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Saharan Dust Explainer

Saharan dust can affect hurricane activity in several ways:

  1. Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow.
  2. The Saharan Air Layer (SAL) is a 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.
  3. Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as "condensation nuclei"--centers around which raindrops can form and grow--the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)
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 2012 (Figure 2), precipitation across the northwestern Sahel was much above average, which should result in less dust than usual over the Atlantic during the 2013 hurricane season.

The future of African dust: highly uncertain

A September 2013 paper in the Bulletin of the American Meteorological Society by Joseph Prospero and Olga Mayol-Bracero, "Understanding the Transport and Impact of African Dust on the Caribbean Basin," discusses the large uncertainties on how African dust may change due to climate change. Over the past decade, there has been no clear relationship between African dust and climate indices such as rainfall in the Sahel or the El Niño/La Niña cycle, which "makes it difficult to predict how dust emissions and transport might change over the coming decades as climate changes. The problem is exacerbated by the inability of models (IPCC 2007) to agree on future rainfall trends over large areas of North Africa (including the Sahel) that are known to be major dust sources today and in the recent past."

Learn More:

Saharan Air Layer Analysis from the University of Wisconsin

The Saharan Air Layer (SAL) was first described in 1972, in this classic paper: Carlson, T. N., and J. M. Prospero (1972), The Large-Scale Movement of Saharan Air Outbreaks over the Northern Equatorial Atlantic, Journal of Applied Meteorology, 11(2), 283-297

Dr. Amato Evan published a study in Science magazine March 2009 showing that 69% of the increase in Atlantic sea surface temperatures over the past 26 years could be attributed to decreases in the amount of dust in the atmosphere.

Kaufman, Y. J., I. Koren, L. A. Remer, D. Rosenfeld, and Y. Rudich, 2005a: The effect of smoke, dust, and pollution aerosol on shallow cloud development over the Atlantic Ocean. Proc. Natl. Acad. Sci. USA, 102, 11 207–11 212.

Wang, Chunzai, Shenfu Dong, Amato T. Evan, Gregory R. Foltz, Sang-Ki Lee, 2012, Multidecadal Covariability of North Atlantic Sea Surface Temperature, African Dust, Sahel Rainfall, and Atlantic Hurricanes, J. Climate, 25, 5404–5415. doi: http://dx.doi.org/10.1175/JCLI-D-11-00413.1

West Africa (Sahel) Accumulated Precipitation Actual vs. Normal Daily Precipitation (mm) Last 365 days.

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