Silne wyże nad Europą (1951-2010)

Abstract

The aim of this study was to determine the temporal and spatial variability of strong and very strong highs in an area of the Euro-Atlantic sector limited by the coordinates 20°N—85°N and 50°W—75°E. The research was based on a rich set of data including, at its core, the average daily air pressure values at sea level at 1377 grid points of a 2.5° × 2.5° grid derived from NCEP/NCAR reanalyses. That data set was supplemented by the values of the 500 hPa isobaric level’s height at the grid points mentioned above. Additionally, a wide range of synoptic maps for different pressure surfaces was used. The study covers the period 1951—2010. The significant spatial and temporal variability of the air pressure over Europe and the Atlantic required the finding of a suitable method to identify strong or very strong anticyclonic systems. The results obtained during this study suggest that defining criteria are highly dependent on both the geographical location and the season of the year. Finally, it was assumed that a pressure system would qualify as a strong high if its pressure was equal or higher than 1030 hPa and as a very strong high if the pressure was at least 1035 hPa. The annual course of the number of days meeting a given criterion became the basis for the division of the Euro-Atlantic region into four main anticyclonic regions of the Euro-Atlantic sector and a number of subregions. The most characteristic differences between regions involved pressure values in strong and very strong highs, their annual patterns and long-term variability of the number of days with strong and very strong highs. The highest pressure values were found in regions featuring the strongest continental highs (more than 1060 hPa) and the lowest values in areas of the most active cyclonicity, i.e. the Iceland region (IIIA) and the Southern region (IV). The Azores High region (IIAW) stands out with the highest average pressure (1018 hPa) at the centre. Areas of oceans and the Southern Region (IV), which covers nearly the entire area below 50°N, stand out from the rest of the sector. They are dominated by cyclonic activity and strong highs only occur for 30—40 days per year. Strong and very strong anticyclones were mainly recorded during the cool half of the year (Oct.—Mar.). The annual maximum of the occurrence of strong and very strong highs is in winter with the exception of the Arctic where it is shifted to springtime. During the final decades of the study period, there has been a change in the timing of the annual maximum of the number of days with these systems, as well as a change in the timing of the onset and end of the season of the greatest frequency of strong and very strong highs. Where long-term variability of strong highs were clear-cut they occurred primarily in the cool half of the year (Oct.—Mar.). They mainly included a decrease in the annual number of strong and very strong highs in the Greenland region (I) by more than four days per 10 years and an increase in that number in the Azores High (IIAW), Asian (IIAE) and Southern (IV) region. The increase began in the 1970s and was the greatest in the Southern region (up to 6 days per 10 years). Air pressure spatial pattern types on days with strong and very strong highs were identified for each region in order to account for the role of atmospheric circulation as a climate control and for relationships existing between individual elements of circulation. Each of the types involved two main distinctive features, namely the spatial distribution and the strength of the most important Euro-Atlantic highs, i.e. Greenland, Azores and Asian, as well as the Iceland low. A comparison of the types of pressure pattern on days with strong and very strong highs reveals that some of them are so similar to each other that they might be regarded as common to the entire Euro-Atlantic sector. This is particularly true of the types occurring during the cool half of the year. Also some of the pressure pattern types are similar to general circulation types identified in this sector by other authors. This would mean that they should be counted among significant features of this area’s atmospheric circulation. Duration is an important characteristic describing the occurrence of pressure systems. Among the 8406 strong or very strong highs that occurred in the Euro-Atlantic sector during the study period, the dominant duration was 1—3 days and slightly more than 50% of all of them were shorter than 7 days. It was also found that the duration of these systems increased with their strength to the point that for systems with a pressure 1045 hPa it averaged at 6 days. High pressure systems in the cool half of the year (Oct.—Mar.) lasted longer than the high pressure systems in the warm half of the year. The most persistent highs develop over Greenland and in the Asian High region (IIAE). In contrast to the rapid movement of cyclones, high pressure systems travel at a much slower pace but cover a far greater area. For this reason, when investigating the occurrence of strong and very strong high pressure systems, the author also identified areas of the most frequent occurrence of their centres, thus indirectly identifying the paths of these systems. The areas so-far identified formed two distinct zones: • A belt stretching from the Azores High area on the Atlantic Ocean across Europe to the East European Lowland (Voyeykov’s axis) and • Greenland. During the cool half of the year, the number of strong and very strong highs of a local nature clearly increases. These anticyclones occur over the Iberian interior, the Alps, a part of the Balkan Peninsula, the Anatolian Upland, the Caucasus and the Scandinavian Peninsula. An analysis of the geographical extent of these two zones revealed that their location was related strongly to the annual cycle of change in atmospheric circulation and, consequently, of the highs in question. For the first time, the results of the research project presented in this paper allowed the researchers to identify changes in the occurrence of strong and very strong highs, one of the most important elements of atmospheric circulation, over such a large spatial scale and time. The results clearly showed that in the Euro-Atlantic sector weather conditions were influenced more by the annual pattern than the long-term variability of these two pressure systems and by the great deal of regional variation discovered in their frequency of occurrence.