Relationship between atmospheric circulation indices and climate variability in Estonia

A simple method for calculating circulation indices by using gridded data of mean sea level pressure is applied to Estonia. The main objective of the research is to analyse the relationship between the developed indices and climate variability in Estonia. The results indicated that correlation between the indices and air temperature varies significantly between different months and seasons. The air temperature is positively correlated with the zonal circulation index during the period from September to March and has no correlation from April to August. The meridional index (positive values correspond to the higher-than-normal southerly airflow) has a positive correlation with the air temperature from April to September and no significant correlation in other months. In case of the higher-than-normal zonal and SW–NE index, the duration of the snow cover in Estonia is shorter than normal because the snow cover duration is related to air temperature conditions. The correlation between the indices and precipitation is low. Several statistically significant trends in monthly and seasonal circulation indices were detected using a linear regression analysis and a non-parametric Mann-Kendall test for trend. During 1946–1997, the mean zonal circulation has intensified in February and winter (DJF), and decreased in April, June and September. The southerly airflow has increased in March. The southwesterly circulation has increased in February and March and decreased in April, June, September, summer (JJA) and autumn (SON). The airflow from southeast has intensified in spring (MAM).