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Article

Weakened Impacts of the East Asia-Pacific Teleconnection on the Interannual Variability of Summertime Precipitation over South China since the Mid-2000s

by
Wei Lu
1,
Yimin Zhu
1,2,
Zhong Zhong
1,2,
Yijia Hu
1 and
Yao Ha
1,2,*
1
College of Meteorology and Oceanography, National University of Defense Technology, Changsha 410003, China
2
School of Atmospheric Sciences, Jiangsu Collaborative Innovation Center for Climate Change, Nanjing University, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
Remote Sens. 2022, 14(20), 5098; https://doi.org/10.3390/rs14205098
Submission received: 19 August 2022 / Revised: 6 September 2022 / Accepted: 7 September 2022 / Published: 12 October 2022
(This article belongs to the Special Issue Remote Sensing for Climate Change)
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Abstract

:
The current study concentrates on the interdecadal shift in the interannual variability of summertime precipitation (IVSP) over South China (SC). Possible causes for the interdecadal shift are explored. The IVSP on a decadal time scale presents a significant weakening after the mid-2000s. The results show that the variances of the interannual precipitation variability over the SC region between 1993 and 2004 (hereafter S1) and 2005 and 2020 (hereafter S2) are 1.40 mm   d − 1 and 0.58 mm   d − 1 , respectively. The variance of the IVSP has decreased by 58.6% since the mid-2000s. The current study reveals that the reduction in the IVSP over SC after the mid-2000s is prominently attributed to the weakened impact of the East Asia-Pacific (EAP) teleconnection. Before the mid-2000s, the interannual variation of the east-west movement of the western Pacific subtropical high was more significant. The warming over the tropical central-eastern Pacific (CEP) and cooling over the western Pacific (WP) suppress the Walker cell in the tropical Pacific and induce anomalous Hadley cell with its descending branch over the WP in the wet years. The anomalies of SST and atmospheric circulation show opposite phases in the dry years. This SSTA pattern enhances the northward propagation of the EAP teleconnection through a Rossby-wave-type response, which triggers an ascending/descending branch with active/suppressed convection over the northwestern Pacific in the wet/dry years. Therefore, the cooling WP and El Niño in its developing phase provide an ideal condition for more precipitation over SC. However, the above ocean–atmosphere interactions changed after the mid-2000s. The significant SST changes in the tropical CEP and the WP weaken the EAP teleconnection and atmospheric circulation anomalies over SC, leading to a significant interdecadal reduction in the IVSP over SC after the mid-2000s.

Graphical Abstract

1. Introduction

East Asian summer monsoon (EASM) exhibits significant interannual and interdecadal variabilities, which have a great influence on summertime precipitation in East Asia [1]. Huang et al. gave a comprehensive and systematic summary of the spatiotemporal features, processes, and causes of the EASM variability and pointed out that the EASM has significant variabilities on interannual and interdecadal time scales [2]. Moreover, the EASM experienced an interdecadal weakening around the late 1970s, while summertime precipitation increased in the Yangtze-Huai River Basin (YHRB) and decreased in North China correspondingly. Summertime precipitation in South China (SC) increased after 1992 and decreased in YHRB after 1999 [3]. Correspondingly, the correlation between interannual variations of the EASM and ENSO became unstable [4]. South China (SC) is located at the east of the Qinghai-Tibet Plateau and south of YHRB. Due to the interactions between tropical and middle- to high-latitude weather systems, the weather and climate in SC are complicated and diverse. SC is the area with the amplest precipitation and the longest rainy season in China. Severe drought and flood disasters associated with abnormal precipitation cause huge losses of lives and properties and greatly influence local economies [5,6,7,8].
Influenced by the EASM, summertime precipitation over eastern China shows multi-scale variability, including seasonal and intraseasonal variability, as well as interannual and interdecadal variability [2,9,10,11,12,13,14,15]. Previous studies have shown that there were three interdecadal variations of summertime precipitation that occurred in the mid-1970s and the early and late 1990s [16]. The interdecadal decrease of summertime precipitation in SC in the 1970s [17,18] is attributed to the weakening of the EASM [19] and the warming of the tropical Indian Ocean (IO) and the tropical central-eastern Pacific (CEP) [20,21].
Summertime precipitation in SC exhibited an interdecadal intensification after the early 1990s [22] and a weakening after the late 1990s [23,24]. Most studies pointed out that uninterrupted warming in the tropical IO and CEP in summertime can cause anomalous Walker Cell and Hadley Cell circulations [25,26]. The interdecadal intensification of precipitation over SC in the early 1990s is attributed to joint effects of the Walker Cell and Hadley Cell anomalies and the reduction in spring snowfall in northern Eurasia [25,27]. Due to the strengthening of the western Pacific subtropical high (WPSH) caused by the tropical Pacific SSTA [23] and the significant reduction in tropical cyclone activity in the western Pacific [28], summertime precipitation in SC decreased in the late 1990s.
Previous studies have shown that there is a link between the East Asian climate and the Northwest Pacific climate, which can be established through atmospheric teleconnection [29]. On the basis of previous studies, Nitta found that there is a seesaw pattern in the atmospheric circulations between regions near Japan and the Philippine Sea and proposed the Pacific-Japan (PJ) teleconnection mode for the first time [30]. Based on the analysis of observations, Huang and Li also found that there is a tele-connected wave train similar to the “tripole” structure of PJ in the atmosphere extending from the Philippines to the Sea of Okhotsk, which is named East Asia-Pacific (EAP) teleconnection type [31]. Many studies focused on the formation mechanism of this teleconnection type. Based on numerical experiments using a barotropic model, Kurihara and Tsuyuki found that convective activities near the Philippines can trigger two-dimensional northward propagating Rossby waves with a horizontal structure similar to the observed PJ/EAP teleconnection [32]. Wakabayashi and Kawamura defined the PJ index based on the decomposition of the orthogonal function through the distribution position of the central node of the graph [33]. Later, the PJ index was widely applied to find the connection between the Northwest Pacific climate anomalies and the East Asian climate anomalies. It is also used in climate forecast, especially the prediction of summertime precipitation anomalies in eastern China. Similar to summertime precipitation in South China, the position and intensity of the EAP teleconnection also underwent significant interdecadal variations. After the late 1970s, the position of the EAP teleconnection pattern changed significantly with an obvious shift to the west and south [34]. Yin et al. pointed out that rainfall anomalies over YHRB associated with the EAP teleconnection were weaker and less evident after the 1980s [35]. Xu et al. revealed that the PJ/EAP (hereafter EAP) teleconnection shifted to the east after the late 1990s, and its intensity weakened significantly [36].
The above studies are helpful for understanding the interdecadal change in the mean state of summertime precipitation over SC. However, less attention has been paid to the study of the interdecadal change of the IVSP in SC [37]. Many studies revealed that the factors which influence the interannual variation of the East Asian summertime climate have changed a lot on the interdecadal time scale in the past decades [38,39,40,41,42]. Therefore, the IVSP could have enormous contributions to the occurrence of extreme precipitation events and should be further investigated. Moreover, interannual changes in various meteorological elements could improve the precision of seasonal prediction by statistical climate prediction models [43]. Therefore, it is necessary to study the interdecadal shift of the IVSP over SC and explore the possible mechanisms behind.

2. Materials and Methods

The precipitation data used in this study are the Precipitation Reconstruction dataset on 2.5 × 2.5° grids provided by NOAA [44,45,46]. The monthly atmospheric circulation data are derived from the NCEP/NCAR Reanalysis on a 2.5 × 2.5° resolution [47]. The daily outgoing longwave radiation (OLR) dataset produced based on NOAA polar-orbiting satellite remote sensing data with a horizontal resolution of 2.5 × 2.5° grids is used in this study [48]. In addition, the NOAA OI SST V2 High Resolution Dataset on a 0.25 × 0.25° resolution is put to use [49].
The time span of this study is the summertime season (JAS, the second rainy season in SC, which usually has a strong relationship with tropical systems) from 1979 to 2020. A 10-year running F-test is performed on the time series of summertime precipitation to assess the interdecadal change. The Student’s t test is used to examine the confidence level of the composites. The wave activity flux (WAF) is applied to describe the energy propagation of wave trains [50]. The conversion of local kinetic energy (CK) is calculated to depict the conversion of kinetic energy from mean flow to turbulence [51].

3. Results

3.1. Weakened Interannual Variability of Summer Precipitation over SC since the Mid-2000s

To describe the interdecadal shift of the IVSP in the mid-2000s, Figure 1 shows the regional mean time series, interannual difference, and 10-year running F-test values of summertime precipitation over SC (20–35°N, 100–125°E) from 1979 to 2020. The time series and interannual difference both reveal a significant weakening of the interannual variability after the mid-2000s. To describe the interdecadal shift of the IVSP over SC in the mid-2000s and determine the interdecadal turning point, the 10-year running F-test and 11-year running standard deviation of summertime precipitation over SC are analyzed (Figure 1b). The results suggest that the IVSP over SC experienced a significant weakening around2004/05. Hence, the year 2004 is taken as the shift point and the stage (1993–2004/2005–2020) with strong/weak IVSP is defined as S1/S2. It is found that the variance in S1 is 1.40 mm   d − 1 and that in S2 is 0.58 mm   d − 1 , and the mean values of the interannual difference in summertime precipitation are 1.82 mm   d − 1 in S1 and 0.81 mm   d − 1 in S2. Furthermore, the mean value of the 11-year running standard deviation is 1.33 mm   d − 1 in S1 and the value reduces to 0.64 mm   d − 1 in S2. All the above results support the conclusion that the interdecadal shift occurred around2004/05.
The time series of summertime precipitation with the linear trends in S1 and S2 being removed is shown in Figure 2. It is found that the variance in S1 is 1.32 mm   d − 1 and the value reduces to 0.37 mm   d − 1 in S2. The variance of the detrended time series in S2 decreases by 72.0% compared to that in S1, which is larger than the value of 58.6% for the original time series that includes the linear trend. Based on the detrended time series, the years with detrended summertime main precipitation larger (smaller) than 0 are determined to be wet (dry) years. The wet and dry years during S1 and S2 are listed in Table 1. Then the differences between the wet and dry years are composited to explore the reasons for the interdecadal shift of the IVSP over SC. Linear trends are eliminated for all environmental variables before the composite analysis is conducted.
The composite differences in summertime precipitation between wet and dry years during S1 and S2 are shown in Figure 3b,d, respectively. It is found that summertime precipitation over SC shows positive anomalies for the two periods but presents different spatial distributions and intensities. In S1, the positive anomalies center is located at (26°N, 113°E), and the maximum intensity of anomaly reaches 3.2–3.4 mm   d − 1 (Figure 3b). In S2, the two centers of positive anomalies are located at (29°N, 113°E) and (23°N, 115°E), which are to the north and south of the positive center in S1, respectively. The maximum value at the two positive centers in S2 is around 1.2–1.8 mm   d − 1 , weaker than that in S1 (Figure 3d). Consistent with the composite differences of summertime precipitation, the center of the maximum standard deviation is located at (26°N, 113°E) in S1 but it splits into two centers, which shift to the north and south respectively in S2. The values of standard deviation at the two positive anomaly centers also decrease in S2 compared to that in S1, which agrees with the interdecadal weakening of the IVSP over SC (Figure 3a,c).

3.2. Atmospheric Circulation Anomalies Responsible for the Interdecadal Shift of Summertime Precipitation over SC

To find out the reasons why there are large differences in the IVSP between S1 and S2, atmospheric circulation characteristics are analyzed in this section. Figure 4 shows composite differences of 200 and 500 hPa geopotential height as well as 200 hPa zonal wind between wet and dry years. The center of positive geopotential height anomalies occurs at 45°N in both S1 and S2, but the center in S2 is located at the east of that in S1. The negative geopotential height anomalies are smaller in S2 compared to that in S1 (Figure 4a,b,d,e). Composites of the western Pacific subtropical high (WPSH) feature line (5880 gpm contour) in wet and dry years are displayed in Figure 4b,e, respectively. In S1, west extension of the WPSH is significant in dry years; in contrast, it retreats to the east in wet years (Figure 4b). In S2, the location and intensity of the WPSH show little difference between wet and dry years (Figure 4e). This suggests that the interannual variability of the WPSH significantly weakened in S2 compared to that in S1, which is similar to the variation in summertime precipitation. Consistent with geopotential height anomalies, the positive and negative anomalies centers of zonal wind at 200 hPa are located more eastward in S1 and more westward in S2. The intensity of positive and negative anomalies centers of zonal wind is weaker in S2 than in S1 (Figure 4c,f).
The influence of atmospheric circulation anomalies on the IVSP is analyzed from the perspective of large-scale circulation above. To explore the impact of water vapor transport and local convection on the IVSP, composite differences in moisture flux and its divergence in the middle and lower levels, OLR, and omega in the middle and lower levels, 850 hPa relative vorticity and horizontal wind between wet years and dry years are analyzed. Results are displayed in Figure 5. In S1, negative OLR and omega anomalies are centered over SC (Figure 5a); in S2, however, the centers of the negative OLR and omega anomalies are located more eastward and become weak (Figure 5d). This result indicates that the intensity of convection is weaker in S2 than in S1. From the perspective of moisture transport, strong easterly anomalies pulled lots of moisture from the Northwestern Pacific (NWP) to SC in S1, resulting in significant moisture convergence over SC (Figure 5b). In S2, however, no significant convergence and divergence of moisture flux anomalies can be found over SC (Figure 5e). Positive relative vorticity anomalies are centered over SC in S1 with strong easterly anomalies (Figure 5c). The meridionally distributed centers of anomalies of positive and negative relative vorticity resemble the EAP teleconnection. Compared to that in S1, the EAP teleconnection moves eastward and becomes weak in S2 (Figure 5f). The changes in the location and intensity of the EAP teleconnection could be a key factor that affects the interdecadal shift of the IVSP over SC.
It is found that the composite differences of summertime precipitation between wet and dry years and standard deviations in the corresponding years over the SC show highly consistent spatial distribution characteristics (Figure 3). To inspect the consistency of the composite differences in moisture transport and local convection between dry and wet years and their standard deviations in the corresponding years, Figure 6 shows the standard deviations of moisture flux divergence, vertically integrated omega from 850 to 500 hPa and OLR in S1 and S2, and the respective differences. Consistent with the results shown in Figure 5, moisture flux divergence, omega, and OLR have larger standard deviations over SC in S1 (Figure 6a–c) than in S2 (Figure 6d–f). Moreover, the differences in moisture flux divergence, omega, and OLR between S1 and S2 (Figure 6g–i) display a similar spatial distribution pattern to that of the standard deviation of summertime precipitation over SC in S1.

3.3. Weakened Impacts of the EAP Teleconnection on Summertime Precipitation over SC since the Mid-2000s

As referred to in Section 3.2, the EAP teleconnection is of vital importance for the interdecadal shift of the IVSP over SC. This section explores the possible mechanisms of the interdecadal shift of the EAP teleconnection impact on summertime precipitation over SC around the mid-2000s. The composite differences of OLR and 850 hPa relative vorticity between wet and dry years are shown in Figure 5, which can reflect some features of the EAP teleconnection. In S1, negative differences of OLR (Figure 5a) and positive differences of vorticity (Figure 5c) are centered over SC, while the positive OLR difference center and negative vorticity difference center are distributed to the north and south, respectively. In S2, the centers of negative OLR (Figure 5b) and positive vorticity (Figure 5d) anomalies both move eastward and their intensity weakens. These results suggest the EAP teleconnection may have experienced an interdecadal weakening process after the mid-2000s.
Since the EAP teleconnection is characterized by meridionally distributed centers of relative vorticity anomalies with alternating signs [36], the 200 and 850 hPa relative vorticity anomalies connected with the EAP teleconnection in wet years during S1 and S2 are shown in Figure 7. In S1, the centers of positive and negative relative vorticity anomalies are meridionally distributed along 105–120°E with the maximum positive center in lower levels located over SC (Figure 7b). In S2, the centers of positive and negative relative vorticity anomalies move eastward and are meridionally distributed along 115–130°E with the intensity weakened in lower levels (Figure 7d). Following the approach proposed by Huang to define the EAP index using 500 hPa geopotential height anomalies [52], the EAP index is defined in the present study using 850 hPa relative vorticity anomalies. The EAP index can be expressed as:
I P J = − 0.25 V o r ’ R 1 + 0.5 V o r ’ R 2 − 0.25 V o r ’ R 3
where V o r ’ R 1 , V o r ’ R 2 , and V o r ’ R 3 represent averaged 850 hPa relative vorticity anomalies over (30–36°N, 105–120°E), (16–29°N, 105–120°E), and (8–15°N, 105–120°E), respectively. The above three areas are denoted by the red boxes in Figure 7b,d. The EAP index is 1.187 in S1 and −0.002 in S2, respectively. The large difference in the EAP index between S1 and S2 suggests that the intensity of the EAP teleconnection experienced a significant weakening since the mid-2000s. Furthermore, the weakening of the EAP teleconnection is partly attributed to the weakening of its intensity and partly attributed to its eastward shift.
In order to illustrate the propagation of Rossby waves, the wave activity flux (WAF) is also composited. In S1, the WAF connected with the EAP teleconnection emanates from the tropical WP, propagates to the north, and reaches and converges SC and the adjacent sea to its east in the lower levels (Figure 7b). In the higher levels, EAP of the WAF predominantly propagates along the westerly jet stream at the middle- to high- latitudes (Figure 7a). In S2, the propagation paths of WAF in lower (Figure 7d) and higher levels (Figure 7c) both are similar to that in S1, but the intensity of WAF is weaker than that in S1. A previous study indicated that the energy of the EAP teleconnection in the wet years of S1 predominantly hails from a Rossby-wave-type response [53].
Traditional studies regard the EAP teleconnection as a free Rossby wave train propagating from the subtropics to the mid latitudes in the lower troposphere [29,31]. To explain in a clearer way the interdecadal variation of the EAP teleconnection from the perspective of energy, the composite differences in CK vertically integrated from 1000 to 100 hPa between wet years and dry years are displayed in Figure 8. Note that positive CK indicates that fluctuations are easier to develop by extracting kinetic energy from basic flow. In S1, positive CK is centered at 150–160°E, 35–40°N and 105–115°E, 15–20°N (Figure 8a). In S2, the positive CK shift to the east and are centered at 170–180°E, 35–45°N and 125–135°E, 25–35°N (Figure 8b). Traditional studies regard the EAP teleconnection as a free Rossby wave train propagating from the subtropics to the mid latitudes in the lower troposphere [29,31]. Therefore, it is obvious that the CK shift to the east in S2 contributes significantly to the eastward shift of the EAP teleconnection.
It has been found that the eastward shift of EAP teleconnection is mainly forced by the shift of related SSTA [54,55]. Figure 9 shows the composite differences of SSTA (unit: K) between wet and dry years from the prior winter (DJF) to the subsequent summertime (JAS). In S1, the EAP teleconnection is connected with the transitional phase from La Niña to El Niño and from “North-warm-South-cold” to “North-cold-South-warm” in the Indian Ocean (IO), as well as a significant cooling phase over the western Pacific (WP) (Figure 9a–c). In S2, the EAP teleconnection is connected with the El Niño Modoki [56] decaying and a cooling phase in the IO, as well as a warming phase over WP (Figure 9d–f).
The patterns of composite SSTA in S1 and S2 are significantly different, which is at least partly responsible for the interdecadal shift of the EAP teleconnection. Looking at the situation in the summertime during S2, it is clear that the cooling in the tropical central-eastern Pacific (CEP) and the warming in the WP (Figure 9f) are favorable for the development of convective activity over WP [57,58]. Strong convective activities in the WP strengthen the WPSH by affecting the Hadley cell due to the weakening of EAP teleconnection. Since the enhancement of the IVSP in S1 attracts more attention, the present study focuses on the mechanism for the strengthening of summertime precipitation in S1.
It is found that the CEP warming can induce anomalous convection activities in the NWP by a Rossby-wave-type response and simultaneously destabilize the atmosphere in lower levels [59,60,61]. To further explore the influence of SSTA pattern on atmospheric circulation anomalies in S1, we analyze the composite differences (shown in Figure 10) in the Walker cell (Figure 10a) and Hadley cell (Figure 10b) between wet and dry years. In the summertime of S1, the updraft in the tropical CEP and downdraft in the tropical WP are significantly enhanced (Figure 10a) due to the warming in the CEP and the cooling in the WP (Figure 9c). This kind of Walker Cell induces an anomalous Hadley cell over the northern WP (Figure 10b). Descending branches appear in the WP around 10S–10N and North China around 35N. An ascending branch occurs near 25°N, which is located at the center of the largest IVSP area (Figure 3a,b). The updraft there suppresses the development of the WPSH (Figure 4b) and promotes more summertime precipitation in the wet years of S1. The Walker Cell and Hadley Cell accompanied by SSTA have opposite impacts on summertime precipitation in SC in the dry years of S1. These two distinctly different processes are responsible for the more significant IVSP in S1.
In a nutshell, the enhanced EAP teleconnection is a Rossby-wave-type response to the warming in the CEP and the cooling in the WP, which result in anomalous Walker Cell and Hadley Cell in the wet years of S1. The northward propagation of Rossby wave energy promotes convective activities and suppresses the WPSH in the NWP, providing an advantageous condition for the increase of summertime precipitation over SC.

4. Conclusions

Many studies revealed that the factors that influence the interannual variation of the East Asian summertime climate have changed a lot on interdecadal time scale in the past decades [38,39,40,41,42]. This study reveals that the IVSP over SC experienced a significant weakening after the mid-2000s. Taking 2004 as the shift point, the periods 1993–2004 and 2005–2020 are defined as S1 and S2, respectively. By composite differences of various atmospheric elements between wet and dry years in S1 and S2, the reasons why the interdecadal shift of the IVSP over SC occured are explored.
The composite differences of summertime precipitation between wet and dry years in S1 and S2 are compared. It is found that summertime precipitation over SC shows positive anomalies for both periods but presents distinctly different spatial distributions and intensities. In S1, the center of positive anomalies is located at (26°N, 113°E) and the intensity of the anomalies reaches 3.2–3.4 mm   d − 1 . In S2, two centers of positive anomalies are distributed to the north and south of the positive center in S1. The two centers are respectively located at (29°N, 113°E) and (23°N, 115°E). The intensity of summertime precipitation anomalies in S2 is 1.2–1.8 mm   d − 1 at the two positive centers, weaker than that in S1. Consistent with the composite differences of summertime precipitation, the center of the standard deviation is located at (26°N, 113°E) in S1 but it splits into two centers that shift to the north and south respectively in S2. The magnitude of standard deviation at the positive anomaly centers also decreases in S2 compared to that in S1, which agrees well with the interdecadal weakening of the IVSP over SC. Consistent with summertime precipitation, local moisture flux divergence, omega, and OLR all have larger standard deviations over SC in S1 than in S2.
To find out the reasons for the enormous differences of the IVSP between S1 and S2, atmospheric circulation characteristics are analyzed. From the perspective of large-scale circulation, the center of positive geopotential height anomalies is located at 45N for both S1 and S2, yet the center in S2 is located further westward compared to that in S1. The local negative geopotential height anomalies are weakening from S1 to S2. Moreover, the WPSH elongates to the west in dry years and moves back to the east in wet years during S1. The location and intensity of the WPSH have little difference between wet and dry years during S2. This suggests that the WPSH interannual variability significantly weakens in S2 compared to that in S1, which is similar to the variation in summertime precipitation. Consistent with geopotential height anomalies, the centers of positive and negative anomalies of zonal wind at 200 hPa are located relatively eastward in S1 and westward in S2. The intensity at centers of positive and negative anomalies of zonal wind is weaker in S2 than in S1. From the perspective of moisture transport and local convection, negative OLR and omega anomalies are centered over SC in S1, while the centers of negative anomalies are located eastward and become weak in S2. This result indicates that the intensity of convection weakens from S1 to S2. Moreover, there are strong easterly anomalies that pull huge amounts of moisture from the Northwestern Pacific (NWP) in S1, resulting in significant moisture convergence over SC. However, no significant convergence and/or divergence of moisture flux anomalies can be found over SC in S2. Positive relative vorticity anomalies are centered over SC in S1 accompanied by strong easterly anomalies. The meridional centers of positive and negative relative vorticity anomalies resemble the EAP teleconnection, which moves eastward and becomes weak from S1 to S2.
Since the EAP teleconnection is of vital importance for the interdecadal shift of the IVSP over SC, the reasons for the interdecadal shift of the EAP teleconnection impact on summertime precipitation over SC around the mid-2000s are investigated. The weakening of the EAP teleconnection is partly attributed to the weakening of its intensity and partly attributed to its eastward shift. It was found that the eastward shift of EAP teleconnection is mainly forced by the shift of related SSTA [54,56]. In S1, the EAP teleconnection is connected with the transitional phase from La Niña to El Niño, from the “North-warm-South-cold” pattern to “North-cold-South-warm” pattern in the IO and a significant cooling phase in the WP. In S2, the EAP teleconnection is connected with the Central Pacific El Niño decaying and a cooling phase in the IO, as well as a warming phase over the western Pacific. The patterns of composite SSTA are significantly different between S1 and S2, which is at least partly responsible for the interdecadal shift of the EAP teleconnection. Looking at the situation in the summertime of S2, the cooling in the tropical CEP and the warming in the western Pacific are favorable for the development of convective activity over WP [57,58]. Strong convective activities in the WP strengthen the WPSH by affecting the Hadley cell due to the weakening of EAP teleconnection. In the summertime of S1, the enhanced EAP teleconnection is a Rossby-wave-type response to the warming in the CEP and the cooling in the WP, which results in anomalous Walker Cell and Hadley Cell in the wet years of S1. The northward propagation of Rossby wave energy promotes convective activities and suppresses the WPSH in the NWP, providing an advantageous condition for the increase of summertime precipitation over SC.
The present study concentrates on the interdecadal shift of the IVSP over SC and the possible mechanisms behind it. However, the present study mainly performs some dynamic and statistical analysis on observational datasets. In the following work, it would be better to do numerical experiments to verify the reliability of the results. This study reveals that the interdecadal shift of the EAP teleconnection is significant around the mid-2000s, which is caused by the interdecadal shift of the SSTA patterns in the CEP and the western Pacific. However, there may exist multiple factors responsible for the interdecadal shift of the EAP teleconnection. It is found that the interdecadal change of the PDO phase in the late-2000s could result in the interdecadal shift of the EAP teleconnection and variations of summertime precipitation in eastern China [61]. Further studies are necessary to explore the complex feedback loops that affect summertime precipitation in China.

Author Contributions

Conceptualization, Y.H. (Yao Ha) and Y.Z.; methodology, Y.H. (Yao Ha); validation, Z.Z. and Y.Z.; formal analysis, Y.H. (Yijia Hu); investigation, W.L.; data curation, Y.H. (Yijia Hu); writing—original draft preparation, W.L.; writing—review and editing, Y.H. (Yao Ha); supervision, Z.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

This work is sponsored jointly by the National Natural Science Foundation of China (41975090); the Natural Science Foundation of Hunan Province, China (2022JJ20043); the Scientific Research Program of National University of Defense Technology (18/19-QNCXJ); and the Jiangsu Collaborative Innovation Center for Climate Change in Nanjing University.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) Time series (black solid line) and interannual difference (blue bars) of summertime precipitation over SC. (b) F-test values (red solid line) and 11-year running standard deviations (blue solid lines) of summertime precipitation over SC (black dashed lines in (a) are averaged values of interannual variability in S1 and S2, respectively; blue dashed lines in (b) are averaged values of running standard deviation in S1 and S2, respectively; the red dashed line in (b) denotes the significant value exceeding the 95% confidence level; unit: mm   d − 1 ).
Figure 1. (a) Time series (black solid line) and interannual difference (blue bars) of summertime precipitation over SC. (b) F-test values (red solid line) and 11-year running standard deviations (blue solid lines) of summertime precipitation over SC (black dashed lines in (a) are averaged values of interannual variability in S1 and S2, respectively; blue dashed lines in (b) are averaged values of running standard deviation in S1 and S2, respectively; the red dashed line in (b) denotes the significant value exceeding the 95% confidence level; unit: mm   d − 1 ).
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Figure 2. Time series of detrended summertime precipitation over SC during S1 and S2.
Figure 2. Time series of detrended summertime precipitation over SC during S1 and S2.
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Figure 3. Composite differences in summertime precipitation between wet years and dry years (unit: mm   d − 1 ; (b,d) for S1 and S2, respectively; dots indicate the regions exceeding 95% confidence level.) and the standard deviation of summertime precipitation (unit: mm   d − 1 ; (a,c) for S1 and S2).
Figure 3. Composite differences in summertime precipitation between wet years and dry years (unit: mm   d − 1 ; (b,d) for S1 and S2, respectively; dots indicate the regions exceeding 95% confidence level.) and the standard deviation of summertime precipitation (unit: mm   d − 1 ; (a,c) for S1 and S2).
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Figure 4. Composite differences of (a,d) 200 hPa geopotential height (unit: gpm), (b,e) 500 hPa geopotential height (unit: gpm), and (c,f) 200 hPa zonal wind (unit: m   s − 1 ) between wet years and dry years. (a–c) for S1 and (d–f) for S2; dots indicate the regions exceeding 95% confidence level; red and black lines in (b,e) indicate composites of the WPSH feature line for wet and dry years, respectively).
Figure 4. Composite differences of (a,d) 200 hPa geopotential height (unit: gpm), (b,e) 500 hPa geopotential height (unit: gpm), and (c,f) 200 hPa zonal wind (unit: m   s − 1 ) between wet years and dry years. (a–c) for S1 and (d–f) for S2; dots indicate the regions exceeding 95% confidence level; red and black lines in (b,e) indicate composites of the WPSH feature line for wet and dry years, respectively).
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Figure 5. Composite differences of (a,d) vertically integrated omega from 850 to 500 hPa (contours with the interval of 2, unit :   kg   ( m     s ) − 3 ) and OLR (shading, unit :   W   m − 2 ); (b,e) vertically integrated moisture flux from 1000 to 500 hPa (vectors, unit :   kg   m − 1   s − 1 ) and its divergence (shading, unit : 10 − 6   kg   m − 2   s − 1 ); and (c,f) 850 hPa wind (vectors, unit: m   s − 1 ) and relative vorticity (shading, unit :   10 − 6   s − 1 ) between wet years and dry years. ((a–c) for S1; (d–f) for S2; significant values exceeding the 95% confidence level are displayed; the dashed/solid lines suggest values larger/smaller than zero in (a,d)).
Figure 5. Composite differences of (a,d) vertically integrated omega from 850 to 500 hPa (contours with the interval of 2, unit :   kg   ( m     s ) − 3 ) and OLR (shading, unit :   W   m − 2 ); (b,e) vertically integrated moisture flux from 1000 to 500 hPa (vectors, unit :   kg   m − 1   s − 1 ) and its divergence (shading, unit : 10 − 6   kg   m − 2   s − 1 ); and (c,f) 850 hPa wind (vectors, unit: m   s − 1 ) and relative vorticity (shading, unit :   10 − 6   s − 1 ) between wet years and dry years. ((a–c) for S1; (d–f) for S2; significant values exceeding the 95% confidence level are displayed; the dashed/solid lines suggest values larger/smaller than zero in (a,d)).
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Figure 6. Standard deviations of vertically integrated moisture flux divergence from 1000 to 500 hPa ((a,d,g); unit : 10 − 6   kg   m − 2   s − 1 ), vertically integrated omega from 850 to 500 hPa ((b,e,h); unit :   kg   ( m     s ) − 3 ), OLR ((c,f,i); unit :   W   m − 2 ) in S1 (a–c); and S2 (d–f) and the respective difference between S2 and S1 (g–i).
Figure 6. Standard deviations of vertically integrated moisture flux divergence from 1000 to 500 hPa ((a,d,g); unit : 10 − 6   kg   m − 2   s − 1 ), vertically integrated omega from 850 to 500 hPa ((b,e,h); unit :   kg   ( m     s ) − 3 ), OLR ((c,f,i); unit :   W   m − 2 ) in S1 (a–c); and S2 (d–f) and the respective difference between S2 and S1 (g–i).
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Figure 7. Composite anomalies of relative vorticity (shading, unit :   10 − 6   s − 1 ) and associated WAF (vectors, unit :   m 2   s − 2 ) at (a,c) 200 and (b,d) 850 hPa in wet years of S1 (a,b) and S2 (c,d) (significant values exceeding the 95% confidence level are displayed).
Figure 7. Composite anomalies of relative vorticity (shading, unit :   10 − 6   s − 1 ) and associated WAF (vectors, unit :   m 2   s − 2 ) at (a,c) 200 and (b,d) 850 hPa in wet years of S1 (a,b) and S2 (c,d) (significant values exceeding the 95% confidence level are displayed).
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Figure 8. Composite differences of the conversion of local kinetic energy (CK) vertically integrated from 1000 to 100 hPa between wet and dry years in (a) S1 and (b) S2 (dots indicate the regions exceeding 95% confidence level; black curves in (a,b) represent the EAP teleconnection).
Figure 8. Composite differences of the conversion of local kinetic energy (CK) vertically integrated from 1000 to 100 hPa between wet and dry years in (a) S1 and (b) S2 (dots indicate the regions exceeding 95% confidence level; black curves in (a,b) represent the EAP teleconnection).
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Figure 9. Composite differences of SSTA (unit: K) between wet and dry years (a,d)/(b,e)/(c,f) for pre-winter (DJF)/spring (MAM)/summer (JAS); (a–c)/(d–f) for S1/S2; dots indicate the regions exceeding 95% confidence level).
Figure 9. Composite differences of SSTA (unit: K) between wet and dry years (a,d)/(b,e)/(c,f) for pre-winter (DJF)/spring (MAM)/summer (JAS); (a–c)/(d–f) for S1/S2; dots indicate the regions exceeding 95% confidence level).
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Figure 10. Composite differences of (a) Walker cell (averaged over 10°S~10°N) and (b) Hadley cell (averaged over 100~120°E) between wet and dry years in S1 (shading regions suggest composite differences of vertical velocity (unit: 10 − 3   m   s − 1 ) between wet and dry years. Significant values exceeding the 95% confidence level are displayed).
Figure 10. Composite differences of (a) Walker cell (averaged over 10°S~10°N) and (b) Hadley cell (averaged over 100~120°E) between wet and dry years in S1 (shading regions suggest composite differences of vertical velocity (unit: 10 − 3   m   s − 1 ) between wet and dry years. Significant values exceeding the 95% confidence level are displayed).
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Table
Table 1. Wet and dry years picked out in S1 and S2 based on the detrended summertime main precipitation larger and smaller than 0, respectively.
Table 1. Wet and dry years picked out in S1 and S2 based on the detrended summertime main precipitation larger and smaller than 0, respectively.
S1S2
Wet years1994, 1996, 1997, 1999, 2002, 20042006, 2008, 2010, 2013, 2015, 2016, 2018, 2020
Dry years1993, 1995, 1998, 2000, 2001, 20032005, 2007, 2009, 2011, 2012, 2014, 2017, 2019
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Lu, W.; Zhu, Y.; Zhong, Z.; Hu, Y.; Ha, Y. Weakened Impacts of the East Asia-Pacific Teleconnection on the Interannual Variability of Summertime Precipitation over South China since the Mid-2000s. Remote Sens. 2022, 14, 5098. https://doi.org/10.3390/rs14205098

AMA Style

Lu W, Zhu Y, Zhong Z, Hu Y, Ha Y. Weakened Impacts of the East Asia-Pacific Teleconnection on the Interannual Variability of Summertime Precipitation over South China since the Mid-2000s. Remote Sensing. 2022; 14(20):5098. https://doi.org/10.3390/rs14205098

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Lu, Wei, Yimin Zhu, Zhong Zhong, Yijia Hu, and Yao Ha. 2022. "Weakened Impacts of the East Asia-Pacific Teleconnection on the Interannual Variability of Summertime Precipitation over South China since the Mid-2000s" Remote Sensing 14, no. 20: 5098. https://doi.org/10.3390/rs14205098

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