Professor Tiffany Shaw, Professor, Department of Geosciences, University of Chicago
        The southern hemisphere is a very turbulent place. Winds at various latitudes have been described as “roaring forty degrees”, “furious fifty degrees”, and “screaming sixty degrees”. Waves reach a whopping 78 feet (24 meters).
        As we all know, nothing in the northern hemisphere can match the severe storms, wind and waves in the southern hemisphere. Why?
       In a new study published in the Proceedings of the National Academy of Sciences, my colleagues and I uncover why storms are more common in the southern hemisphere than in the northern.
       Combining several lines of evidence from observations, theory, and climate models, our results point to the fundamental role of global oceanic “conveyor belts” and large mountains in the northern hemisphere.
        We also show that, over time, storms in the southern hemisphere became more intense, while those in the northern hemisphere did not. This is consistent with climate model modeling of global warming.
       These changes matter because we know that stronger storms can lead to more severe impacts such as extreme winds, temperatures and rainfall.
        For a long time, most observations of the weather on Earth were made from land. This gave scientists a clear picture of the storm in the northern hemisphere. However, in the Southern Hemisphere, which covers about 20 percent of the land, we did not get a clear picture of storms until satellite observations became available in the late 1970s.
       From decades of observation since the beginning of the satellite era, we know that storms in the southern hemisphere are about 24 percent stronger than those in the northern hemisphere.
        This is shown in the map below, which shows the observed average annual storm intensity for the Southern Hemisphere (top), Northern Hemisphere (center) and the difference between them (bottom) from 1980 to 2018. (Note that the South Pole is at the top of the comparison between the first and last maps.)
        The map shows the persistently high intensity of storms in the Southern Ocean in the Southern Hemisphere and their concentration in the Pacific and Atlantic Oceans (shaded in orange) in the Northern Hemisphere. The difference map shows that storms are stronger in the Southern Hemisphere than in the Northern Hemisphere (orange shading) at most latitudes.
       Although there are many different theories, no one offers a definitive explanation for the difference in storms between the two hemispheres.
        Finding out the reasons seems to be a difficult task. How to understand such a complex system spanning thousands of kilometers as the atmosphere? We cannot put the Earth in a jar and study it. However, this is precisely what scientists who study the physics of climate are doing. We apply the laws of physics and use them to understand the Earth’s atmosphere and climate.
        The most famous example of this approach is the pioneering work of Dr. Shuro Manabe, who received the 2021 Nobel Prize in Physics “for his reliable prediction of global warming.” Its predictions are based on physical models of the Earth’s climate, ranging from the simplest one-dimensional temperature models to full-fledged three-dimensional models. It studies the response of the climate to rising levels of carbon dioxide in the atmosphere through models of varying physical complexity and monitors emerging signals from underlying physical phenomena.
        To understand more storms in the Southern Hemisphere, we have collected several lines of evidence, including data from physics-based climate models. In the first step, we study observations in terms of how energy is distributed across the Earth.
        Since the Earth is a sphere, its surface receives solar radiation unevenly from the Sun. Most of the energy is received and absorbed at the equator, where the sun’s rays hit the surface more directly. In contrast, poles that light hits at steep angles receive less energy.
        Decades of research have shown that the strength of a storm comes from this difference in energy. Essentially, they convert the “static” energy stored in this difference into “kinetic” energy of motion. This transition occurs through a process known as “baroclinic instability”.
        This view suggests that incident sunlight cannot explain the greater number of storms in the Southern Hemisphere, since both hemispheres receive the same amount of sunlight. Instead, our observational analysis suggests that the difference in storm intensity between south and north could be due to two different factors.
        First, the transport of ocean energy, often referred to as the “conveyor belt.” Water sinks near the North Pole, flows along the ocean floor, rises around Antarctica, and flows back north along the equator, carrying energy with it. The end result is the transfer of energy from Antarctica to the North Pole. This creates a greater energy contrast between the equator and the poles in the Southern Hemisphere than in the Northern Hemisphere, resulting in more severe storms in the Southern Hemisphere.
        The second factor is the large mountains in the northern hemisphere, which, as Manabe’s earlier work suggested, dampen storms. Air currents over large mountain ranges create fixed highs and lows that reduce the amount of energy available for storms.
        However, analysis of observed data alone cannot confirm these causes, because too many factors operate and interact simultaneously. Also, we cannot exclude individual causes to test their significance.
       To do this, we need to use climate models to study how storms change when different factors are removed.
        When we smoothed out the earth’s mountains in the simulation, the difference in storm intensity between the hemispheres was halved. When we removed the ocean’s conveyor belt, the other half of the storm difference was gone. Thus, for the first time, we uncover a concrete explanation for storms in the southern hemisphere.
       Since storms are associated with severe social impacts such as extreme winds, temperatures and precipitation, the important question we must answer is whether future storms will be stronger or weaker.
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        A key tool in preparing societies to cope with the effects of climate change is the provision of forecasts based on climate models. A new study suggests that average southern hemisphere storms will become more intense towards the end of the century.
        On the contrary, changes in the average annual intensity of storms in the Northern Hemisphere are predicted to be moderate. This is partly due to competing seasonal effects between warming in the tropics, which makes storms stronger, and rapid warming in the Arctic, which makes them weaker.
        However, the climate here and now is changing. When we look at changes over the past few decades, we find that average storms have become more intense over the course of the year in the southern hemisphere, while changes in the northern hemisphere have been negligible, consistent with climate model predictions over the same period.
        Although the models underestimate the signal, they indicate changes occurring for the same physical reasons. That is, changes in the ocean increase storms because warmer water moves toward the equator and colder water is brought to the surface around Antarctica to replace it, resulting in a stronger contrast between the equator and the poles.
       In the Northern Hemisphere, ocean changes are offset by the loss of sea ice and snow, causing the Arctic to absorb more sunlight and weakening the contrast between the equator and the poles.
        The stakes of getting the right answer are high. It will be important for future work to determine why the models underestimate the observed signal, but it will be equally important to get the right answer for the right physical reasons.
       Xiao, T. et al. (2022) Storms in the Southern Hemisphere due to landforms and ocean circulation, Proceedings of the National Academy of Sciences of the United States of America, doi: 10.1073/pnas.2123512119
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