|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
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.|