Impact of Euro-Atlantic blocking on the occurrence of heat waves and cold spells in Poland

The spatiotemporal variability of atmospheric blockings over the Euro-Atlantic region and their influence on the occurrence of the persisting extreme temperature conditions in Poland namely heat waves and cold spells during the period 1978-2023 was analysed. Blockings were identified at 500 hPa geopotential level, using the meridional geopotential gradient method, supplemented with the quantile filter and persistence filter. Heat waves (HWs) and cold spells (CSs) were defined as sequences of at least 3 days with the maximum air temperature above 30°C or below -10°C, respectively.

The climatology of Euro-Atlantic blocking occurrence in the zonal belt between 45 and 75 degrees in the northern hemisphere exhibits high spatiotemporal variability. Blocking structures are most frequent in the spring (MAM) occurring with a 10% frequency in the arched belt extending from the west of the British Islands through the North Sea, south Scandinavia, to the east of the Baltic Sea. In summer (JJA) they are most frequent in the area spanning from south Finland to west Russia, as well as over Greenland. During winter (DJF) blockings are most commonly located over the southeast Atlantic (west of France) and extending northeastward up to southern Scandinavia with their occurrence being least frequent in autumn (SON).

The mean annual number of HW days ranges from one day in the southern mountainous regions and in northern Poland to more than four days in the central and western parts. However, during the extremely hot summer of 2015, the number of HW days exceeded 20 in central and southern Poland. CSs have become rarer, with the average seasonal number of CS days amounting to 1-2 days in recent decades, and only in singular seasons such as 1986/1987, more than 10 days fulfilled the criteria of CS in northeastern Poland. The occurrence of HWs in Poland is constantly accompanied by blocking situations, most often located northeast of Poland, while the winter CSs are associated with the blockings located over the North Atlantic and northern Scandinavia.