Hot Days and Heat Waves in Poland in the Period 1951-2019 and the Circulation Factors Favoring the Most Extreme of Them

The aim of the study is to analyze the occurrence of hot days and heat waves in Poland, their intra-annual distribution, and their long-term variability, and to present the circulation factors favoring the appearance of extensive waves in the country. Hot days were days with Tmax not lower than the threshold value defined by the 95th percentile of summer Tmax in the period 1961-1990. Atmospheric circulation was described using sea level pressure, geopotential of 700 and 500 hPa level, and horizontal and vertical wind on these levels. A statistically significant increase in the number of hot days in the entire study period and a significant acceleration in growth after 1980 were shown. In the entire analyzed period, only 11 waves were found covering at least 25% of the country area and lasting no less than a week. Among them, only one occurred before 1990, and more than half were observed in the last decade. Four circulation patterns favoring the extensive heat waves were distinguished differing the location of main baric center location. Spatial and temporal distribution of vertical velocity anomalies allows distinguishing clear phases of strengthening, stabilization, and weakening of anticyclone accompanying the occurrence of a heat wave.

Automated summary

The study showed a significant increase in the number of hot days in Poland throughout the entire analysis period, with a noticeable acceleration in growth after 1980. Only one heat wave was found before 1990, while over half occurred in the last decade. Four circulation patterns favoring extensive heat waves were identified, characterized by differences in the location of the main baric center.