by University of Chicago

The Southern Hemisphere is stormier than the Northern, and we finally know why1 / 1A new study by the University of Chicago and University of Washington lays out for the first time why the Southern Hemisphere is stormier than the Northern.

Above: An extratropical cyclone off the coast of Australia in 2012. Credit: NASA

For centuries, sailors who had been all over the world knew where the most fearsome storms of all lay in wait: the Southern Hemisphere. “The waves ran mountain-high and threatened to overwhelm [the ship] at every roll,” wrote one passenger on an 1849 voyage rounding the tip of South America.

Many years later, scientists poring over satellite data could finally put numbers behind sailors’ intuition: The Southern Hemisphere is indeed stormier than the Northern, by about 24%, in fact. But no one knew why.

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

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

The findings 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 very much about the weather in the Southern Hemisphere: Most of the ways we observe weather are land-based, and the Southern Hemisphere has much more ocean than the Northern Hemisphere does.

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

Ideas had been 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 the University of Washington’s Aaron Donohoe—had hypotheses from their own and other previous studies, but they wanted to take the next step. This meant bringing together multiple lines 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 instead we use climate models built on the laws of physics and run experiments to test our hypotheses.”

They used a numerical model of Earth’s climate built on the laws of physics that reproduced the observations. Then they removed different variables one at a time, and quantified each one’s impact on storminess.

The first variable they tested was topography. Large mountain ranges disrupt air flow in a way that reduces storms, and there are more mountain ranges in the Northern Hemisphere.

Indeed, when the scientists flattened 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. This creates an energy difference between the two hemispheres. When the scientists tried eliminating this conveyor belt, they saw the other half of the difference in storminess disappear.[…]

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