Using ECWMF ERA-40 and Interim reanalysis data, the planetary wave fluxes associated with the February extreme stratospheric polar vortex were studied. Using the three-dimensional Eliassen-Palm (EP) flux as a measure of the wave activity propagation, the authors show that the unusual warm years in the Arctic feature an anomalous weak stratosphere-troposphere coupling and weak downward wave flux at the lower stratosphere, especially over the North America and North Atlantic (NANA) region. The extremely cold years are characterized by strong stratosphere-troposphere coupling and strong downward wave flux in this region. The refractive index is used to examine the conception of planetary wave reflection, which shows a large refractive index (low reflection) for the extremely warm years and a small refractive index (high reflection) for the extremely cold years. This study reveals the importance of the downward planetary wave propagation from the stratosphere to the troposphere for explaining the unusual state of the stratospheric polar vortex in February.
Interannual variations in the number of winter extreme warm and cold days over eastern China （EC） and their relationship with the Arctic Oscillation （AO） and E1 Nifio-Southern Oscillation （ENSO） were investigated using an updated temperature dataset comprising 542 Chinese stations during the period 1961- 2011. Results showed that the number of winter extreme warm （cold） days across EC experienced a significant increase （decrease） around the mid-1980s, which could be attributed to interdecadal variation of the East Asian Winter Monsoon （EAWM）. Probability distribution functions （PDFs） of winter temperature extremes in different phases of the AO and ENSO were estimated based on Generalized Extreme Value Distribution theory. Correlation analysis and the PDF technique consistently demonstrated that interannual variation of winter extreme cold days in the northern part of EC （NEC） is closely linked to the AO, while it is most strongly related to the ENSO in the southern part （SEC）. However, the number of winter extreme warm days across EC has little correlation with both AO and ENSO. Furthermore, results indicated that, whether before or after the mid-1980s shift, a significant connection existed between winter extreme cold days in NEC and the AO. However, a significant connection between winter extreme cold days in SEC and the ENSO was only found after the mid-1980s shift. These results highlight the different roles of the AO and ENSO in influencing winter temperature extremes in different parts of EC and in different periods, thus providing important clues for improving short-term climate prediction for winter temperature extremes.
Responses of the East Asian winter monsoon （EAWM） in future projections were studied based on two core future projections of CMIP5 in coordinated experiments with the IAP-coupled model FGOALS2-s. The projected changes of EAWM in climatology, seasonality, and interannual variability are reported here; the projections indicated strong warming in winter season. Warming increased with latitude, ranging from 1°C to 3°C in the Representative Concentration Pathways simulation RCP4.5 projection （an experiment that results in additional radiative forcing of 4.5 W m-2 in 2100） and from 4° to 9°C in the RCP8.5 projection （an experiment that results in additional radiative forcing of8.5 W m-2 in 2100）. The northerly wind along the East Asian coastal region became stronger in both scenarios, indicating a stronger EAWM. Accordingly, interannual variability （described by the standard deviation of temperature） increased around the South China Sea and lower latitudes and decreased over eastern China, especially in North China. The two EAWM basic modes, defined by the temperature EOF analysis over East Asia, were associated with the Arctic Oscillation （AO） and stratospheric polar vortex. The future projections revealed more total variance attributable to the secondary mode, suggesting additional influences from the stratosphere. The correlation between AO and the leading mode decreased, while the correlation between AO and the secondary mode increased, implying increased complexity regarding the predictability of EAWM interannual variations in future projections.
