Saharan dust intrusions are a recurrent problem threatening public health and economic well-being of a large population in Southern Europe and West Africa. Forecasts of hazardous air pollution caused by dust aerosols are crucial to help reduce personal exposure to outdoor air pollution, and would help decision makers to restrict anthropogenic emissions of domestic, traffic and industrial pollutants during predicted periods of high pollution. Today, most operational dust forecasts extend only 2-5 days. Providing predictive information at longer lead times would give communities more time to prepare for risks associated with airborne dust.
Here we show that on timescales of weeks to months, North African dust emission and transport are impacted by sudden stratospheric warmings (SSWs), implying the potential for longer-term dust and air pollution predictions. SSWs lead to a weakening of the climatological high pressure ridge over Northern Africa, shifting the patterns of dust emission and transport for up to two months. Chemical transport models show a large-scale dipolar dust response to SSWs, with the burden in the Eastern Mediterranean enhanced up to 30% and a corresponding reduction in West Africa. Observations of inhalable particulate (PM10) concentrations and aerosol optical depth confirm this dipole. The enhanced dust pollution over the Eastern Mediterranean is linked to increased northward African dust transport, while the reduction in dust pollution within West Africa arises from a combination of suppressed African dust emission and decreased westward dust transport.
The dust pollution changes caused by SSWs can further impact premature deaths from exposure to fine particulate (PM2.5). According to our estimate, a single SSW causes on average 680-2460 additional premature deaths in the Eastern Mediterranean via dust-source PM2.5 enhancement and prevents 1180-2040 premature deaths in West Africa through dust-source PM2.5 reduction. The impact of SSWs on dust pollution mortality suggests that skillful forecasts of dust pollution during SSWs may alleviate pressure on health and social care systems, on condition that personal exposure to outdoor air pollution is reduced and/or that anthropogenic emissions of domestic, traffic and industrial pollutants are restricted during predicted periods of high dust pollution.
Currently, individual SSW events can be predicted about 1-2 weeks in advance and are easily observed when they do occur. Furthermore, once an SSW event occurs, it can lead to changes at surface air pollution that can last for weeks to months, providing a window of opportunity for advanced air quality warnings at longer lead times. Altogether, our results demonstrate that the stratosphere represents a non-negligible source of sub-seasonal to seasonal predictability for surface air pollution, particularly for Southern Europe and West Africa and call for consideration of the stratospheric variability in air quality forecasting systems.
Header Image Credit: NASA Worldview / Terra MODIS