Of dust and hurricanes

The Saharan Air Layer (SAL) is an layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion--a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this "inversion" acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm's updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane. The dust may also act to interfere with the formation of cloud drops and rain drops that these thunderstorms need to grow, but little is known about such effects.


Figure 1. Map of the mean summer dust optical thickness derived from satellite measurements between 1979 and 2000. Maximum dust amounts originate in the northern Sahel (15 to 18 N) and the Sahara (18 to 22 N). The Bodele depression in Chad is also an active dust source. Image credit: Evidence of the control of summer atmospheric transport of African dust over the Atlantic by Sahel sources from TOMS satellites (1979-2000) by C. Moulin and I. Chiapello, published in January 2004 in Geophysical Research Letters.

The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18 - 22 N) and the northwestern Sahel (15 - 18 N) (Figure 1). The dust that originates in the Southwest Sahara stays relatively constant from year to year. However, the dust from the northwestern Sahel varies significantly from year to year, and understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is. The level of drought experienced in the northwestern Sahel during the previous year is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. In 2005 (Figure 2), precipitation across the northwestern Sahel averaged near normal, so I'm a bit surprised we saw so much dust over the Atlantic. So far in 2006, precipitation in the northwestern Sahel has been lower than in 2005. If the research cited above is any indication, we should have at least as much dust over the Atlantic during the 2007 hurricane season as the 2006 hurricane season had, which should act to hamper hurricane formation in the region between Africa and the Lesser Antilles Islands.



Figure 2. Departure of precipitation from normal for the African rainy season. Precipiation was near normal averaged across the northwestern Sahel region. Image credit: NOAA.


Jeff Masters