Properties and changes in empirical orthogonal components of temperature fields in Northern Eurasia in the 20th century

Using data from 55 meteorological stations in the Northern Hemisphere for the 20th century as a whole and separately for 1901–1950 and 1951–2000, the norms and empirical orthogonal components (EOCs) of the fields of the annual mean surface air temperature were calculated. These fields were found to have two components: smooth and nonsmooth. The field of differences between the temperature norms of the first and second halves of the 20th century was found to be projected almost completely on the very first component of fixed sign; i.e., the current climate trend of the Northern Hemisphere is spatially homogeneous. No substantial changes were found in the transition from the first half to the second half of the 20th century, either in the form of components or in the spectrum of eigenvalues, with the only exception being a small growth (in magnitude) in part of the eigenvalues corresponding to the nonsmooth component. This can be explained by the progressing urbanization of northern Eurasia. All coefficients of the temperature-field expansion by natural components are distributed normally, and their temporal correlation functions for smooth components of fields have a form characteristic of processes with long-term memory. The latter manifests itself particularly in the motions of the two main waves of the smooth component and reflects two stages of the current climate warming.