Rainfall from fierce tropical cyclones in the Atlantic Ocean may have a surprising cause - dust from the Sahara.
Fine desert dust blown across the ocean can make the rain heavier by changing the size of cloud droplets, a new study says.
Peak dusty conditions, however, can have the opposite effect and lessen rainfall, especially in waters closer to the shores of Africa.
This 'boomerang-shape relationship' was discovered by Stanford University scientists, who say global warming could change the natural picture.
Scientists expect less Saharan dust to blow in future due to changed ocean winds, one of the Stanford authors, Yuan Wang, explained to The National.
How this affects the rainfall from tropical cyclones may depend on which part of the Atlantic is at the eye of future storms.
A typical Atlantic hurricane season, from June to November, brings more than a dozen named storms and about seven hurricanes, according to US government meteorologists.
Rain effect
The amount of rain is down to various factors but the new study suggests the dust transported from the Sahara plays a significant role.
Further from the Sahara's edge, the background level of 'dust loading' is relatively low and a wind from the desert can lead to heavier rain.
The study suggests this could be because the dust acts as an aerosol that encourages water particles to form in the atmosphere.
"When background dust loading is relatively low, which is more observed in the western Atlantic, dust tends to increase the rainfall," Prof Wang said.
"However, when background dust loading is higher, more dust will reduce the precipitation."
This may be because the dusty air keeps moisture at a low level, or blocks solar radiation that can influence storms.
An example of this was during the 1970s and 1980s, when there was a downturn in Atlantic hurricane activity, the paper says.
It coincided with high dust emissions coming from Africa in that period, believed to be linked to drought and air pollution over land.
Sprinkled dust
The mineral-rich Saharan dust blown westward by 'trade winds' can also feed tiny marine life in the Atlantic, such as phytoplankton.
Even further west, the iron and phosphorus airlifted from the Sahara acts as a fertiliser for the Amazon rainforest.
The dust can also cause haze, poor air quality, navigation problems and dramatic red and orange skylines far from the desert.
In the opposite direction, the warm current of the Gulf Stream keeps the Atlantic's eastern shores, such as Britain and Europe, warmer than they would otherwise be.
A separate study this month found that global warming could weaken the Gulf Stream and, paradoxically, cool down parts of Europe.
