The southern hemisphere is stormier than the north, and we finally know why

For centuries, sailors who sailed the world knew where the most fearsome storms lay in wait: the southern hemisphere. “The waves rose up to the mountains and threatened to overwhelm [the ship] with every roll,” wrote one passenger on an 1849 trip around the tip of South America.

Many years later, scientists poring over satellite data were finally able to put some numbers behind the sailors’ hunch: the southern hemisphere is indeed stormier than the north, by around 24%, in fact. But no one knew why.

A new study by University of Chicago climatologist Tiffany Shaw presents the first concrete explanation for this phenomenon. Shaw and his colleagues found two major culprits: ocean circulation and large northern hemisphere mountain ranges.

The study also found that this storm asymmetry has increased since the beginning of the satellite era in the 1980s. They found that the increase was qualitatively consistent with predictions of climate change 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 weather are land-based, and the Southern Hemisphere has a lot more ocean than the Northern Hemisphere. .

But with the advent of global satellite 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 had circulated, but no one had established a definitive explanation for this asymmetry. Shaw – along with Osamu Miyawaki (now at the National Center for Atmospheric Research) and Aaron Donohoe at the University of Washington – had hypotheses drawn from their own studies and other previous studies, but they wanted to take the next step. This involved bringing together multiple sources 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 the Earth’s climate built on the laws of physics that reproduced the observations. Then they removed different variables one at a time and quantified the impact of each on the storm.

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.

Indeed, when scientists flattened all of Earth’s mountains, about half of the storm difference between the two hemispheres disappeared.

The other half was related to ocean circulation. Water moves around the globe like a very slow but powerful treadmill: it flows in the Arctic, skirts the bottom of the ocean, rises near Antarctica and then rises near the surface, carrying energy with her. This creates an energy difference between the two hemispheres. When scientists tried to eliminate this treadmill, they saw the other half of the storm difference disappear.

Get even stormier

After answering the fundamental question of why the Southern Hemisphere is stormier, the researchers then looked at how the storm has changed since we were able to track it.

Looking at the past decades of observations, they found that storm asymmetry had increased during the satellite era beginning in the 1980s. In other words, the southern hemisphere is getting even stormier, as the change on average in the northern hemisphere was negligible.

Storm changes in the Southern Hemisphere were tied to changes in the ocean. They found that a similar oceanic influence occurs in the Northern Hemisphere, but its effect is negated by absorption of sunlight in the Northern Hemisphere due to loss of sea ice and snow.

Scientists checked and found that the models used to predict climate change as part of the Intergovernmental Panel on Climate Change’s assessment report showed the same signals – an increase in storms in the southern hemisphere and negligible changes in the north – an important independent check on the accuracy of these models.

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

It was not until after World War II that scientists began to build large-scale physical models of weather and climate (the main contributions to which were made at the University of Chicago by Professor Carl-Gustaf Rossby).

But having a thorough understanding of the physical mechanisms driving climate and its response to human-induced changes, such as those presented in this study, is crucial to predicting and understanding what will happen as climate change continues. ‘accelerated.

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