Successive cold winters of severely low temperatures in recent years have had critical social and economic impacts on the mid-latitude continents in the Northern Hemisphere. Although these cold winters are thought to be partly driven by dramatic losses of Arctic sea-ice, the mechanism that links sea-ice loss to cold winters remains a subject of debate. Here, by conducting observational analyses and model experiments, we show how Arctic sea-ice loss and cold winters in extra-polar regions are dynamically connected through the polar stratosphere. We find that decreased sea-ice cover during early winter months (NovemberDecember), especially over the Barents-Kara seas, enhances the upward propagation of planetary-scale waves with wavenumbers of 1 and 2, subsequently weakening the stratospheric polar vortex in mid-winter (January-February). The weakened polar vortex preferentially induces a negative phase of Arctic Oscillation at the surface, resulting in low temperatures in mid-latitudes.

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The variations in the wintertime precipitation over East Asia and the related large-scale circulation associated with the Madden-Julian oscillation (MJO) are examined. By analyzing the observed daily precipitation for the period 1974-2000, it is found that the MJO significantly modulates the distribution of precipitation over four East Asian countries; the precipitation rate difference between wet and dry periods over East Asia, when the centers of MJO convective activities are located over the Indian Ocean and western Pacific, respectively, reaches 3-4 mm day Ϫ1 , which corresponds to the climatological winter-mean value.Composite analysis with respect to the MJO suggests that the MJO-precipitation relation is mostly explained by the strong vertical motion anomalies near an entrance region of the East Asia uppertropospheric jet and moisture supply in the lower troposphere. To elucidate different dynamic origins of the vertical motion generated by the MJO, diagnostic analysis of a generalized omega equation is adopted. It is revealed that about half of the vertical motion anomalies in East Asia are induced by the quasigeostrophic forcings by the MJO, while diabatic heating forcings explain a very small fraction, less than 10% of total anomalies.

[1] The variations of wintertime surface air temperature (SAT) and the occurrences of cold surges over east Asia in association with the Madden and Julian Oscillation (MJO) are examined. The MJO is defined as a combination of the two leading empirical orthogonal function modes of 850 and 200 hPa zonal winds and outgoing longwave radiation in the tropics. The spatial pattern and magnitude of SAT anomalies over east Asia significantly change with respect to MJO phases. Composite analysis reveals that the MJO-SAT relationship is mainly affected by strong temperature advection in the lower troposphere. In addition, occurrences of cold surges are identified using objectively determined synoptic criteria, and their relation to the MJO is analyzed. Most extreme cold surges occur when the MJO convection center is located over the Indian Ocean, and it is suggested that MJO-induced circulation anomalies may reinforce the amplification of normal cold surges to extreme cold surges.