispheric imbalance. When the researchers removed this conveyor like effect in the model, the remaining split in storminess collapsed as well.
Southern hemisphere, increasingly stormy
With the basic causes identified, the team reviewed how storms have evolved over the past several decades. Observations indicate that storm asymmetry has grown in the satellite era beginning in the 1980s. The northern hemisphere shows little to no average change, while storms intensify in the south.
Changes in southern storms were linked to ocean dynamics. A similar oceanic influence appears in the northern hemisphere, but it is largely offset by the loss of sea ice and snow which reduces sunlight absorption in the north.
Climate models used in major assessments echo these results. They show a consistent rise in southern storms with little change in the north, which serves as an important check on the reliability of climate projections.
It is surprising that a question so seemingly simple has remained unanswered so long. Shaw noted that weather and climate physics is a relatively young science compared with many others. The field began to mature after World War II when physics based models started to drive large scale weather and climate research.
Gaining a deep understanding of the mechanisms behind climate change helps scientists project how patterns will shift as human emissions continue to influence the climate. The study reinforces the value of physical principles in predicting future weather extremes and their hemispheric distribution.
The reference work is published in the Proceedings of the National Academy of Sciences. The study highlights a clear, physics grounded explanation for why one hemisphere experiences more frequent and intense storms than the other, while underscoring the ongoing role of ocean circulation and topography in shaping global weather patterns.
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Note: this summary draws on peer reviewed findings and marked citations including attribution to the National Academy of Sciences and related researchers.