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Of tempests, barren ground and a thousand furlongs of sea

Filed under: — group @ 21 May 2009

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.

So how did they do this calculation? Firstly, they use a relatively simple model to relate SST to the reduction in net radiation into the ocean surface, prior to any climatic response. This forcing is calculated using the total aerosol amount inferred from the AVHRR data. Variations in SST due to variations in heat transport by ocean currents or diffusion into the thermocline are neglected while contributions by changes in evaporation, turbulent transfer, and surface radiation are estimated as being proportional to the anomalous air-sea temperature difference. Cooling of the ocean by aerosols must therefore be offset by a reduction in heat lost from the ocean to the atmosphere.

They note a key simplification is their neglect of any change to the surface air temperature when calculating anomalous air-sea temperature difference. This would require an atmospheric model along with a consideration of aerosol forcing at the top of the atmosphere (TOA). There is a strong relationship between surface air temperature and TOA forcing (at least at large spatial scales). As a consequence, the ocean-atmosphere flux depends upon not only forcing at the surface but the forcing at the TOA. By neglecting the effect of the changes in surface air temperature upon SST, Evan et al. may be underestimating the impact of the aerosols on their calculated trend. This is especially important for volcanic aerosols, whose TOA forcing is large and comparable to the surface forcing, as opposed to absorbing aerosols like dust where the surface forcing is larger than at TOA. However, balancing this effect is the neglect of heat diffusion into the thermocline which would reduce the ocean cooling. It is not a priori obvious which effect is more important, especially since the atmosphere can balance the forcing by adjusting lateral heat transport, which would also influence the anomalous surface air temperature.

Another way to test the importance of atmospheric changes would be to calculate both the TOA and surface forcing using the satellite measurements, and then impose this transient forcing in a general circulation model that calculates both the atmosphere and ocean response. That too would have problems, given that the models are not perfect, but it would be a useful check on the order of magnitude of the inferred effects. Indeed, assessments of the causes of tropical Atlantic trends using the IPCC AR4 models (Santer et al, 2006) come up with a much larger component due to anthropogenic effects, though those models did not include dust forcing changes.

Using their methodology, Evan et al. find that a decline in total aerosols contributed around two-thirds of the observed warming in NH tropical Atlantic SST between 1982 and 2007. Most of this is due to the two major volcanic eruptions (El Chichon and Pinatubo) that cooled the ocean early on in this period (and so lead to a warming once they were no longer present). However, the attributed aerosol trend would have been smaller had the satellite record extended a decade earlier. The estimated contribution of dust changes to the observed trend is small, roughly one-quarter of the total trend.

Whatever its impact upon SST, dust might impact other factors contributing to cyclone intensity (Emanuel, 1995), in particular, the reduction of the air-sea heat flux and temperatures in the upper troposphere. Unfortunately, global models don’t quite have the resolution to explicitly calculate all these effects.

Ultimately, the effect of dust upon hurricanes is important because, like ocean temperatures, African dust export is expected to change during the 21st century in response to global warming and changes in African rainfall. One study shows that dust production is expected to decrease (Mahowald and Luo, 2003), though given the diversity of Sahel rainfall projections and the preliminary state of vegetation models, this is not necessarily going to be a universal response.

The calculation by Evan et al. is an interesting first step to quantifying the effect of dust changes on SST, but there plenty of issues left to investigate.

Footnote: For some presumably poetic reason, the Bard neglected to note that the Main Development Region is more like 25,000 furlongs across and the Sahara is about 2 billion acres.

69 Responses to “Of tempests, barren ground and a thousand furlongs of sea”

  1. 51
    Hank Roberts says:

    Chuckle. He’s found a cozy place over at WTF; not worth more time here.

  2. 52
    Jim Norvell says:

    Google pyrocumulonimbus and you will get many hits. See From the abstract, “Very recent investigations into pyroCb have revealed that this
    phenomenon has occurred with surprising regularity, and in both northern and southern
    hemispheres. Moreover, it has been learned that the historical record in the satellite
    era includes several cases of stratospheric aerosol layers, formerly attributed to volcanic
    eruptions, of pyroCb polluting the stratosphere”

  3. 53
    Hank Roberts says:

    Yes, it’s interesting to read these things:

    2009: “Washington-based scientist Michael Fromm agrees the phenomenon is unprecedented. ‘We have seen aerosols higher in the stratosphere by several kilometres than we have ever observed anywhere on the earth,’ he said.”

    2005: Polluting the stratosphere: an assessment of the impact
    of pyro-cumulonimbus
    M. Fromm (1), R. Servranckx (2), R. Bevilacqua (1), G. Nedoluha (1)
    1. Naval Research Laboratory, Washington DC, USA
    2. Canadian Meteorological Centre, Dorval, Quebec, Canada

    Review: “… higher in the stratosphere by several kilometres than we have ever observed anywhere on the earth,” he said.” (2009)

  4. 54
    Hank Roberts says:

    PS, to be fair, I only warn against mistakes I make myself (grin). I read “reached Antarctica” above and assumed “reached Antarctica” meant reached the continent, so the precedent would be the paleo record. Not so, obvious as soon as I read and quoted from the first link. Dr. Fromm is quoted, and wrote in 2005, on how high in the stratosphere smoke has been observed over the satellite record.

    Is smoke found in the paleo cores? Will this year show a layer? Dunno.

  5. 55
    Hank Roberts says:

    PPS, here’s how it works, though none of these pyrocumulus continue to rise — you can see the smoke rising and how when the smoke cools sufficiently the water in it condenses — it’s releasing heat of vaporization, and that heat energy lifts the water cloud much higher than the smoke is rising. A few times you can see, with the collapse of the vapor, what looks like rain or ice falling out. (This must have been a day with a significant inversion layer — warmer above — so the pyrocumulus did not keep rising into the higher level air)

    On a sufficiently big pyrocumulus in the right conditions, these would continue to climb and grow and carry smoke up with them instead of collapsing.

    Quite a video (time lapse, speeded up), from a California fire: hat tip to ‘GoletaBrian’

  6. 56
    Antonio San says:

    Chris S. I guess Jim Norvell responded for me… and that too was priceless.

  7. 57
    David Horton says:

    Um yes, the unprecedented part was reaching Antarctica. Even relatively small fires can, in the right conditions, create cumulus clouds above the smoke. In some circumstances (notably in central Australia) the clouds can even generate rain. The difference here was the immense height achieved because of the enormous temperatures and in turn getting into the Antarctic circulation. If massive bushfires are going to be increasingly common in Australia, does ash reaching Antarctic present a future problem either in terms of aerosols or blackening of snow?

  8. 58
    Hank Roberts says:

    I don’t find anything about the smoke, or ash, reaching the surface.

    Here’s the report:

  9. 59
    Brian Dodge says:

    According to “The Bodélé Depression, Chad is the planet’s largest single source of dust.” Much of the dust is the silica shells of diatoms which were deposited when the area was a shallow lake. Because the shapes of the diatom particles are far from compact spheres, the ratio of aerodynamic drag to mass is higher, so I would expect this kind of particle to settle more slowly and become a larger fraction of the dust over time(or distance from the source).
    On the other hand, “Hygroscopicity and cloud condensation nucleus (CCN) activity were measured for three mineral dust samples: one from the Canary Islands, representing North African dust transported across the Atlantic…” and “Only the Canary Island sample generated from aqueous suspension showed appreciable hygroscopic growth at subsaturated conditions…” (, which may lead to higher rates of cloud formation and rainout of Bodélé sourced dust, and a different fractionation profile with time. I couldn’t find whether amorphous(diatom) silica is specifically more hygroscopic than crystalline silica and other minerals; question for Scott Robertson – did you look at the amorphous silica content of the dust, or other diatom identifiers? Could “hygroscopic growth at subsaturated conditions…” act to dry tropical waves and suppress hurricane formation?
    Other interesting abstracts i found (full articles paywalled) are:
    doi:10.1016/j.gca.2008.05.037 “We thus define an iberulite as a coassociation with axial geometry, constituted by well-defined mineral grains together with non-crystalline compounds, structured on a coarse-grained core and a smectite rind, with only one vortex and pinkish color, formed in the troposphere by complex aerosol–water–gas interactions.”

    On a completely different off topic question, could the sinuous lead/open water in the arctic ice from ~120E to 165E and 70-75N represent a hydrate instability contour where methane fountaining is causing localized ice melting?

  10. 60
    David Horton says:

    Hank, I’m assuming the aerosols will settle out eventually?

  11. 61
    Tom Woods says:

    Re #59:

    On a completely different off topic question, could the sinuous lead/open water in the arctic ice from ~120E to 165E and 70-75N represent a hydrate instability contour where methane fountaining is causing localized ice melting?

    What you are seeing is the main pack ice beginning to draw away from the shorefast ice along the Russian Coast.

  12. 62
    Scott Robertson says:

    Re #59


    We were simply analyzing the aerosol data generated by the IMPROVE network so we were only looking at elemental silicon, but the only silicon-containing source we identified was very characteristic of mineral dust (i.e. had common elements Ca, Al, Mg, Fe etc in ratios we expected for dust)I would expect any form a silica to be hydrophobic. My adviser published this paper (Perry, K. D., S. S. Cliff, and M. P. Jimenez-Cruz (2004), Evidence for hygroscopic mineral dust particles from the Intercontinental Transport and Chemical Transformation Experiment, J. Geophys. Res., 109, D23S28, doi:10.1029/2004JD004979.) The transport was from China and they think the hygroscopic nature of the dust is from pollution interaction (pretty likely in China) prior to transport.

  13. 63
    Hank Roberts says:

    David, re
    > assuming the aerosols will settle out eventually?

    I just looked briefly for mention of such layers in the ice cores from the past and didn’t find mention of smoke apart from volcanic ash in layers; I’d guess someone’s collecting what falls out each year and perhaps someone knows more. Believing it’s there is reasonable; being able to show it’s there is publishable (grin).

  14. 64
    David Horton says:

    Hank – I wasn’t suggesting it is there, on the ice surface, already. And my impression is that there isn’t any evidence of such an event in the past, but I may be wrong (could volcanic ash be distinguished from bush fire ash, I guess so). But it just seems to me that this event and the arrival of aerosols in Antarctic air (at least), as well as the movement of desert dust into the Atlantic, may be other unexpected consequences of global warming, with potential feedback (positive or negative) effects. Dust from central Australia reaches well into the Tasman Sea (and has been found in sediments) and I think all the way to New Zealand. Pollution from China reaches America. Smoke from burning Malaysian rainforest covers huge areas. I hadn’t seen any discussion of such processes as part of the climate change modelling process, and I just wonder if this might begin now.

  15. 65

    There is a whole literature on transatlantic transport of fungal spores, and bacteria some of which have been suggested as sources of Caribbean coral stress . Çile, ben, aerosolların, sonunda dışarı yerleştirecek olduğunu farz ediyorum? Thank you

  16. 66
    Ike Solem says:

    For the other (closed) post, see this for fun:

    It does apply to ocean cycles, ENSO influence, etc.

    David Horton:
    I hadn’t seen any discussion of such processes as part of the climate change modeling process, and I just wonder if this might begin now.

    See the most recent IPCC reports. Here’s the most recent one, the modeling section:

    Aerosols play an important role in the climate system. Interactive aerosol parametrizations are now used in some models (HADGEM1, MIROC-hi, MIROC-med). Both the ‘direct’ and ‘indirect’ aerosol effects have been incorporated in some cases (e.g., IPSL-CM4). In addition to sulphates, other types of aerosols such as black and organic carbon, sea salt and mineral dust are being introduced as prognostic variables (Takemura et al., 2005; see Chapter 2)

    Here’s the discussion of the data that goes into those assessments:

    Also see Chapter Seven: Global Climate Model Estimates of the Total Anthropogenic Aerosol Effect

    Then, you had all the Pinatubo test case studies, which showed that for volcanic aerosols, the model predictions were pretty accurate.

    Finally, on black carbon reaching Antarctica:

    Continuous high-temporal resolution black carbon ice core records from Antarctica, Edwards et al. 2008

    Hope that helps. For more, see this recent realclimate post.

    Some of the most interesting conclusions of the study include those relating to the Arctic. For example, we estimate that black carbon contributed 0.9 +/- 0.5ºC to 1890-2007 Arctic warming (which has been 1.9ºC total), making BC potentially a very large fraction of the overall warming there. We also estimated that aerosols in total contributed 1.1 +/- 0.8ºC to the 1976-2007 Arctic warming.

    All told, there’s nothing involving aerosols that changes the basic relationships between fossil fuel combustion, deforestation, atmospheric CO2 content and global warming.

  17. 67
    David Horton says:

    Thanks for the references Ike, very useful. But “All told, there’s nothing involving aerosols that changes the basic relationships between fossil fuel combustion, deforestation, atmospheric CO2 content and global warming” – I never thought there was. Just wondered if the increase in bushfires and dust storms, with increasing severity spreading material far and wide, as the climate warms was yet another positive feedback mechanism.

  18. 68
    Mark says:

    Thank you very much for the references and links in the article/comments.

    In regards to the Australian brush-fires, the droughts have become so severe that much of west Australia has lost all vegetation for miles…it’s a tragic thing to see first hand.

    If you have strong feelings on water efficiency and/or flooding from global warming, then watch these and send them to people you know. Everyone can help turn climate change around.

  19. 69
    ed hardy says:

    A smoothed time series of northern tropical Atlantic dust cover (Fig. 1) shows a maximum and minimum in dust activity that occurred in 1985 and 2005, respectively, and a downward trend in dust optical depth over the record.ed hardy
    buy ed hardy
    Many gases including water vapor are transparent to visible light, but some like water vapor condense into particles/droplets. Those as they get larger, including water clouds in our atmosphere, scatter light in various ways.