Detecting changes in winter seasons in Latvia : the role of arctic air masses

Empirical climate variability and change studies may be particularly beneficial when they combine a search for dynamic causes with the examination of trends in meteorological variables. By using an air-mass-based methodology, arctic air masses over Latvia were identified for the 1950-2005 period according to the classification used by Berliner Wetterkarte, which defines air mass types by their origin and the extent of continental or maritime influence. The frequencies of maritime, transformed and continental arctic air masses and the class of arctic air masses were examined to evaluate whether and to what extent these account for changes and variations in the surface temperature. Trends in the frequency of arctic air, monthly-average temperature and monthly lowest minimum temperature series at seven observation sites were determined by using the non-parametric Mann-Kendall test. The results indicate that the frequency of arctic air masses during winter seasons decreased significantly during this period, with the majority of the decrease being associated with maritime arctic air, and that the frequency of the bitterly cold continental arctic air has also demonstrated a decrease. This trend in the monthly frequency of arctic air was the greatest in February. The increase in the winter, near-surface air temperature was partially attributable to a decrease in maritime arctic air mass frequency and, at a seasonal scale, these changes tended to smooth the peaks in the monthly temperature time series.