Circulation Features Associated with the Record-Breaking Rainfall over South China in June 2017

Jianqi Sun Nansen-Zhu International Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, and Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, and University of Chinese Academy of Sciences, Beijing, China

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Jing Ming Nansen-Zhu International Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China

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Mengqi Zhang Nansen-Zhu International Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China

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Shui Yu Nansen-Zhu International Research Center, Institute of Atmospheric Physics, Chinese Academy of Sciences, and University of Chinese Academy of Sciences, Beijing, China

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Abstract

In June 2017, south China suffered from intense rainfall that broke the record spanning the previous 70 years. In this study, the large-scale circulations associated with the south China June rainfall are analyzed. The results show that the anomalous Pacific–Japan (PJ) pattern is a direct influence on south China June rainfall or East Asian early summer rainfall. In addition, the Australian high was the strongest in June 2017 during the past 70 years, which can increase the equatorward flow to northern Australia and activate convection over the Maritime Continent. Enhanced convection over the Maritime Continent can further enhance local meridional circulation along East Asia, engendering downward motion over the tropical western North Pacific and enhancing the western Pacific subtropical high (WPSH) and upward motion over south China, which increases the rainfall therein. In addition, a strong wave train pattern associated with North Atlantic air–sea interaction was observed in June 2017 at Northern Hemispheric mid- to high latitudes; it originated from the North Atlantic and propagated eastward to East Asia, resulting in an anomalous anticyclone over the Mongolian–Baikal Lake region. This anomalous anticyclone produced strong northerly winds over East Asia that encountered the southerly associated with the WPSH over south China, thereby favoring intense rainfall over the region. Case studies of June 2017 and climate research based on data during 1979–2017 and 1948–2017 indicate that the extremities of the atmospheric circulation over south Europe and Australian high and their coupling with the PJ pattern could be responsible for the record-breaking south China rainfall in June 2017.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Jianqi Sun, sunjq@mail.iap.ac.cn

Abstract

In June 2017, south China suffered from intense rainfall that broke the record spanning the previous 70 years. In this study, the large-scale circulations associated with the south China June rainfall are analyzed. The results show that the anomalous Pacific–Japan (PJ) pattern is a direct influence on south China June rainfall or East Asian early summer rainfall. In addition, the Australian high was the strongest in June 2017 during the past 70 years, which can increase the equatorward flow to northern Australia and activate convection over the Maritime Continent. Enhanced convection over the Maritime Continent can further enhance local meridional circulation along East Asia, engendering downward motion over the tropical western North Pacific and enhancing the western Pacific subtropical high (WPSH) and upward motion over south China, which increases the rainfall therein. In addition, a strong wave train pattern associated with North Atlantic air–sea interaction was observed in June 2017 at Northern Hemispheric mid- to high latitudes; it originated from the North Atlantic and propagated eastward to East Asia, resulting in an anomalous anticyclone over the Mongolian–Baikal Lake region. This anomalous anticyclone produced strong northerly winds over East Asia that encountered the southerly associated with the WPSH over south China, thereby favoring intense rainfall over the region. Case studies of June 2017 and climate research based on data during 1979–2017 and 1948–2017 indicate that the extremities of the atmospheric circulation over south Europe and Australian high and their coupling with the PJ pattern could be responsible for the record-breaking south China rainfall in June 2017.

© 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Jianqi Sun, sunjq@mail.iap.ac.cn

1. Introduction

Heavy rainfall occurs frequently during the summer in south China, causing significant economic losses. In particular, under the background of global warming, heavy rainfall events have been and will continue to be more frequent and intense (Chen et al. 2012a,b). Thus, understanding the influencing factors and physical processes responsible for the variability in the summer rainfall is of great socioeconomic importance for south China where there is an enormous population and developed industry and agriculture.

Among the possible influencing factors, the contribution of the tropical sea surface temperature (SST) has been the subject of a number of previous studies. It was found that the El Niño–Southern Oscillation (ENSO) plays an important role in the variability of summer rainfall over south China, and the physical process indicates that changes in the western Pacific subtropical high (WPSH) serve as a bridge connecting the ENSO with summer rainfall over south China (e.g., Huang and Wu 1989; Zhang et al. 1996, 1999; B. Wang et al. 2000; Chang et al. 2000a,b; Zhang et al. 2013; Qiang and Yang 2013). While subsequent studies suggested that the impact of the ENSO on summer rainfall in south China is modulated by the Pacific decadal oscillation (PDO) (e.g., Mantua et al. 1997; Zhang et al. 1997), in phase (out of phase) variabilities in ENSO and PDO result in significant (weak) responses in the south China summer rainfall (Chan and Zhou 2005; Zhou et al. 2006). Recent studies showed that, as one part of the ENSO continuum (Lai et al. 2015), the El Niño Modoki (e.g., Larkin and Harrison 2005; Ashok et al. 2007; Kug et al. 2009) also has an influence on south China summer rainfall (Wang and Wang 2013). From a prediction aspect, Duan et al. (2013) suggested that the year-to-year variability in June rainfall over south China is more predictable in a PDO positive phase than in a negative phase. In addition, the SST variability over the Indian Ocean also exhibits a contribution to the south China summer rainfall anomalies. For example, previous studies showed that the summer rainfall in south China experienced a strong decadal variability around 1992/93 (Ding et al. 2008; Yao et al. 2008; Wu et al. 2010), during which time an increase in the SST within the tropical Indian Ocean played an important role (Wu et al. 2010). In addition, the Indian Ocean dipole pattern has contributed to the summer rainfall over south China (Tang and Sun 2005). The dipolar SST tendency over the Indian Ocean and the western North Pacific can modify the Philippine Sea anticyclone and ultimately result in anomalous early summer rainfall events over south China (Yim et al. 2014).

In addition to the impacts of tropical systems, some extratropical systems generate contributions to summer rainfall in south China. Regarding the boundary factors, the increase (decrease) in Arctic sea ice during the boreal spring is related to more (less) summer rainfall over south China (Wu et al. 2009b). Less spring Eurasian snow cover and excessive Tibetan Plateau snow cover are associated with more summer rainfall over south China (Wu et al. 2009a). Winter North Atlantic tripolar SST anomalies and a warming process over Siberia coupled with the decreasing tendency of snowfall are closely connected with the early summer rainfall over south China, which have been used to develop the statistical prediction model of south China early summer rainfall (Yim et al. 2014). In addition, the mid- to high-latitude atmospheric factors, such as the Okhotsk high (Yim et al. 2014), the Antarctic Oscillation (AAO) (Sun et al. 2009), the Australian high (Xue et al. 2004), and cross-equatorial flows (Zhu 2012), also contribute to the summer rainfall variability over south China.

Although heavy rainfall occurs annually over south China, the rainfall in June 2017 was unusually intense. Two rainfall datasets show consistently greater rainfall over south China during that time (Figs. 1a,b), although there are some visible differences in the spatial distribution, which may be attributed to the difference in the resolution and sources of the two rainfall datasets (the rainfall datasets are introduced in section 2). A statistical analysis conducted by the China Meteorological Administration (CMA) showed that the rainfall record over the past 60 years had been broken at numerous stations throughout south China, leading to widespread landslides and flooding; these events constituted the most damaging natural hazard in south China with significant economic losses over RMB 30 billion. The rainfall in June 2017 was even more intense than that in June 1998 and has thus become the first peak recorded over the past 30 and even 70 years (Fig. 1c; the south China June rainfall index is defined as the averaged rainfall over the region of 22°–30°N, 108°–125°E). The intense rainfall in June 1998 resulted from the strong ENSO in 1997/98, which has been well revealed by observational analyses and numerical simulations (e.g., H. J. Wang et al. 2000; Huang et al. 2000; Shen et al. 2001). In addition, as reviewed in the abovementioned content, previous studies have indicated that the tropical SST plays an important role in the south China June rainfall variability. However, for the intense rainfall in June 2017, there was no strong signal in the tropical SSTs from the boreal winter to the following spring and June (figure not shown). Thus, the intense rainfall in June 2017 could be attributed to other influences.

Fig. 1.
Fig. 1.

The anomalous rainfall (units: mm day−1) over south China in June 2017 based on the (a) PREC/L and (b) CMAP datasets and the (c) normalized south China rainfall index.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

In this study, the possible mechanism for record-breaking intense rainfall in June 2017 is investigated. It is found that extratropical wave activity downstream from the North Atlantic air–sea coupling system and cross-equatorial influence from the Southern Hemisphere contributed to the severe rainfall over south China in June 2017. The organization of this paper is as follows. The datasets used in this study are described in section 2. Sections 3 and 4 present the atmospheric circulation and SST patterns, respectively, associated with the south China June rainfall. A summary of the results is given in section 5.

2. Datasets

To investigate the rainfall anomalies, two precipitation datasets are used in this study: 1) the monthly mean precipitation reconstruction data over land (PREC/L) produced by the National Oceanic and Atmospheric Administration (NOAA)/Climate Prediction Center (CPC; Chen et al. 2002), which are available in a 1° × 1° grid from January 1948 to the present and obtained by interpolation of gauge observations over land (PREC/L) from over 17 000 stations collected in the Global Historical Climatology Network (GHCN), version 2, and the Climate Anomaly Monitoring System (CAMS) datasets; and 2) the monthly mean CPC Merged Analysis of Precipitation (CMAP; Xie and Arkin 1997), which are available in a 2.5° × 2.5° grid from January 1979 to the present and obtained from five kinds of satellite estimates (GPI, OPI, SSM/I scattering, SSM/I emission, and MSU) and gauge data.

The monthly SST dataset is the NOAA Extended Reconstruction SST, version 4 (Huang et al. 2015), which has a resolution of 2.0° × 2.0° and is available from 1854 to the present. Horizontal wind, geopotential height, vertical velocity, and heat flux are from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis products (Kalnay et al. 1996; Kistler et al. 2001), which are available in a 2.5° × 2.5° grid from 1948 to the present.

All of the datasets used in this study are provided by the NOAA/OAR/ESRL Physical Science Division, Boulder, Colorado (and are downloaded from the website http://www.cdc.noaa.gov/). In this paper, the anomalies in June 2017 are computed relative to the climatology of 1981–2010. Because of concerns regarding the data quality before the incorporation of satellite data, particularly over the Southern Hemisphere (Kistler et al. 2001), our analysis is confined over the period of 1979–2017 to obtain a more reliable result. Some indices are shown over the periods of both 1979–2017 and 1948–2017 to reflect the extremity of the climate systems.

The wave activity flux is a good indicator to reflect the propagation of stationary waves in the atmosphere. Therefore, the wave activity flux, which is calculated using the formulation of Takaya and Nakamura (2001), is used here to diagnose the horizontal propagation of quasi-stationary Rossby waves. The wave activity flux (W) formulation is as follows:
eq1
where is the streamfunction; perturbations are denoted by primes; the subscripts are partial derivatives; U and V are the basic zonal and meridional wind components, respectively; and p is the normalized pressure [pressure (1000 hPa)−1]. Here, the June mean field over the period of 1981–2010 is referred to as the basic state.

3. Atmospheric circulations associated with the south China June rainfall

To study the causes for the June 2017 rainfall anomaly over south China, the atmospheric circulation anomalies in June 2017 are analyzed first. These anomalies are then compared with those associated with the south China June rainfall during 1979–2017. Because a highly consistent variability is observed in the two south China June rainfall indices with a correlation coefficient of 0.94 over the period of 1979–2017, the result calculated from the PREC/L dataset is used for the following analysis.

a. Anomalous atmospheric circulation in June 2017

Figure 2 shows the anomalous horizontal winds at 850 hPa in June 2017. There is a meridional triple pattern over East Asia, with two anomalous anticyclones over the tropical western Pacific–South China Sea and the Mongolian–Baikal Lake regions as well as an anomalous cyclone between these two regions. Such a teleconnection pattern is the Pacific–Japan (PJ)- or East Asia–Pacific (EAP)-like pattern (Nitta 1987; Huang and Sun 1992). The anomalous anticyclone over the tropical western Pacific–-South China Sea indicates the enhanced and westward shift of the WPSH, which enhances the East Asian early summer monsoon flow and consequently transports more moisture to south China, providing a favorable moisture condition for rainfall over the region. In addition, the pronounced warm and moist southwesterly encounters the anomalous cold and dry northerly from the anomalous anticyclone over the Mongolian–Baikal Lake region over south China, which leads to strong convergence and upward motion and consequently provides a favorable dynamical condition for rainfall within the region. All of these changes in the atmospheric circulation over East Asia are directly responsible for the intense rainfall over south China in June 2017.

Fig. 2.
Fig. 2.

The anomalous horizontal winds (vectors) and wind velocity (shading) at 850 hPa in June 2017 (units: m s−1). “A” and “C” indicate anticyclone and cyclone, respectively.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Meanwhile, in addition to the anomalous signal over East Asia, a wave train can be detected over Northern Hemispheric mid- to high latitudes. This wave train is related to the significant anticyclone over the Mongolian–Baikal Lake region, further coupling well with the center of pronounced PJ pattern over East Asia (Fig. 2). According to the zonal wave train, its signal and propagation is clearer at the upper level (e.g., Watanabe 2004; Ding and Wang 2005; Sun et al. 2008; Sun and Wang 2012). Thus, the anomalous 200-hPa geopotential height in June 2017 and the related Rossby wave source (RWS) and flux are shown in Fig. 3. The RWS is calculated from the related term in the potential vorticity equation (Sardeshmukh and Hoskins 1988). The figure suggests that, over the Iceland low region, there is a strong positive wave source that can excite a zonal wave train pattern propagating eastward to the Mongolian–Baikal Lake region and then migrating southeastward to low-latitude East Asia, concurrent with an anomalous PJ pattern. Previous studies mainly focused on the influences of tropical SSTs and convection on the PJ pattern (Nitta 1987; Huang and Sun 1992). However, in June 2017, the tropical SST anomaly is weak (figure not shown). Thus, the mid- to high-latitude systems can also exert an impact on the variability in the PJ pattern and WPSH via wave activity in 2017 and some other years (Orsolini et al. 2015).

Fig. 3.
Fig. 3.

The anomalous geopotential height at 200 hPa (contours, units: gpm), the related wave activity flux (vectors, units: m2 s−2), and the wave source (shading, units: 10−11 s−2; only the part over longitudes between 70°W and 5°E are plotted in order to reflect clearer the wave source over the North Atlantic), in June 2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

In addition to the Northern Hemisphere, the AAO (Gong and Wang 1999; Thompson and Wallace 2000) was also abnormal in June 2017 in a strong positive phase. Although the AAO is located over mid- to high latitudes in the Southern Hemisphere, it can impact East Asian spring dust storm and summer rainfall events (e.g., Gao et al. 2004; Nan and Li 2003; Xue et al. 2004; Fan and Wang 2004; Wang and Fan 2005; Fan 2006), West African and North American summer monsoon rainfall (Sun et al. 2010; Sun 2010), and western Pacific tropical cyclone variability (Ho et al. 2005; Wang and Fan 2007). In particular, Sun et al. (2009) suggested that the AAO could impact the convection activity over the Maritime Continent, further strengthening the WPSH and consequently resulting in anomalous rainfall over south China. Based on the definition of Gong and Wang (1999), the AAO index is calculated. The normalized (relative to 1948–2017) AAO index values are both greater than one standard deviation in the boreal spring and June of 2017. In a positive AAO phase, the Southern Hemispheric midlatitude shows positive SLP anomalies, which can enhance the Australian high (e.g., Gong and Wang 1999; Xue et al. 2004). The enhanced Australian high will strengthen the equatorward flow and then increase the convection over the Maritime Continent. Figure 4 shows strong upward motion over the Maritime Continent, indicating that convection over this region is enhanced. In June 2017, the SST anomaly over the Maritime Continent is less than 0.5°C (figure not shown). Thus, strong convection over the Maritime Continent is more attributable to atmospheric variability, for example, the Australian high and the related equatorward flow (Fig. 2). Enhanced convection over the Maritime Continent can strengthen the meridional circulation and lead to downward motion over the western Pacific–South China Sea region (Fig. 4). The downward motion can result in positive geopotential high over the region and consequently enhance the WPSH (as shown in Fig. 2) with an anomalous anticyclone over the tropical western North Pacific. Thus, in June 2017, the anomalous Southern Hemispheric circulation has also contributed to the WPSH and the PJ pattern. The meridional circulation could also lead to strong upward motion over south China and consequently contribute to the intense rainfall over south China.

Fig. 4.
Fig. 4.

Latitude–pressure cross section of anomalous meridional circulation (vectors, meridional wind units: m s−1; vertical motion units: 10−2 Pa s−1) and vertical motion (shading) in June 2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

b. Atmospheric circulations associated with the south China June rainfall from a climate perspective

The case study in the last section indicates that the Northern Hemispheric mid- to high-latitude wave train pattern and the Southern Hemispheric atmospheric circulation are strongly coupled with the PJ pattern over East Asia and consequently contributed to the intense rainfall over south China in June 2017. In this section, the atmospheric circulations related to the south China June rainfall are further explored from a climate perspective over the period of 1979–2017 to identify the specialty of the reasons for south China intense rainfall in June 2017.

Figure 5 shows the regressed pattern between the sea level pressure (SLP) and the south China rainfall index in June during 1979–2017. The figure suggests an enhanced WPSH over the western North Pacific. Generally, an enhanced WPSH can increase the southwesterly over south China and consequently advect more water vapor into the region, thereby favoring additional rainfall over south China. Thus, the WPSH is the direct influence on the south China summer rainfall in both June 2017 and other anomalous years.

Fig. 5.
Fig. 5.

Regression map of the sea level pressure (shading, units: hPa) against the south China rainfall index in June for the period of 1979–2017. Dotted areas denote the 95% confidence level based on the Student’s t test.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

A positive AAO-like pattern exists over the Southern Hemisphere. In particular, the Australian high is significantly strengthened. Further the Australian high index is defined as the averaged SLP over the region of 30°–45°S, 90°–150°E. The index analysis indicates a covariability between the south China rainfall and Australian high, with a correlation coefficient of 0.40 over the period of 1979–2017, significant at the 95% confidence level. An enhanced Australian high can strengthen the equatorward flow to northern Australia, which can increase the convection over the Maritime Continent as reflected by the rainfall and vertical motion anomalies. As shown in Fig. 6, more rainfall and strengthened upward motions over the Maritime Continent correspond to an enhanced Australian high. The enhanced convection can further strengthen the local meridional circulation, leading to downward motion over the South China Sea region that can enhance the WPSH and engender upward motion over south China, thereby increasing the rainfall over the region (Fig. 7). In addition, index analysis indicates that the Australian high is currently the strongest within the past 70 years (Fig. 8). The similarity between the south China rainfall-related atmospheric circulation over the past several decades and the circulation in 2017, and the extremity of the Australian high suggest that the anomalous Australian high was one of factors that contributed to the record-breaking south China rainfall in June 2017.

Fig. 6.
Fig. 6.

Regression maps of the rainfall (contours, units: mm day−1) and 500-hPa vertical velocity (shading, units: Pa s−1) against the Australian high index in June for the period of 1979–2017. Diagonal (dotted) areas denote the 95% confidence level for the rainfall (omega) based on the Student’s t test.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Fig. 7.
Fig. 7.

Latitude–pressure cross section of the regression maps of the meridional circulation (vectors, meridional wind units: m s−1; vertical motion units: 10−2 Pa s−1) and vertical motion (shading) against the Australian high index in June for the period of 1979–2017. Dotted areas denote the 95% confidence level for omega based on the Student’s t test.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Fig. 8.
Fig. 8.

Normalized time series of the Australia high in June for the period of 1948–2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Figure 9 shows the regressed pattern of the 200-hPa geopotential height on south China rainfall index. The south China rainfall-related circulation shows a wave train–like pattern over the Eurasian continent, with a significant negative center over central China, two positive centers over south Europe and the Mongolian–Baikal Lake region, and two weak negative centers over the North Atlantic and northern Europe. This atmospheric circulation pattern is highly consistent with that observed in June 2017 (as shown in Fig. 3), indicating that the wave train pattern over the Eurasian continent is an important factor for the south China June rainfall. The anomalous negative center over central China associated with the wave train pattern results in an enhanced East Asian upper-level jet that is favorable for summer rainfall over east China, as was revealed by previous studies (e.g., Gong and Ho 2003; Lu 2004; Zhang et al. 2006; Lin and Lu 2008; Li and Zhang 2014).

Fig. 9.
Fig. 9.

Regression map of the geopotential height at 200 hPa (units: gpm) against the south China rainfall index in June for the period of 1979–2017. Dotted areas denote the 95% confidence levels based on the Student’s t test.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Figure 9 shows the significant upstream circulation of the wave train pattern over south Europe. Previous studies have shown that the anomalous circulation variability over the region can excite Rossby waves along the Asian upper-level jet (e.g., Sun et al. 2008). Thus, to investigate the variability of the pronounced wave train pattern over the Eurasian continent from the climate perspective, an anticyclone index is defined as the averaged mean of the 200-hPa geopotential height over the region of 40°–55°N, 5°–30°E. Figure 10 shows the anticyclone index-related circulation pattern. The results suggest that there is a significant zonal wave train pattern over the Eurasian continent along the Asian upper-level jet corresponding to anomalous circulation over south Europe, which leads to an anomalous anticyclone over the Mongolian–Baikal Lake region. The wave train pattern over the Eurasian continent is similar to the Silk Road and polar wave trains that have been studied by Kosaka et al. (2009) and Orsolini et al. (2015). The strong anticyclone over the Mongolian–Baikal Lake region can further result in a strong northerly over East Asia, which converges with the southwesterly over south China and consequently results in intense rainfall over the region.

Fig. 10.
Fig. 10.

Regression maps of the horizontal winds at (a) 500 hPa (units: m s−1) and (b) the geopotential height at 200 hPa (units: gpm) against the south European anticyclone index in June for the period of 1979–2017. Light (dark) blue shaded areas in (a) denote the 90% (95%) confidence levels, and the dotted areas in (b) denote the 95% confidence levels based on the Student’s t test.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

The index analysis further indicates that the south European anticyclone index is the strongest in June 2017 for the past half century (Fig. 11). Thus, the extremity of the anticyclone over south Europe and the similarity between the south China–related upper-level circulation over the past several decades and that in 2017 both indicate that the anomalous wave train pattern over the Eurasian continent could have contributed to the extremity of the south China rainfall in June 2017.

Fig. 11.
Fig. 11.

The standardized time series of the south European anticyclone index in June for the period of 1948–2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Comparing Figs. 8 and 11 with Fig. 1c, we can also find that there are still large values of the two atmospheric circulation indices in some years those are not followed by large rainfall in south China. As reviewed in the introduction, the variability of summer rainfall over south China is complicated and impacted by a number of factors. Therefore, in the pronounced years, the impact of the Australian high and/or south European anticyclone could be offset by some other factors (i.e., tropical SST anomalies, Okhotsk high, etc.) consequently resulting in small changes of rainfall over south China. During 2017, the Australian high and south European anticyclone are strong, whereas other factors, like tropical SST and the Okhotsk high are normal (figure not shown); therefore, the combined effect of the Australian high and the south European anticyclone contributes to the record-breaking June rainfall over south China.

4. SST conditions over the North Atlantic associated with the south China June rainfall

In contrast to the normal tropical SST situation, relatively stronger SST anomalies were detected over the North Atlantic region in June 2017. Did North Atlantic SST anomalies contribute to the pronounced anomalous atmospheric circulation? To answer this question, the anomalous SST, low-level wind, meridional thermal gradient, heat flux, and K index in 2017 are analyzed.

Figure 12 shows the SST and 1000-hPa wind anomalies in May and June 2017. From May to June, the SST anomalies exhibited a horseshoe-like SST pattern with a negative center over the central North Atlantic and positive values surrounding it. Previous studies indicated that extratropical anomalous SSTs are primarily controlled by atmospheric conditions (e.g., Frankignoul and Reynolds 1983; Miller et al. 1994). In May, there is a strong anomalous cyclone over the central North Atlantic. An anomalous northerly west of the cyclone brings cold air from the high latitudes, favoring SST cooling over the central North Atlantic. The anomalous southerly east of the cyclone brings warm air from low latitudes, favoring SST warming along the western coast of Europe. In addition, the anomalous westerly can decrease the climatological easterly at low latitudes, thereby reducing the wind evaporation process, favoring SST warming over the low-latitude North Atlantic region to the west of the West African coast. The warmer SSTs to the east of the North American coast are closely related to the advection of relatively warm air from the tropics by the anomalous southerly. Figure 13 shows an anomalous turbulent heat flux over the North Atlantic in May and June 2017. In May, the heat flux shows negative values over the warmer SST regions, indicating a reduction of the heat loss from the ocean. Over the central North Atlantic, the heat flux shows positive values, consistent with the strong zonal wind over the regions, which can increase the wind evaporation process and take away the heat from the ocean, consequently favoring SST cooling over the region.

Fig. 12.
Fig. 12.

Anomalous 1000-hPa horizontal winds (vectors, units: m s−1) and SSTs (shading, units: °C) over the North Atlantic region in (a) May and (b) June of 2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

Fig. 13.
Fig. 13.

Anomalous turbulent heat flux (units: W m−2) over the North Atlantic region in (a) May and (b) June of 2017. Positive (negative) indicates the upward (downward) heat flux.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

On the other hand, oceanic anomalies can also provide feedback to the atmosphere, consequently contributing to the variability and persistence of the atmospheric patterns (e.g., Kushnir et al. 2002; Gulev et al. 2013). The cold SSTs over the central North Atlantic can enhance (weaken) the meridional temperature gradient along the 35°N (55°N) in May 2017 (Fig. 14a). The enhanced (weakened) meridional temperature gradient can increase (decrease) the zonal winds along the 35°N (55°N). As shown in Fig. 14b, the zonal wind along the 35°N (55°N) is enhanced (decreased) over the troposphere, which can further persist the anomalous cyclone over the central North Atlantic.

Fig. 14.
Fig. 14.

Anomalous (a) meridional temperature gradients (units: K m−1) at 1000–500 hPa and (b) latitude–pressure section of zonal wind (units: m s−1) averaged between 60° and 20°W in May 2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

From May to June, the North Atlantic circulation system shows a northeastward shift, as shown in Figs. 15a–c. Such a northeastward shift favors northeastward movement of the central North Atlantic anomalous cyclone in May 2017 to the northern North Atlantic in June 2017. In addition, the anomalous southerly to the west of Europe can transport warm air to the northern North Atlantic, and the accumulation of warm advection increases the SSTs in the northern North Atlantic with the anomalous center value approximately 1.2°C. In addition, there is an anomalous northerly over the northern North Atlantic warm SST region in June of 2017 (Fig. 12b). Warm SSTs and overlying cold air currents would cause release of the heat flux from the ocean to the atmosphere (Fig. 13b), resulting in an instability of the atmosphere in situ, which is reflected by the K index (Fig. 15d). The K index is used to represent the atmosphere stability and is defined as
eq2
where T is air temperature and Td is dewpoint temperature. The unstable atmosphere is favorable for the formation of the cyclone circulation over the region. Therefore, both the May-to-June northeastward shift of the circulation system and the reduced atmospheric stability over the northern North Atlantic favor the northeastward shift of the cyclone anomaly over the central North Atlantic in May to the northern North Atlantic in June, which would deepen the Iceland low. Thus, coupled air–sea interactions contributed to the anomalous Iceland low in June 2017. The anomalous Iceland low further excited the wave train pattern (as shown in Fig. 3) originating from the Iceland low and propagating southeastward to south Europe and then eastward to East Asia.
Fig. 15.
Fig. 15.

Climatological zonal wind at 850 hPa (units: m s−1) in (a) May and (b) June; (c) difference of climatological zonal winds (units: m s−1) between June and May; and (d) anomalous K index in June 2017.

Citation: Journal of Climate 31, 18; 10.1175/JCLI-D-17-0903.1

5. Summary

In this study, the intense rainfall over south China in June 2017 is analyzed. The results show that this rainfall is the record-breaking event during the past 70 years. Previous studies mainly focused on the role of tropical SSTs on the south China summer rainfall (e.g., Huang and Wu 1989; Zhang et al. 1996, 1999; B. Wang et al. 2000; Chang et al. 2000a,b; Chan and Zhou 2005; Zhou et al. 2006; Duan et al. 2013; Zhang et al. 2013). However, in June 2017, the tropical SST anomalies are weak, and therefore, the tropical SSTs could not be important for the intense rainfall in June 2017.

In contrast to the tropical SSTs situation, strong anomalies exist in the extratropical climate systems. The Australian high located over the Southern Hemisphere becomes extremely enhanced, and it breaks the record of the previous 70 years. The enhanced Australian high further increases the equatorward flow north of Australia, thereby activating convection over the Maritime Continent. Maritime Continent convection can further enhance meridional circulation, leading to downward motion over the tropical western North Pacific and further enhancing the WPSH and upward motion over south China, thereby providing dynamical conditions for the severe rainfall over the region. Such physical processes are also valid from a climate perspective. Thus, the extremity of the Australian high over the past 70 years could be responsible for the record-breaking rainfall over south China in June 2017.

In addition to the Southern Hemisphere, strong anomalies also exist over the Northern Hemisphere mid- to high latitudes. A strong wave train–like pattern is observed in June 2017; it originates from the North Atlantic and propagates eastward to East Asia over the Eurasian continent. This wave train pattern results in a strong anomalous anticyclone over the Mongolian–Baikal Lake region. Such a strong anticyclone would produce an anomalous northerly over East Asia, bringing cold and dry air to south China, which encounters the warm and wet air transported by the WPSH over south China, contributing to the intense June 2017 rainfall over south China. The south China June rainfall-related circulation indicates that the variability in the upper-level atmospheric circulation over south Europe and its related wave train part have close relationship with the south China June rainfall. In addition, the atmospheric circulation anomalies over south Europe are the strongest in the previous 70 years, which could also contribute to the extremity of the rainfall over south China in June 2017. To further understand the atmospheric circulation anomaly over south Europe in June 2017, the air–sea interaction over the North Atlantic is investigated. Actually, the evaluation of the atmospheric and oceanic anomalies begins in May. There was a strong anomalous cyclone over the central North Atlantic in May that can result in SST anomalies over the North Atlantic and the anomalous SST feedback to the overlying atmospheric circulation, consequently persisting the atmospheric and oceanic anomalies. From May to June, the North Atlantic atmospheric circulation has a northeastward shift. In addition, the changes in the SST and atmosphere decrease the stability of the atmosphere over the northern North Atlantic, favoring an anomalous cyclone over the region and deepening the Iceland low. The enhanced Iceland low further excites the wave train pattern southeastward and enhances the atmospheric circulation over south Europe, which further propagates eastward to East Asia. Comparing Figs. 3 and 9, we can find that in June 2017, the Iceland low is strong and benefit for the wave train pattern from the North Atlantic to East Asia. However, from a climate perspective, we can find that the atmospheric circulation anomalies over south Europe and its downstream wave train pattern have a closer connection with the south China June rainfall anomaly and the connection between the Iceland low and south China June rainfall is weak. This result indicates that the anomalous circulation variability over south Europe is generally important for exciting Rossby wave along the Asian upper-level jet and therefore influencing the East Asian summer climate, which is similar to the findings of previous studies (e.g., Sun et al. 2008). And the anomalous Iceland low just impacts the East Asian summer climate via the wave train pattern in some years (e.g., in 2017).

Climate variability is very complicated; the mechanisms revealed in this study may not represent all of the processes that control the intense June rainfall over south China. Some intraseasonal processes could also contribute to the June rainfall over south China, and thus, these other factors should be explored, the results of which may produce many valuable findings.

Acknowledgments

This work was jointly supported by the National Natural Science Foundation of China (41522503 and 41421004), and the External Cooperation Program of BIC, Chinese Academy of Sciences (134111KYSB20150016).

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