3.1.1. Variation Characteristics of the Plateau Monsoon
The onset and retreat times of the plateau summer and winter monsoon are important indicators for determining the characteristics of the monsoon over the plateau. If the indices are greater than zero, the plateau summer monsoon starts when the plateau is controlled by the thermal low. Indices of less than zero indicate the onset of the plateau winter monsoon. Figure 2
a shows the annual variations of the standardized plateau monsoon indices. Although these indices all present distributions of a unimodal type, there are some differences among them in terms of seasonal variation. The TPMI is greater than zero after March, whereas the ZPMI and DPMI are greater than zero after April and May, respectively. All of these indices reach their peak values in June, when the summer monsoon prevails over the plateau, before gradually decreasing. The TPMI is less than zero after September, whereas the DPMI and ZPMI are less than zero around October. This suggests that the onset and retreat of the plateau summer monsoon differ among the various monsoon indices. How to describe the onset and retreat times of the summer and winter monsoons correctly is key to analyzing the characteristics of the plateau monsoons. Overall, the onset and retreat of the TPMI are 1–2 months earlier than those of the DPMI and ZPMI. The TPMI is significantly weaker than the DPMI and ZPMI from July to September. In addition, monsoon precipitation increase and wind shifts over the plateau are the main features of the summer monsoon onset. The plateau receives more precipitation from April to October during which the plateau is controlled by a warm low (the ZPMI is greater than zero) (Figure 2
a). Compared with the seasonal variation of precipitation over the plateau, the annual variation characteristics given by the DPMI and ZPMI are better than that of TPMI. In addition to the climatological monthly evolution of precipitation over the plateau, a good monsoon index should also indicate the evolution of precipitation in each year. Considering that the rainy season over the plateau is mainly confined to summer, the inter-annual variation of plateau summer precipitation is given in Figure 2
b. The inter-annual and inter-decadal variation tendency of the plateau summer precipitation is consistent with these of the plateau summer monsoon indices. Compared with the correlation between the plateau summer monsoon indices and precipitation (Table 1
), the inter-annual variation characteristics given by the ZPMI and QPMI are better than those of the TPMI and DPMI. Overall, the ZPMI can better reflect the seasonal and inter-annual characteristics of the plateau monsoonal precipitation compared with the other indices.
shows the inter-annual variation of the plateau summer (b) and winter (c) monsoon indices from 1979 to 2014. The plateau summer and winter monsoons display similar inter-annual and inter-decadal variation characteristics, with a rising trend that is more significant for the summer monsoon. Table 2
presents the Z statistic of the MK trend test for the plateau summer and winter monsoon indices. For the prescribed significance level,
, the critical value of Z
is ±1.96. Without exception, the Z
values of the summer monsoon indices are greater than zero, which shows that the summer monsoon presents a rising trend for the period 1979–2014. The MK trend test Z statistic of the ZPMI, TPMI, and QPMI is greater than 1.96, indicating that the increasing trend is significant. For the winter monsoon indices, the Z
values are greater than zero but fail to pass the 95% confidence test, demonstrating that the increasing trend of the winter monsoon is not significant.
To further reveal the variation characteristics of the plateau monsoon, the SQMK test is applied to study the abrupt changes in the plateau summer and winter indices. Figure 3
a shows the UF and UB curves of the summer ZPMI. During the period 1979–2014, a sudden change appears in 1998, when the variation tendency of the summer monsoon and the warm low over the plateau shift from weak to strong. After 2004, the UF values exceed the significance level of 0.05, indicating that the plateau summer monsoon is enhanced significantly after this time. The UF and UB curves of the winter ZPMI are shown in Figure 3
b. Before 1990, the variation characteristics of the plateau winter monsoon display small fluctuations with no significant trend. They show an increasing trend in the subsequent 24 years, with the most significant trend from 1995 to 2008. For the summer TPMI (Figure 3
c), the plateau summer monsoon shifts from weak to strong in 2002, and the variation trend is significant after 2009. Meanwhile, for the winter TPMI (Figure 3
d), the plateau winter monsoon fluctuates with a small amplitude and no significant changes. The UF and UB curves of the summer DPMI are shown in Figure 3
e; the UF values are greater than zero except in a small number of specific years, and the curves of UF and UB do not exceed the critical significance level. The results depicted in Figure 3
e indicate that the plateau summer monsoon displays an increasing trend, although the tendency is not obvious. For the winter DPMI (Figure 3
f), the variation trends are similar to that of the winter TPMI. The UF and UB curves of the summer QPMI are displayed in Figure 3
g; in 1997, the plateau summer monsoon shifts from weak to strong, and the ascending trend is significant after 2003.
The annual cycle of the monsoon is a very important aspect of the monsoon climate [10
]. Through the above analysis, it is found that the onset and retreat of the plateau summer and winter monsoons reflected by the ZPMI and DPMI are better than that of the TPMI. Additionally, the inter-annual and inter-decadal variability of the plateau summer and winter monsoon indices generated by the different calculation methods are very consistent. Meanwhile, the plateau summer and winter monsoons both present an ascending trend, but the increasing trend of the summer monsoon is more significant. As seen from the SQMK analysis, the plateau summer monsoons reflected by the ZPMI, TPMI, and QPMI shift from weak to strong in 1998, 2002, and 1997, respectively. In contrast, no significant increasing or decreasing trends are detected in the plateau summer monsoon reflected by the DPMI. The plateau winter monsoon reflected by the different indices appears similar, and the winter ZPMI is enhanced more significantly during 1995–2008. Previous studies [38
] have showed that the annual mean temperature in the plateau experienced two abrupt changes from low to high in 1987 and 1998, respectively. The changing points to an abrupt increase in the annual precipitation and runoff in the plateau occur in 1996 and 1997, respectively [40
]. The shifts of the plateau monsoon are bound to cause changes in meteorological elements. Based on the changing point of the meteorological elements mentioned above, the ZPMI has a reasonable performance compared to other indices.
3.1.2. Correlation Analysis between the Plateau Monsoon and Meteorological Elements
A reasonable monsoon index is capable of not only reflecting monsoon circulation characteristics but also explaining relevant weather phenomena [10
]. In the present study, the linkages between the plateau summer monsoon and meteorological elements over the plateau are discussed.
The correlation between the summer ZPMI and precipitation over the plateau is presented in Figure 4
a. A significant positive correlation between the ZPMI and precipitation was detected in the main body of the plateau. The positive correlation center is located in Nagqu (31.47°N, 92.1°E) and its adjacent regions, where the correlation coefficients reach 0.77. Significant negative correlations were located in the northwest of the plateau. The results depicted in Figure 4
a indicate that when the plateau summer monsoon is strong (weak), precipitation in the main body of the plateau, especially in Nagqu and its adjacent areas, is higher (lower) than normal, whereas the precipitation over the northwestern plateau is lower (higher). For the TPMI (Figure 4
b), the significant positive correlations were confined to the area of (30°–35°N, 85°–102°E), with the coefficients reaching 0.7. The significant negative correlations were located in the western plateau. For the DPMI (Figure 4
c), the regions of significant positive correlation over the main body of the plateau decrease obviously compared to those of the ZPMI and TPMI. The distribution characteristics of the correlation between the QPMI and precipitation (Figure 4
d) are similar to that of the ZPMI. The positive correlation center is closer to the northern plateau, with coefficients reaching 0.73.
The correlation coefficients between the plateau summer monsoon indices and the summer air temperature are given in Figure 5
. The distribution characteristics are similar to these depicted in the Figure 4
. Significant positive correlations were located in the main body of the plateau, whereas significant negative correlations were located at the edge of the western plateau. The results depicted in Figure 5
indicate that when the plateau summer monsoon is strong (weak), the air temperature in the main body of the plateau is higher (lower) than normal, while the air temperature over the western marginal areas of plateau is lower (higher).
Through contrast analysis of the correlations between different indices and meteorological elements, similar spatial patterns were detected among the various monsoon indices. Corresponding to stronger plateau summer monsoon, more (less) precipitation and a higher (lower) air temperature appear over the eastern (western) plateau. This result is consistent with that proposed by Tang et al.
]. In strong (weak) plateau summer monsoon years, there exists an anomalous cyclone (anticyclone) over the plateau, which results in an anomalous ascending (descending) motion. The plateau and its adjacent regions are under the effect of an anomalous southwesterly (northwesterly) wind that can transport more (less) moisture there. All of these factors contribute to more (less) precipitation and a higher (lower) temperature over the eastern and central plateau [10
]. Apparently, the ZPMI is capable of effectively reflecting meteorological elements.