Why are there more storms in the southern hemisphere?

Oddly enough, the cumulative energy of depressions and storms in mid-latitudes (between the tropics and the poles) is 24% higher in the Southern Hemisphere than in the Northern Hemisphere—in other words, storms there are more numerous and more intense. How can this discrepancy be explained? The question remains unanswered to this day. Using a digital climate model, Tiffany Shaw and her colleagues from the University of Chicago have identified the factors that seem to explain this phenomenon.

Mid-latitude (or extratropical) depressions and storms are complex meteorological phenomena involving the convergence of many air masses of different temperatures and pressures. Its intensity depends on the amount of energy available in the atmosphere. The difference between the northern and southern hemispheres necessarily comes from the factors that affect the transfer of energy between the surface of the continents or the ocean and the atmosphere, or within the atmosphere (by acting on the phenomenon of convection or phase change for example).

As it is quasi impossible d’isoler l’effet de chaque facteur dans les observations de l’atmosphère, les chercheurs ont utilisé des simulations numeriques, et plus précisément le modèle numerique de climat ECHAM6, developed by the Max-Planck institute in Hamburg , In Germany. The team thus evaluated the effect of modifying some factors, which made it possible to isolate the causes of the observed asymmetry.

What are these factors? The Tiffany Shaw team was initially interested in the etching effect. In fact, the majority of the emerging lands, and thus the petroglyphs, are found in the northern hemisphere. However, according to Ludivine Arroba, a lecturer at the Sorbonne University, “patterns have an effect on the circulation of air masses and thus indirectly on the genesis of hurricanes. On the one hand, they act as obstacles that slow horizontal currents in the atmosphere; on the other hand, they can They act as springboards and trigger vertical air movements.The air rises and if it is moist enough, the water it contains condenses at a height on contact with the cold air, causing precipitation and releasing energy in the form of heat.

By artificially smoothing the hemisphere’s topography into the numerical model, the researchers observed an increase in the energy of storms in the two hemispheres and a shrinking difference between them. The researchers therefore suggest that on a global scale, northern hemisphere elevations limit the occurrence of extreme weather events. However, this factor explains only half of the difference between the two hemispheres.

To explain the rest, the researchers turned to ocean currents. The differences in temperature and salinity between the different oceans feed a whole network of currents that form a giant ring of convection that connects the oceans together. However, this thermohaline loop is asymmetric: it transfers more energy from the Southern Hemisphere to the Northern Hemisphere, contributing to higher temperatures in the Arctic, than in the Antarctic. However, the smaller temperature difference between the equator and the pole is less favorable for storm formation at mid-latitudes. Storms derive their energy from the temperature contrast between the equator and pole. via A mechanism known as “baroclinic instability”, Ludivine Oruba identifies.

By imposing an asymmetry of energy exchange between the two hemispheres—always by removing the heights—in the numerical simulation, the researchers effectively observed that the asymmetry of storms between the two hemispheres disappeared, although their overall frequency then increased by report to observations.

Since the beginning of the era of satellite observations, the north-south asymmetry appears to have widened further, with storm energy in the south increasing. The researchers suggest a possible explanation: Climate change is slowing the thermohaline cycle, which is moving in the direction of increasing the energy of identical storms in both hemispheres. However, in the northern hemisphere, melting of the polar cap and glaciers favors the absorption of solar radiation at the pole. This latter effect would counteract the modulating effect of ocean currents and protect the northern hemisphere from an increase in storm surge.