# Why the Increasing Trend of Summer Rainfall over North China Has Halted since the Mid-1990s

Authors: Haiwen Liu; Jiarui Miao; Kaijun Wu; Mengxing Du; Yuxiang Zhu; Shaoyu Hou
Journal: Advances in Meteorology
 (2020)

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## Abstract

Previous studies indicate that the summer (July-August) rainfall over North China has decreased since the mid-1970s due to the weakening of East Asian summer monsoon (EASM). However, this study firstly discovers the new evidences that the summer rainfall over North China had a significant increasing tendency during 1979–1996; since 1997, this increasing tendency has halted while more summer droughts occurred over North China. One important cause for the halted increasing tendency over North China is the interdecadal decrease of the westerly water vapor transport during 1997–2016 in addition to the weakened EASM. The decrease of the westerly water vapor transport during 1997–2016 was due to the interdecadal warming over Lake Baikal. The interdecadal warming in the upper troposphere at 200 hPa forced the weakening of the upper-level zonal winds since 1997, which resulted in the anomalous descending flow over the north side of North China and the halted precipitation trend in North China.


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## Body

## 1. Introduction

North China, a highly populated region, is located in northern China [1]. The summer (July-August) mean rainfall over North China has exhibited strong interdecadal variability [2–8]. North China experienced a relatively wet period from the 1950s to 1964 and a dry period since the end of 1970s [9, 10] when the droughts have greatly affected local agriculture, industry, and even the drinking water [11]. Except the interdecadal shift of summer rainfall over eastern China in the late 1970s [10, 12–14], another decadal shift over East Asia in the mid-1990s has also been investigated [15–19]. For example, Chen et al. [20] found that southern China summer rainfall experienced a remarkable increase in the early 1990s. Wu et al. [21] discovered a pronounced interdecadal change in summer rainfall over southern China around 1992/93, with persistent negative anomalies during 1979–1992 and positive anomalies during 1993–2002.Many possible causes of the interdecadal shift of the eastern China rainfall pattern in the late 1970s are investigated [10, 12–14], for example, the thermal contrast between the land and ocean by the large-scale temperature variations [2, 22], the sea surface temperature (SST) variations in the equatorial central and eastern Pacific [5], the role of the air-sea interaction in the middle latitudes [23], the Arctic sea-ice variations in winter [24], the North Atlantic Oscillation (NAO) and North Pacific Oscillation (NPO) variation [25], the interdecadal change of EASM [12], the western Pacific subtropical high (WPSH) over the subtropical regions of East Asia [4, 26], the interdecadal cooling in the upper troposphere and lower stratosphere over East Asia [27], the Pacific decadal oscillation (PDO) [28–30], and the thermal effect of black carbon aerosols over Asia [31].Although the consensuses on the interdecadal summer drought over North China have been made in the mentioned studies above, along with the development of the updated rainfall data with higher spatial resolutions, how did the summer rainfall over North China change in the recent decades is still of great concern in the climate research community. It is well known that there are four vapor inflow corridors, the southwest corridor, the South China Sea, the southeast corridor, and northwest corridor, from the mid-latitude westerlies to China [32, 33]. In addition to the great importance of the summer monsoon water vapor transport to North China [34], which boundary is also crucial to the moisture budget over North China associated with interdecadal variations of the summer rainfall there? To address these issues, this study will investigate a new interdecadal characteristic of the summer rainfall over North China using the latest precipitation data and analyze the possible causes.The organization of the paper is as follows. The datasets and methodology are described in Section2. Section 3 presents the interdecadal variability of summer rainfall over North China between the periods of 1979–1996 and 1997–2016 and further analyzes the interdecadal variations of atmospheric circulation, water vapor budget, and surface air temperature (SAT). A summary is given in Section 4.


## 2. Datasets and Methods

The existing criteria of the precipitation in North China [9], the averaged summer (July-August) precipitation of the 17 gauge-based stations chosen from 160 stations in China from 1951 to 2016, are used to reveal the interdecadal variability of summer rainfall over North China. The 17 meteorological stations are Chengde, Beijing, Tianjin, Shijiazhuang, Dezhou, Xingtai, Anyang, Yantai, Qingdao, Weifang, Jinan, Linyi, Heze, Zhengzhou, Changzhi, Taiyuan and Linfen, which are widely used to represent rainfall over North China [9] and mostly located within the domain (35°N–41°N, 110°N–122°E) in Figure 1.Figure 1
Locations of the 17 weather stations (blue circles) in North China and the region of North China (35°N–41°N, 110°E–122°E, blue rectangle). The shading represents the surface elevation surrounding North China in meters.To further demonstrate the robustness of the interdecadal variability of the summer rainfall over North China, the full data monthly product version 2018 of Global Precipitation Climatology Centre (GPCC) is also used. The GPCC with a spatial resolution of 1.0° × 1.0° is derived from quality-controlled station data from 1979 to 2016 and is widely used in the study of the interdecadal variation of local or regional rainfall [35–37].To investigate the interdecadal variations of the summer rainfall over North China using two types of the precipitation data, two indexes of summer precipitation over North China (ISPNC) are defined. One index is the normalized 17-station averaged summer precipitation, referred to as the ISPNC17 [9]. The other one is the normalized regional averaged summer precipitation in 35°N–41°N, 110°N–122°E using GPCC data, defined as ISPNCGPCC.To investigate the interdecadal variabilities of the atmospheric circulation, the European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) is used [38]. The gridded (0.75° × 0.75°) monthly ERA-Interim data during 1979–2016 are applied in this study. Variables include horizontal winds in 850 hPa, geopotential height in 500 hPa, 2 m surface air temperature (SAT), and vertical integral of water vapor flux. To verify the interdecadal variabilities of the atmospheric circulation, the monthly JRA-55 data during 1979–2013 are also used [39]. Two re-analysis datasets are highly consistent. For brevity, we only show the ERA-Interim results. Mann-Kendall (M-K) method is used to detect the trend and abrupt point of the time series [40, 41]. The statistical significance of the composite analysis and trend analysis is tested using Student’s t-test [42].The water vapor transport (M) via each boundary is calculated by(1)M=∫LQ⟶×n⟶dl.Q
⟶ is vertical integral of water vapor flux, while L is boundary line and dl is the unit boundary length. n⟶ is the inward-pointing normal vector of the boundaries of the target region [43]. The net budget of the regional water vapor transport is calculated by each boundary. The positive regional water vapor budget indicates the net atmospheric water vapor flux from outside and the abundant precipitable water within the region.


## 3. Results

### 3.1. Interdecadal Variability of Summer Rainfall over North China

Figure2(a) shows time series of ISPNC17 during 1979–2016. Obviously, summer rainfall of North China had an increasing tendency from 1979 to 1996. With the rate of 0.101/year, the tendency during 1979–1996 passed the significance test at the 95% confidence level, suggesting that summer rainfall of North China had a significant increasing tendency during the period. This interdecadal characteristic is very different from the previous studies [2–8], which mainly focused on the interdecadal decrease of the summer rainfall in North China since the mid-1980s [5, 6, 9]. Few studies discover that there was an obviously increasing tendency during 1979–1996. Unfortunately, the increasing tendency has halted since 1997. Then North China entered the drought period, with several persistent droughts accompanied by severe effects on industry and agriculture over North China [5, 44].Figure 2
(a) The time series of ISPNC17 during 1979–2016. The horizontal dashed lines indicate the interdecadal mean, while solid line from 1979 to 1996 indicates the interdecadal trend, and the vertical dashed lines indicate the interdecadal shift points. (b) The Mann-Kendall test of ISPNC17 (green dotted line shows backward statistic rank series, and blue dotted line shows forward statistic rank series). Two black lines show 95% confidence level.
(a)
(b)The time series of ISPNCGPCC in Figure 3(a) also show similar increasing tendency during 1979–1996 with the trend rate of 0.094/year, which also reaches the 95% confidence level according to Student’s t-test. This increasing tendency has also halted since 1997. So the gauge-based station precipitation data and the grid precipitation data of GPCC both show that summer rainfall over North China had an obvious increasing tendency during 1979–1996 and has halted since 1997.Figure 3
(a) Same as Figure2(a) but for the ISPNCGPCC. (b) Same as Figure 2(b) but for the ISPNCGPCC.
(a)
(b)To further investigate the interdecadal shift of the summer rainfall over North China, Mann-Kendall test method was used to detect the abrupt point of ISPNC17 in Figure 2(b). There is a cross point between the backward statistic rank series and forward statistic rank series, suggesting that summer rainfall over North China experienced a distinct interdecadal change around 1996. Since 1997, there have been more droughts occurring in North China. A severe drought attacking North China in 2014 also indicated that North China has become drier in recent years [44]. Figure 3(b) further verifies the interdecadal shift of the summer rainfall over North China using grid precipitation data of GPCC.The differences of summer rainfall by 1997–2016 mean minus 1979–1996 mean over Northeast Asia further verify the interdecadal variability of summer rainfall over North China in Figure4. Whether using gauge-based station data or using the grid precipitation data of GPCC, the significant negative summer rainfall differences are both found over North China, which is statistically significant at the 95% confidence level by Student’s t-test. Meanwhile, the main wetter regions are located in the Huaihe River valley, which suggests that the wetter belt has moved northwards from southern China [45]. Meanwhile, the spatial pattern of summer rainfall variation since 1997 has been different from the so-called pattern of southern flood and northern drought since the end of 1970s [26, 27, 46, 47].Figure 4
Difference of summer rainfall by 1997–2016 mean minus 1979–1996 mean for (a) 160 stations and (b) GPCC. The shading indicates statistically significant difference at the 95% confidence level based on Student’st-test. The box shows North China.
(a)
(b)


### 3.2. The Interdecadal Variability of the Atmospheric Circulation

East Asia is dominated by a typical monsoon climate [45, 48]. The summer precipitation change over eastern China, affected by the EASM greatly, is very significant on the interannual and interdecadal timescales [10]. To further study the causes of drier North China, Figure 5(a) shows the difference of the 850 hPa wind during 1997–2016 with respect to the period of 1979–1996. As Figure 5(a) shows, North China is dominated by the anomalous northerly wind from Lake Baikal, which suggests that the EASM is weaker in 1997–2016 compared with 1979–1996. Weaker summer monsoon favors the less precipitation over North China [12, 44].Figure 5
The difference by 1997–2016 mean minus 1979–1996 mean for (a) 850 hPa horizontal winds (units: m/s) and (b) vertical integral of water vapor flux (unit: kg/m/s). Shaded areas are statistically significant at the 95% confidence level according to Student’st-test. The red box shows North China. The green box shows the region of Lake Baikal (40°N–60°N, 80°E–120°E). The white region outlines surface pressure lower than 850 hPa.
(a)
(b)Meanwhile, an anomalous anticyclonic circulation dominates the region of Lake Baikal (40°N–60°N, 80°E–120°E) significantly. The anticyclonic circulation system over Lake Baikal is also vital for the variation of summer rainfall over North China [8, 49]. The anomalous easterly flow and northerly flow from the anomalous anticyclone are closely associated with the water vapor flux anomaly over North China. In Figure 5(b), an anomalous anticyclonic center of the water vapor flux is also observed over Lake Baikal. North China is dominated by the anomalous northeast water vapor flux. The difference of water vapor flux over Lake Baikal and North China during two periods is significant and reaches the 95% confidence level. Obviously, the anomalous northeast water vapor flux from Lake Baikal is beneficial to the interdecadal drought over North China.To further investigate which boundary of water vapor transport is critical for interdecadal drought over North China, the water vapor transports via four boundaries are calculated. As shown in Figure6, the anomalous input water vapor transports are via east and north boundary, which is consistent with the fact that North China is dominated by the northeast flow and northeast water vapor flux in Figure 5. The interdecadal differences of the water vapor transport via the east boundary and north boundary are input 9.14 × 106 kg/s and 21.70 × 106 kg/s, and the interdecadal differences of the water vapor transport via the west boundary and south boundary are output 14.62 × 106 kg/s and 18.47 × 106 kg/s, respectively.Figure 6
Difference of water vapor budget via each boundary of North China by 1997–2016 mean minus 1979–1996 mean (unit: kg/s). Dark arrows indicate the direction of horizontal water vapor transport difference across each boundary. Number in box indicates the difference of water vapor budget for North China.Because the sum of output water vapor transport is greater than the sum of input water vapor transport, the difference of water vapor budget over North China is negative (-2.25 × 106 kg/s). Numerous studies focused on the fact that weaker summer monsoon flow is responsible for less water vapor transport via south boundary and less summer rainfall in North China [34]. This study further points that water vapor transport via west boundary is also important for the less summer rainfall in North China. The water vapor transport via the west boundary is crucial for the summer rainfall over eastern China [32, 33]. In the JRA-55 data, the differences of water vapor transport via four boundaries of North China have same directions with those in ERA-Interim data but there is a weak positive difference of net water vapor budget, which is not consistent with the results in ERA-Interim data. The weak positive difference of water vapor budget is against the fact of less rainfall over North China. The reason for the weak positive difference of water vapor budget over North China in JRA-55 data is worthy of further study.There are also obvious interdecadal variations in low and middle troposphere. As Figure7(a) shows, the anomalous positive geopotential height at 500 hPa is observed over Lake Baikal. The anomalous positive geopotential height benefits the anomalous northerly wind in eastern China. The anomalous positive geopotential height at 500 hPa is associated with the interdecadal warming over Lake Baikal. The interdecadal warming over Lake Baikal happens not only in the surface in Figure 7(b) but also in 200 hPa in Figure 8(a). The interdecadal warming in 200 hPa reduces the meridional contrast of air temperature nearby Lake Baikal. Thereby, the negative difference of zonal wind exists around 40°N, suggesting the weakening 200 hPa zonal winds in the interdecadal time scales. As Figure 8(b) shows, the northward and westward movement of the 200 hPa zonal winds center over the northwest of North China during 1997–2016, compared with 1979–1996, suggests the interdecadal weakening of the 200 hPa zonal winds. The strength of the upper-level zonal winds has great contribution to the precipitation over North China [50, 51]. The anomalous descending motion over North China in Figure 9 further demonstrates the weakened pumping role of the 200 hPa zonal winds over North China. The interdecadal weakening of ascending motion and interdecadal reduction of the water vapor flux over North China result in the interdecadal reduction of precipitation over North China.Figure 7
Same as Figure5 but for (a) 500 hPa geopotential height (unit: gpm) and (b) SAT (unit: °C).
(a)
(b)Figure 8
(a) Same as Figure5 but for 200 hPa zonal winds (contours, unit: m/s) and temperature (shading, unit: °C). The difference of temperature in blue box with black dots is statistically significant at the 95% confidence level according to Student’s t-test. The difference of zonal winds with black cross is statistically significant at the 95% confidence level according to Student’s t-test. (b) 200 hPa zonal winds (unit: m/s) in JA of 1979–1996 mean (black contours) and 1997–2016 mean (white contours). The shading is the climatic mean of 200 hPa zonal winds during 1979–2016. The black box shows North China. The green box shows the region of Lake Baikal.
(a)
(b)Figure 9
Latitude-height cross section of difference of vertical velocity along 116.25°E in JA by 1997–2016 mean minus 1979–1996 mean (unit: 10−2 Pa/s). Shaded areas are statistically significant at the 95% confidence level according to Student’s t-test. The red line shows North China.


### 3.3. The Possible Mechanism of the Interdecadal Variability of Summer Rainfall over North China

The interdecadal warming of Lake Baikal has a significant impact on the summer rainfall over North China [8, 48, 52], which is also confirmed in Figure 7(b). To discover the abrupt point of the warming of Lake Baikal, the standardized time series of the averaged summer SAT over Lake Baikal (40°N–60°N, 80°E–120°E) is shown in Figure 10(a). The feature of the interdecadal and interannual variability of SAT over Lake Baikal is apparent. Mann-Kendall test method is used to investigate accurate abrupt point of the SAT over Lake Baikal in Figure 10(b). The abrupt year of the SAT over Lake Baikal is close to the abrupt year of the summer rainfall over North China.Figure 10
(a) Same as Figure2(a) but for the summer SAT over Lake Baikal. (b) Same as Figure 2(b) but for the summer SAT over Lake Baikal.
(a)
(b)The SAT over Lake Baikal and the summer rainfall over North China have experienced the interdecadal changes since the mid-1990s. The negative correlation existing between the SAT over Lake Baikal and the summer rainfall over North China [49] suggests that the interdecadal warming of Lake Baikal contributes to the interdecadal less summer rainfall over North China. The interdecadal warming of Lake Baikal is observed not only in the lower troposphere in Figure 7(b) but also in the upper troposphere in Figure 8(a). The interdecadal warming of Lake Baikal results in not only the anomalous anticyclonic circulation and anomalous positive geopotential height over Lake Baikal in the lower and middle troposphere but also the weakening of the zonal wind in the upper troposphere. Obviously, the anomalous anticyclonic circulation over Lake Baikal is beneficial to less water vapor transport from the monsoon flow and the westerlies. Meanwhile, the weakening of the zonal wind in the upper troposphere favors the weakening of the ascending motion and further results in the weakening of the pumping effects of the zonal winds in the upper troposphere. The interdecadal weakening of the ascending motion and interdecadal reduction of the water vapor transport to North China directly lead to the interdecadal drought over North China.


## 3.1. Interdecadal Variability of Summer Rainfall over North China

Figure2(a) shows time series of ISPNC17 during 1979–2016. Obviously, summer rainfall of North China had an increasing tendency from 1979 to 1996. With the rate of 0.101/year, the tendency during 1979–1996 passed the significance test at the 95% confidence level, suggesting that summer rainfall of North China had a significant increasing tendency during the period. This interdecadal characteristic is very different from the previous studies [2–8], which mainly focused on the interdecadal decrease of the summer rainfall in North China since the mid-1980s [5, 6, 9]. Few studies discover that there was an obviously increasing tendency during 1979–1996. Unfortunately, the increasing tendency has halted since 1997. Then North China entered the drought period, with several persistent droughts accompanied by severe effects on industry and agriculture over North China [5, 44].Figure 2
(a) The time series of ISPNC17 during 1979–2016. The horizontal dashed lines indicate the interdecadal mean, while solid line from 1979 to 1996 indicates the interdecadal trend, and the vertical dashed lines indicate the interdecadal shift points. (b) The Mann-Kendall test of ISPNC17 (green dotted line shows backward statistic rank series, and blue dotted line shows forward statistic rank series). Two black lines show 95% confidence level.
(a)
(b)The time series of ISPNCGPCC in Figure 3(a) also show similar increasing tendency during 1979–1996 with the trend rate of 0.094/year, which also reaches the 95% confidence level according to Student’s t-test. This increasing tendency has also halted since 1997. So the gauge-based station precipitation data and the grid precipitation data of GPCC both show that summer rainfall over North China had an obvious increasing tendency during 1979–1996 and has halted since 1997.Figure 3
(a) Same as Figure2(a) but for the ISPNCGPCC. (b) Same as Figure 2(b) but for the ISPNCGPCC.
(a)
(b)To further investigate the interdecadal shift of the summer rainfall over North China, Mann-Kendall test method was used to detect the abrupt point of ISPNC17 in Figure 2(b). There is a cross point between the backward statistic rank series and forward statistic rank series, suggesting that summer rainfall over North China experienced a distinct interdecadal change around 1996. Since 1997, there have been more droughts occurring in North China. A severe drought attacking North China in 2014 also indicated that North China has become drier in recent years [44]. Figure 3(b) further verifies the interdecadal shift of the summer rainfall over North China using grid precipitation data of GPCC.The differences of summer rainfall by 1997–2016 mean minus 1979–1996 mean over Northeast Asia further verify the interdecadal variability of summer rainfall over North China in Figure4. Whether using gauge-based station data or using the grid precipitation data of GPCC, the significant negative summer rainfall differences are both found over North China, which is statistically significant at the 95% confidence level by Student’s t-test. Meanwhile, the main wetter regions are located in the Huaihe River valley, which suggests that the wetter belt has moved northwards from southern China [45]. Meanwhile, the spatial pattern of summer rainfall variation since 1997 has been different from the so-called pattern of southern flood and northern drought since the end of 1970s [26, 27, 46, 47].Figure 4
Difference of summer rainfall by 1997–2016 mean minus 1979–1996 mean for (a) 160 stations and (b) GPCC. The shading indicates statistically significant difference at the 95% confidence level based on Student’st-test. The box shows North China.
(a)
(b)


## 3.2. The Interdecadal Variability of the Atmospheric Circulation

East Asia is dominated by a typical monsoon climate [45, 48]. The summer precipitation change over eastern China, affected by the EASM greatly, is very significant on the interannual and interdecadal timescales [10]. To further study the causes of drier North China, Figure 5(a) shows the difference of the 850 hPa wind during 1997–2016 with respect to the period of 1979–1996. As Figure 5(a) shows, North China is dominated by the anomalous northerly wind from Lake Baikal, which suggests that the EASM is weaker in 1997–2016 compared with 1979–1996. Weaker summer monsoon favors the less precipitation over North China [12, 44].Figure 5
The difference by 1997–2016 mean minus 1979–1996 mean for (a) 850 hPa horizontal winds (units: m/s) and (b) vertical integral of water vapor flux (unit: kg/m/s). Shaded areas are statistically significant at the 95% confidence level according to Student’st-test. The red box shows North China. The green box shows the region of Lake Baikal (40°N–60°N, 80°E–120°E). The white region outlines surface pressure lower than 850 hPa.
(a)
(b)Meanwhile, an anomalous anticyclonic circulation dominates the region of Lake Baikal (40°N–60°N, 80°E–120°E) significantly. The anticyclonic circulation system over Lake Baikal is also vital for the variation of summer rainfall over North China [8, 49]. The anomalous easterly flow and northerly flow from the anomalous anticyclone are closely associated with the water vapor flux anomaly over North China. In Figure 5(b), an anomalous anticyclonic center of the water vapor flux is also observed over Lake Baikal. North China is dominated by the anomalous northeast water vapor flux. The difference of water vapor flux over Lake Baikal and North China during two periods is significant and reaches the 95% confidence level. Obviously, the anomalous northeast water vapor flux from Lake Baikal is beneficial to the interdecadal drought over North China.To further investigate which boundary of water vapor transport is critical for interdecadal drought over North China, the water vapor transports via four boundaries are calculated. As shown in Figure6, the anomalous input water vapor transports are via east and north boundary, which is consistent with the fact that North China is dominated by the northeast flow and northeast water vapor flux in Figure 5. The interdecadal differences of the water vapor transport via the east boundary and north boundary are input 9.14 × 106 kg/s and 21.70 × 106 kg/s, and the interdecadal differences of the water vapor transport via the west boundary and south boundary are output 14.62 × 106 kg/s and 18.47 × 106 kg/s, respectively.Figure 6
Difference of water vapor budget via each boundary of North China by 1997–2016 mean minus 1979–1996 mean (unit: kg/s). Dark arrows indicate the direction of horizontal water vapor transport difference across each boundary. Number in box indicates the difference of water vapor budget for North China.Because the sum of output water vapor transport is greater than the sum of input water vapor transport, the difference of water vapor budget over North China is negative (-2.25 × 106 kg/s). Numerous studies focused on the fact that weaker summer monsoon flow is responsible for less water vapor transport via south boundary and less summer rainfall in North China [34]. This study further points that water vapor transport via west boundary is also important for the less summer rainfall in North China. The water vapor transport via the west boundary is crucial for the summer rainfall over eastern China [32, 33]. In the JRA-55 data, the differences of water vapor transport via four boundaries of North China have same directions with those in ERA-Interim data but there is a weak positive difference of net water vapor budget, which is not consistent with the results in ERA-Interim data. The weak positive difference of water vapor budget is against the fact of less rainfall over North China. The reason for the weak positive difference of water vapor budget over North China in JRA-55 data is worthy of further study.There are also obvious interdecadal variations in low and middle troposphere. As Figure7(a) shows, the anomalous positive geopotential height at 500 hPa is observed over Lake Baikal. The anomalous positive geopotential height benefits the anomalous northerly wind in eastern China. The anomalous positive geopotential height at 500 hPa is associated with the interdecadal warming over Lake Baikal. The interdecadal warming over Lake Baikal happens not only in the surface in Figure 7(b) but also in 200 hPa in Figure 8(a). The interdecadal warming in 200 hPa reduces the meridional contrast of air temperature nearby Lake Baikal. Thereby, the negative difference of zonal wind exists around 40°N, suggesting the weakening 200 hPa zonal winds in the interdecadal time scales. As Figure 8(b) shows, the northward and westward movement of the 200 hPa zonal winds center over the northwest of North China during 1997–2016, compared with 1979–1996, suggests the interdecadal weakening of the 200 hPa zonal winds. The strength of the upper-level zonal winds has great contribution to the precipitation over North China [50, 51]. The anomalous descending motion over North China in Figure 9 further demonstrates the weakened pumping role of the 200 hPa zonal winds over North China. The interdecadal weakening of ascending motion and interdecadal reduction of the water vapor flux over North China result in the interdecadal reduction of precipitation over North China.Figure 7
Same as Figure5 but for (a) 500 hPa geopotential height (unit: gpm) and (b) SAT (unit: °C).
(a)
(b)Figure 8
(a) Same as Figure5 but for 200 hPa zonal winds (contours, unit: m/s) and temperature (shading, unit: °C). The difference of temperature in blue box with black dots is statistically significant at the 95% confidence level according to Student’s t-test. The difference of zonal winds with black cross is statistically significant at the 95% confidence level according to Student’s t-test. (b) 200 hPa zonal winds (unit: m/s) in JA of 1979–1996 mean (black contours) and 1997–2016 mean (white contours). The shading is the climatic mean of 200 hPa zonal winds during 1979–2016. The black box shows North China. The green box shows the region of Lake Baikal.
(a)
(b)Figure 9
Latitude-height cross section of difference of vertical velocity along 116.25°E in JA by 1997–2016 mean minus 1979–1996 mean (unit: 10−2 Pa/s). Shaded areas are statistically significant at the 95% confidence level according to Student’s t-test. The red line shows North China.


## 3.3. The Possible Mechanism of the Interdecadal Variability of Summer Rainfall over North China

The interdecadal warming of Lake Baikal has a significant impact on the summer rainfall over North China [8, 48, 52], which is also confirmed in Figure 7(b). To discover the abrupt point of the warming of Lake Baikal, the standardized time series of the averaged summer SAT over Lake Baikal (40°N–60°N, 80°E–120°E) is shown in Figure 10(a). The feature of the interdecadal and interannual variability of SAT over Lake Baikal is apparent. Mann-Kendall test method is used to investigate accurate abrupt point of the SAT over Lake Baikal in Figure 10(b). The abrupt year of the SAT over Lake Baikal is close to the abrupt year of the summer rainfall over North China.Figure 10
(a) Same as Figure2(a) but for the summer SAT over Lake Baikal. (b) Same as Figure 2(b) but for the summer SAT over Lake Baikal.
(a)
(b)The SAT over Lake Baikal and the summer rainfall over North China have experienced the interdecadal changes since the mid-1990s. The negative correlation existing between the SAT over Lake Baikal and the summer rainfall over North China [49] suggests that the interdecadal warming of Lake Baikal contributes to the interdecadal less summer rainfall over North China. The interdecadal warming of Lake Baikal is observed not only in the lower troposphere in Figure 7(b) but also in the upper troposphere in Figure 8(a). The interdecadal warming of Lake Baikal results in not only the anomalous anticyclonic circulation and anomalous positive geopotential height over Lake Baikal in the lower and middle troposphere but also the weakening of the zonal wind in the upper troposphere. Obviously, the anomalous anticyclonic circulation over Lake Baikal is beneficial to less water vapor transport from the monsoon flow and the westerlies. Meanwhile, the weakening of the zonal wind in the upper troposphere favors the weakening of the ascending motion and further results in the weakening of the pumping effects of the zonal winds in the upper troposphere. The interdecadal weakening of the ascending motion and interdecadal reduction of the water vapor transport to North China directly lead to the interdecadal drought over North China.


## 4. Summary and Discussion

Using the 17-station rainfall and the new GPCC full data monthly product precipitation data sets, the interdecadal variations of the summer rainfall over North China since the mid-1990s are firstly discovered in this paper. The possible causes such as the interdecadal variations of the atmospheric circulation and the water vapor budget are discussed. The major mechanisms are shown in Figure11 and summarized as follows.Figure 11
Schematic diagram of mechanism for the halt of the increasing trend of summer rainfall over North China since the mid-1990s.Summer rainfall over North China had an increasing tendency during 1979–1996; since 1997, this increasing tendency has halted, and more summer droughts occurred over North China.The SAT over Lake Baikal and the summer rainfall over North China have had interdecadal abrupt since the mid-1990s. The interdecadal warming of Lake Baikal is beneficial to the interdecadal less summer rainfall over North China.The intense interdecadal warming of Lake Baikal results in not only the anomalous anticyclonic circulation and anomalous positive geopotential height over Lake Baikal in the lower and middle troposphere, but also the weakening of the zonal wind in the upper troposphere. The anomalous anticyclonic circulation in the lower troposphere over Lake Baikal results in less water vapor transport from the monsoon flow and the westerly flow. The weakening of the zonal wind in the upper troposphere favors the weakening of the ascending motion and the pumping effects of the zonal winds in the upper troposphere. The interdecadal weakening of the ascending motion and the less interdecadal water vapor transport result in the interdecadal drought in North China.In addition to the contribution of the less water vapor transport from weakened monsoon flow to the summer rainfall over North China, the interdecadal reduction of the water vapor transport from western boundary to North China is also responsible for the halted rainfall tendency over North China since the mid-1990s, which is usually ignored.There are many other factors that also contribute to the interdecadal variation of the summer rainfall over North China [6], such as Pacific decadal oscillation [52], arctic sea ice [44], and Tibet Plateau snow cover [53]. These factors and climate model verification are worthy of further study.



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*Source: 9031796-2020-01-27.xml*