NAULEA Wave Train: A New Teleconnection Pattern Affecting Precipitation in the Yangtze River Basin

Previous studies have found that abnormal wave trains in middle and high latitudes lead to precipitation anomalies in China (e.g., Zhang et al., 2019). Zhang et al. (1998) pointed out that abnormal wave trains in middle and high latitudes lead to abnormal precipitation in the middle and lower reaches of the Yangtze River. To explore the causes of extremely geopotential height near Siberia and Mongolia, we analyzed the correlation distribution of ASO in March and geopotential height at 200hPa in April. The most interesting finding in Figure 6(b) is that there are four abnormal centers in the middle and high latitudes, namely the North Atlantic, northwest Europe, near the Ural Mountains, Siberia, and Mongolia, which correspond to the conclusion of Li et al. (2008). Li et al. (2008) defined the four abnormal centers as 'North Atlantic-Ural Mountain-East Asia' (i.e., NAULEA) and noted that the abnormal wave train in middle and high latitudes is another teleconnection type related to NAO.

As shown in Figures 6(a) and 6(b), the geopotential height of 200hPa in March has an AO-like pattern but transforms into a NAULEA wave train in April. Consequently, we defined the NAULEA index as NAULEA = 1/4 [H (30ﾰN, 30ﾰW) - H (50ﾰN, 10ﾰE) + H (60ﾰN, 35ﾰE) - H (50ﾰN, 90ﾰE)]. The polar region and the Mongolian region are both components of the meridional wave train from the polar region to the middle latitude. The meridional wave train persists until April, leading to the strong strength of the centers C and D of NUALEA wave train. Figures 6(b) and 6(c) show that the NAULEA wave train is a quasi-barotropic structure; hence, the barotropic model of LBM will be adopted. Figures 6(b) and 6(d) demonstrate that NAULEA is located on the north side of the East Asian jet stream (EAJS) (e.g., Figures 6(d)), and impinges on the eastern part of China through the EAJS. Many studies have proven that the Antarctic ozone hole causes the mid-latitude jet to move towards the pole (e.g., Polvani et al., 2011; Son et al., 2008; Gerber et al., 2014; Waugh et al., 2015). As shown in Figures 6(d) and 6(e), there is a significant positive correlation between the abnormal change of ASO in March and the EAJS in April, providing a plausible interpretation for how ASO affects the intensity of the EAJS but has little effect on the shift of the EAJS. The middle and lower reaches of the Yangtze River are located in the entrance of the East Asian jet stream. Besides, the ascending branch on the south side of the jet stream entrance provides sufficient dynamic conditions for precipitation. The high-altitude jet carries a significant amount of dry and cold air in the sinking branch, but the ascending branch transports the warm and humid flow from the South Sea to the Yangtze River Basin, forming the intersection of cold and warm flow in the middle and lower troposphere. This is conducive to the formation of precipitation, increasing precipitation in the Yangtze River Basin and decreasing precipitation in other regions.

It is noteworthy that the QBO signal appears in Figure 6(e). However, the NAULEA wave train exists in both east and west wind phase years of QBO, showing that the appearance of the NAULEA wave train has nothing to do with QBO phase transition (not shown). Hu et al. (2003) present a mechanism in which Arctic ozone depletion leads to an enhanced meridional temperature gradient near the subpolar stratosphere and strengthened westerly winds. The strengthened winds refract planetary waves towards the low latitudes and cause the reduction of wave activity in high latitudes. Comparing Figure 7(a) and Figure 7(b), it is not difficult to find that the planetary waves in the troposphere were obviously upward in March, but in April, the planetary waves turned abnormally downward, which was caused by stratospheric ozone loss. As for Figure 7(d)(e), we selected 90ﾰE and 30ﾰW to verify that the downward propagation of planetary waves from the stratosphere to the troposphere is mainly concentrated in the North Atlantic Ocean and 90ﾰE. The energy propagates to the east of China along the NAULEA wave train and the meridional wave train (as shown in Figure 7(c)). Li et al. (2017) find a teleconnection between the North Atlantic Ocean and the Eurasian continent, suggested by statistical and dynamical analysis of the northern summer 500 hPa geopotential height field. This teleconnection, termed the Atlantic-Eurasian (AEA) teleconnection, has five centers of action, in the subtropical North Atlantic Ocean, northeastern North Atlantic Ocean, Eastern Europe, the Kara Sea, and north China. It may be due to the difference in season selection that there are only four centers in NAULEA during spring.