Why are there more storms in the Southern Hemisphere?

Ironically, the cumulative energy of depressions and storms in the mid-latitudes (between the tropics and the poles) is 24% higher in the Southern Hemisphere than in the Northern Hemisphere – in other words, there are more and more intense storms there. How can this asymmetry be explained? The question remained unanswered until today. Using a numerical climate model, Tiffany Shaw of the University of Chicago and her colleagues pinpointed the factors that explain the phenomenon.

Mid-latitude (or extratropical) depressions and thunderstorms are complex meteorological phenomena involving the meeting of several air masses at different temperatures and pressures. Their intensity depends on the amount of energy available in the atmosphere. The difference between the northern and southern hemispheres is necessarily due to factors affecting the transfer of energy between the surface of the continents or between the ocean and the atmosphere or within the atmosphere (for example, by influencing the phenomena of convection or phase change).

Since it is almost impossible to isolate the influence of each factor in atmospheric observations, the researchers used numerical simulations, namely the ECHAM6 numerical climate model developed by the Max Planck Institute in Hamburg, Germany. Thus, the team evaluated the effect of changing certain factors, which allowed them to isolate the causes of the observed asymmetry.

What are these factors? Tiffany Shaw’s team was first interested in the effect of reliefs. Indeed, the majority of emergent soils, and therefore landforms, are found in the northern hemisphere. However, according to Sorbonne University teacher Ludivine Oruba, “reliefs affect the circulation of air masses and, consequently, the formation of cyclones. On the one hand, they act as barriers that slow down horizontal atmospheric flows; on the other hand, they can act as a springboard and cause vertical air movements. The air rises and if it is humid enough, the water it contains condenses at altitude in contact with the cold air, causing rain and releasing energy in the form of heat. .

By artificially smoothing the Earth’s topography in a numerical model, the researchers observed that storm energy in both hemispheres increased and the difference between them decreased. Therefore, researchers suggest that on a global scale, the topography of the Northern Hemisphere limits the occurrence of severe weather events. However, this factor explains only half of the difference between the two hemispheres.

To explain the rest, researchers turned to ocean currents. Differences in temperature and salinity between different oceans feed a network of currents that form a giant convection ring connecting the oceans. However, this so-called “thermohaline” cycle is asymmetric: it transfers more energy from the Southern Hemisphere to the Northern Hemisphere, which contributes to warmer temperatures in the North Pole than in the South Pole. However, the lower temperature difference between the equator and the pole is less conducive to the formation of storms in the midlatitudes. “Storms get their energy from the temperature contrast between the equator and the poles. through a mechanism known as ‘baroclinic instability'”, reports Ludivine Oruba.

By imposing the symmetry of energy exchange between the hemispheres in numerical simulations – and always removing terrain – the researchers effectively observed the asymmetry of storms between the two hemispheres disappear, although their overall frequency increases with later reporting. observations.

Since the beginning of the era of satellite observations, the asymmetry between North and South has been widened by increasing storm energy in the South. The researchers suggest a possible explanation: climate change is slowing down the thermohaline circulation, which tends to increase the energy of the same storms in the two hemispheres. However, in the northern hemisphere, the melting of the polar cap and glaciers contributes to the absorption of solar radiation at the pole. This latter effect would counteract the modification of ocean currents and protect the Northern Hemisphere from increased storms.

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