The hurricane seasons of 2005 and 2006 have posed a bit of a mystery to climatologists recently. After a whopping 15 hurricanes were forged out of the North Atlantic in 2005, including the devastating Katrina, only 5 hurricanes appeared a year later.
Most scientists will lay the blame for 2005’s record amount of hurricanes at the feet of a warming ocean. Heat from oceans increases the amount of hurricanes produced as well as their ferocity. So it seems only likely that a temporary cooling of the waters is the reason there were so few the following season.
Now, William Lau and Kyu-Myong Kim at NASA’s Goddard Space Flight Center, Greenbelt, Md., have pointed to airborne Saharan dust that made its way out over the North Atlantic as the reason for this cooling.
The pair reported Dec. 8, 2007 in the American Geophysical Union’s Geophysical Research Letters that the westerly driven sands of the Sahara had probably ended up blocking the sunlight above the water, effectively dropping the temperature of the North Atlantic. This is the first time that quantitative evidence showing the role that dust plays in the North Atlantic, and dusts role in hurricane activities.
“Previous studies have looked at how hot, dry air associated with a Saharan dust outbreak affects an individual storm, but our study is the first to focus on dust’s radiative effect on sea surface temperatures, which may affect storms for the entire season. Nobody had suggested that link before,” Lau says.
To investigate the link between dust and sea surface temperatures, Lau and Kim calculated the cooling pattern that Saharan dust should produce on temperatures as a result of its sun blocking. They then compared the results to observed temperature patterns. Using data collected by the National Center for Environmental Prediction, the pair found out how much solar energy enters Earth’s atmosphere, and subsequently reaches Earth’s surface free of any interference, such as dust and clouds.
With the concentration of atmospheric dust as measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument onboard a NASA satellite, Lau and Kim went about calculating the level of heat that could penetrate the dust and reach the surface of the North Atlantic waters. All of these calculations were found to closely match the observable evidence, thus supporting a link between the Saharan dust and the cooling levels of the North Atlantic.
The pair also looked back as well, examining historical dust and sea surface temperature records, to see if there was a link. What they found only further confirmed their original findings, with cooler North Atlantic surface temperatures occurring in more dusty years.
“We’re not saying that El Nino does not have a substantial influence on sea surface temperatures, but rather that dust is an important factor that we cannot ignore anymore. The 2007 hurricane season appears to be another one in which forecasts for an above normal hurricane season have failed,” Lau says.