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African Winds may Change With Climate, Possibly Affecting Hurricanes, Drought and Dust

By Stephanie Paige Ogburn, E&E reporter
May 4, 2014; 1:37 AM ET

Scientists have discovered that yet another weather phenomenon is likely to be affected by climate change.

Called the African easterly wave (AEW), it's a summertime system that originates in North Africa but has links to Atlantic hurricanes and dust transported as far afield as North and South America.

A new paper published yesterday in the journal Proceedings of the National Academy of Sciences, found that a warming climate is likely to increase the strength of these waves, which flow like a jet stream from eastern Africa toward the west.

"In the future climate simulations, we find that the AEWs increase in strength within a specific region of northern Africa," said Christopher Skinner, the paper's lead author and a doctoral candidate in Stanford University's Department of Environmental Earth System Science.

Waves can lead to intense hurricanes

One reason researchers care about the waves is because they are linked to rainfall in the Sahel, a band of semiarid land across Africa that has experienced famine-causing megadroughts as well as devastating floods.

Most of the Sahel's inhabitants rely on rain-fed agriculture and livestock.

"Any weather system that influences rainfall in the Sahel is critical for the well-being of those living in the region," Skinner said.

AEWs are also associated with strong Atlantic hurricanes. While only a small fraction of AEWs become hurricanes, many of the most intense hurricanes form from them.

"Interestingly, close to 80 percent of the most intense hurricanes in the Atlantic form from African easterly waves," Skinner said.

While not all climate models can simulate AEWs, the researchers found 17 that could and ran them in a global warming scenario that matches our current emissions trajectory. Called RCP 8.5, this scenario is the most extreme warming the Intergovernmental Panel on Climate Change has outlined to date.

The scientists compared climate model simulations from 1980 to 2005 and how AEWs behaved over that time period, with a future time where CO2 concentrations are twice what they are now.

They found surprising agreement among the models, Skinner said, showing that the AEWs strengthen in the north.

Selection of favourite pictures from West African countries, including the Sahel and Coastal states, by various ECHO staff. ©EC/ECHO/Christophe Valingot)

Change in gradients strengthens waves

The reason for this strengthening, said Noah Diffenbaugh, a Stanford University professor who co-authored the article, is a change in the temperature gradient between air over the Sahara Desert and air over the tropical region of Africa, which is what leads to the formation of the waves.

Although IPCC reports have documented this likely change in the temperature gradient, scientists had not looked at how that would affect African easterly waves.

"And so we have done that in this paper," Diffenbaugh said.

The researchers were quick to note that although the models agree that the waves will strengthen, the ripple effects to Sahel rainfall and flooding, Atlantic hurricanes, and dust transport are still unknown.

Previous research on the impact of climate change on Atlantic hurricanes has largely indicated that there may be fewer, more intense hurricanes, although some researchers have also published research indicating there will be more hurricanes overall.

The strongest link might be to that of dust transport, since the area where the waves strengthen the most is also the source for much of the dust carried westward, Diffenbaugh said.

"For the tropical cyclones, that's a really complicated question, and certainly the AEWs are one piece of a puzzle and it is one piece that hasn't received as much attention, so I think that we will see how the community responds to this piece of the puzzle," the researcher said.

"We can't infer changes in tropical cyclones just from these changes in AEW activity," Diffenbaugh added.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.

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