Ruidan CHEN,Zhiping WEN,Riyu LU,and Chunzai WANG

1Center for Monsoon and Environment Research/Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies/School of Atmospheric Sciences,Sun Yat-sen University,Guangzhou 510275,China

2Department of Atmospheric and Oceanic Sciences/Institute of Atmospheric Sciences,Fudan University,Shanghai 200438,China

3State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing 100029,China

4State Key Laboratory of Tropical Oceanography,South China Sea Institute of Oceanology,Chinese Academy of Sciences,Guangzhou 510301,China

5University of the Chinese Academy of Sciences,Beijing 100049,China

6Jiangsu Collaborative Innovation Center for Climate Change,Nanjing 210023,China

ABSTRACT This study investigates why an extreme hot midsummer occurred in Central and South China(CSC)during 2017.It is shown that the western North Pacific subtropical high(WNPSH)was abnormally intensified and westward-extending,resulting in anomalous high pressure and consequent extreme heat over CSC.The abnormal WNPSH was favored by the warming of the western tropical Pacific(WTP),which was unrelated to ENSO and manifested its own individual effect.The WTP warming enhanced the convection in-situ and led to anomalous high pressure over CSC via a local meridional circulation.The influence of the WTP was confirmed by CAM4 model experiments.A comparison between the 2017 midsummer and 2010 midsummer(with a stronger WNPSH but weaker extreme heat)indicated that the influence of the WNPSH on extreme heat can be modulated by the associated precipitation in the northwesternflank.The role of the WTP was verified by regression analyses on the interannual variation of the WTP sea surface temperature anomaly(SSTA).On the other hand,the WTP has undergone prominent warming during the past few decades,resulting from decadal to long-term changes and favoring extreme warm conditions.Through a mechanism similar to the interannual variation,the decadal to long-term changes have reinforced the influence of WTP warming on the temperature over CSC,contributing to the more frequent hot midsummers recently.It is estimated that more than 50%of the temperature anomaly over CSC in the 2017 midsummer was due to the WTP warming,and 40%was related to the decadal to long-term changes of the WTP SSTA.

Key words:hot midsummer,Central and South China,western tropical Pacific,decadal to long-term changes

1.Introduction

The globally averaged temperature in 2017 was the highest on record without an El Ni?no,according to the World Meteorological Organization(WMO;https://public.wmo.int/en/media/press-release/wmo-confirms-2017-among-threewarmest-years-record).In China,a destructive extreme hot midsummer attacked Central and South China(CSC)during July–August 2017,with many observational stations witnessing unprecedented high temperature.For example,in Shanghai,Xujiahui station recorded a maximum tempera-ture of 40.9°C on 21 July 2017,breaking the record since 1873.In fact,CSC is the region where high temperature extremes occur most frequently in China(Wei and Chen,2009).The occurrence of an extreme hot midsummer leads to huge socioeconomic threats,due to the high density of the population and concentration of the economy in these regions.Therefore,understanding the causes of extreme hot midsummers in CSC is of great concern to both the government and public.

Many previous studies have investigated the variations in temperature extremes in CSC(e.g.,Yan et al.,2011;Wang et al.,2015a,b;Qian et al.,2018a).It has been revealed that the intensity and frequency of warm extremes in CSC have witnessed an obvious increasing trend in the past,and are projected to continue increasing in the future(Wang et al.,2015a,b;Qian,2016).Multiple factors are responsible for this increasing trend,including anthropogenic forcing(Sun et al.,2014;Ma et al.,2017),multi-decadal variability(Xia et al.,2016;Qian et al.,2018a),and local urbanization effects(Qian,2016).On the other hand,the occurrence of extreme heat in a specific year is prominently modulated by the interannual climate variability,such as ENSO events(Su et al.,2017).Thecurrentstudyfocusesontheinternalforcingofthe climate system,and investigates the atmospheric circulation and sea surface temperature(SST)anomalies responsible for the extreme hot midsummer in CSC during 2017.

The atmospheric circulation responsible for high temperature is typically characterized by anomalous high pressure over the corresponding regions(Baldi et al.,2006;Loikith and Broccoli,2012;Chen and Lu,2015;Ma et al.,2017).Anomalous high pressure results in stronger subsidence,lower humidity and less cloud cover,all of which would enhance both the vertical adiabatic heating and the diabatic heating caused by more solar radiation reaching the surface(Sun et al.,1999;Black et al.,2004).For CSC,which is located in the East Asian monsoon region,anomalous high pressure is usually related to the intensification and westward extension of the western North Pacific subtropical high(WNPSH)(Zhang et al.,2004;Lin et al.,2005;Wang et al.,2016;Luo and Lau,2017).For instance,Wang et al.(2016)studied three hot summer cases,including the summers of 2003,2006 and 2013,with high temperatures mainly occurring in southeastern,southwestern and eastern China,and found that all three cases were associated with the intensification and westward extension of the WNPSH.Luo and Lau(2017)performed composite analyses on heat waves in Guangdong Province,South China,and concluded that the westward displacement of the WNPSH is a primary factor responsible for their occurrence.

However,the intensification and westward extension of the WNPSH can also lead to above-normal precipitation,particularly in the northwesternflank,through strengthening the southwesterly water vapor transport(e.g.,Yang and Sun,2003;Lee et al.,2013).Precipitation would further reduce the surface air temperature via evaporative cooling.Therefore,the influence of the WNPSH on regional extreme heat is meanwhile modulated by the associated precipitation,suggesting that a stronger WNPSH does not necessarily result in a higher occurrence of extreme heat.In the current study,a comparison between the 2017 hot midsummer and the midsummer of the strongest WNPSH during the study period(year 2010)helps test this hypothesis.

The East Asian climate is remarkably modulated by the SST anomaly(SSTA)over the tropical Pacific(e.g.,Wang et al.,2000;Sui et al.,2007;Chen et al.,2016).The extreme heat over CSC tends to be enhanced during the summer following a mature El Ni?no phase(Chen and Zhou,2018;Freychet et al.,2018;Luo and Lau,2018)and during the developing stage of the central Pacific El Ni?no-like pattern(Qian et al.,2015,2018b).In midsummer,both the SSTA over the western tropical Pacific(WTP)and central–eastern tropical Pacific(CETP)are addressed as crucial factors influencing the East Asian monsoonal circulation and thus the temperature over China.Wang et al.(2017)analyzed the atmospheric circulation and SST anomalies responsible for the leading patterns of heat wave variation in China,and indicated that the dipole and tripole variation patterns are associated with a Rossby wave train triggered by the enhanced diabatic heating over the WTP.The enhanced diabatic heating associated with convection over the WTP is favored by an anomalous Walker circulation over the tropical Pacific,accompanied by a positive SSTA and negative SSTA over the WTP and CETP,respectively.Focusing on the midsummer temperature in South China,Chenetal.(2018a)suggestedthatananomalouswarm midsummer in South China is associated with the El Ni?no to La Ni?na transition phase evolving from the preceding winter to the simultaneous summer.In the summer,the anomalous warm WTP strengthens the local meridional circulation and results in abnormal high pressure and high temperature over South China,while the anomalous cold CETP further favors the convection over the WTP via the Walker circulation.In these studies,the SSTAs over the WTP and CETP were observed simultaneously,and thus it is hard to distinguish their individual effects without the help of numerical experiments.Wang et al.(2017)employed a linear baroclinic model to detect the forcing effect of idealized adiabatic heating over the WTP,and Chen et al.(2018a)used CAM4 to compare the forcing effects of different idealized SSTA patterns.Both results underline the critical role of the WTP warming on the extreme heat in China.

The hot midsummer in 2017 was unrelated to ENSO,characterized by an obvious positive SSTA over the WTP but weak SSTA over the CETP in the midsummer.This makes the 2017 midsummer a unique case to study the individual effect of the WTP after excluding the CETP SSTA.The current study investigates the influence of the WTP SSTA on recent extreme hot midsummers in China from the perspective of both a case study and climate statistics,and the warming trend over the WTP is emphasized.The rest of the paper is organized as follows:Section 2 describes the data,methods and model experiments.Section 3 analyzes the hot midsummer case in 2017,and compares it with the midsummer in 2010,which was characterized by the most intensified and westward-extending WNPSH during the study period.Both cases were found to be associated with the anomalies of the WNPSH and WTP SST.Section 4 further verifies the influence of the WTP SSTA from a climate perspective.Section 5 presents our conclusions.

2.Data,methods and model experiments

2.1.Data and methods

The daily maximum surface air temperature over 824 observational stations in China was obtained from the China Meteorological Data Service Center(http://data.cma.cn).After excluding stations with missing data,740 stations were used for the analysis.The monthly mean data from Reanalysis-1(Kalnay et al.,1996)were employed to analyze the atmospheric circulation,including the geopotential height,windfield,specific humidity and surface air temperature.The horizontal resolution of these data is 2.5°×2.5°and there are 17 vertical levels extending from 1000 hPa to 10 hPa.The monthly mean SST was extracted from ERSST.v5,which has a horizontal resolution of 2°×2°(Huang et al.,2017).The monthly mean precipitation was derived from CMAP,with a horizontal resolution of 2.5°×2.5°(Xie and Arkin,1997).The midsummer period in this study refers to July–August.The data during 1979–2017 were used for the analysis,and the average from 1981–2010 was computed as the climatology.This selection of base period complies with the suggestion of using an updated 30-year baseline as advocated by the WMO,making the results comparable with others.

An extreme heat day over a specific station was defined as when the daily maximum temperature exceeded 35°C,which is identical to the definition adopted by the China Meteorological Administration.In order to depict the status of the WNPSH,the monthly mean 500-hPa geopotential height was used,with the 5880-gpm contour denoting the domain of the WNPSH.A westward extension index and intensity index of the WNPSH were employed.The westward extension index was defined as the most westward position of the 5880-gpm contour over the region between 90°E and 180°E.Regarding the intensity index,firstly,the grids with geopotential height greater than 5880 gpm were selected,then the differences between the geopotential height over each of these grids and 5870 gpm were calculated,and the accumulated difference of all the selected grids was defined as the intensity of the WNPSH.These definitions are the same as those endorsed by the National Climate Center in China,and have been adopted in previous studies to describe the WNPSH(Li et al.,2003;Tan and Sun,2004;Zhang and Zhi,2010).The vertically in-tegrated water vaporflux is calculated asq VVVdp,where g is the acceleration of gravity,q the specific humidity,V the horizontal wind vector,Psthe surface pressure,and Ptis set at 300 hPa.

For the case studies,the anomalies were computed by subtracting the climatological mean(1981–2010).The standardized anomalies upon the interannual variability were also computed to unify the anomalous magnitudes of different variables and different regions,for the convenience of comparison.Regression analyses were further performed by using the annual series,so as to verify the mechanisms deduced from the case studies.In distinguishing the effects of different time scales,a nine-year high-passfilter was employed to extract the interannual time scale,and compared with the original series including the interannual variability and decadal to long-term changes.The Student’s t-test with a significance level of 90%was used for estimating the significance of regression analyses,based on the effective degrees of freedom(EDOF)taking the autocorrelation of the time series into account.The EDOF is computed as Nedof=N(1-r1r2)/(1+r1r2),where N denotes the original sample size and r1and r2denote the lag-1 autocorrelation of the two time series,respectively.

2.2.Model experiments

In order to validate the influence of the WTP SSTA on the atmospheric circulation during the midsummer of 2017,CAM4 was adopted to conduct numerical experiments.This model can reproduce the mean climate state well and has been widely used in relevant studies.A detailed description of the model and simulation is provided by Neale et al.(2013).The horizontal resolution is approximately 1.9°latitude×2.5°longitude and the vertical direction contains 26 levels from the surface to near 3.5 hPa.

Two numerical experiments were performed in the current study.One was the control run(hereafter,CTL run),which was forced by the observed climatological mean seasonal cycle of global SST.The other was a sensitivity experiment forced by the climatological SST overlapped by the observed SSTA over the WTP during July–August in 2017(hereafter,WTPrun).TheSSTAovertheregion(10°S–10°N,105°–180°E)was added to the WTP run,because the tropical SSTA within these latitudes plays an active role in influencing the atmosphere,as implied by the precipitation anomaly(see Section 3.1 for details).Each simulation was integrated for 30 years.Thefirst year was regarded as the spin-up,with the last 29 years extracted for analysis.The composite differences between the WTP run and the CTL run were calculated to detect the forcing effect of the WTP SSTA in the midsummer of 2017.The Student’s t-test with a significance level of 90%was used for estimating the significance.

3.Atmospheric circulation and SSTAs associated with extreme hot midsummers

3.1.Hot midsummer in 2017

Figure 1 shows the frequencies of extreme heat days and the corresponding anomalies during the 2017 summer.Above-normal extreme heat days occurred in CSC during July and August,while the anomaly was weak during June(Figs.1a–c).The accumulated frequency of extreme heat days from July to August reached 30 days in CSC,about 10 days more than the climatology(Fig.1d).Therefore,this study will focus on the high temperature during July to Au-gust in CSC,which could represent the extreme hot midsummer in 2017.

Fig.1.Occurrence frequencies of extreme heat days(contours;contour interval:10)and the corresponding anomalies(color shading)during(a)June,(b)July,(c)August,and(d)July–August in 2017.Units:days.

Figure 2 demonstrates the anomalies of mid-and lowertropospheric geopotential height and horizontal winds in the midsummer of 2017.The 500-hPa geopotential height increased obviously over China,with a largest amplitude of 20 gpm and 2 standard deviations(Fig.2a).The standardized anomaly centers occurred to the south of 35°N,which were obviously more southward compared to the original anomaly centers extending northwards to the north of 40°N.This is because the interannual standard deviation of geopotential height in the low-latitude region was generally smaller compared to the high-latitude region.The positive geopotential height anomaly over CSC was still obvious at 850 hPa,with an amplitude of 15 gpm and 1.5 standard deviations(Fig.2b).The anomalous high pressure over CSC was associated with the abnormal intensification and westward extension of the WNPSH,which is denoted by the 5880-gpm contour of the 500-hPa geopotential height(Fig.2c).The climatological western edge of the WNPSH is located near 130°E,but anomalously extended to 112.5°E in the midsummer of 2017(17.5°westwards).It was reported in some previous studies that WNPSH indices based on geopotential height contain a pronounced increasing trend,since the geopotential height in the tropical and subtropical regions generally enhances against the background of global warming,and thus other indices based on the relative vorticity of the horizontal windfield have been proposed(Yang and Sun,2003;He et al.,2015).Therefore,we also analyzed the horizontal wind to examine the anomaly of the WNPSH.It was found that both the 500-hPa and 850-hPa wind anomalies presented an obvious anticyclone over the southeastern coast of China(Figs.2c and d),confirming that the WNPSH was abnormally strong and westward-extended in the midsummer of 2017.

In order to quantitatively evaluate the anomaly of the WNPSH in the midsummer of 2017,we analyzed the time series of the WNPSH intensity index and westward extension index during the past few decades.Figure 3 displays the standardized anomalies of the indices from 1979 to 2017.The anomaly of the intensity index exceeds 2 standard deviations in 2017,which is the second highest next to 2010.The anomaly of the westward extension index is about 1.2 standard deviations in 2017,ranking it fourth during the past 39 years.It is illustrated that the WNPSH was extremely strong and extended westwards in the midsummer of 2017,leading totheanomaloushighpressureoverCSCandfavoringtheoccurrence of extreme high temperature.Notably,the WNPSH was the most anomalous in the midsummer of 2010,with the largest intensity and the most extreme westward extension.Therefore,we also analyze the case of the 2010 midsummer later.

Fig.2.Anomalies(contours and vectors)of the(a,b)geopotential height(units:gpm)and(c,d)horizontal wind(units:m s-1)and their standardized anomalies(color shading)at(a,c)500 hPa and(b,d)850 hPa in the midsummer of 2017.In(c),the 5880-gpm contours depict the position of the WNPSH,with the blue contour denoting the climatology and the red one denoting the midsummer of 2017.For the horizontal winds,the color shading denotes the standardized anomalies of meridional wind.The black shading denotes the Tibetan Plateau.

Fig.3.Standardized anomalies of the(a)intensity and(b)westward extension indices of the WNPSH during 1979 to 2017.

Fig.4.The SSTA(contours;units:°C)and standardized SSTA(color shading)in the(a)preceding winter and(b)midsummer of 2017.(c)Standardized SSTA(contours),standardized precipitation anomaly(color shading)and anomaly of 850-hPa wind(green vectors;units:m s-1;only vectors greater than 1 m s-1are plotted)over the western Pacific in the midsummer of 2017.The red box covering(10°S–10°N,105°–160°E)is used to define the WTP SSTA index.(d)Anomaly of the meridional vertical circulation averaged between 110°E and 140°E(vectors;units:m s-1for the horizontal velocity and-10-4hPa s-1for the vertical velocity omega)and the standardized anomaly of the vertical velocity omega(color shading).

Furthermore,the tropical SSTA was analyzed to detect the external forcing of the anomalous atmospheric circulation.Figures 4a and b show the SSTA in the preceding winter and simultaneous midsummer for the case 2017.There was obvious warming over the western Pacific but a weak anomaly over the CETP,manifesting a non-ENSO pattern(Figs.4a and b).The warming amplitude in the midsummer over the western Pacific exceeded 0.6°C and 2 standard deviations(Fig.4b).The positive SSTA over the WTP between 10°S and 10°N was favorable for a positive precipitation anomaly(Fig.4c),suggesting that the SSTA over this region plays an active role in influencing the atmospheric circulation.In contrast,the positive SSTA north of 10°N was generally accompanied by negative precipitation anomaly,suggesting that the underlying SSTA is a response to the atmospheric circulation.The positive SSTA over the WTP led to an anomalous local meridional circulation with ascent over the tropics and descent over the subtropics,as demonstrated by the meridional vertical circulation averaged between 110°E and 140°E(Fig.4d).The descending branch was favorable for the intensification and westward extension of the WNPSH.These results are consistent with our previous work showing that the WTP warming during midsummer is vital for the occurrence of anomalous high pressure over South China(Chen et al.,2018a).

In order to assess the strength of the WTP SSTA in 2017,we defined an intensity index of the WTP SSTA as the midsummer SSTA averaged over the region(10°S–10°N,105°–160°E),denoted by the red box in Fig.4c.This definition is the same as that in Chen et al.(2018a).Figure 5a shows the standardized series of the WTP SSTA intensity index during 1979–2017(black line).The index is remarkably abovenormal in 2017,exceeding 1.4 standard deviations and ranking fourth.Although ranking fourth,the WTP SSTA in 2017 is unique since it is unrelated to an ENSO event.Figures 5b and c illustrate the evolutions of the SSTA averaged over the WTP and CETP(5°S–5°N,170°–120°W)for the top four years of the WTP SSTA index,i.e.,1998,2010,2016 and 2017.For 1998,2010 and 2016,the CETP SSTA evolves from a remarkable positive phase to a negative phase from theprecedingwintertothesimultaneousmidsummer,accompanied by the WTP SSTA evolving from a negative or nearnormal phase to a positive phase.It is shown that the top three WTP SSTA midsummers were related to the transition phase from a strong El Ni?no to La Ni?na.By contrast,for 2017,the CETP SSTA evolved from a negative phase to positive phase with much weaker amplitude in the preceding winter than the other three years,and the WTP SSTA was persistently positive from the preceding autumn to the simultaneous midsummer.The WTP warming in the 2017 midsummer was unrelated to the La Ni?na phase,and thus could manifest the individual effect of the WTP warming.

The individual effect of the WTP SSTA on the atmospheric circulation in 2017 was further verified by numerical experiments performed using CAM4.In response to a WTP SSTA pattern identical to the observed SSTA during July–August 2017(Figs.6a and b),there was anomalous high pressure at 500 hPa and an anticyclone at 850 hPa over the western North Pacific and CSC(Fig.6c).The high-pressure anomaly was favored by the local meridional circulation,which resulted from the abnormal convection over the tropics(Fig.6d).Overall,these atmospheric responses in the WTP run were similar to the anomalies in the observation,as shown in Figs.2a,2d and 4d,although the simulated anomalies in the WTP run appear to be weaker.It is confirmed that the WTP warming was important for the abnormally intensifi ed and westward-extending WNPSH in the midsummer of 2017.

Fig.5.(a)Standardized anomalies of the WTP SSTA index during 1979 to 2017.The black line denotes the original anomaly and the red line denotes the interannual component.Evolutions of the(b)WTP and(c)CETP SSTA from the preceding autumn to the simultaneous midsummer for the top four years of the WTP SSTA index,i.e.,1998,2010,2016 and 2017.

3.2.Hot midsummer in 2010

The above analysis reveals the important role of the WNPSH and WTP warming in the occurrence of the extreme hot midsummer over CSC during 2017.In fact,the anomalies of the WNPSH and WTP were even stronger in the midsummer of 2010.The WNPSH intensity and westward extension indices reached 2.7 and 1.8 standard deviations in the midsummer of 2010,both rankingfirst during the analyzed period(Fig.3).In addition,the WTP SSTA intensity index exceeded 2 standard deviations and ranked second in the midsummer of 2010,second only to the super El Ni?no year of 1997/98(Fig.5a).Therefore,we further examined the midsummer of 2010 and compared it with the 2017 case.

Figure 7 exhibits the frequency of extreme heat days in July–August 2010 and compares it with the midsummer of 2017.Above-normal extreme heat days occurred over CSC in 2010,with about 10 days more than the climatology(Fig.7a).Similar to 2017,the hot midsummer in 2010 was due to the anomalous high pressure and anticyclone associated with the intensification and westward extension of the WNPSH(not shown).The anomaly of the WNPSH in 2010 was larger than in 2017,manifested by the indices as shown in Fig.3.Associated with the stronger dominance of the WNPSH,more extreme heat days occurred over the regions south of the Yangtze River in 2010 than in 2017(Fig.7c).However,the frequency of extreme heat days over Central China appears to have been less in 2010 than in 2017.The most obvious difference lay to the east of the Tibetan Plateau(denoted by the blue boxes,and the topography is depicted in Fig.8),characterized by frequent extreme heat days in 2017 but far fewer in 2010.

One possible reason for the fewer extreme heat days to the east of the Tibetan Plateau in 2010 might be the precipitation anomaly.Figure 8 displays the anomalies of the vertically integrated water vaporflux and its divergence and the precipitation during the midsummers of 2010 and 2017.In the northwesternflank of the abnormally strong WNPSH,there was anomalous southwesterly water vapor transport over CSC.Overlapping the climatological southwesterly summer monsoon,more water vapor was transported northwards and above-normal moisture convergence occurred in the northwesternflank of the WNPSH(Figs.8a and b),favoring above-normal precipitation in-situ(Figs.8c and d).In 2010,the anomalous southwesterly water vaporflux extended westwards to the east of the Tibetan Plateau and led to above-normal moisture convergence and precipitation over the Sichuan Basin(Figs.8a and c).The precipitation anomaly reached 1.5 standard deviations,which would have inhibited the extreme heat through evaporative cooling.In comparison,the anomalous water vapor transport in 2017 was located more eastwards over the plain areas and the southwesterly water vaporflux thrust further northeastwards,leading to above-normal precipitation around North-east China(Figs.8b and d).On the contrary,the precipitation anomaly over CSC was much weaker and extreme heat days occurred frequently in 2017.These differences between 2010 and 2017 indicate that the influence of the WNPSH on extreme heat is modulated by the associated precipitation.

Fig.6.(a)July and(b)August SSTA imposed in the WTP run,which are identical to the SSTA observed in the midsummer of 2017.The anomalous region is(10°S–10°N,105°–180°E).Composite differences between the WTP run and the CTL run:(c)500-hPa geopotential height(color shading;units:gpm;significant areas are dotted)and 850-hPa wind(vectors;units:m s-1;black vectors are significant);(d)meridional vertical circulation averaged over 110°–140°E(units:m s-1for the horizontal velocity and-10-4hPa s-1for the vertical velocity omega;significant areas are shaded).

Fig.7.Occurrence frequencies of extreme heat days(contours)and the corresponding anomalies(color shading)during July–August in(a)2010 and(b)2017.(c)Difference in extreme heat frequency between 2010 and 2017.Units:days.The blue boxes denote the areas with obviously fewer extreme heat days in 2010 than in 2017.

The SSTA associated with the 2010 case was also analyzed.There was a positive SSTA over the CETP in the preceding winter and a negative(positive)SSTA over the CETP(WTP)in the simultaneous midsummer,presenting an obvious transition phase from El Ni?no to La Ni?na(Figs.9a and b).The amplitudes of the SSTAs over the CETP and WTP reached 1.5 standard deviations.In the midsummer,the positive(negative)SSTA over the WTP(CETP)coincided with above-normal(below-normal)precipitation,suggesting that the SSTA over both the WTP and CETP play an active role in influencing the atmospheric circulation(Fig.9c).On the one hand,an anomalous warm WTP would trigger an abnormal meridional circulation,with the descending branch over the western North Pacific(Fig.9d).The descending branch would be constrained to the south of 25°N while anoma-lous ascending motion occurs to the north,which is consistent with the abnormal precipitation over Central China(Fig.9c).On the other hand,an anomalous cold CETP might enhance the anticyclonic anomaly over the western North Pacific via a Rossby wave response to the northwest of the cooling center.The WTP and CETP worked together to reinforce the WNPSH,leading to an extremely intensified WNPSH in 2010.Combiningthe2017 and2010cases,bothmidsummers show a positive SSTA over the WTP,indicating the important role of WTP warming in the occurrence of high temperature over CSC.

Fig.8.Anomalies of the vertically integrated water vaporflux(vectors;units:kg m-1s-1)and(a,b)the corresponding divergence(color shading;units:10-5kg m-2s-1),and(c,d)the standardized anomalies of precipitation(color shading;units:mm d-1),for the midsummers of 2010 and 2017.The blue boxes are the same as those in Fig.7.

Fig.9.As in Fig.4 but for the 2010 case.

4.Role of the WTP SSTA in hot midsummers on interannual and longer time scales

This section further investigates the influence of the WTP SSTAontheatmosphericcirculationandairtemperatureover CSC from a climate perspective.It is noticeable that the WTP SSTA index presents an obvious warming trend during the analyzed period(Fig.5a),which is regarded as a combination of both decadal variability and the long-term trend associated with global warming(Cravatte et al.,2009;Kidwell et al.,2017).Therefore,we performed regression analyses onto both the interannual component and the original series of the WTP SSTA(denoted by the red line and black line in Fig.5,respectively),so as to clarify the anomalies associated with the WTP SSTA.

Figure 10a shows the regressed anomalies of 500-hPa geopotential height,850-hPa horizontal wind,and surface air temperature against the interannual component of the WTP SSTA(named the interannual regression pattern).Associated with the interannual variation of the WTP SSTA,there are significant anomalies over China(Fig.10a).A positive WTP SSTA is favorable for anomalous high pressure and an anticyclone over CSC,indicating an abnormally strong and westward-extending WNPSH.The anomalous high pressure favors higher temperature,and thus prominent warming occurs over CSC.Therefore,the interannual variation of the WTP SSTA could significantly influence the atmospheric circulation and subsequently the air temperature over CSC.The mechanisms highlighted in the 2017 and 2010 cases are in agreement with the climate processes.

Figure 10b shows the regressed anomalies against the original series of the WTP SSTA,which contains the signals of interannual variability and the decadal to long-term changes(named the original regression pattern).Over CSC,there is anomalous high pressure,an anticyclone,and high temperature associated with a positive WTP SSTA,presenting patterns similar to the interannual variability.It is thus implied that the decadal to long-term variability work together with the interannual variability to modulate the climate in CSC through a similar mechanism.Therefore,the obvious warming of the WTP SSTA in recent decades would reinforce the influence of the positive-phase WTP SSTA.For the 2017 case,the positive WTP SSTA(1.4 standard deviations)was largely due to the decadal to long-term changes,since the interannual component was nearly normal(0.4 standard deviations)(Fig.5a).Regarding the 2010 case,the interannual variability was pronounced(1.4 standard deviations),which was intimately related to ENSO,as illustrated in Figs.9a and b.Nevertheless,the decadal to long-term component of the WTP SSTA made a positive contribution and enhanced the SSTA to 2 standard deviations(Fig.5a).Therefore,the decadal to long-term changes favor the recent occurrence of extremewarmconditionsintheWTP,contributingtoextreme hot midsummers over CSC.

In order to quantify the impact of the WTP SSTA on the hot midsummer in 2017 and assess the relative contribution of the interannual and decadal to long-term components,the above regression patterns were further employed to reconstruct the corresponding temperature anomalies.The reconstructed temperature anomaly based on the interannual(original)regression pattern,computed as the corresponding regression coefficient multiplied by the interannual component of the WTP SSTA(the original WTP SSTA)in July–August 2017,was used to represent the contribution of the interannual component of the WTP SSTA(the original WTP SSTA)(Figs.10c and d).The difference between the reconstructed temperature anomaly based on the original and the interannual regression patterns was used to represent the contribution from the decadal to long-term components of the WTP SSTA(Fig.10e).It was found that the original WTP SSTA led to obvious warming over CSC,with a large-value center located over the Yangtze River(Fig.10d).The reconstructed temperature anomaly based on the original WTP SSTA exceeded 0.4°C over CSC,accounting for more than 50%of the actual warming(0.5°C–1.0°C).The contribution from the interannual component of the WTP SSTA was weak,with the corresponding reconstructed temperature anomaly less than 0.1°C(Fig.10c).In contrast,the reconstructed temperature anomaly based on the decadal to long-term components of the WTP SSTA reached 0.3°C–0.4°C,accounting for more than 40%of the actual warming and 80%of the total WTP SSTA contribution(Fig.10e).It is thus confirmed that the WTP SSTA was vital for the hot midsummer over CSC in 2017,and the decadal to long-term component of the WTP SSTA played a dominant role.

It is noteworthy that the regressed anomalies against the interannual component and the original series of the WTP SSTA present some obvious differences,especially in the mid–high latitudes.Regression upon the original series demonstrates significant abnormal high pressure and high temperature around Lake Baikal(Figs.10b and d),which is not apparent for the regression upon the interannual component(Figs.10a and c).The implication is that the circulation anomaly around Lake Baikal is not influenced by the WTP SSTA.Instead,the circulation anomaly around Lake Baikal and the WTP SSTA are both modulated by other forcings on the decadal to long-term time scale,leading to their significant relationship.In fact,the anomalous high pressure and high temperature around Lake Baikal have been noticed by previous studies,and the anomalies were found to be related to both global warming and the decadal changes in the East Asian summer monsoon circulation(Kwon et al.,2007;Zhu et al.,2012;Chen and Lu,2014).

Fig.10.Regressed anomalies of 500-hPa geopotential height(color shading;units:gpm),850-hPa horizontal wind(vectors;units:m s-1),and surface air temperature(contours;units:°C;contour interval:0.3)onto the(a)interannual component and(b)original series of the WTP SSTA index.Only areas significant at the 90%significance level are plotted.(c–e)Original(contours;contour interval:0.5)and reconstructed(color shading)surface air temperature in July–August 2017 based on the(c)interannual and(d)original regression patterns,and(d)the difference between them(original minus interannual,to represent the contribution of the decadal to long-term components).The dotted(slashed)areas denote the ratios of the reconstructed temperature to the original temperature are more than 40%(50%).

5.Conclusions and discussion

The causes of hot midsummers in CSC were investigated in this study.The hot midsummer in 2017 wasfirstly analyzed,which witnessed up to 30 extreme heat days(about 10 days more than the climatology)during July–August.The extreme hot midsummer was found to result from the anomalous intensification and westward extension of the WNPSH.The intensity and westward extension indices of the WNPSH in 2017 exceeded 2 and 1.2 standard deviations,ranking them second and fourth during the analyzed period,respectively.The abnormal WNPSH was accompanied by anomalous high pressure over CSC,favoring high temperature and the occurrence of a hot midsummer.At the same time,the WTP was anomalously warm in the midsummer,with the anomaly amplitude exceeding 1.4 standard deviations and ranking fourth.The WTP warming was not clearly related to the SSTA over the CETP,which appeared to be weak in the 2017 midsummer and the preceding period.This configuration of the WTP and CETP SSTA is unique during the analyzed period,and distinct from other warming years that are characterized by the transition phase from a strong El Ni?no to La Ni?na.Therefore,the 2017 midsummer could stand as a unique case to study the individual effect of the WTP.The positive SSTA within 10°S–10°N over the WTP led to abnormally strong convection in-situ,which further favored the anomalous high pressure over CSC through a local meridional circulation.Moreover,the influence of WTP warming on the anomalous high pressure over CSC during the midsummer of 2017 was verified by numerical experiments performed using CAM4.

The anomalies of the WNPSH and WTP SST were even larger in the midsummer of 2010,with the WNPSH intensity and westward extension indices rankingfirst and the WTP SSTA index ranking second in the past 39 years.The same as in the 2017 case,CSC experienced above-normal extreme heat days in the midsummer of 2010,which also resulted from the anomalous high pressure associated with an intensifi ed and westward-extended WNPSH.Despite the anomaly of the WNPSH in 2010 having been stronger than that in 2017,the number of extreme heat days east of the Tibetan Plateau appeared to be fewer in 2010.This was related to the above-normal precipitation east of the Tibetan Plateau in 2010,which was favored by the anomalous southwesterly water vapor transport in the northwesternflank of the intensified WNPSH.It is thus suggested that the influence of the WNPSH on the occurrence of extreme heat is modulated by the associated precipitation.Moreover,the SSTA in 2010 was also different from that in 2017,characterized by an El Ni?no to La Ni?na transition phase.The anomalous warm WTP and cold CETP in the midsummer worked together to reinforce the WNPSH,via a local meridional circulation and a Rossby wave response,respectively.The common SSTA in 2017 and 2010 indicates that the WTP warming is important for hot midsummers in CSC.

The impact of the WTP SSTA on the atmospheric circulation and air temperature over CSC was further investigated from a climate perspective.The results of regression analyses showed that the interannual variation of the WTP SSTA modulates the air temperature in China through the same mechanism as proposed by the case studies,i.e.,through favoring a high pressure anomaly over CSC.On the other hand,the WTP has witnessed pronounced warming during the past few decades,which is due to the decadal to long-term changes.Actually,the WTP warming in 2017 was largely due to the decadal to long-term changes,since the interannual component was nearly normal.The WTP warming in 2010 was also enhanced by the decadal to long-term changes,overlapping the prominent interannual component associated with ENSO.Furthermore,the circulation anomalies over CSC associated with the original series of the WTP SSTA(including the interannual variability and decadal to long-term changes)are similar to the anomalies associated with the interannual variability of the WTP SSTA.Therefore,it is implied that the decadal to long-term changes would reinforce the influencing process of the WTP warming on the atmospheric circulation and consequently the air temperature over CSC.The contribution of the WTP SSTA to the surface air temperature enhancement over CSC in the 2017 midsummer was evaluated to be more than 50%,and 40%was due to the decadal to long-term components of the WTP SSTA.

In fact,the prominent warming of the WTP during recent decades has been revealed by various studies,despite being based on different data and analysis periods(Liu and Huang,2000;Cravatte et al.,2009).This warming trend has been documented as resulting from both decadal variability and the long-term trend associated with global warming(Cravatte et al.,2009;Kidwell et al.,2017).In contrast,the trend in the CETP SST is questionable,as either a warming or cooling trend has been proposed(Cane et al.,1997;Liu and Huang,2000).Moreover,our previous work shows that the relationship between the midsummer temperature in South China and the WTP SSTA experienced an obvious interdecadal enhancement in the early 1990s(Chen et al.,2018a).It follows that more attention should be paid to the effect of the positive-phase WTP SSTA,which might have enhanced and contributed to the more frequent occurrence of extreme hot midsummers in CSC recently.One the other hand,the current study mainly focuses on the seasonal mean background favorable for hot midsummers in CSC,but the sub-seasonal variation of the East Asian midsummer monsoon is also prominent(e.g.,Zhou and Chan,2005;Ren et al.,2013).Previous studies indicate that extreme heat over CSC is significantly influenced by intraseasonal oscillations(Chen et al.,2016;Gao et al.,2018;Chen et al.,2018b).The specific role played by intraseasonal oscillations in the hot midsummer of 2017 needs further investigation.

Acknowledgements.We thank the two reviewers for their helpful comments and suggestions.This work was jointly supported by National Key R&D Program of China(Grant No.2016YFA0600601),the National Natural Science Foundation of China(Grant Nos.41605027,41721004,41530530 and 41731173),the Leading Talents of Guangdong Province Program,the Pioneer Hundred Talents Program of the Chinese Academy of Sciences,and the Fundamental Research Funds for the Central Universities.