Thermal effects of the North Atlantic Oscillation on the cold period of the year in Lithuania

Abstract

The impact of the North Atlantic Oscillation (NAO) on long-term air temperature fluctu- ations in Lithuania was studied for the period 1865 to 2000. Mean monthly values of 2 NAO indices (NAO-1, NAO-2) and air temperatures at 9 Lithuanian meteorological stations were used as refer- ences. Synoptic situations, as well as the mean sea level pressure (SLP) and 500 hPa geopotential height field of a total of 265 months, were analysed. NAO conditions were classified as positive, neg- ative or neutral. Air temperature fluctuations in Lithuania during the cold period of the year (Novem- ber to March) are positively correlated with fluctuations of the NAO indices. During periods of extreme NAO indices (| I | > ±1.5σ), a negative NAO phase is usually correlated (75 to 88% of the cases) to negative temperature anomalies, whereas a positive NAO phase is correlated to positive temperature anomalies. In the other cases, reverse impact of positive and negative NAO patterns, the relation between NAO phases and regional temperature anomalies is opposite or not detectable. Also, this study includes the analysis of atypical periods of NAO-like circulation: when positive NAO phase was related to negative temperature anomalies and negative NAO phase to positive tempera- ture anomalies in Lithuania.