The impacts of the East Asian subtropical westerly jet on weather extremes over China in early and late summer

Ying Zhou,Jiacan Yuan,Zhiping Wen,Sihua Huang,Xiaodan Chen,Yuanyuan Guo,Qiyan Lin

Department of Atmospheric & Oceanic Sciences and Institute of Atmospheric Sciences, Fudan University, Shanghai, China

Keywords:East Asian subtropical westerly jet Early and late summer Precipitation extremes Humid heat extremes

ABSTRACT Summer weather extremes (e.g.,heavy rainfall,heat waves) in China have been linked to anomalies of summer monsoon circulations.The East Asian subtropical westerly jet (EASWJ),an important component of the summer monsoon circulations,was investigated to elucidate the dynamical linkages between its intraseasonal variations and local weather extremes.Based on EOF analysis,the dominant mode of the EASWJ in early summer is characterized by anomalous westerlies centered over North China and anomalous easterlies centered over the south of Japan.This mode is conducive to the occurrence of precipitation extremes over Central and North China and humid heat extremes over most areas of China except Northwest and Northeast China.The centers of the dominant mode of the EASWJ in late summer extend more to the west and north than in early summer,and induce anomalous weather extremes in the corresponding areas.The dominant mode of the EASWJ in late summer is characterized by anomalous westerlies centered over the south of Lake Baikal and anomalous easterlies centered over Central China,which is favorable for the occurrence of precipitation extremes over northern and southern China and humid heat extremes over most areas of China except parts of southern China and northern Xinjiang Province.The variability of the EASWJ can influence precipitation and humid heat extremes by driving anomalous vertical motion and water vapor transport over the corresponding areas in early and late summer.

1.Introduction

Weather extremes,such as heavy rainfall and heat waves,have recently begun to attract broad attention because of the catastrophic impacts they can have on societies,economies,and ecology.Certain atmospheric circulations,such as the westerly jet stream and its interaction with cyclones and anticyclones,have been suggested to cause local variability in weather extremes (Yuan et al.,2015,2018).As an important component of the summer monsoon circulations,the East Asian subtropical westerly jet (EASWJ) is closely related to regional weather in China(Kuang and Zhang,2006).Its intraseasonal variability in summer may have a substantial impact on weather extremes,but its characteristics and underlying mechanisms have not been closely investigated.

The EASWJ locates in the upper troposphere over subtropical East Asia and the western Pacific.Previous studies have investigated the interannual variability of the EASWJ (Lin and Lu,2005),while some recent studies have paid attention to the intraseasonal variability of the EASWJ (Chu and Yao,2020).Chu and Yao (2020) suggested the variabilities of the EASWJ in summer are mainly characterized by northto-south and east-to-west oscillation.While most studies have investigated the variability of the EASWJ based on the three summer months(i.e.,June–July–August),Lin and Lu (2005) noticed that the anomalous zonal wind associated with the EASWJ index at 200 hPa can only be observed in July and August,rather than in June.In addition,certain systems that are related to the EASWJ (e.g.,the South Asian high and the western Pacific Subtropical high (WPSH)) present different characteristics between early and late summer at the intraseasonal time scale(Ge and Ren,2019 ;Guan et al.,2019).However,it remains unclear as to whether the EASWJ exhibits different characteristics between early and late summer.

Closely related to the East Asian summer monsoon,the EASWJ can influence the local climate and weather.Many studies have suggested that the ascending motion on the southern side of the EASWJ corresponds to the divergence and negative vorticity there,and a meridional shift in the EASWJ will change the location of vertical motion,which further influences the local weather and climate (Kuang and Zhang,2006 ;Chu and Yao,2020).Wang et al.(2016) indicated that a southward displacement of the EASWJ induces anticyclonic anomalies and meridional circulation anomalies to the south,which lead to extreme high temperatures in the region south of the Yangtze River.Kuang and Zhang (2006) found that the location of the WPSH shifted north and an anomalous anticyclonic circulation appeared in the lower troposphere when the jet was situated abnormally north.The circulation anomalies caused by a meridional shift in the EASWJ can influence the vertically integrated water vapor flux related to the moisture supply over mainland China (Xuan et al.,2013).An anomalous southward (northward) displacement of the EASWJ can induce increased precipitation over the Yangtze River Basin (North China) (Kuang and Zhang,2006).Given the close relationship between the EASWJ and precipitation,the impacts of the variability of the EASWJ on precipitation extremes at the intraseasonal time scale in early and late summer are still unclear.

Beside precipitation extremes,heat waves are another kind of extreme weather that can have substantial socioeconomic impacts.However,most previous studies have measured heat waves based solely on the high temperatures involved,without consideration of the fact that human thermal comfort is affected not only by temperature but also by other ambient variables,among which humidity is an important contributor.Recently,increased attention has been paid to the humid heat extremes accounting for the combined effects of both high temperatures and high humidity on thermal comfort (Li et al.,2020).While many previous studies have focused on the impacts of circulation variabilities on high-temperature extremes,little attention has been paid to the influence on humid heat extremes.Accordingly,this study investigated the impacts of the EAWSJ on heat waves considering both humidity and temperature,as well as their potential mechanisms.

In this paper,we try to address two questions: (1) What are the characteristics of the intraseasonal variability of the EASWJ in early summer and late summer? (2) At the intraseasonal time scale,what is the connection between the EASWJ and weather extremes,and what are the potential mechanisms involved? Following this introduction,Section 2 describes the data and methods.The basic features of the EASWJ at the intraseasonal time scale and its influence on the variability of weather extremes are reported in Section 3.Conclusions and some further discussion are given in Section 4.

2.Data and methods

2.1. Data

This study used daily (0000 UTC) data from ERA5 (Hersbach and Dee,2016) for the summer season (May–August (MJJA)) from 1979 to 2018.May and June were defined as early summer,and July and August as late summer.The horizontal resolution of the data was 1°×1°in longitude and latitude.Variables from the daily data included the zonal wind,meridional wind,surface pressure,2 m temperature (T2m),specific humidity,vertical velocity,and vorticity.The daily data of precipitation were obtained from the Climate Prediction Center Global Unified Precipitation dataset provided by NOAA/OAR/ESRL PSL (Chen et al.,2008),with a horizontal resolution of 0.5° × 0.5°.To obtain the intraseasonal component,the anomalies for daily variables at each grid point were preprocessed by removing the seasonal cycle from the data.The seasonal cycle was defined as the average for each calendar day in MJJA throughout the 40 years.Therefore,the seasonal cycle was composed of 123 averages corresponding to 123 days in MJJA.

2.2. Methods

To obtain the intraseasonal variability of the EASWJ,empirical orthogonal function (EOF) analysis was applied to the zonal wind anomalies at 200 hPa over East Asia (20°–60°N,80°–140°E) during 1979–2018 for early and late summer,respectively.The EOF analysis was insensitive to area selection (Fig.S1).Before the EOF analysis,the data were area-weighted by the square root of the cosine of latitude.The Monte Carlo test was used to test the significance.

2.2.1.Calculationofwetbulbglobetemperature

Wet bulb globe temperature (WBGT) is a commonly used metric to measure the intensity of heat stress considering the combined effect of both temperature and humidity (Li et al.,2020).Following Li et al.(2020),we used a simplified WBGT to measure the humid heat:

whereTais the dry air temperature andTwis the wet bulb temperature–the lowest temperature to which an air parcel can be cooled by evaporating water into the air at a constant pressure level.The dry air temperature is the 2 m temperature and the wet bulb temperature is calculated by the 2 m temperature,relative humidity,and surface pressure.More details on the calculation of wet bulb temperature can be found in the supporting information of Li et al.(2020).

2.2.2.Occurrenceprobabilityofextremes

An extreme day was defined as a day when the daily value of a specific variable exceeded the 95th percentile of the historical distribution during 1979–2018.EASWJ extreme positive (negative) days were defined as days when the PC1 for EOF1 was greater (smaller) than or equal to 1.0 (-1.0).The results were not sensitive to this threshold (Figs.S2 and S3).Because the extreme negative days displayed a similar pattern as the extreme positive days,except for the opposite sign,we focused on extreme positive days in this study.Inactive days were defined as days when the PC1 was smaller than 1.0 but greater than 0.The occurrence probability of extreme days during extreme positive or inactive days of the EASWJ was calculated as follows:

where the subscript “k” denotes a positive or inactive phase of the EASWJ,nk(e(i,j)) indicates the amount of extremes during the EASWJ extreme positive or inactive days,andnk(i,j) indicates the number of EASWJ extreme positive or inactive days.And the percentage change of the occurrence probability was calculated as follows:

3.Results

3.1. Dominant modes of the EASWJ in early and late summer

The EOF1s of daily zonal wind over East Asia in early and late summer both feature a meridional dipole pattern with anomalous easterlies and westerlies,indicating a meridional oscillation of the EASWJ.However,the pattern of the dipole in early summer is quite different from that in late summer (Fig.1 (a,b)).Specifically,the anomalous westerly is centered over North China and the anomalous easterly over the south of Japan in early summer,while the anomalous westerly is centered over the south of Lake Baikal and the anomalous easterly over Central China in late summer.Compared with early summer,the locations of the anomaly centers in late summer extend more to the west and north.The zero line of the wind anomalies is around 36°N in early summer,while it is around 42°N in late summer.In addition,the pattern of the anomaly centers in early summer is more southwest–northeast tilted than in late summer.Consistent with the zonal wind anomalies,the 200-hPa circulation anomalies related to EOF1 present an anomalous anticyclone centering around Bohai Bay in early summer,and centering around Inner Mongolia in late summer (Fig.1 (c,d)).The circulation related to the EASWJ at 850 hPa is different to that at 200 hPa.In particular,anomalous southwesterlies are observed around southern Northeast,central and southern China in early summer (Fig.1 (e)),while there are anomalous southerlies around central and northern China and anomalous easterlies around southern China (Fig.1 (f)).

Fig.1.Spatial distribution of (a) the first EOF mode of zonal wind in May–June (MJ;shading) and (b) the first EOF mode of zonal wind in July–August (JA;shading).The thicker black solid lines indicate the zero line,and the white symbols mark the maximum or minimum values of the mode.The seasonal cycle of zonal wind was removed before the EOF analysis.The explained variance is shown in the top-right of panels (a) and (b).(c,d) Regression of 200-hPa wind fields onto the PC1 of(c) MJ and (d) JA.(e,f) Regression of 850-hPa wind fields onto the PC1 of (e) MJ and (f) JA.Only the wind anomalies that are statistically significant at the 90%confidence level are shown.

Fig.2.The percentage change in the occurrence probability (units: %) of (a,b) humid heat extremes and (c,d) precipitation extremes between extreme positive days and inactive days during (a,c) May–June (MJ) and (b,d) July–August (JA).The dots indicate the regions where the anomalies are statistically significant at the 90% confidence level.

3.2. Relationships between the EASWJ and extreme weather in early and late summer

To investigate whether the variation in the EASWJ is linked to the different patterns of weather extremes between early and late summer,the occurrence probabilities of precipitation and humid heat extremes are compared between the extreme positive and inactive EASWJ days(Fig.2).For early summer,the percentage change in the occurrence probability of extreme humid heat is dominated by a dipole pattern with negative anomalies over Northeast and Northwest China and positive anomalies over central and southern China (Fig.2 (a)),where the occurrence probability of humid heat extremes on extreme positive days is over 75% more than that on inactive days.The percentage change in the occurrence probability of extreme precipitation is dominated by a dipole pattern with negative anomalies over South China and positive anomalies over Central and North China,where the occurrence probability of precipitation extremes on extreme positive days is over 50% more than on inactive days (Fig.2 (c)).For late summer,the occurrence probability of extreme humid heat on positive days is over 75% more than that on inactive days over most areas of China,except for parts of southern China and northern Xinjiang Province (Fig.2 (b)).The percentage change in the occurrence probability of extreme precipitation is dominated by a tripole pattern with negative anomalies over central China and positive anomalies over northern and southern China (Fig.2 (d)).Over these two regions,the occurrence probability on extreme positive days is over 50% more than on inactive days.In addition,the maximum percentage change in extreme precipitation (humid heat) exceeds 100% (300%).These results indicate significant relationships between the EASWJ and precipitation/humid heat extremes.

Fig.3.Composites of the (a,b) WBGT anomalies (shading;units: K) and (c,d) precipitation anomalies (shading;units: mm) for the extreme positive days: (a,c)May–June (MJ);(b,d) July–August (JA).The dots indicate the regions where the anomalies are statistically significant at the 90% confidence level.

3.3. Potential mechanism by which the EASWJ influences extremes

To investigate whether and how the EASWJ influences extremes,we further examine composites of the precipitation and WBGT anomalies in early and late summer based on the extreme positive days (Fig.3).The patterns of anomalies in Fig.3 are similar to the corresponding patterns in Fig.2 except for some slight differences.For WBGT,the occurrence probability decreases without passing the statistical significance test over eastern Inner Mongolia in early summer and over northern Heilongjiang Province in late summer (Fig.2 (a,b)),where the composite anomalies are negative but statistically significant (Fig.3 (a,b)).For precipitation,the occurrence probability in early summer decreases in central Northeast China (Fig.2 (c)),where the composite anomalies are weakly positive (Fig.3 (c)).The occurrence probability in late summer increases in western Xinjiang Province (Fig.2 (d)),where the composite anomalies are weakly positive but statistically significant (Fig.3 (d)).In summary,at the intraseasonal time scale,a shift in the EASWJ may induce a shift in the precipitation and WBGT anomalies,which further induce different patterns of the occurrence probability of extremes from early to late summer.

WBGT is a combination of temperature and humidity,and precipitation is usually influenced by anomalous ascending motion and abundant water vapor.To detect the possible mechanisms by which the intraseasonal variability of the EASWJ influences the precipitation and WBGT anomalies,the results of a composite analysis of multiple factors related to the EASWJ based on the extreme positive EASWJ days are shown in Fig.4.For early summer,the anomalies of T2m generally resemble the anomalies of WBGT (Fig.4 (a)),except in North China where the T2m anomalies are negative but the WBGT anomalies are positive.The significantly positive anomalies of specific humidity may account for the positive WBGT anomalies over North China (Fig.4 (c)),while the significantly positive T2m may be the dominant contributor to the positive WBGT anomalies over South China where the specific humidity is diminished.As the surface temperature is connected with the vertical motion (Chen and Lu,2015),the positive (negative) anomalies of T2m over South (North) China may be attributable to anomalous descending motion via adiabatic warming and enhanced solar radiation (via anomalous ascending motion through inducing solar radiation)(Fig.4 (e)).The positive anomalies of specific humidity over Central and North China can be explained by the moisture fluxes as well as their convergence (Fig.4 (g)).For late summer,the anomalies of T2m resemble the anomalies of WBGT well (Fig.4 (b)).The specific humidity is anomalously positive over northern China but anomalously negative over the Yangtze River Basin (Fig.4 (d)).This indicates that high temperature is the main contributor to the positive anomalies of WBGT over the Yangtze River Basin,where anomalous descending motion associated with the EASWJ is observed (Fig.4 (f)).Over northern China,the positive anomalous humidity is related to increased water vapor flux from the south and enhanced water vapor flux convergence related to the EASWJ.

Fig.4.Composites of the (a,b) T2m anomalies (shading;units: K),(c,d) vertically integrated specific humidity anomalies from 1000 hPa to 200 hPa (shading;units: 105 g m-2),(e,f) vertical velocity anomalies (shading;units: Pa s-1) at 500 hPa and relative vorticity anomalies at 200 hPa (contours;units: s-1),and (g,h)vertically integrated water vapor flux anomalies (vectors;units: 105 g (m s)-1) and water vapor flux divergence anomalies from 1000 hPa to 200 hPa (shading;units:g (m2 s)-1) for extreme positive days.The water vapor flux divergence anomalies have been taken the opposite sign.The left-hand column is for May–June (MJ),and the right-hand column for July–August (JA).The dots indicate the regions where the anomalies are statistically significant at the 90% confidence level.Only the water vapor flux anomalies and relative vorticity anomalies that are statistically significant at the 90% confidence level are shown.

For precipitation,the anomalies of vertical motion and water vapor flux divergence are generally consistent with the pattern of precipitation anomalies (Fig.4 (e–h)).The anomalies of ascending (descending) motion and anomalous water vapor flux convergence (divergence) appear over Central and North China (South China) in early summer.Meanwhile,the anomalies of ascending (descending) motion and anomalous water vapor flux convergence (divergence) appear over northern and southern China (central China) in late summer.The ascending (descending) anomalies,which can lead to positive (negative) precipitation anomalies,are observed over the southern side of the anomalous westerlies (easterlies),corresponding to the negative (positive) vorticity anomalies at 200 hPa (Fig.4 (e,f)).Meanwhile,the anomalies of southerly moisture flux associated with the EASWJ indicate increased water vapor being transported to Central and North China in early summer,and to northern China in late summer,further leading to the convergence of water vapor over that region (Fig.4 (g,h)).As the moisture is generally concentrated in the low-level troposphere,the southerly moisture transport may largely be due to the low-level southerly wind anomalies associated with the EASWJ.

In summary,the westerly (easterly) anomalies of the EASWJ correspond to negative (positive) vorticity on the southern side,which leads to the anomalous ascending (descending) motion over the corresponding area.Furthermore,the anomalies of low-level southerlies associated to the EASWJ may enhance the water vapor transportation and lead to a convergence of moisture under the upper-level anticyclone.These physical processes are consistent with previous studies(Kuang and Zhang,2006 ;Chu and Yao,2020).The combination of the anomalous vertical motion and additional water vapor related to the EASWJ provide favorable conditions for positive anomalies of precipitation and WBGT in early and late summer.

4.Summary and discussion

This study investigated the different characteristics of the EASWJ variability and their effects on precipitation/humid heat extremes between early and late summer at the intraseasonal time scale.The EOF1 of early summer is characterized by anomalous westerlies centered over North China and anomalous easterlies centered over the south of Japan.The EOF1 of late summer is characterized by anomalous westerlies centered over the south of Lake Baikal and anomalous easterlies centered over Central China.The variability of the EASWJ in either early or late summer corresponds to an anomalous anticyclone,the center of which extends more to the west and north in late summer than in early summer.

The differences in the variability of the EASWJ between early and late summer lead to different patterns in weather extremes.The dominant mode in early summer is related to an increase in the occurrence probability of extreme precipitation over Central and North China,and extreme humid heat over most areas of China except Northwest and Northeast China.Meanwhile,the dominant mode in late summer is associated with an increase in the occurrence probability of precipitation extremes over northern and southern China,and an increase in the occurrence probability of humid heat extremes over most areas of China except parts of southern China and northern Xinjiang Province.In general,compared with early summer,the location of increased weather extremes is more to the west and north in late summer.The anomalies of precipitation (WBGT) resemble the occurrence probability of extreme precipitation (humid heat) both in early and late summer.This indicates that the EASWJ may influence the local anomalies of precipitation and WBGT that provide favorable conditions for an increase in the probability of precipitation and humid heat extremes over the corresponding areas.

The potential mechanisms through which the variability of the EASWJ influences weather extremes were also examined.It was found that negative (positive) vorticity anomalies locate around the southern side of the westerly (easterly) anomalies of the EASWJ,corresponding to anomalous ascending (descending) motion over Central and North China (South China) in early summer and northern China (central China) in late summer.The ascending motion contributes to the positive precipitation anomalies over Central and North China in early summer,and over southern and northern China in late summer.The descending motion contributes to positive T2m anomalies over South China in early summer,and over central China in late summer.In addition,the anomalous anticyclone of water vapor flux,corresponding to the anomalous anticyclone at upper levels,can also influence the precipitation and WBGT anomalies.In particular,the southerly wind anomalies from the anticyclone can transport more water vapor to the Central and North China in early summer and northern China in late summer.This water transportation can lead to an increase in specific humidity and water vapor convergence,which is conducive to positive anomalies of precipitation and WBGT.However,there are still some physical processes by which the EASWJ influences weather extremes that are not very clear.For instance,as a major contributor of the positive WBGT anomalies over North China in late summer,positive anomalies of T2m are consistent with ascending motion over that region.Generally speaking,ascending motion is related to an increase in cloud and a decrease in shortwave radiation.The decreasing shortwave radiation can cool the surface,but the cloud cover can warm the surface through downward infrared radiation (Lee et al.,2012).Further studies are needed to elucidate the other factors and physical processes involved in the influence of the EASWJ on weather extremes.

Funding

This research was supported by the National Natural Science Foundation of China [grant numbers 42175066,41875087,42030601,and 42105017 ],the Shanghai Municipal Natural Science Fund [grant number 20ZR1407400 ],and the Shanghai Pujiang Program [grant number 20PJ1401600].

Acknowledgments

We would like to thank the European center for Medium-Range Weather Forecasts for the reanalysis data,and the Climate Prediction Center for the precipitation data.

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi: 10.1016/j.aosl.2022.100212.