1.State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China 2.University of the Chinese Academy of Sciences, Beijing 100049, China Manuscript received: 2017-12-12 Manuscript revised: 2018-02-11 Manuscript accepted: 2018-03-21 Abstract:There is a well-known seesaw pattern of precipitation between the tropical western North Pacific (WNP) and the Yangtze River basin (YRB) during summer. This study identified that this out-of-phase relationship experiences a subseasonal change; that is, the relationship is strong during early summer but much weaker during mid-summer. We investigated the large-scale circulation anomalies responsible for the YRB rainfall anomalies on the subseasonal timescale. It was found that the YRB rainfall is mainly affected by the tropical circulation anomalies during early summer, i.e., the anticyclonic or cyclonic anomaly over the subtropical WNP associated with the precipitation anomalies over the tropical WNP. During mid-summer, the YRB rainfall is mainly affected by the extratropical circulation anomalies in both the lower and upper troposphere. In the lower troposphere, the northeasterly anomaly north of the YRB favors heavier rainfall over the YRB by intensifying the meridional gradient of the equivalent potential temperature over the YRB. In the upper troposphere, the meridional displacement of the Asian westerly jet and the zonally oriented teleconnection pattern along the jet also affect the YRB rainfall. The subseasonal change in the WNP-YRB precipitation relationship illustrated by this study has important implications for the subseasonal-to-seasonal forecasting of the YRB rainfall. Keywords: Yangtze River basin, tropical western North Pacific, precipitation, subseasonal change 摘要:长江流域夏季降水与热带西北太平洋夏季降水之间存在显著的反相关关系。本文的研究结果表明,两者的反相关关系经历了显著的次季节变化:反相关关系在前夏较强,而在中夏减弱。我们进而分析了前夏和中夏影响长江流域降水的大尺度环流异常。结果表明,前夏长江流域降水主要受热带环流异常的影响,即热带西北太平洋降水异常所激发的反气旋或气旋式异常。中夏,长江流域降水主要受热带外环流异常的影响。在对流层低层,长江流域北部的东北风异常通过增加相当位温的经向梯度使长江流域降水增多。在对流层高层,亚洲急流的经向偏移以及沿急流传播的纬向波列也对长江流域降水有重要影响。本研究对长江流域降水的次季节-季节预测具有重要的指示意义。 关键词:长江流域, 热带西北太平洋, 降水, 次季节变化
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4.1. Lower-tropospheric anomalies
Figure 4 shows the regression of precipitation with respect to the normalized YRBPI in early summer and mid-summer. In early summer, precipitation anomalies expectedly exhibit the seesaw relationship between the YRB and the tropical WNP. There are positive anomalies over the YRB and the positive anomalies also extend eastward to southern Japan and there are negative anomalies over the tropical WNP (Fig. 4a). These anomalies are similar to those for the JJA mean (Fig. 1). The correlation coefficient between the YRBPI and the WNPPI during this period is -0.66, significant at the 99% confidence level. By contrast, the anomalies over the tropical WNP are sharply reduced in mid-summer and the weak negative and positive anomalies tend to offset one another (Fig. 4b). The correlation coefficient between the YRBPI and the WNPPI during this period is only 0.01. These results are consistent with Fig. 2a and confirm that the impact of the convection anomalies over the tropical WNP on the YRB rainfall is weak during mid-summer. Figure4. Regression of precipitation with respect to the normalized YRBPI during (a) early summer (ES) and (b) mid-summer (MS). The highlighted region represents the averaging area for the WNPPI. The contour interval is 0.5 mm d-1. The zero line is not shown. The shading denotes the 95% confidence level based on the Student’s t-test.
Figure5. Regression of 850-hPa horizontal winds (vectors; units: m s-1) with respect to the normalized YRBPI during (a) early summer (ES) and (b) mid-summer (MS). Shading indicates that either zonal or meridional wind anomalies are significant at the 95% confidence level.
Figure 5 shows the wind anomalies at 850 hPa regressed onto the normalized YRBPI during early summer and mid-summer. The wind anomalies in early summer show a significant anticyclonic anomaly over the subtropical WNP (Fig. 5a). The anomalous anticyclone indicates a westward extension of the WNP subtropical high and favors enhanced rainfall over the YRB by transporting water vapor along the northwest flank of the high. The anticyclonic anomaly also corresponds to the negative precipitation anomalies over the tropical WNP (Fig. 4a) and this correspondence has become well known as the Pacific-Japan pattern (e.g., Nitta, 1987; Lu, 2001a, 2001b; Lu, 2001a; Kosaka et al., 2011). On the other hand, the extratropical circulation anomalies are weak and insignificant. There are slight northeasterly anomalies north of the YRB, but with marginal significance. These results indicate that the YRB rainfall is mainly affected by the tropical circulation anomalies in early summer. In mid-summer, the wind anomalies are characterized by a cyclonic anomaly over East China (Fig. 5b). Associated with this cyclonic anomaly are the northeasterly anomalies north of the YRB and westerly south of the YRB. (Li and Lu, 2017) reported that the northeasterly anomalies play a vital role in precipitation anomalies over the YRB by increasing the meridional gradient of equivalent potential temperature (θ e) and water vapor accumulation. On the other hand, the anticyclonic anomaly over the subtropical WNP disappears. These results indicate that the YRB rainfall is mainly affected by the extratropical circulation anomalies during mid-summer. To better illustrate the subseasonal change in the impact of the northeasterly anomalies on the YRB rainfall, we use the northeasterly anomalies averaged over the region (32.5°-40°N, 107.5°-120°E) to define a northeasterly index (NEI): \begin{equation*} {\rm NEI}=A_{\rm ave}\left[u\cos\left(-\dfrac{3\pi}{4}\right)+v\sin\left(-\dfrac{3\pi}{4}\right)\right] . \end{equation*} Here, A ave is the area average, and u and v are the zonal and meridional wind anomalies at 850 hPa, respectively. This definition is similar to that in (Li and Lu, 2017), who used the 700-hPa wind anomalies. We also defined the NEI at 700 hPa and obtained similar results. Figure 6 shows the 15-day running correlation coefficient between the YRBPI and the NEI. The YRBPI-NEI relationship is weak and insignificant during early summer and becomes strong and significant during mid-summer. The correlation coefficient between the YRBPI and the NEI is 0.27 during early summer and 0.51 during mid-summer, confirming that the impact of the northeasterly anomalies on the YRB rainfall is enhanced during mid-summer. It is well known that the mei-yu rainfall over the YRB is characterized by a strong humidity gradient in the lower troposphere. Therefore, the equivalent potential temperature (θ e), which combines the temperature and humidity, is widely used in mei-yu rainfall analyses (e.g., Zhu et al., 2000; Ninomiya, 2000; Zhou et al., 2004; Ding, 2005; Park et al., 2015; Chen and Zhai, 2015). A higher θ e value indicates a warmer and more humid weather condition. Climatologically, the θ e decreases with latitude over East China and shows a strong meridional gradient over the YRB during summer (not shown), which favors the enhanced precipitation there by inducing convection instability (e.g., Ninomiya, 1984; Ninomiya and Shibagaki, 2007). Figure6. Fifteen-day running correlation between the YRBPI and the NEI (see text for definition). The horizontal dashed line represents the significance of the correlation coefficient at the 95% confidence level according to the Student’s t-test.
Figure7. Regression of 700-hPa θ e anomalies with respect to the normalized YRBPI during (a) early summer (ES) and (b) mid-summer (MS). The contour interval is 0.2 K and the zero line is not shown. Shading denotes the 95% confidence level based on the Student’s t-test. The scopes (red lines) of the significant positive precipitation anomalies and the significant wind anomalies at 700 hPa are also given for comparison. Precipitation anomalies are based on the station data. The curves in the right-hand panels show the corresponding meridional gradient anomalies (units: 10-3 K km-1) of θ e, which are averaged over 107.5°-120°E.
Figure 7 shows the θ e anomalies at 700 hPa with respect to the normalized YRBPI during early summer and mid-summer. The significant precipitation and wind anomalies are also shown to facilitate comparison. The right-hand panels show the corresponding meridional gradient anomalies of θ e, averaged over 107.5°-120°E. A positive value indicates the decrease in θ e in the northward direction, i.e., the intensified θ e gradient. For early summer (Fig. 7a), there are positive θ e anomalies over the YRB and negative θ e anomalies north of the YRB, which enhance the meridional gradient of the θ e over the YRB. The difference in θ e anomalies between the southern edge [averaged over (27.5°N, 107.5°-120°E)] and the northern edge [averaged over (33.75°N, 107.5°-120°E)] of the YRB is 0.82 K, which is 9% of the climatological difference during this period (9.34 K). The enhanced θ e gradient anomalies (right-hand panel of Fig. 7a) agree well with the enhanced precipitation anomalies, but show a slight northward shift compared to that of precipitation. This northward shift, as explained by (Li and Lu, 2017), is due to the northward tilt of the enhanced gradient with height in the lower troposphere. In addition, the anomalous southwesterlies correspond to the positive θ e anomalies over the YRB, which suggests that the tropical circulation anomalies are responsible for the intensified θ e gradient over the YRB and the associated enhanced precipitation. In mid-summer (Fig. 7b), there are positive anomalies south of the YRB. A sharp contrast to the early summer is the enhancement of the negative anomalies north of the YRB. These positive and negative anomalies enhance the meridional gradient of θ e over the YRB. The difference in θ e anomalies between the southern and northern edges of the YRB is 1.67 K, and this increase is about 68% of the climatological difference during this period (2.47 K). The enhanced θ e gradient is also in agreement with the enhanced precipitation. In addition, the circulation anomalies also correspond well to the θ e anomalies. Enhanced northeasterlies north of the YRB correspond to the negative θ e anomalies, and southwesterlies south of the YRB correspond roughly to the positive θ e anomalies. To better illustrate the correspondence between the rainfall and θ e, we choose the years in which the absolute values of the YRBPI are greater than 0.7 standard deviations and perform composite analyses based on these years, for both early summer and mid-summer. Table 1 shows the positive and negative cases based on this criterion. Besides the criterion of 0.7 standard deviations shown here, we also used some other criteria, such as 0.8 standard deviations or 1.0 standard deviation, and obtained similar results (not shown). Figure 8 shows the 700-hPa θ e and rainfall for positive and negative YRBPI cases during early summer and mid-summer. For all categories, the high θ e values stretch from southwest to northeast in a tongue-shaped pattern, which is induced by the warm and moist air transported by the southwesterlies associated with the subtropical high. The θ e decreases with latitude over the north of the tongue and shows a maximum meridional gradient over the northeast of the tongue, corresponding to the heavy rainfall there. Figure8. Composite 700-hPa θ e (contour interval: 2.0 K) and precipitation (shading; units: mm d-1) over East China in the (a, c) positive and (b, d) negative YRBPI cases for (a, b) early summer (ES) and (c, d) mid-summer (MS). Precipitation anomalies are based on the station data.
Figure9. Regression of the 850-hPa horizontal winds (vectors; units: m s-1) and precipitation (shading; units: mm d-1) with respect to the normalized negative WNPPI during (a, c) early summer (ES) and (b, d) mid-summer (MS). The color shading in (a, b) is based on the CMAP data. The anomalies over East China are magnified in the bottom panels (c, d) and color shading is based on the station precipitation data. The black shading represents mountains higher than 1500 m. Red lines refer to 30°N. The dots denote the 95% confidence level for precipitation anomalies based on the Student’s t-test, and only the vectors of either zonal or meridional wind anomalies significant at the 95% confidence level are shown.
However, the distributions of θ e are distinct between the positive and negative cases. For early summer, the warm and moist tongue is located over South China in the positive cases and the maximum meridional gradient of θ e appears over the YRB (Fig. 8a), which corresponds well to the heavy precipitation there. For the negative cases (Fig. 8b), the tongue retreats southward and the meridional gradient of θ e over the YRB is reduced, in agreement with the rain band appearing over South China and precipitation being less over the YRB. In contrast to early summer, the meridional scope of the high θ e, or the warm and moist tongue, becomes wider and moves northward during mid-summer for both positive and negative cases (Figs. 8c and d versus Figs. 8a and b), which is consistent with the subseasonal march of the θ e over East Asia during summer. As a result, the θ e gradient over the YRB is greatly reduced, corresponding to the reduced precipitation over the YRB and increased precipitation over North China. This phenomenon is associated with the northward advance of the subtropical high. During mid-summer, the distributions of θ e are also distinct between the positive and negative cases. For the positive cases, the high θ e appears over South China, and large θ e gradient appears over the YRB (Fig. 8c), corresponding to the high precipitation there. By contrast, the scope of high θ e is extended, occupying both South China and the YRB, and the location of the high θ e gradient moves northward for the negative cases (Fig. 8d), corresponding to the heavier rainfall over North China but less rainfall over the YRB. These θ e distributions are consistent with the lower-tropospheric circulations: the anomalous northeasterlies prevent the subseasonal march over East China for the positive cases in comparison with the negative cases. The above results demonstrate that the relationship in precipitation between the tropical WNP and the YRB rainfall is strong during early summer but weak during mid-summer. In the following, we analyze the anomalies associated with the rainfall over the tropical WNP, in an attempt to illustrate the reason for the weakened impact of the precipitation anomalies over the tropical WNP on the YRB rainfall during mid-summer. Figure 9 shows the 850-hPa horizontal wind and precipitation anomalies regressed onto the normalized negative WNPPI during early summer and mid-summer. Here, the anomalies associated with the negative WNPPI are shown to facilitate comparison with the anomalies that are favorable for the enhanced YRB rainfall (Fig. 5). The anomalies over East China are highlighted in the bottom panels. For both early summer and mid-summer, there are negative precipitation anomalies over the tropical WNP (Figs. 9a and b). The negative precipitation anomalies induce the meridional teleconnection pattern over the WNP and East Asia, i.e., the anticyclonic/cyclonic/anticyclonic anomalies over the region from the tropics to high latitudes, which has been well reported in previous studies (Nitta, 1987; Huang and Sun, 1992; Lau et al., 2000; Lu, 2001b; Lu, 2001a; Kosaka et al., 2011). However, there is an appreciable difference between early summer and mid-summer: the WNP anticyclonic anomaly trenches further northward during mid-summer than in early summer (Figs. 9b and d versus Figs. 9a and c). The northern boundary of the anomalous anticyclone is located around 30°N during early summer (Figs. 9a and c) and is similar to the location of the WNP anticyclonic anomaly that favors the YRB rainfall (Fig. 5a), which indicates there is water vapor convergence over the YRB. Accordingly, significantly enhanced precipitation appears over the East Asian rainy belt, including the YRB. Meanwhile, the northern boundary of the WNP anticyclonic anomaly reaches around 35°N during mid-summer (Figs. 9b and d), which suggests that water vapor is transported further northward into the Huaihe River region, and there is less water vapor convergence over the YRB during this period. Accordingly, the enhanced rainfall anomalies tend to move northward in East China and are much weaker over the YRB (Figs. 9b and d). It should be mentioned that the rainfall anomalies over the tropical WNP during early summer and mid-summer are roughly similar, for both the scope and amplitude (Figs. 9a and b). The northward extension of the subtropical WNP anticyclonic anomaly during mid-summer can be explained by the results of (Kosaka and Nakamura, 2010), who indicated that identical tropical heat forcing can trigger a northward-extended cyclonic anomaly over the subtropics when the upper-tropospheric westerly jet axis, as the basic flow, is located northward. During summer over East Asia, the upper-tropospheric westerly jet experiences a northward shift (Lin and Lu, 2008). Therefore, the subtropical WNP anticyclonic anomaly extends northward during mid-summer, in comparison with early summer. A similar northward extension of the WNP anticyclonic anomaly, associated with the preceding wintertime ENSO, has been found in previous studies (Ye and Lu, 2011; Hu et al., 2017).
2 4.2. Upper-tropospheric anomalies -->
4.2. Upper-tropospheric anomalies
Figure 10 shows the zonal and meridional wind anomalies at 200 hPa regressed onto the normalized YRBPI during early summer and mid-summer. In early summer, there are significant anomalies over the tropical WNP for both zonal and meridional winds (Figs. 10a and c), and the extratropical anomalies are very weak and insignificant, consistent with the lower-tropospheric circulation anomalies (Fig. 5a). These results confirm that the precipitation anomalies over the YRB are mainly affected by the tropical circulation anomalies in early summer. Figure10. Regression of the 200-hPa (a, b) zonal and (c, d) meridional winds with respect to the normalized YRBPI during (a, c) early summer (ES) and (b, d) mid-summer (MS). The bold black lines represent the corresponding climatological jet axis for ES and MS. The contour intervals are 0.5 and 0.3 m s-1 for the zonal and meridional wind anomalies, respectively. The zero contour lines are not shown. Shading denotes the 95% confidence level based on Student’s t-test.
By contrast, the anomalies in mid-summer mainly appear over the extratropical regions (Figs. 10b and d). For the zonal wind anomalies, there are significant positive anomalies south of the jet axis and significant negative anomalies north of the jet axis over West Asia and East Asia (Fig. 10b), suggesting a southward displacement of the westerly jet in these regions. The zonal wind anomalies are remarkably weaker over Central Asia. On the other hand, the meridional wind anomalies show a zonally oriented teleconnection pattern along the westerly jet, or the Silk Road Pattern (Fig. 10d). These zonal and meridional wind anomalies resemble the JJA-mean results of (Hong and Lu, 2016), who suggested the connection between the jet axis displacement and the Silk Road Pattern. Furthermore, these upper-tropospheric circulation anomalies suggest a cyclonic anomaly over East Asia, and this cyclonic anomaly shifts poleward compared to its counterpart in the lower troposphere (Fig. 5b). The meridional displacement of the EAJ and the Silk Road Pattern have been documented as the mainly extratropical circulation anomalies that affect the YRB rainfall for the JJA mean (Li and Lu, 2017). Here, we show that these anomalies mainly affect the YRB rainfall during mid-summer. The anomalies over the tropical WNP are very weak for both the zonal and meridional wind, consistent with the weak relationship in rainfall between the tropical WNP and the YRB.