IGU Regional Conference, Kraków, Poland 18-22 August 2014 IGU 2014 Book of Abstracts IGU2014 – 1175 Changes and variability in centres of action of the atmosphere in the Northern Hemisphere in January and July Falarz M. Department of Climatology, Faculty of Earth Sciences, University of Silesia There were analysed long-term changes and variability of centres of action of the atmosphere (CAA) in the Northern Hemisphere in January and July in the second half of the 20th century. The geographical coordinates, baric extent and the atmospheric pressure value in the centre of each baric system (Icelandic Low (IL), Azores High (AH), Siberian High (SH), North American High (NAH), Aleutian Low (AL) and Hawaii High (HH) in January, IL, AH, Siberian Low (SL), HH in July) were read in maps of mean monthly sea level pressure basing on gridded 2,5°x 2,5 data of Reanalysis Project of the National Centre for Atmospheric Research for the period 1948-2003. The baric extent of the CAA was defined as the absolute value of the difference between the sea level pressure value in the centre of baric system and the value of pressure of the last closed isobar not including the other CAA. There were investigated trends of each CAA features and these of differences of sea level pressure values in the centres of AH and IL (D1), SH or SL and AH (D2), SH or SL and IL (D3), AH and HH (D4), HH and AL (D5), IL and AL (D6). Moreover indices of location changing of CAA centres in 4 directions (E-W/W-E, N-S/S-N, SW-NE/NE-SW, NW-SE/SE-NW) were created and analysed in the investigated period. Following statistically significant at the 0.05 level trends were detected: a) the upward trend of the sea level pressure in the centre of AH in January (+0.8 hPa per 10 years), b) the downward trend of the sea level pressure in the centre of NAH in January (-0.5 hPa per 10 years), c) the upward trend of the sea level pressure in the centre of SL in July (+1 hPa per 10 years), d) the downward trend of the baric extent of SL in July (-0.9 hPa per 10 years), e) the downward trend of D2 in January, f) the upward trend of D4 in January, g) the upward trends of D2 and D3 in July. Moreover the increasing tendencies (close to the significance level) of the latitude (meaning the northward move) and the baric extent of AH in January were observed. The indices of location changing show: moving of IL and AH from SW to NE in January, moving of AH from N and NE to S and SW in July, moving of SL from NE to SW in July, moving of AL from NW to SE in January, moving of HH southward after 1960 in January. The significant change of the sea level pressure of the AH in January is the reason for the increasing tendency of the D1 and for the decreasing trend of the D2; these two IGU Regional Conference, Kraków, Poland 18-22 August 2014 trends cause the intensification of the western advection and the diminishing of S-N air flow over Europe. So, the upward trend of the Azores High in the second half of the 20th century is probably one of the reasons of the observed winter climate warming in Europe. .