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Understanding the Transport and Impact of African Dust on the Caribbean Basin

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Understanding the Transport and Impact of African Dust on the Caribbean Basin UNDERSTANDING THE TRANSPORT AND IMPACT OF AFRICAN DUST ON THE CARIBBEAN BASIN BY JOSEPH M. PROSPERO AND OLGA L. MAYOL-BRACERO The huge quantities of African dust carried into the Caribbean Basin warrant the formation of an observational program that tracks the impacts of this transport and future changes linked to climate variability. FIG. 1. A National Aeronautics and Space Administration (NASA) Sea-viewing Wide-Field Sensor (SeaWiFS) satellite composite image of an African dust outbreak on 3 Apr 2010. Dust extends from the coast of West Africa to the western Caribbean. The dust outbreak first emerged from Africa in late March; heavy dust transport continued across the region for several weeks. [Image courtesy of Norman Kuring, NASA Goddard Space Flight Center, and GeoEye.] FRICAN DUST TRANSPORT, METEOROLOGY, AND CLIMATE. The arid regions of North Africa are estimated to emit about 800 Tg yr–1 of soil dust each year, 70% of A the global total and six times more than the next largest source, Asia (Huneeus et al. 2011). A large fraction of these emissions are carried to the west over the Atlantic Ocean. Satellite images of individual dust outbreaks (Fig. 1) often show dust extending in a continuous plume from the coast of Africa into the Caribbean Basin. Satellite measurements of aerosol properties (e.g., aerosol optical depth; Hsu et al. 2012) clearly show the temporal and spatial distribution of dust AMERICAN METEOROLOGICAL SOCIETY SEPTEMBER 2013 | 1329 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC over this huge region. While satellites serve as a climate indices, such as El Niño–Southern Oscilla- valuable tool for studying dust transport, our detailed tion (ENSO) (Prospero and Lamb 2003), the Atlantic understanding of the long-term record of this trans- multidecadal oscillation (AMO) (Evan et al. 2011; port, its link to climate, and the specific impact on the Wang et al. 2012), and the North Atlantic Oscilla- Caribbean Basin rests largely on the aerosol records tion (NAO) (Ginoux et al. 2004; Evan et al. 2006). accumulated at Barbados (Prospero and Lamb 2003), Since the late 1980s, rainfall has improved in the Puerto Rico (Gioda et al. 2013; Reid et al. 2003a), Sahel, and in some years has been plentiful. Indeed, and Miami (Prospero 1999) . The longest records are there is evidence of “greening” across the region (de from Barbados, where studies began in 1965, and Jong et al. 2011). Nonetheless, Barbados dust con- from Miami, starting in 1974. These measurements centrations have remained higher than pre-drought continue to this day. A portion of these records, from levels. Moreover, in contrast to the first 25 years of 2004 to 2009, is shown in Fig. 2. Concentrations are the record, there is no clear relationship to Sahel greatest in Barbados and show a strong seasonal cycle rainfall or to the cited (and other) climate indices. with a summer maximum and winter minimum. The absence of such linkages makes it difficult to Miami dust concentrations track those in Barbados predict how dust emissions and transport might in a general way. The Miami record differs in that change over the coming decades as climate changes. concentrations are generally less than those in The problem is exacerbated by the inability of models Barbados and the dust transport season is shorter. (Seneviratne et al. 2012) to agree on future rainfall There are large year-to-year differences between trends over large areas of North Africa (including the two records, which are attributed to changes in the Sahel) that are known to be major dust sources large-scale winds and removal by precipitation. The today and in the recent past. similarities and differences between these two sites However, there are factors other than rainfall provide a sense of what we might expect to see across and dust source activity in Africa that could impact the entire basin. transport to the Caribbean (Engelstaedter et al. 2009). The Barbados measurements show that large If we wish to understand present-day variability and changes have occurred over the 48-yr record. Initially to predict future trends, we need a better under- these changes seemed to be linked to African cli- standing of the entire dust cycle, starting with the mate (Prospero and Lamb 2003). Concentrations processes that affect dust mobilization in Africa, the were low in the late 1960s, at the end of a long wet meteorological environment that controls transport phase in West Africa. Transport dramatically in- including the transit time, and finally the removal en creased in the early 1970s in apparent response to route, especially by precipitation. drought and again in the early 1980s when drought There is continuing debate about the nature of was most intense. Over this period trade-wind dust the most active dust sources and the relative im- concentrations were highly anticorrelated to Sahel portance of small-scale sources (dust “hot spots”) precipitation, used as a proxy for general changes in (Ginoux et al. 2012; Okin et al. 2011) such as dry source-region climate (Prospero and Lamb 2003). lakes, playas, and wadis as contrasted to larger-scale There were also suggestions of relationships to major terrain characteristics and climate (Bullard et al. 2011; Muhs 2013; Prospero et al. 2002). Of particular importance, and still highly uncertain, is the role AFFILIATIONS: PROSPERO—Cooperative Institute for Marine of anthropogenic land disturbance, which could and Atmospheric Studies, Rosenstiel School of Marine and account for a large fraction of current emissions Atmospheric Science, University of Miami, Miami, Florida; MAYOL- (Ginoux et al. 2012; Mahowald et al. 2010). There BRACERO—Institute for Tropical Ecosystem Studies, University of have been a number of major field campaigns in Puerto Rico, Río Piedras Campus, San Juan, Puerto Rico North Africa during the past decade that have led to CORRESPONDING AUTHOR: Joseph M. Prospero, Rosenstiel School of Marine and Atmospheric Science, University of Miami, a better grasp of dust mobilization processes, most 4600 Rickenbacker Causeway, Miami, Florida 33149-1031. notably the African Monsoon Multidisciplinary E-mail: [email protected] Analyses program (AMMA; Mari et al. 2011) and the Saharan Mineral Dust Experiment (SAMUM; The abstract for this article can be found in this issue, following the table of contents. Heintzenberg 2009). While these and other studies DOI:10.1175/BAMS-D-12-00142.1 have greatly broadened our knowledge of dust pro- cesses in North Africa, we nonetheless continue to In final form 24 January 2013 ©2013 American Meteorological Society lack a good understanding of the meteorological process that are the most important drivers of dust 1330 | SEPTEMBER 2013 Unauthenticated | Downloaded 09/24/21 04:11 PM UTC emissions (e.g., frontal systems, haboobs, mesoscale cyclonic sys- tems, and small-scale turbulent processes; Knippertz and Todd 2012). The Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Obser- vations (CALIPSO) dust product shows a strong link to the seasonal migration of the intertropical con- vergence zone over West Africa and monsoon dynamics (Adams et al. 2012). All these processes could play a fundamental role in dust emissions; the relative importance of these processes could change with season and with changing climate (Engelstaedter and Washington 2007; Williams 2008). Moreover, dust can itself impact FIG. 2. Monthly mean dust concentrations at Barbados and Miami, atmospheric and ocean processes to 2004–09. a degree that could affect weather and climate over the Atlantic and the Caribbean most notably the annual dust cycle at Barbados and Basin. Of particular interest is the evidence that high Miami and the southward shift of the plume in boreal dust concentrations and the meteorological environ- winter that results in transport to South America ment associated with Saharan air outbreaks could (Prospero et al. 1981) and the Amazon basin (Swap modulate the growth of tropical cyclones (Dunion et al. 1992). Model development is hampered by the and Velden 2004). Evan et al. (2011) suggest that dearth of dust measurements over the oceans. Given variations in dust transport over time are negatively the wide impact of dust over the Atlantic and the correlated to sea surface temperature changes over Caribbean Basin, measurements in this region would the tropical Atlantic. These changes, in turn, could contribute greatly to model development both to bet- be linked to the suggested negative correlation ter constrain emission estimates in North Africa and between Atlantic dustiness and hurricane activity. to better characterize transport and deposition over Cloud microphysics could also be affected by African the receptor region. dust, which can serve as both condensation (Twohy Improved model performance will require a more et al. 2009) and freezing nuclei (Cziczo et al. 2013; accurate description of all processes controlling dust Heymsfield et al. 2009). Because of the complexity of mobilization and distribution including the evolution the ocean–atmosphere processes in this region and and persistence of the strong stratification of dust the impact on cloud processes (Rauber et al. 2007) it layers. Early research (Carlson and Prospero 1972; is difficult to quantify the role of dust in this context Prospero and Carlson 1972) postulated the existence at this time. Nonetheless, there is a clear need to focus of the Saharan air layer (SAL)—a hot, dry, elevated on these relationships. layer within which the highest dust concentrations are usually found (Reid et al. 2003a). The SAL is a DUST MODELS. Models are essential to the persistent feature in sonde profiles over the region development of an understanding of the entire dust in summer (Dunion 2010). Long-term lidar studies cycle. However, dust models are in an early stage of on Barbados carried out by the University of Miami development. A recent intercomparison of 15 global (http://mplnet.gsfc.nasa.gov/index.html; select models in the Aerosol Comparisons between Obser- “Ragged Point”) and the Max Planck Institute for vations and Models (AEROCOM) project (Huneeus Meteorology in Hamburg (http://barbados.zmaw et al.
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