Climate change to intensify important African weather systems

Weather systems that bring rainstorms to many drought-prone areas of northern Africa, carry Saharan dust across the ocean and seed Atlantic hurricanes could grow stronger as a result of human-caused climate change, a new analysis by Stanford scientists suggests.

Known as African easterly waves, or AEWs, these weather systems form above northern Africa during the summer season and travel east to west, toward the Atlantic Ocean.

"Not only are AEWs important for rainfall in West Africa, they also play a role in climate across the Atlantic, including here in the United States," said Noah Diffenbaugh, an associate professor of environmental Earth system science and a senior fellow at the Stanford Woods Institute for the Environment.

The climate of West Africa varies sharply from the wet tropical region along the equator to the very dry Sahara desert in the north. The strip of land that lies between these two extremes, called the Sahel, has experienced some of the most prolonged and severe droughts in the world over the past half century.

AEWs travel from east to west across northern Africa along two tracks. One track lies along the southern Sahel and Guinea coast region. The other track follows the border between the northern Sahel and southern Sahara Desert. Along the northern track, the strength of the AEWs is driven largely by the difference in the ground temperature of the Sahara and the relatively cooler surface temperatures over the Sahel and Guinea Coast farther south. The greater the temperature difference, the more potential energy there is for storm systems such as AEWs to draw from.

Because AEWs have such a strong influence on the climate in Africa and the Atlantic basin, Diffenbaugh and a graduate student in his lab, Christopher Skinner, wanted to understand how a warming atmosphere might affect the strength and track of AEWs. Their research is detailed in the April 28 issue of the Proceedings of the National Academy of Sciences.

Computing a consensus

The pair began by analyzing simulations from 17 computer models of interactions between Earth's ocean and atmosphere. Each model was produced by a different research institute, and each one simulates physical processes in a slightly different way.

"For example, all models need a component that simulates rainfall. There are multiple ways to represent rainfall in a model, and each model does it slightly differently," Skinner said. "By using multiple models we are able to get a better sense of what the possible range of climate responses will be for a given level of greenhouse gases in the atmosphere."

Diffenbaugh and Skinner focused on simulations of AEWs during the period from 1980 to 2005 and simulations of AEWs during a projected future period in which the concentration of atmospheric carbon dioxide is roughly twice what it is today. Although some of the models differed in their simulation of AEWs during the 20th century, nearly all agreed that the winds associated with AEWs would grow stronger by the late-21st century if increases in greenhouse gas emissions continue along their current trajectory.

Additionally, all of the models predicted that as greenhouse gases rise, both the Sahara Desert and the Guinea coast region to the south will heat up, but the desert will warm more than the Guinea region.

"The temperature difference between the desert and the region farther south actually becomes larger than it is today," Skinner said. "Because the strength of the African easterly waves is influenced by the temperature difference between these two regions, we would expect the energy of the AEWs to become larger, and that's what the simulations show."

More dust, increased rain

In particular, the models predict a strengthening in the AEWs that travel near the border of the Sahara and the Sahel. This strengthening could have important impacts on precipitation in the drought-prone Sahel region.

"This is a region that has experienced some of the most severe humanitarian disasters from droughts," Diffenbaugh said. "But there has also been a lot of uncertainty about how global warming could impact rainfall in that region. To see such clear agreement in the response of AEWs to climate change opens the door for increasing our understanding of Sahel precipitation."

A strengthening of waves in this region could also mean more uplift and transport of dust out of Africa and across the Atlantic. In the current climate, these dust plumes deliver life-sustaining nutrients to the ocean but also can affect rainfall and air quality as far away as the Caribbean.

The authors also note that stronger AEWs could influence hurricanes that form in the Atlantic. The African easterly waves themselves don't become hurricanes, but a wave can create a protective environment in which significant rainfall and vertical wind motion can develop. "This convection can serve as the seed for a hurricane," Skinner said.

Not all Atlantic hurricanes are tied to AEWs, but studies have indicated that about 80 percent of the most intense hurricanes are associated with the African disturbances. A stronger AEW could conceivably influence the likelihood that the AEW generates a tropical cyclone, but the authors urge caution in jumping to conclusions.

"Hurricanes will be affected by global warming through changes in sea surface temperature, wind shear, and other environmental variables," Skinner said. "This is just one piece of a very complicated puzzle, but it's an interesting piece that hasn't really been looked at before."