Low-frequency oscillations and associated wave motions over Eurasia

Abstract

Based on 5-day-mean grid point data for both 500-hPa geopotential height (Z500) and sea level pressure (SLP) fields, and focusing on periods of 4 weeks to a year, surface and mid-tropospheric low-frequency oscillations over Eurasia are extracted by using multi-channel singular spectrum analysis (MCSSA), combined with the maximum entropy method. Besides the permanent annual cycle (365~366 days) and its second harmonic (181 days), low-frequency oscillations with periods of 55—65 and 35—45 days were found both in the mid-troposphere and at sea level, explaining a substantial number of the total variances in the relevant frequency bands. These quasi-periodicities are robust, since they are reproducible when different window lengths are used and when MCSSA is applied to two equal-length subsets of the basic data series, and since they are qualitatively different from those obtained from randomly resampled basic datasets. Moreover, no periodicities were reproduced from scrambled basic datasets. The spatial-temporal structures of these low-frequency oscillations, i.e., the associated wave motions, are presented, in order to display evolutionary processes of circulation systems probably related to the Asian monsoon variability. Some reliable information about the amplitudes of the oscillations is also obtained by using composite analysis. The 55—65 day oscillation is, in the mid-troposphere, associated with transitions between flow regimes of intense anomalies over the polar cap-Ural mountains, matched by anomalies of the opposite sign over the mid-latitudes of the Atlantic and Asia-Pacific; the anomaly centres seem to shift southwestward over Europe, westward over Asia, and poleward along the Ural mountains and over the Pacific. At sea level, it is associated with decaying anomaly centres moving southeastward over Asia-Pacific, and southwestward over Europe; its high-amplitude phases are characterized by a strong action centre dominating over northern Eurasia, matched by anomalies of opposite sign occurring over the middle-lower latitudes of the European and Pacific sectors. The 35—45 day oscillation is associated with a flow pattern characterized by a blocking structure in the mid-troposphere located over either Europe or Lake Baikal; the related anomaly centres appear to move southwestward over the Pacific, westward over Asia, poleward along the Ural mountains, and southwestward over Europe. At sea level, the anomaly centres associated with this 35—45 day oscillation appear to displace equatorward over Asia, eastward over the Pacific, and southwestward over Europe; the high-amplitude phases are characterized by a strong action centre dominating over northern Eurasia, with weakening anomalies of the opposite sign occurring simultaneously over the subtropics of east Asia-Pacific and Europe. In terms of the wave structures and wave propagation over the Eurasia sector, these two oscillations coincide with previously reported low-frequency oscillations with similar periods at the 700 hPa level. Moreover, the high-amplitude phases of the 35—45 day oscillation are intimately associated with conspicuous anomalies of precipitation over China.