Global warming is altering the position of the tropical rain belt

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In response to global warming, the position of the tropical rain belt is changing. But this movement is not homogeneous. It varies according to the regions of the globe as detailed in a new study. Given the structural dependence many intertropical countries to these rains, any change threatens the food security of billions of people. Results published in the journal Nature’s climate change this January 18.

The atmospheric circulation is remarkably structured on a large scale. Also, at any time, a satellite image of the globe reveals the presence of a narrow band of storms near the equator. We are talking about Inter-Tropical Convergence Zone (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 elements such as volcanism, solar activity or greenhouse gas emissions by 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.

Satellite image of April 23, 2020. Note the Zcit near the equator and areas generally poor in clouds on both sides. Finally, at medium and high latitudes we find more or less complex cloudy windings. Credits: Worldview / NASA.

A significant shift in the tropical rain belt

In a new study, researchers detail how global warming affects the position of the Zcit. In particular, they show that the effect is unequal depending on the longitude considered. As previous work has often focused on the mean per circle of latitude, regional particularities 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 at the level of two longitudinal sectors covering almost two thirds of the globe”, relates Antonios Mamalakis, main author of the paper. “A process that will have cascading effects on water availability and food production in the world “.

In fact, eastern Africa and the Indian Ocean are expected to experience a northward shift. Conversely, the eastern Pacific and the Atlantic Ocean are expected to see a southward shift. Therefore, Southeast Africa, Madagascar and Central America would be promised a drier climate. On the contrary, the rains would become more frequent in southern Asia and the Arabian Peninsula.

tropical rains
Thresholded image on visible channel. Credits: Eumetsat.

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 toisolate 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 Zcit to 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.

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