In response to global warming, the position of the tropical rain belt is changing. But this shift is not homogeneous. It varies according to the regions of the globe as detailed in a new study. Given the structural dependence of many intertropical countries on these rains, any change threatens the food security of billions of people. Results published in the journal Nature climate change on January 18.
Atmospheric circulation is remarkably structured on a large scale. Also, at any moment, a satellite image of the globe reveals the presence of a narrow band of thunderstorms near the equator. We speak of Zone de Convergence Inter-Tropicale (ZCIT). In addition, we identify much calmer regions on both sides – corresponding to the belt of the great deserts – and the presence of large cloudy windings at mid and high latitudes. This arrangement results from a very fine balance between the pressure force linked to the differential heating and the Coriolis effect linked to the rotation of the globe.
If the constraints mentioned above impose a preferential organization on the atmosphere, they nevertheless allow external factors to modulate this structuring a little. By external we mean things like volcanism, solar activity or the release of greenhouse gases from human activities. As a reminder, the latter exert a forcing on the climate system by limiting the amount of energy leaving the Earth. As a result, the planet is heating up.
A significant shift in the tropical rain belt
In a new study, researchers detail how global warming affects the position of ITCZ. In particular, they show that the effect is unequal depending on the longitude considered. As previous work had often focused on the mean per circle of latitude, regional peculiarities were not easily identifiable. These changes have major implications since we know that several billion people depend on these rains.
“Our work shows that climate change will cause the position of the tropical rain belt to shift in opposite directions in two longitudinal sectors covering almost two thirds of the globe,” says Antonios Mamalakis, lead author of the paper. “A process that will have cascading effects on water availability and food production around the world.”
In fact, eastern Africa and the Indian Ocean are expected to see a northward shift. Conversely, the eastern Pacific and the Atlantic Ocean are expected to see a southward shift. As a result, Southeast Africa, Madagascar as well as Central America would be promised a drier climate. On the contrary, the rains would become more frequent in southern Asia and the Arabian Peninsula.
Global warming and ZCIT: an innovative approach
These results were obtained through the detailed analysis of 27 high-resolution climate simulations. And this, in a scenario where the emissions of greenhouse gases continue until the end of the century. The innovative point of the study being the separation made between the western and eastern hemispheres. In particular, such an approach makes it possible to isolate the differentiated responses of the two sectors.
“In Asia, predicted reductions in aerosol emissions, melting glaciers in the Himalayas and loss of snow cover in northern regions caused by climate change will cause the atmosphere to warm faster than in other regions, ”explains James Randerson, co-author of the paper. “We know that the rain belt is moving towards this warming, and that its northward movement in the eastern hemisphere is consistent with the expected impacts of climate change.” Conversely, in the Western Hemisphere, the relative cooling present in the south of Greenland induces a reflux of the ITCZ towards the south.
“This study combines the engineering approach of systems thinking with data analysis and climate science to reveal subtle, previously unrecognized manifestations of global warming on precipitation dynamics,” he adds. .
