Abstract
In this study, recent variability of the Eurasian (EU) pattern during 1978-94 and its association with wave activity flux are investigated. The sign of the wintertime EU index abruptly reversed from positive to negative in winter 1988/89, concurrent with the intensified polar vortex. To investigate the mechanism of the EU pattern in detail, we computed the wave activity flux. When the EU index is positive, the Rossby wave propagation appears from North Europe to East Asia directly. When the EU index is negative the wave activity flux is directed more southeastward to the Middle East. An empirical orthogonal function (EOF) analysis is performed for the wave activity flux to clarify the variations of the Rossby wave propagation over Eurasia. Two dominant modes associated with the EU pattern were detected. The first mode is closely linked with the height anomaly over arctic regions and contributes to a shift in the 1988/89 winter atmosphere. The second mode is related to the other principal teleconnection patterns: the North Atlantic Oscillation (NAO), and Pacific/North American (PNA) patterns. The appearance of the modes are interpreted in terms of wave forcing to the zonal wind, and the meridional wave propagation of Rossby waves. The Rossby wave with wavenumber three, plays an important role in the zonal wind changes of the first mode. The meridional profile of the zonal wind is consistent with the structure of wave activity flux. The positive feedback between the wave activity and the zonal wind is responsible for the dominance of the first EU mode.
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