Interannual Variations Of Early And Late Winter Temperatures In China And The Mechanisms Underlying Their Linkage

Based on the daily mean and minimum surface air temperature(SAT) data from 549 Chinese stations during 1960-2007, boreal winter is divided into early winter(EW, November 16 th to January 15 th in the following year) and late winter(LW, January 16 th to March 15th) by analyzingthe date of the coldest days. Following this definition, SAT from 549 Chinese stations, atmospheric circulation data provided by the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR), and the monthly mean sea surface temperature(SST) derived from National Oceanic and Atmospheric Administrator(NOAA) are analyzed by using empirical orthogonal function(EOF), season-reliant EOF(SEOF), regression and composite methods to examine the interannual variations of early and late winter temperatures in China and their evolutions. We also analyze the linkage between interannual variations of early and late winter temperatures and the mechanisms underlying these links by investigating the related anomalies of atmospheric circulation and external forcing.The first EOF modes ofthe both early and late winter depict a pan-China temperature variation, and the second EOF modes ofthe both early and late winter feature a temperature oscillation between northern and southern parts of China. An inspection of their corresponding principal components(PCs) indicates that if a certain SAT mode is observed in early winter, the possibility of observing the same SAT mode with the same sign in late winter is approximately 50%, which is nearly equal to that of observing the same SAT mode with the opposite sign. These results indicate that averaging the SAT for the entire winter is suitable for the former case but unsuitable for the latter case. Therefore, it is necessary to divide winter into early and later winters when investigatingthe variability of wintertime SAT.To capture the major modes of the interannual variability of SAT and the relationship between early and late winter, SEOF analysis is performed on the covariance matrix constructed with SATs from 549 Chinese stations. The first SEOF mode(SEOF1) captures an in-phase evolution of SAT from early to late winter. The corresponding PC time series displays an obvious interdecadal change around the mid-1980 s. The second SEOF mode(SEOF2) features an out-of-phase evolution of SAT from early to late winter. The corresponding PC time series is dominatedby interannual variability.In the middle troposphere, the SEOF1 related atmospheric circulation anomaly is closely similar to the Eurasia-like teleconnection of the same sign throughout the winter, which consequently leads to changes in the intensity of the East Asian winter monsoon by changing the land–sea thermal contrast between East Asia and surrounding areas, and the intensity and the meridional location of the East Asian upper-tropospheric jet stream. In early winter, the SEOF2 mode is correlated to the Eurasian-like teleconnection, and the associated circulation anomalies are significant throughout the entire troposphere. In late winter, it is dominated by the Scandinavian-like teleconnection of the opposite phase, and the significant circulation anomalies are observed only in the middle and lower troposphere. These results suggest that the formations of SEOF1 and SEOF2 are mainly attributed mainly to the mid-latitude atmospheric internal dynamical processes, particularly atmospheric teleconnection.Meanwhile, the positive SAT anomaly in China appeared in the in-phase evolution mode is related to the positive SST anomaly in the middle and east parts of the north Atlantic(35-45oN,33-20oW) in early winter and to the west of Spain and Morocco in late winter. These SST anomalies can modify the activity of atmospheric transient wave by changing the baroclinicity in lower atmosphere, which further contributes to the eastward wave over the Eurasian continent by the dynamic and thermal feedback of the transient waves. When the early winter is colder and the late winter is warmer in the out-of-phase evolution mode, the related positive SST anomaly in early winter is located in the middle of north Atlantic(40-53oN,25-2oW), similar to the letter “U”, while negative SST anomaly in late winter is observed around the Labrador Seato the northwestward of North Atlantic(56-68oN,65-15oW). To a certain extent, these SST anomalies can also influence the activity of atmospheric transient wave and explain the associated atmospheric anomaly via the dynamic and thermal feedback of the transient waves.The monthly mean SAT data from 160 Chinese stations during 1951-2013 are also analyzedto confirm the results derived from the 549 stations., The boreal winter is divided into early winter(ND, November and December) and late winter(JFM, January to March of the following year) in 160 Chinese stations data. Results show that the interannual variations of early and late winter and their linkage can also be captured by 160 Chinese stations.