MODES OF VARIABILITY IN THE NORTHERN HEMISPHERES MIDTROPOSPHERIC LARGE-SCALE CIRCULATION

In this paper the preferential modes of non-seasonal variation of the 500 hPa height fields and the temporal variations thereof are explored at pentad time steps using S-mode Principal Component Analysis, maximum entropy spectrum analysis, single-channel singular spectrum analysis and the Kendall test of randomness. In a pentad analysis the modes similar to the well-known circulation regimes identified by applying different methods on monthly mean or 1-month mean series (Wallace and Gutzler, 1981; Barnston and Livezey, 1987) are identified and also a number of new modes. Unlike in the previous studies, we obtain some information about the amplitudes of the preferential circulation modes by using composite analysis; significant trends were detected and extracted by performing singular spectrum analysis, in combination with the Kendall test of randomness, and low-frequency fluctuations of about 30-50 days are revealed as being intimately associated with blocking activities. Moreover, a quasi-cycle of about 3.86-year is detected, in association with the so-called PNA teleconnection, which can be related to the low-frequency component of the ENSO phenomenon.