Summertime warming in the European part of Russia and extreme heat in 2010 as manifestation of large-scale atmospheric circulation trends in the late 20th-early 21st centuries

Considered is a relationship between long-term variations of the surface air temperature and of the field of geopotential at the level of 500 hPa in the middle of summer in the European part of Russia and the variations of large-scale atmospheric circulation described by the indices of North Atlantic and North Pacific centers of atmospheric action. The considerable inhomogeneity in the course of average temperature in the European part of Russia in July divides it into two periods, before and after the 1980s. Unlike stationary fluctuations of temperature before the 1980s, the 6-10-year fluctuations are typical of the period of 1981-2010 against a background of its considerable rise by 0.8A degrees C per 10 years with the contribution of 20% to the total variance. During this period, about 80% of temperature variability are caused by the circulation variations, and 55% of them are associated with the North Pacific centers of atmospheric action, in particular, with the WP index in July, May, and April. Revealed regression dependences between circulation indices and the air temperature in the European part of Russia explain rather accurately the linear trend, the fluctuations with the period of 6-10 years, and the extremes in 1988, 2001-2003, and 2010. The analysis of the time series (1950-2012) of WP, PDO, and SOI indices demonstrates that changes in the atmospheric circulation took place in the extratropical zone of the Northern Hemisphere at the turn of the 1980s and this caused the formation of blocking situations and, hence, steady heat and drought in summer in the mid-latitude zone including the European part of Russia. These variations can be interpreted as a change in the regimes of strong (1950-1980) and weak (1981-2012) circulation. The heat in July 2010 was an extreme display of weak circulation, and this is indicated by the unprecedented low values of the WP index in July and May.