New research  provides the first clear explanation for the difference in storms and demonstrates that storms are becoming more severe over time.

For centuries, sailors who had traveled the world knew that the most powerful storms were located in the Southern Hemisphere. It also has a stronger jet stream and more intense weather events.

Years later, scientists analyzing satellite data confirmed sailors’ observations that the Southern Hemisphere experiences about 24% more storms than the Northern Hemisphere. However, the reason for this discrepancy was not known.

A new study by climate scientists  lays out the first concrete explanation for this phenomenon. They 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. The increase was shown to be qualitatively consistent with climate change forecasts from physics-based models.

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 airflow 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.

Having answered the fundamental question regarding why the southern hemisphere is stormier, the researchers moved on to examine how storminess has changed since we’ve been able to track it.

Looking over past decades of observations, they found that the storminess asymmetry has increased over the satellite era beginning in the 1980s. That is, the Southern Hemisphere is getting even stormier, whereas the change on average in the Northern Hemisphere has been negligible.

The Southern Hemisphere’s storminess changes were connected to changes in the ocean. They found a similar ocean influence is occurring in the Northern Hemisphere, but its effect is canceled out by the absorption of sunlight in the Northern Hemisphere due to the loss of sea ice and snow.

The scientists checked and found that models used to forecast climate change as part of the Intergovernmental Panel on Climate Change assessment report were showing the same signals—increasing storminess in the Southern Hemisphere and negligible changes in the Northern—which serves as an important independent check on the accuracy of these models.

Having a deep understanding of the physical mechanisms behind the climate and its response to human-caused changes, such as those laid out in this study, are crucial for predicting and understanding what will happen as climate change accelerates.

“Stormier Southern Hemisphere induced by topography and ocean circulation” by Tiffany A. Shaw, Osamu Miyawaki and Aaron Donohoe, 5 December 2022, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2123512119