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Bispectral analysis of geopotential‐height anomalies for synoptic‐intraseasonal time‐scales
Author(s) -
Efimov V. V.,
Shokurov M. V.
Publication year - 2001
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.49712757513
Subject(s) - geopotential height , geopotential , bispectrum , asymmetry , physics , synoptic scale meteorology , moment (physics) , nonlinear system , spectral density , geodesy , mathematics , meteorology , geology , geophysics , classical mechanics , statistics , precipitation , quantum mechanics
Spectral decomposition of the third moment of geopotential‐height fluctuations, bispectral analysis, is considered as a convenient and straightforward method of studying the nonlinear three‐wave interactions between spectral constituencies of anomalies. The real part of the bispectrum directly indicates an asymmetry of the probability density function (PDF), and the imaginary part indicates lack of time reversability of the time series. NCEP reanalysis data for 1982–96 and NMC analysis data for 1946–89 are used. Different frequency domains of bispectra are considered, namely, the synoptic frequencies, 2–6 days, lower intraseasonal frequencies, 32–128 days, and joint synoptic‐intraseasonal frequency ranges. The spatial distribution of the integral over all frequencies of the real part of the bispectra, characterizing asymmetry of the PDF, on the whole, is the same as that obtained earlier by Holtzer. A similar integral of the imaginary part essentially differs from zero and has more non‐zonal behaviour. The first of them, asymmetry, accounts for different contributions of nonlinear and linear terms in the balance equation for geopotential anomalies. The spatial distribution of the bispectra in the low‐frequency intraseasonal domain is nearly the same as for their integrals, because of the predominant contribution of low frequencies in the spectra of geopotential oscillations. The distribution of the real and imaginary parts of the bispectra in the synoptic frequency range indicates an asymmetry between positive and negative deviations of high‐frequency transients, and a spatial asymmetry between the front and back slopes of the eastward‐moving synoptic cyclones. This behaviour is clearly evident in storm‐track regions. Most interesting is the synoptic‐intraseasonal frequency domain. Non‐zero values of real and imaginary parts of the bispectra imply an existence of low‐frequency oscillations of geopotential heights bounded by envelopes of synoptic fluctuations. This phenomenon is seen for all regions in both hemispheres and during both seasons, but takes place especially clearly in the storm‐track regions over the Pacific and Atlantic oceans. Phase shifts between the low‐frequency component and the variance of synoptic transients in most regions are nearly out of phase, but in the regions of storm‐track activity the phase relationship changes regularly along the tracks. Meaningful treatment of many of the results of the bispectral analysis needs special consideration.