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

A new study by the University of Chicago and the University of Washington reveals for the first time why the Southern Hemisphere is more stormy than the Northern Hemisphere. Above: An unusual hurricane off the coast of Australia in 2012. Credits: NASA

For centuries, sailors around the world knew where the most terrible storms lurked: the Southern Hemisphere. “The waves reached the top of the mountains and threatened to crush them. [the ship] A traveler touring the tip of South America in 1849, at every turn,” he wrote.

Years later, scientists studying satellite data were finally able to put numbers behind sailors’ intuition: The Southern Hemisphere is indeed about 24% more stormy than the Northern Hemisphere. But no one knew why.

A new study led by University of Chicago climate scientist Tiffany Shaw reveals the first concrete explanation for this phenomenon. Shaw and his colleagues found two major culprits: ocean circulation and vast mountain ranges in the Northern Hemisphere.

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

The findings were published in the journal Proceedings of the National Academy of Sciences.

“The Tale of the Two Hemispheres”

For a long time we didn’t know much about the weather in the Southern Hemisphere: Many of the ways we observe weather are land-based, and the Southern Hemisphere has much more ocean than the Northern Hemisphere.

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

Ideas were circulating, but no one had been able to provide 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 work and other previous work, but wanted to take the next step. This meant putting together a wealth of evidence from observations, theory, and physics-based simulations of Earth’s climate.

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

They used a numerical model of Earth’s climate, based on the laws of physics that reproduced the observations. They then removed the different variables one at a time and measured the impact of each on the storm.

The first variable they tested was topography. Wide mountain ranges disrupt airflow to reduce storms, and there are more mountain ranges in the Northern Hemisphere.

Indeed, as scientists leveled every mountain on Earth, about half of the storm differential between the two hemispheres disappeared.

The other half was about ocean circulation. Water moves around the earth like a very slow but powerful conveyor belt: it sinks at the North Pole, moves at the bottom of the ocean, rises near Antarctica, and then flows to the surface, carrying energy with it. This creates an energy difference between the two hemispheres. When the scientists tried to remove this conveyor belt, they found that the other half of the difference in the storm also disappeared.

It’s getting stormier

After the researchers answered the fundamental question of why the Southern Hemisphere is more stormy, they moved on to examine how the storm has changed since we’ve been able to track it.

Looking at observations over the past decades, they found that storm asymmetry increased in the satellite era, which began in the 1980s. That is, the average change in the Northern Hemisphere has been insignificant, while the Southern Hemisphere has become even more stormy.

Southern Hemisphere storminess changes were linked to oceanic changes. They found that a similar oceanic effect occurred in the Northern Hemisphere, but its effect was canceled by the absorption of sunlight in the Northern Hemisphere due to loss of sea ice and snow.

The scientists checked and found that the models used to predict climate change as part of the Intergovernmental Panel on Climate Change assessment report show the same signals – increasing storms in the Southern Hemisphere and negligible changes in the Northern Hemisphere – which is an important independent control task. he sees. accuracy of these models.

It may be surprising that such a deceptively simple question—why is one hemisphere more stormy than the other—has remained unanswered for so long, but Shaw explained that the field of weather and climate physics is relatively young compared to many other fields.

Scientists only II. After World War II, they began to build large-scale models of physics that drive weather and climate (significant contributions were made by Prof. Carl-Gustaf Rossby at the University of Chicago).

But having a deep understanding of the physical mechanisms behind climate and its response to human-induced changes such as those revealed in this study is crucial to predicting and understanding what will happen as climate change accelerates.

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

More information:
Tiffany A. Shaw, Stormier Southern Hemisphere, triggered 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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