If not NAO then what?\u2014regional circulation patterns governing summer air temperatures in Poland

Abstract

The correlation between the daily/monthly North Atlantic Oscillation (NAO) index and the daily/monthly anomalies of the summer (JJA) surface air temperatures in Poland is close to 0.00, being positive in most of the country (but hardly exceeding 0.15) and negative in the southeast. Therefore, the regional scale circulation patterns other than NAO were determined using principal component analysis (PCA) to establish the relationship between the air circulation and the daily anomalies of the mean, maximum and minimum air temperatures in Poland. Regression and correlation analysis were applied to define the strengths and spatial distributions of these relationships. The best results were obtained for the mean and maximum daily temperature anomalies, for which the Pearson correlation coefficient (r) between the modelled and observed values exceeded 0.75 in the vast areas of central and southeastern Poland, while, over the rest of the country, it amounted to 0.7. The weaker influence of the circulation on air temperatures is observed in the northwestern part of the country, along the Baltic Sea shore (r < 0.65). The weakest results were obtained for the anomalies of the daily minimum temperatures (r equals 0.5–0.6 over most of the country and drops below 0.4 in the northwest). Furthermore, the influences of each PCA-based regional circulation pattern on the variabilities of the surface temperatures were analysed. The circulation pattern recognized as the second principal component, distinguished in the positive phase by the high-pressure conditions with the anticyclonic centre located right over central and southeastern Poland, revealed the strongest positive relationship with the air temperature.