Winter synoptic circulation types and influenza epidemics in the Czech Republic

Influenza is an acute respiratory infection that causes recurring seasonal epidemics and is associated with substantial morbidity and mortality. In temperate climates, the influenza season usually peaks in winter; however, the timing and severity of epidemics vary considerably from year to year. Previous research has shown that this variability relates to meteorological factors such as air temperature and absolute humidity. It remains uncertain whether synoptic circulation types can serve as a useful parameter for evaluating the impact of weather on the timing and dynamics of influenza epidemics.

The Grosswetterlagen reanalysis classification of daily synoptic circulation types over Central Europe is used to characterise winter weather (December–March) in the Czech Republic from 1982 to 2020. The average frequency of synoptic types is calculated over two-week periods aligned with the start, peak, and end of epidemics. These are categorised by their effects on excess mortality (mild, moderate, or severe) and by the dominant influenza virus (A/H3N2, A/H1N1, or B). The relationship between synoptic types and influenza epidemics over time is analysed through bootstrapping. Excess mortality during epidemics is estimated using a quasi-Poisson regression model.

Results show that the onset of influenza epidemics is typically preceded by a higher frequency of westerly types, which is compensated by fewer easterly types. During epidemics, this pattern shifts when westerly types are strongly suppressed, allowing easterly (namely north-easterly and south-easterly) types to predominate and bring cold, continental air into the region. In non-epidemic seasons, the opposite pattern is observed, with predominating westerly types and milder, humid winter conditions. Severe epidemics with high mortality impacts are associated with frequent anticyclonic situations, prevailing north-easterly and south-easterly types, and suppressed westerly and north-westerly types, corresponding to cold, dry winter conditions with frequent temperature inversions. The weather patterns accompanying A/H3N2 and A/H1N1 epidemics differ: A/H3N2 is associated with anticyclonic conditions, with frequent south-easterly types and high pressure over central Europe, while westerly types are suppressed, representing a stable, colder-than-average, and dry winter regime. During A/H1N1 epidemics, the occurrence of westerly types is at the seasonal average, while south-westerly types are more frequent, indicating more variable weather, including milder temperatures and more frequent precipitation. Influenza B epidemics are associated with a higher frequency of easterly and northerly types and a lower frequency of westerly and southerly types, reflecting stable, dry continental conditions with below-average temperatures.

Synoptic circulation represents a complex indicator of meteorological conditions that influence the timing and severity of influenza epidemics and provides an integrated view of the effects of individual meteorological variables, which may contribute to improved seasonal risk assessment.