Tuesday, June 6, 2023

Study unveils why Southern hemisphere is stormier than the Northern

A first concrete explanation for difference, and show it is getting even stormier over time.

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Storms and extreme weather events are stronger in the Southern Hemisphere than in the Northern Hemisphere. The Southern Hemisphere has a stronger jet stream and more extreme weather events than the Northern Hemisphere. Understanding the relative importance of land–ocean contrast, including topography, radiative processes, and ocean circulation, for determining this asymmetry is essential and may help interpret projections of future storminess.

Using an energetic perspective, observations, and climate model simulations, a new study by the University of Chicago offers a first-string explanation for this phenomenon. They found two significant culprits: ocean circulation and the large mountain ranges in the Northern Hemisphere.

The study also discovered that this storminess imbalance had grown since the 1980s when the satellite age began. They found the increase was qualitatively in line with climate change forecasts made by physics-based models.

For a long time, significantly less was known about the weather in the Southern Hemisphere. Most of the ways for observing weather were land-based. 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 extreme weather events.

Thoughts had been shared, but no one had found a conclusive cause for this asymmetry. Shaw, Osamu Miyawaki (Ph.D. ’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.

University of Chicago climate scientist Tiffany Shaw said, “You can’t put the Earth in a jar, so instead, we use climate models built on the laws of physics and run experiments to test our hypotheses.”

They applied a numerical model 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 to lessen storms. Indeed, when the scientists flattened every mountain on Earth, about half the difference in storminess between the two hemispheres disappeared.

The other part concerned the circulation of the ocean. Water circulates across the world akin to a 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 disappeared when the scientists attempted to remove this conveyor belt.

After addressing the fundamental query of why the southern hemisphere experiences more storms, the scientists looked at how storminess has evolved.

They discovered that the storminess asymmetry has grown over the satellite era, which started in the 1980s, by analyzing observations from previous decades. That is, while the average change in the Northern Hemisphere has been minimal, the Southern Hemisphere is becoming even 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. Still, this influence is canceled out by the Northern Hemisphere’s increased solar absorption due to the melting of snow and sea ice.

As an essential independent check on the accuracy of these models, the scientists examined the models that were used to forecast climate change as part of the Intergovernmental Panel on Climate Change assessment report and discovered that they all displayed the same signals—increasing storminess in the Southern Hemisphere and minor changes in the Northern.

Scientists noted“It may be surprising that such a deceptively simple question—why one hemisphere is stormier than another—went unanswered for so long, but Shaw explained that the field of weather and climate physics is relatively young compared to many other fields.”

Journal Reference:

  1. Tiffany A. Shaw et al. Stormier Southern Hemisphere induced by topography and ocean circulation. PNAS. DOI: 10.1073/pnas.2123512119
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