Melba Lara: You're listening to WBEZ. And it's time for our weekly climate conversation. The Earth's Southern Hemisphere is more stormy than the Northern Hemisphere. But why? What's causing the difference? Well, new research led by University of Chicago Professor Tiffany Shaw offers an explanation and shows the asymmetry is only growing. Professor Shaw joins us now to talk about this study. Thanks for joining us.

Tiffany Shaw: Thank you for having me. 

Melba Lara: First of all, how big of a difference are we talking about here between the two hemispheres?

Tiffany Shaw: So the southern hemisphere is a much stormier place than the north. It has more extreme weather events, stronger winds that are often referred to as the roaring forties and ocean waves driven by storms that can reach an astounding 78 feet.

Melba Lara: And when we're measuring something like storminess, does it mean more storms, stronger storms? Both?

Tiffany Shaw: Yeah, so in our study, we measured the storminess using winds that we then used to quantify the kinetic energy of the storms, which is the energy of motion. And we did this for both hemispheres. And that's how we came up with the fact that the Southern Hemisphere is 24% stormier than the north.

Melba Lara: And your team found actually two main variables that are making the Southern Hemisphere more stormy. What's the first one?

Tiffany Shaw: The first variable was mountains. The large mountain ranges disrupt air flow in a way that reduces storms, and there are more mountain ranges in the Northern Hemisphere.

Melba Lara: So we've got the topography of the mountains as the first reason. What's the second?

Tiffany Shaw: The second was the ocean. The movement of water in the ocean can be described as a conveyor belt. It sinks in the arctic. It travels along the bottom of the ocean rises up near Antarctica and the result is that there's more energy available to fuel the storms in the Southern Hemisphere.

Melba Lara: And why do you think the question of why one hemisphere is stormier than the other went unanswered for so long?

Tiffany Shaw: Well, sailors had known about the stormier southern hemisphere for centuries, but we've only been able to truly quantify it since the advent of satellite based global observing in the 1980s. Because before that, most of our observations were land based and the Southern Hemisphere is 80% ocean. Now lots of ideas had been circulated, but no one had established a definitive answer. In our study, we had hypotheses that we wanted to test and we took the next step by compiling multiple lines of evidence from not only observations, but our own theories and physics based simulations of our climate. So when we flatten every mountain, we found that the storminess difference was cut in half, and when we eliminated the ocean conveyor belt, the other half of the difference disappeared.

Melba Lara: Tell us a bit more about the impact climate change is having on storms in the two hemispheres.

Tiffany Shaw: So, having answered the fundamental question of why the Southern Hemisphere is stormier, we went on to examine how storminess has changed. And looking over the past decades of observations, we found storminess asymmetry has increased because the Southern hemisphere is getting even stormier over time, whereas the change in the Northern Hemisphere, on average has been negligible. We know from previous research examining the forecasts of climate change from physics based climate models that when you increase carbon dioxide in the atmosphere by burning fossil fuels, the Southern Hemisphere will get stormier and there should be muted changes in the north. So that's what we expect under climate change. Now in our work, when we compared these physics based predictions over the same time period as observations, they show the same signals – increasing storminess in the Southern hemisphere and negligible change in the Northern Hemisphere. And this is really important because, you know, revealing the accuracy of these projections helps give us confidence in these models, which are used to inform society of the impacts of climate change.

Melba Lara: I've been speaking with Professor Tiffany Shaw from the University of Chicago's Department of Geophysical Sciences. This is WBEZ. 

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