Of tempests, barren ground and a thousand furlongs of sea

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.

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