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We analyzed spatial and temporal growth variations of Qilian Junipers over the northeastern Tibetan Plateau (TP) during the period 1450—2001 by applying the empirical orthogonal function (EOF) technique to seven moisture-sensitive tree ring-width chronologies. The first three principal components together explain 78.21% of the total variance. The first principal component (PC1) accounts for 51.58% of the total variance, and represents consistent tree-growth variations in the northeastern TP. The second EOF mode reveals an opposite structure for the southern and northern parts of the northeastern Tibetan Plateau, reflecting the effect of huge topography and associated atmospheric circulation on tree-growth anomalies. The third EOF mode represents an opposite structure for the eastern and western parts of the northeastern Tibetan Plateau, possibly indicating the influence of different branches of atmospheric circulation system in this area. The EOF analysis results of instrumental precipitation data resemble those derived from the seven regional chronologies, providing further support that tree growth is a good indicator of regional precipitation variations. According to the PC1, dry conditions occurred in AD 1450—1510, 1631—1739, 1765—1833, 1866—1883 and 1921—1947, whereas periods of relatively wet years are identified for AD 1511—1630, 1740—1764, 1834—1865, 1884—1920 and 1948—2001. During the past 550 years, 28 years with extreme regional growth depressions was identified. These ring-width extremes generally occurred in century-scale dry periods. The years 1467, 1484, 1641, 1721, and 1928 with extreme tree-growth reductions in the northeastern TP are coeval with severe drought events in eastern China. In addition, the dry intervals or extreme growth depressions generally coincided with cold periods in the northeastern TP and vice versa, implying that the probability of occurrence of wet events in the northeastern TP will be higher in the future under the ongoing global warming.

A recent study categorized the Madden-Julian Oscillation (MJO) during boreal winter season into four types including stand, jump, slow and fast MJO. This study focuses on the stand and jump MJO. Based on whether their convection penetrates the Maritime Continent (MC), stand and jump MJO are seen as non-penetrating (NP) MJO, while the rest two are seen as eastward-penetrating (EP) MJO. Results reveal the relative roles of the westward-propagating wave (WPW), as well as the QBO and ENSO, in limiting MJO propagation. Lack of the premoistening over the southern sea surface of the MC stops NP MJO from penetrating the MC. The active convection of the WPWs hinders the descending branch of the NP MJO circulation and therefore leads to the insufficient meridional advective moistening over the southern sea surface of the MC. The independent convection over the Pacific for jump MJO is influenced by a combined effect of the QBO and ENSO. The tropopause instability induced by MJO is found to significantly decouple from its convection over the Pacific in the QBOW winters than in the QBOE winters. For jump MJO, the independent convection over the central Pacific comes from local WPWs whose amplification and further development into deep convection are correlated to jump MJO’s decoupled tropopause instability. For stand MJO, however, the seasonal-mean La Nina-like cool SST anomalies weaken the WPW activity over the central Pacific and confine WPWs within the western Pacific. Therefore, the decoupled tropopause instability of stand MJO is out phase of WPWs and fails to induce an independent convection over the central Pacific.