Concurrent heatwaves and extreme Ozone (O3) episodes: combined atmospheric patterns and impact on human health

BACKGROUND AND AIM: In the context of the global environmental change, more recurrent and/or intense heat waves and extreme ozone episodes are likely to occur during the next decades. The main purpose of this research is to assess how extreme temperature may trigger the appearance of high ozone levels and how the concurrence of these two extremes could be linked to the synoptic general circulation and impacts human health and wellbeing. METHODS: We have used surface maximum temperature and ozone observations during extended summers (April–September) in two cities from Morocco: Casablanca and Marrakech, between 2010 and 2019. A percentile-thresholding method was applied to identify heatwaves and ozone episodes and a statistical approach was used to assess the connection between the observed parameters and climate indexes (North Atlantic Oscillation (NAO), Mediterranean Oscillation (MO), and Saharan Oscillation (SaOI)). The weather types behind the concurrence of extremes were identified and Heat and Air Quality Indexes were assessed to determine their combined health risks. RESULTS:Our findings show that the concurrence of heatwaves and ozone episodes depends both on the specific city and the large-scale atmospheric circulation. The likely synoptic pattern behind the concurrence of these events occurs when the country is under the combined influence of an anticyclonic area in the north and the Saharan trough extending the depression centered in the south. This pattern generates a warm southern flow and may foster photochemical pollution and then impact human health. CONCLUSIONS:Our study is an attempt to assess extreme temperature and ozone episodes combined features inducing large-scale atmospheric circulation and health effects. It explores the hypothesis that particular weather patterns increase the vulnerability of individuals especially those sensitive to air pollution. This work is a first step to the establishment of an alert system and will help to provide recommendations for coping with concurrent heatwaves and air pollution episodes.