Circulation Patterns of Compound Atmospheric Events Associated with Wintertime Mortality Excess in Mainland Spain

Numerous studies have demonstrated the impacts of extreme meteorological events and air pollution on public health. Nonetheless, only a few consider the synergy of both factors (compound events) when analysing mortality rates from an atmospheric perspective. This work aims to establish connections between mortality excesses in mainland Spain during wintertime, extreme atmospheric events, and the circulation patterns involved in these episodes. For that, daily mortality rate data at a provincial level are used for the 2015-2022 period.

Winter mortality extremes were categorised and associated with preceding circulation types (CTs), based on sea-level pressure (SLP), temperature at 850 mb, and geopotential height at 500 mb. This classification uses daily average fields derived from ERA5 reanalysis over a domain covering the entire Iberian Peninsula. This classification uses daily average fields derived from ERA5 reanalysis over a domain covering the entire Iberian Peninsula. For each resulting CT, mean fields of temperature, NO2, and PM10 were computed using CAMS reanalysis data. The results show that most mortality extremes succeed extreme atmospheric conditions, with a time lag that depends on the location and variable considered. The variables that better explain mortality include maximum and minimum temperature, nitrogen dioxide (NO2), particulate matter (PM10), and their combinations. Their influences from previous days are significant for more than half of the cases, especially when the compound event comprising extreme minimum temperature followed by high levels of NO2 occurs, with a median lag of about one week.

Regarding the circulation patterns, the results show that most cases of high mortality are linked to anticyclonic systems and warm temperatures at high levels, which can lead to very cold conditions at the ground level and contribute to air stagnation. The situation is exacerbated with the concatenation of CTs related to cold conditions. Extreme mortality occurs when CTs associated with high NO2 levels join persistent low temperatures. This phenomenon may impact multiple provinces simultaneously or within a short timeframe, suggesting that early warning systems should consider these events to alert vulnerable populations and prevent nationwide high mortality rates.