The work discusses the relationships between the intensity of the North Atlantic thermohaline circulation (NA THC), characterized by the DG3L index, and the number of days with extremely high air temperatures over Poland in the years 1951–2020. The measure of extreme conditions was the number of days in a year with maximum daily temperature ≥25°C (hot days; D5DG) and ≥30°C (very hot days; D5DU), as well as the number of days with average daily temperature ≥25°C (D5D25). Highly significant relationships were found between the variability of the DG3L index and D5DG, D5DU and D5D25, indicating that the more intense the NA THC, the more extremely warm days there are in a year. The long-term variability of the number of extremely warm days clearly refers to the variability of macro-circulation conditions – circulation epochs according to the Wangengejm-Girs classification. The increase in extremely warm days throughout the year is associated with an increase above the longterm average of the zonal macrotype W. The variability of NA THC is the cause of changes in heat resources in the waters of the North Atlantic, which affects the formation of meridional thermal gradients in the mid-troposphere. As NA THC increases, these gradients increase. As a result of the increase in these gradients, in the Atlantic-Eurasian circulation sector, there is an increase in the frequency of long waves with wave number 4 (macrotype W) and a decrease in the frequency of long waves with wave number 5 (macrotypes E and C; meridional circulation). The result is an increase in geopotential height (h500) over western and central Europe, south of 55°N. There is an increase in sea level pressure over this area, which, on the synoptic scale, causes an increase in the frequency of anticyclonic weather, without stratiform clouds (As, Ns and St; frontal), a strong increase in sunshine duration and a reduction in rainfall. In the structure of heat fluxes from land surfaces to the atmosphere, the share of evaporative heat fluxes decreases, and the share of sensible heat fluxes increases, causing a strong increase in air temperature. The strong upward trend observed in the DG3L index after 1988 is reflected in the increasing number of extremely warm days over Poland since then.
