The Southern Hemisphere has long been known to harbor the world’s most terrifying storms, according to mariners who have traveled the globe. On a voyage in 1849 that circled the southernmost tip of South America, a passenger described the waves as “running mountain-high and threatening to overrun [the ship] with every roll.”
After several years, researchers studying satellite data were able to confirm what sailors had long suspected: the southern hemisphere is, in fact, 24% more storm-prone than the northern. However, no one was aware of the underlying cause.
Tiffany Shaw, a climate scientist at the University of Chicago, is the lead author of a new study that provides the first direct explanation for this phenomenon. The vast mountain ranges in the Northern Hemisphere and ocean circulation were the two main offenders that Shaw and her colleagues identified.
The study also discovered that since the 1980s, when the satellite age first began, this storminess imbalance has grown. They discovered the increase was qualitatively in line with projections of climate change made by physics-based models.
Two Hemispheres: A Tale
Since most of the ways we study weather are land-based, and the Southern Hemisphere contains a lot more ocean than the Northern Hemisphere, we didn’t know a lot about the weather in the Southern Hemisphere for a very long time.
In contrast, we were able to measure how extreme the divergence was in the 1980s with the introduction of satellite-based worldwide observation. The jet stream is stronger, and the weather events are more severe in the southern hemisphere.
Thoughts had been shared, but no one had found a conclusive cause for this asymmetry. Shaw, Osamu Miyawaki (PhD ’22, currently at the National Center for Atmospheric Research), and Aaron Donohoe from the University of Washington all had theories from earlier research but wanted to move further. This required combining numerous lines of evidence from observations, theory, and physics-based climate simulations.
Shaw said, “Since you can’t put the Earth in a jar, we utilize climate models based on the principles of physics and conduct experiments to evaluate our assumptions.”
They applied a computerized simulation of Earth’s climate based on physical rules to replicate the data. They then measured the effects of each variable’s removal, one at a time, on storminess.
They initially examined topography as a factor. There are more mountain ranges in the Northern Hemisphere, and large mountain ranges can impede air movement and lessen storms.
In fact, the difference in storminess between the two hemispheres was reduced by nearly half when the scientists leveled every peak on Earth.
The other part concerned the circulation of the ocean. Water circulates across the world in a manner akin to a very sluggish but potent conveyor belt: it descends in the Arctic, travels through the ocean’s floor, rises in Antarctica, and then flows up near the surface, carrying energy with it. The two hemispheres now have an energy differential. The other half of the variance in storminess vanished when the scientists attempted to remove this conveyor belt.
Increasingly stormy
After addressing the fundamental query of why the southern hemisphere experiences more storms, the researchers looked at how storminess has evolved over time.
They discovered that the storminess imbalance has grown over the satellite era, which started in the 1980s, by analyzing observations from previous decades. That is, whereas the average change in the Northern Hemisphere has been minimal, the Southern Hemisphere is becoming considerably more stormy.
Variations in the ocean were linked to changes in storminess in the southern hemisphere. They discovered that the Northern Hemisphere also has a comparable ocean influence, but that this influence is cancelled out by the Northern Hemisphere’s increased solar absorption as a result of the melting of snow and sea ice.
The scientists checked the models and discovered that they all displayed the same signals—increasing storminess in the southern hemisphere and minimal changes in the northern—that were used to predict climate change as part of the Intergovernmental Panel on Climate Change assessment report. This serves as an important independent check on the accuracy of these models.
Why one hemisphere is stormier than the other may seem like an easy issue, but Shaw explained that the subject of weather and climate physics is very new compared to many other fields, which may explain why it stayed unsolved for so long.
After WWII, scientists began developing models of the physics underlying large-scale weather and climate (key contributions were made at the University of Chicago by Prof. Carl-Gustaf Rossby).
But predicting and comprehending what will occur as climate change accelerates depends on having a thorough grasp of the physical mechanisms underlying the climate and its response to changes induced by humans, such as those spelled out in this paper.
To better help society prepare for the effects of climate change, Shaw said, “We enhance confidence in climate change estimates by creating this foundation of understanding.” “Understanding if models are providing us with accurate information now will help us believe what they predict about the future, which is one of the main themes of my research.” “Because the stakes are so high, it’s critical to choose the best course of action.”
