Sailors who traveled the world for centuries knew where the worst of storms lurk: the southern hemisphere. “The waves were as high as a mountain and threatened to crush the ship at every turn,” wrote a traveler who had sailed the tip of South America in 1849.
Years later, scientists studying satellite data were finally able to put numbers on what sailors knew intuitively: The southern hemisphere is stormier than the northern, and about 24% more stormy. But no one knew why.
Now, a study led by climatologist Tiffany Shaw of the University of Chicago (USA) has offered the first concrete explanation for this phenomenon. Shaw and colleagues found two main culprits: ocean circulation and the northern hemisphere’s great mountain range.
Additionally, the research also found that: this tumultuous asymmetry has increased since the 1980s, when the satellite era began. They found that the increase was qualitatively consistent with climate change estimates derived from physically-based models. The findings were published in the journal Proceedings of the National Academy of Sciences.
The story of the two hemispheres
For a long time we didn’t know much about the weather in the southern hemisphere: Most of the ways we look at the weather are ground-based, but The southern hemisphere has much more oceans than the northern hemisphere.
With the advent of satellite-based global 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.
While ideas are circulating on this subject, the truth is that no one has offered a definitive explanation for this asymmetry. Shaw, along with Osamu Miyawaki and Aaron Donohoe of the University of Washington, had hypotheses from their own work and previous work, but they wanted to take the next step. This meant collecting a wealth of evidence from observations, theories, and physics-based simulations of Earth’s climate.
“You can’t put the Earth in a lab jar,” Shaw explained, “so we use climate models based on the laws of physics and do experiments to test our hypotheses.”
They used a numerical model of Earth’s climate, based on the laws of physics that replicated the observations. They then eliminated the different variables one at a time and measured the impact of each on the storms.
mountains affect
The first variable they tested was topography. Wide mountain ranges disrupt airflow to reduce storms, and there are more mountain ranges in the Northern Hemisphere.
Actually, when scientists leveled all the mountains in the world, about half the difference in storms disappeared between the two hemispheres.
The other half was about ocean circulation. Water moves around the earth like a very slow but powerful conveyor belt: it sinks at the North Pole, moves along the ocean floor, rises near Antarctica, and then flows near the surface, carrying energy. This creates an energy difference between the two hemispheres. When the scientists tried to remove this conveyor belt, they found that the other half of the difference was lost in the storm.
Southern hemisphere, increasingly stormy
After the researchers answered the basic question of why the southern hemisphere is more stormy, they moved on to the review. how storms have changed because we were able to trace it.
Looking at observations over the past few decades, they found that storm asymmetry has increased in the satellite era, which began in the 1980s. The average change in the northern hemisphere is negligible, while the storm intensifies in the southern hemisphere.
Changes in storms in the Southern Hemisphere were linked to changes in the ocean. They found that a similar oceanic effect occurred in the Northern Hemisphere, but the effect was canceled in the Northern Hemisphere by the absorption of sunlight due to loss of sea ice and snow.
Scientists discovered that Models used to predict climate change as part of the IPCC assessment report showed the same signs: Increasing storms in the southern hemisphere and negligible changes in the north serve as an important independent check of the accuracy of these models.
It may be surprising that such a deceptively simple question—why is one hemisphere more stormy than the other—has remained unanswered for so long, but Shaw explained: the field of weather and climate physics is relatively young compared to many other fields.
Scientists began to build physics models that drive weather and climate on a large scale only after World War II.
But having a deep understanding of the physical mechanisms of climate and its response to human-induced changes, such as those revealed in this study, is crucial to predicting and understanding what will happen as climate change accelerates.
Reference work: https://www.pnas.org/doi/10.1073/pnas.2123512119
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