In the shimmering deserts of Tunisia and Morocco, an unexpected messenger carries news of disease: the Atlantic Ocean. A new study shows that the climate memory stored in those far-off waters can warn of a skin disease outbreak months before the first lesion appears — giving doctors in North Africa a powerful head start.
The disease is cutaneous leishmaniasis, a parasite spread by the bite of tiny insects called sand flies. It rarely kills, but it leaves painful skin ulcers that can last for years and scar permanently, bringing physical, psychological and social costs. Roughly one million new cases appear worldwide every single year, especially across the Mediterranean Basin, the Middle East, Central Asia and the Americas.
For a long time, scientists thought this disease was simply too unpredictable to forecast in temperate regions. Early warning systems worked well in the tropics, where climate patterns like El Niño let researchers predict outbreaks months ahead. But in temperate zones, the atmosphere was seen as too chaotic to read.
A new study in the journal Science Advances, led by the Barcelona Institute for Global Health (ISGlobal) with the Pasteur Institutes of Tunisia, Morocco and France, shows that gap can be closed. The team tapped into two big Atlantic climate rhythms: the North Atlantic Oscillation (NAO), which steers winds and winter rains across Europe and North Africa, and the Atlantic Multidecadal Variability (AMV), slow shifts in ocean surface temperatures that unfold over years or decades.
Here is how the chain works. Warmer or cooler Atlantic waters change the winds that carry moisture toward North Africa. Those winds bring rain. The rain makes desert plants grow. The plants feed rodents — and rodents are the reservoirs where the parasite hides. When the desert blooms, the disease follows. Because the desert sits under so few competing climate forces, the Atlantic's signal shows through with striking clarity, like a clean mirror reflecting a faraway storm.
"The desert acts as a natural filter that reveals the Atlantic's climate memory," said Adrià San-José, ISGlobal researcher and first author of the study. "We identified a region where rainfall is remarkably predictable, the deserts of Tunisia and Morocco, and demonstrated that this predictability is also transferred to an infectious disease."
Using that insight, the team built a dynamic model of how the parasite moves between humans, sand flies and rodents, weaving together local temperature and rainfall with signals from the atmosphere and the ocean. The model successfully predicted both when and how intense outbreaks of Leishmania major and Leishmania tropica would hit Morocco and Tunisia — up to 12 months in advance. In Tunisia, where the Atlantic's grip is weaker and patchier, the forecasts gained less.
It is a hopeful glimpse of a smarter future for public health: reading the ocean to guard the people who live nearest the desert, and turning distant climate memory into months of extra time to prepare.