The Southern Hemisphere is stormier than the Northern, and we finally know why

A new study from the University of Chicago and the University of Washington explains for the first time why the southern hemisphere is stormier than the northern hemisphere. Above: An extratropical cyclone off the coast of Australia in 2012. Image credit: NASA

For centuries, sailors sailing the world have known where the most fearsome storms lurk: the southern hemisphere. “The waves ran high and threatened to overwhelm [the ship] at every roll,” wrote one passenger on an 1849 voyage that rounded the tip of South America.

Many years later, scientists poring over satellite data were finally able to put numbers to seafarers’ intuition: The Southern Hemisphere is actually stormier than the Northern, actually by about 24%. But nobody knew why.

A new study led by University of Chicago climate scientist Tiffany Shaw provides the first concrete explanation for this phenomenon. Shaw and her colleagues found two main culprits: ocean circulation and the great mountain ranges in the northern hemisphere.

The study also found that this storm asymmetry has increased since the satellite era began in the 1980s. They found that the increase was qualitatively consistent with climate change projections from physics-based models.

The results are published in the journal Proceedings of the National Academy of Sciences.

“A Tale of Two Hemispheres”

For a long time we didn’t know much about the weather in the southern hemisphere: most of the ways we observe the weather are land-based, and the southern hemisphere has a lot more ocean than the northern hemisphere.

But with the advent of satellite-based global observation in the 1980s, we were able to quantify just how extreme the difference was. The southern hemisphere has a stronger jet stream and more intense weather events.

Ideas had been floated, but no one had come up with a definitive explanation for this asymmetry. Shaw — along with Osamu Miyawaki (now at the National Center for Atmospheric Research) and Aaron Donohoe of the University of Washington — had hypotheses from their own and other previous studies, but they wanted to take it to the next step. This meant bringing together multiple lines of evidence from observations, theories and physics-based simulations of Earth’s climate.

“You can’t put the Earth in a jar,” Shaw explained, “so instead we use climate models based on the laws of physics and run experiments to test our hypotheses.”

They used a numerical model of Earth’s climate based on the laws of physics and reproduced the observations. Then they removed different variables one by one and quantified the impact of each one on the storms.

The first variable they tested was topography. Large mountain ranges disrupt airflow in a way that reduces storms, and there are more mountain ranges in the northern hemisphere.

When scientists leveled every mountain on Earth, about half the difference in storminess between the two hemispheres disappeared.

The other half had to do with ocean circulation. Water moves around the globe like a very slow but powerful conveyor belt: it sinks in the Arctic, travels along the bottom of the ocean, rises near Antarctica, and then flows up near the surface, carrying energy with it leads. This creates an energy difference between the two hemispheres. When the scientists tried to eliminate this conveyor belt, they saw the other half of the difference in turbulence disappear.

Will be even more stormy

After answering the fundamental question of why the southern hemisphere is more stormy, the researchers looked at how storms have changed since we’ve been able to track them.

Looking at observations over the past few decades, they found that storm asymmetry has increased over the course of the satellite era, starting in the 1980s. That is, the southern hemisphere will be even more stormy, while the change was negligible on average in the northern hemisphere.

The Southern Hemisphere storms were associated with changes in the ocean. They found that a similar oceanic influence occurs in the northern hemisphere, but its effect is offset by the absorption of sunlight in the northern hemisphere due to loss of sea ice and snow.

The scientists reviewed and found that models used to predict climate change as part of the Intergovernmental Panel on Climate Change assessment report showed the same signals — increasing storms in the southern hemisphere and negligible changes in the northern hemisphere — serving as an important independent verification of the accuracy of these models.

It may come as a surprise that such a deceptively simple question — why one hemisphere is stormier than another — went unanswered for so long, but Shaw explained that the field of weather and climate physics is relatively young compared to many other fields.

It was only after World War II that scientists began building models of the physics that drive large-scale weather and climate (to which Prof. Carl-Gustaf Rossby at the University of Chicago made significant contributions).

However, a deep understanding of the physical mechanisms behind climate and its response to human-induced changes, as outlined in this study, is critical to predicting and understanding what will happen as climate change accelerates.

“By laying this foundation of understanding, we increase confidence in climate change projections, thereby helping society better prepare for the impacts of climate change,” said Shaw. “One of the main aspects of my research is to understand whether models are giving us good information now so we can trust what they say about the future. The stakes are high and it is important to get the right answer for the right reason.”

More information:
Tiffany A. Shaw, Stormier Southern Hemisphere Induced by Topography and Ocean Circulation, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2123512119.

Provided by the University of Chicago

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