Theory West African dust outbreaks and the relationship with North Atlantic hurricanes Amato T. Evan, Christopher S. Velden, Andrew K. Heidinger & Jason Dunion Climatology2006 The Saharan Air Layer forms over the Saharan desert region as a warm, well-mixed, dry, and dusty air mass, which under the right synoptic conditions can advect from West Africa into the tropical North Atlantic Ocean. The Saharan Air Layer rides over the Atlantic marine boundary layer and can be a significant tropospheric feature as it transits across the North Atlantic, often seen as far removed from the source as the Caribbean (~7,000 km west of the Sahara Desert). 1) Dry air intrusion into the storm, promoting downdrafts and disrupting the convective organization within the tropical cyclone vortex. 2) The midlevel jet found within the Saharan Air Layer increases the local vertical wind shear. 3) The radiative effects of the dust in the Saharan Air Layer may enhance the preexisting trade wind inversion. Evan et al. [2006] looked at a 25-year satellite record of dustiness, where dust is used as a tracer for the SAL, and defined as a significant signature of mineral-based aerosols (Saharan dust). They show that hurricane seasons with increased SAL dustiness over the Atlantic basin exhibits less tropical cyclone activity than those seasons with lower levels. Dunion and Velden [2004] suggest that the Saharan Air Layer could inhibit the formation, or reduce the intensity of tropical cyclones in the North Atlantic through three primary mechanisms: Conceptual model of the SAL from Karyampudi et al. [1999] Observations of the interaction between Hurricane Erin and the SAL (depicted as yellow/red colors in the satellite-derived SAL product imagery, available on the CIMSS tropical cyclone web site [Dunion and Velden, 2004]. The images on the left are of tropical cyclone days, and the maps on the right are of dust coverage derived from an AVHRR algorithm. Both displays represent average values over the period August 20 through September 30 for their respective years [Evan et al., 2006] Time series analysis of dust and tropical cyclone activity over the tropical North Atlantic reveals a robust negative correlation between the two. Removing years likely affected by ENSO, and another with a data gap, shows an even more significant linear relationship. According to the AVHRR dust product, 2006 was below average for dust activity relative to the 25-year mean. However, the dust loadings for August and September were higher than they have been for the last several years. Time series of dust over the main development region (10–20N & 15–65W). Dust effects on SST It is well-known that aerosols reflect sunlight, and that aerosol optical thickness in the tropical Atlantic (especially during hurricane season) can be affected by SAL dust. New results are showing that SST changes in the tropical Atlantic can be attributed to alterations of shortwave forcing by SAL aerosols. Lead/Lag correlations between dust and SST over the main development region. A simple radiative transfer model utilizing data from the AVHRR dust algorithm shows that temperatures over the Atlantic TC genesis region can be depressed by more than 1-degree Celsius due to the presence of significant SAL aerosols, and that there is a distinct trend in forcing since the start of the record. Satellite data set information: The plot on the left is the climatological mean monthly forcing by aerosols in the main Atlantic TC development region. The plot on the right shows a 25-yr time series of SST forcing in this region averaged over July, August & September. Long-term satellite studies have been made with data from the Pathfinder Atmospheres Extended AVHRR reprocessing project. Contact: References Information and data from this 25-year climatology can be found at Carlson, T. N., and J. M. Prospero (1972), J. Appl. Meteorol., 11, 283– 297. Dunion, J. P., and C. S. Velden (2004), Bull. Am. Meteorol. Soc., 85(3), 353–365. Evan, A. T., J. Dunion, J. Foley, A. Heidinger & C. Velden (2006), Geophys. Res. Lett., 33, L19813, doi: /2006GL Karyampudi, V. M., et al. (1999), Bull. Am. Meteorol. Soc., 80, 1045– August September Aerosol Optical Thickness