Study of the low-frequency variability of the atmospheric general circulation with the use of time-dependent empirical orthogonal functions

The theory of both time-and space-dependent empirical orthogonal functions (EOFs) is outlined from a common point of view as a discrete version of the Karhunen-Loeve expansion. EOF techniques are used to process the results of a 1300-day-long numerical experiment in the mode of perpetual January with a daily step. As a result, the component determined by internal hydrothermodynamic processes was isolated from the general variability of the atmospheric circulation. To isolate intraseasonal variability (a winter season), the principal components of the 500-hPa height field were processed with a filter close to a π-shaped filter isolating the frequency band 0.1–0.01 day −1 . The spatial principal components were analyzed via the method of time-dependent EOFs. The spectral characteristics obtained can characterize the intraseasonal low-frequency variability and agree partially with empirical data.