Cold surges were unusually active in subtropical East Asia during January-February 2005. These cold surges were preceded by upstream wave trains, which originated in the Mediterranean-Sahara region and propagated eastward along the subtropical jet stream over the Eurasian continent. The northerly of upper-level cyclonic anomaly in East Asia coupled with the low-level northerly upon the arrival of wave activity, followed by the quick southward penetration of cold air mass and surface

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... Diagnostic and numerical results suggest that the anomalously active wave activity affecting the East Asian cold surges may be attributed to the anomalously enhanced jet stream over Middle East and the anomalously westward extended East Asian Jet Stream. The configuration of these two subtropical jet streams established a strong waveguide through which the wave activity forced in the Mediterranean-Sahara region efficiently propagated to East Asia and resulted in more-than-averaged cold surge events in the subtropical East Asia. Wave-like perturbation tended to amplify at the entrance of the East Asian jet through barotropic energy conversion from the mean flow. stream, which has been identified as a Rossby waveguide in previous studies (e.g., Hsu and Lin 1992; Hoskins and Ambrizzi 1993) . Through this jet-stream waveguide, the forcing or perturbation occuring in as far upstream as Europe can affect the East Asian weather in a few days. Despite the knowledge about the relationship with the upstream wave-like disturbances, the mechanism guiding the wave train has not been fully explored. In view of the strong cold surges and upstream wave activity in the 2004/2005 winter, this study is aimed to explore the waveguide effect on the wave-like disturbances in the upper troposphere and the possible effect of upper-level wave activity on the initiation of the cold surges in the 2004/05 winter. In this study, we will demonstrate that the upper-level Rossby wave trains originating in the Mediterranean-Sahara region and propagating through the jet-stream waveguide help trigger the East Asian cold surge. It is also suggested that the East Asian Jet Stream (EASJ) plays an important role in amplifying the wave-like disturbances through wave-mean flow interaction. Numerical simulation will also demonstrate the sensitivity of the wave propagation to the detailed structure of the subtropical jet stream. Data and model The daily NCEP/NCAR reanalysis wind, geopotential height (Kalnay et al. 1996) during the 2004/05 winter are used in this study. The cold surges were identified based on the criteria defined by the Central Weather Bureau, the official meteorological agency in Taiwan. The criteria are: (i) the daily maximum temperature of Taipei (located in northern Taiwan) drops at least 4°within 24-48 hours, and (ii) the daily minimum temperature is less than 10°. To make sure the cold surge is not station dependent, the data of Pengchiayu, an island to the northeast of Taipei, is also used to identify cold surge. Four cold surge events listed in Table 1 were identified. The corresponding time series of daily minimum temperature is also shown in Fig. 1 . The abrupt temperature drop ranges from 4.8° to 9.9° and the minimum temperature fell to 7.3°-9.6° It is also interesting to note that the cold surges occurred regularly every 16-18 days after the warm December. The model used to explore the waveguide effect on the Rossby wave-like disturbances is a linearized perturbation barotropic vorticity equation. It is outlined as: ' ' ' ' 4 ' ' Kalnay, , 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteorol. Soc., 77, 437-471. Lau, N.-C., and K.-M. Lau, 1984: The structure and energetics of midlatitude disturbances accompanying cold-air outbreaks over east Asia. Mon. Wea. Rev., 112, 1309-1327. Sardeshmukh, P. D., and B. J. Hoskins, 1988: The generation of global rotation flow by steady idealized tropical divergence. J. Atmos. Sci., 45, 1228-1251. Simmons, A. J., J. M. Wallace, and G. W. Branstator, 1983: Barotropic wave propagation and instability, and atmospheric tele-connection patterns. J. Atmos. Sci., 40, 1363-1392. Takaya, K. and H. Nakamura, 2001: A formulation of a phase-independent wave-activity flux for stationary and migratory quasigeostrophic eddies on a zonally varying basic flow. J. Atmos. Sci., 58, 608-627. Watanabe, M., 2004: Asian jet waveguide and downstream extension of the north Atlantic oscillation. J. Climate, 17, 4674-4691. Wu, M. C., and J. C. L. Chan, 1995: Surface features of winter monsoon surges over