Of tempests, barren ground and a thousand furlongs of sea

Guest commentary by Ron Miller, NASA GISS

Several studies have shown that hurricane activity is generally reduced during years when there is a thick aerosol haze over the subtropical Atlantic. The haze is comprised mainly of soil particles, stripped by wind erosion from the barren ground over the Sahara and Sahel. These particles are lifted into the atmosphere and carried by the Trade winds as far as the Caribbean and Amazon basin. Plumes of dust streaming off the African coast are easily recognized in satellite imagery, and were even described by Charles Darwin during his voyage on the Beagle.

The amount of dust crossing the Atlantic has been measured at Barbados since the mid 1960s (aptly by Prospero and colleagues). These measurements show a threefold increase in dust between the original part of the record and the mid 1980s at the peak of the Sahel drought, when the region was unusually vulnerable to wind erosion. African dust crosses the tropical Atlantic within the Saharan Air Layer (SAL), an elevated duct of air between about 2 and 5 km in altitude. Because of its continental origin, this air is not only dusty but extremely dry.

Figure 2: Monthly mean dust concentration measured at Barbados. Arrows mark years with large El Niño events, which are irrelevant here (Prospero and Lamb, 2003).

There is an observed anti-correlation between dustiness and tropical cyclone days in the Atlantic (Evan et al, 2006). This anti-correlation might indicate the a direct influence of dust on hurricanes, or a connection between the dry air the dust resides in and hurricanes, or might even be related to a much larger scale pattern which controls both hurricanes and dustiness. If there is a connection, one hypothesis is that entrainment of dry SAL air rapidly strangles a developing cyclone because of the low humidity that accompanies the dusty air, while the dust itself has no direct effect. An alternative hypothesis is that the reduction in sunlight beneath the dust layer cools the ocean surface, whose temperature is a well-known predictor of hurricane activity (at least at the basin scale). Thus it is plausible that decadal variations in dustiness could contribute to decadal variations in hurricane activity, but how big might such an effect be?

A recent article in Science by Evan et al. (2009) is one of the few attempts to quantify the contribution of both dust and volcanic aerosols to the observed warming within the tropical Atlantic. The authors infer the amount of total aerosol using the Advanced Very High-Resolution Radiometer (AVHRR) satellite instrument and screen for locations where dust is present (they note that other aerosols might be mixed with the dust, but neglect this overlap). They also assume that dust has no effect where there are clouds. However, where the SAL extends over low marine clouds, the dust (since it is darker than cloud) might have an opposing effect to that seen in clear sky regions, although this is hard to quantify. They then calculate the contribution by dust and volcanic aerosols to observed changes in sea surface temperature (SST) during the satellite record between 1982 and 2007. During this period, the aerosol amount varied with dust export from Africa, but also from major eruptions by two volcanoes (El Chichon in 1982 and Pinatubo in 1991), each of which left a reflective layer of sulfate droplets in the lower stratosphere for a couple of years.

Evan et al. calculate that between 1982 and 2007 the ocean surface warmed by 0.25°C/decade in the main region of Atlantic hurricane genesis (15-­65°W and 0­-30°N). For comparison, they calculate a warming trend of 0.18°C/decade due to a reduction of dust and volcanic aerosols. That decreasing aerosols account for two-thirds of the observed warming might suggest that other factors like the increase in greenhouse gas concentrations (combined with anthropogenic aerosol changes) made a relatively modest net contribution to the warming (and by implication to observed trends in hurricane activity). For the natural aerosols, they calculate that stratospheric aerosols made roughly twice the contribution of dust over this period.

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