Long-term variability of the frequency and persistence of strong highs over Poland

The paper discusses the spatial and temporal variability in the occurrence of strong highs over Poland in the period 1951-2015. It focuses in particular on the persistence of the systems in question and the changes in their long-term variability. The study was based on the average daily sea-level air pressure values obtained from NCEP/NCAR Reanalyses for 12 grid points. A day with a strong high was defined as a day with daily average pressure equal to or higher than 1030 hPa. Over the study period, a minor increase in the annual air pressure values (0.17-0.32 hPa/10 years) was identified, as well as evidence of an increase in the number of days with strong highs. These changes were the most distinct in December and in southern Poland. A majority of strong high occurrences were recorded during the cool half of the year, they covered less than the whole territory of the country and typically persisted for no more than two days. The longest spells with pressure equal to or greater than 1030 hPa lasted between 15 and 22 days depending on the region. Such long sequences of days with strong highs coincided with years when strong highs were particularly frequent, especially in the 1980s and 1990s. No specific trends in persistence or seasonality were identified.