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    Climatological occurrence frequency of summertime jet core at 300 hPa during 1960–2010. Shaded regions denote where numbers are more than 30. The blue dashed boxes indicate the active regions of the EAPJ and EASJ. The black bold solid line indicates the boundary of the Tibetan Plateau.

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    Normalized time series of the (a) EAPJ and EASJ intensity, (b) the NDI, and (c) the correlation coefficient between the NDI and summer rainfall over eastern China during 1960–2010. Regions over the 95% significant level of the t test are shaded in (c).

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    (left) Composite winds and (right) wind anomalies at 300 hPa in the (a),(b) positive and (c),(d) negative configuration (unit: m s−1). The black dashed boxes in (b) and (d) indicate the active regions of the EAPJ and EASJ.

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    Composite summer rainfall anomalies over eastern China in the (a) positive configuration and (b) negative configuration (unit: mm day−1). Regions over the 95% significant level of the t test are shaded.

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    Composite vertically integrated water vapor flux anomalies from (a),(b) 1000 to 300 hPa (unit: kg m−1 s−1) and latitude–height cross sections averaged over 110°–120°E of the (c),(d) vertical winds (vectors, unit: m s−1) and specific humidity anomalies (contours, unit: 10−3 g kg−1) in the (left) positive and (right) negative configuration. Regions over the 95% significant level of the t test are shaded in (a) and (b).

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    Composite SST anomalies in the (a) previous winter, (b) current spring, and (c) current summer in the positive configuration (unit: °C). Regions over the 95% significant level of the t test are shaded.

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    (a) Normalized time series of SSTI in 1960–2010, (b) the regression of summer winds at 300 hPa (unit: m s−1), and (c) the latitude–height cross sections averaged over 70°–110°E of the regression of summer vertical motion (unit: m s−1, vector) and meridional temperature gradient anomalies (unit:10−5, contour) with respect to the normalized SSTI for the period of 1960–2010. Regions over the 95% significant level of the t test are shaded in (b). The black boxes indicate the active region of the EAPJ and EASJ in (b). The red shaded area indicates the active regions of (left) EASJ and (right) EAPJ in (c). The black shaded area indicates the terrain.

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    Regressions of summer geopotential height (contour, unit: gpm) and winds at 50 hPa (vector, unit: m s−1) with respect to normalized wintertime negative EMI for the period of 1960–2010. Regions over the 95% significant level of the t test are shaded.

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    (a) Composite summer Eady growth rate anomalies between 700 and 850 hPa (unit: day−1), (b) the transient eddy kinetic energy anomalies (unit: m2 s−2), (c) the divergence of E–P vector anomalies (unit: 10−5 m s−2), and (d) the conversion anomalies between time-mean kinetic energy and transient eddy kinetic energy (unit: 10−5 m2 s−3) in the positive configuration. Regions over the 95% significant level of the t test are shaded.

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    (left) Composite SST anomalies (unit: °C) and (right) OLR anomalies (unit: W m−2) in the negative configuration in the (a),(b) previous winter; (c),(d) current spring; and (e),(f) current summer. Regions over the 95% significant level of the t test are shaded.

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    (a) The vertical shear (300 hPa minus 850 hPa) of climatological summer winds during the period of 1960–2010 and (b) the composite vertical shear anomalies in the negative configuration (unit: m s−1). Regions over the 95% significant level of the t test are shaded in (b).

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    (a) Regressions of summer geopotential height (unit: gpm) at 1000 hPa and (b) latitude–height cross sections averaged over 60°–80°E of summer vertical motion (vector, unit: m s−1) with respect to normalized summer negative Niño-3.4 index for the period of 1960–2010. Regions over the 95% significant level of the t test are shaded in (a). The black shaded regions indicate the terrain in (b).

  • View in gallery

    As in Fig. 9, but for negative configuration.

  • View in gallery

    (a) Composite wind anomalies at 300 hPa (unit: m s−1) and (b) the summer rainfall anomalies (mm day−1) over eastern China in the neutral configuration. Regions over the 95% significant level of the t test are shaded in (b).

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The Different Configurations of the East Asian Polar Front Jet and Subtropical Jet and the Associated Rainfall Anomalies over Eastern China in Summer

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  • 1 School of Atmospheric Sciences, Nanjing University, Nanjing, China
  • | 2 State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, China
  • | 3 School of Atmospheric Sciences, Nanjing University, Nanjing, China
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Abstract

To investigate the concurrent impacts of the East Asian polar front jet (EAPJ) and subtropical jet (EASJ) on the summer rainfall over eastern China, positive (strengthened EAPJ with weakened EASJ) and negative (weakened EAPJ with strengthened EASJ) configurations are identified. In the positive configuration, rainfall decreases in the northern part of eastern China and increases in the southern part, vice versa in the negative configuration. The possible mechanisms maintaining the two jet configurations are further proposed from the perspectives of sea surface temperature (SST) and synoptic-scale transient eddy activities (STEA). In the positive configuration, meridional distributed cold–warm SST anomalies over the eastern North Pacific may induce regional circulation and meridional temperature gradient anomalies, which can strengthen the EAPJ and weaken the EASJ. The central Pacific La Niña–like SST anomalies are related with the Arctic vortexlike anomalies in the stratosphere, which may strengthen the EAPJ. Furthermore, the divergence of Eliassen–Palm vectors and the conversion from eddy kinetic energy to mean kinetic energy over the active region of the EAPJ may strengthen the EAPJ, vice versa for the weakened EASJ. In the negative configuration, associated with the warm SST anomalies over the western North Pacific, the enhanced convective activities may lead to a strengthened EASJ via meridional teleconnection. The teleconnection may be intensified by the strengthened easterly vertical shear. Additionally, eastern Pacific La Niña–like SST anomalies may intensify the Walker circulation, which may strengthen the EASJ via the Hadley circulation. The STEA-related anomalies are almost opposite those in the positive configuration, especially for the weakened EAPJ.

Corresponding author address: Dr. Dan-Qing Huang, School of Atmospheric Sciences, Nanjing University, Xianling Dadao No. 163, Nanjing 210023, China. E-mail: huangdq@nju.edu.cn

Abstract

To investigate the concurrent impacts of the East Asian polar front jet (EAPJ) and subtropical jet (EASJ) on the summer rainfall over eastern China, positive (strengthened EAPJ with weakened EASJ) and negative (weakened EAPJ with strengthened EASJ) configurations are identified. In the positive configuration, rainfall decreases in the northern part of eastern China and increases in the southern part, vice versa in the negative configuration. The possible mechanisms maintaining the two jet configurations are further proposed from the perspectives of sea surface temperature (SST) and synoptic-scale transient eddy activities (STEA). In the positive configuration, meridional distributed cold–warm SST anomalies over the eastern North Pacific may induce regional circulation and meridional temperature gradient anomalies, which can strengthen the EAPJ and weaken the EASJ. The central Pacific La Niña–like SST anomalies are related with the Arctic vortexlike anomalies in the stratosphere, which may strengthen the EAPJ. Furthermore, the divergence of Eliassen–Palm vectors and the conversion from eddy kinetic energy to mean kinetic energy over the active region of the EAPJ may strengthen the EAPJ, vice versa for the weakened EASJ. In the negative configuration, associated with the warm SST anomalies over the western North Pacific, the enhanced convective activities may lead to a strengthened EASJ via meridional teleconnection. The teleconnection may be intensified by the strengthened easterly vertical shear. Additionally, eastern Pacific La Niña–like SST anomalies may intensify the Walker circulation, which may strengthen the EASJ via the Hadley circulation. The STEA-related anomalies are almost opposite those in the positive configuration, especially for the weakened EAPJ.

Corresponding author address: Dr. Dan-Qing Huang, School of Atmospheric Sciences, Nanjing University, Xianling Dadao No. 163, Nanjing 210023, China. E-mail: huangdq@nju.edu.cn

1. Introduction

The East Asian summer monsoon is one of the most important components of the earth’s climate system, which significantly affects the weather and climate in East Asia (Wang et al. 2001), particularly in China (Ding et al. 2008). In recent years, severe floods and major droughts in summer have caused great damage to eastern China due to extremely anomalous rainfall, especially in regions without sufficient adaptation strategy (Adger et al. 2007). For example, northeastern China experienced a severe flooding in the summer of 2012 (Zhou et al. 2013), while the Yangtze–Huaihe basin suffered from a major drought at the same time. The flood in northeastern China in summer 2013 (Climate Center News on the extreme events monitoring, available online at http://cmdp.ncc.cma.gov.cn/Monitoring/cn_china_extreme.php) caused great economic and social losses. These destructive events are likely to become more frequent in a warmer climate of the future (Trenberth et al. 2003; Kharin et al. 2013). Understanding the spatial and temporal variation of summer precipitation in eastern China and its possible factors will have great benefits to the society and economy.

Previous studies have suggested a strong linkage between the summer rainfall in eastern China and the two extratropical atmospheric jets over East Asia: the East Asian subtropical jet (EASJ) and the East Asian polar front jet (EAPJ). The EASJ is part of the global subtropical jet that forms along the poleward side of the Hadley cell (Held and Hou 1980), and the EAPJ is part of the global polar front jet (also named as eddy-driven jet), which forms as a result of eddy momentum flux convergence by atmospheric waves that develop in regions of enhanced baroclinicity (Panetta and Held 1988; Panetta 1993; Lee 1997). In summer, the EASJ and EAPJ are zonally located over mid- to high-latitude regions north to the Tibetan Plateau (e.g., Sheng 1986; Zou et al. 1990; Schiemann et al. 2009; Hudson 2012). Although Holton et al. (2002) mentioned that both jets may play a critical role in the development of extratropical cyclones and the organization of mesoscale convective systems, most studies in China focus on the impact of the EASJ location and intensity on the summer rainfall (e.g., Liang and Wang 1998; Dong et al. 2009; Du et al. 2009; Dong et al. 2010; Lu et al. 2013). Liao et al. (2004) indicated that the meridional shift of the EASJ has great impact on the formation of summer floods and droughts in eastern China. The southward shift of the EASJ favored the enhanced precipitation over the Yangtze–Huaihe River (Kuang and Zhang 2006; Ma et al. 2011; Xuan et al. 2011), whereas the northward shift of the EASJ may increase the rainfall in the northeastern China (Shen et al. 2011). Moreover, the intensification of the EASJ can lead to decreased precipitation over the Yangtze River valley, which is also one major reason for the severe drought over that area in the summer of 2003 (Lin and Lu 2004). On the contrast, little attention has been paid to the EAPJ in China. The EAPJ behaves like a boundary separating cold polar air to the north from warm subtropical air to the south. Because of the cold air mass associated with the EAPJ, Zhang et al. (2008) indicated that the change of the EAPJ intensity might be used as a precursor for the prediction of the EASM onset and the beginning of mei-yu.

Several factors have been contributed to the variability of the summertime EASJ and EAPJ; for example, sea surface temperature (SST) and synoptic-scale transient eddy activities (STEA). The SST variation, particularly those over the eastern Pacific (Zhang et al. 1996; Dong et al. 1999; Wang et al. 2000; Lin and Lu 2009) and the Indian Ocean (Xie et al. 2009; Qu and Huang 2012), affects the EASJ variation via the convective activities over the tropical oceans (Lu 2005). Significant relationship between tropical SST anomalies in the previous winter–spring and the EASJ location in current summer is revealed. The eastern tropical Pacific warming persists into the following summer, and results in the southward shift of the EASJ based on the thermal wind balance (Lin and Lu 2009). The relationship also exists between the SST anomalies in simultaneous summer and EASJ variation (Lin 2010), but with different mechanism. In the current summer, a meridional teleconnection exists between the western North Pacific (WNP) and East Asia, and affects the EASJ variations (Nitta 1987; Tsuyuki and Kurihara 1989; Huang and Sun 1992). Recently, Qu and Huang (2012) demonstrated that warming (cooling) over the tropical Indian Ocean SST forced a Kelvin wave wedge, which induced the Pacific–Japan/East Asian Pacific teleconnection, and resulted in the southward (northward) shift of the EASJ.

In addition to the SST variation, some studies have proposed that the interaction between the transient eddy activities (STEA) and the mean flow can reinforce zonal wind anomaly (Ren and Zhang 2007; Xiang and Yang 2012) via the convergence of eddy heat and momentum fluxes. The eddy development is supported by energy transferred from available potential energy to the atmospheric time-mean flow (Ren et al. 2008). In the study of the relationship between the EASJ and the STEA over the North Pacific, Ren et al. (2008) indicated that the northward (southward) shift of STEA was favorable for the northward (southward) shift of the EASJ. They further found that the occurrence of the EASJ and EAPJ along the northern flank of the Tibetan Plateau was often accompanied by a weakened STEA over the mid- to high latitudes of East Asia in the warmer seasons.

Thus far, previous studies mainly focused on the climatic effects and their maintaining mechanisms for the EASJ and the EAPJ, individually. Actually, on the synoptic scale, concurrent impacts of the two jets on the surface cyclogenesis have been revealed, mostly focusing on cold seasons (e.g., Uccellini and Kocin 1987; Prezerakos 1990; Hakim and Uccellini 1992; Thorncroft and Flocas 1997; Kaplan et al. 1998; Prezerakos et al. 2006). Recently, Liao and Zhang (2013) indicated that the concurrent variation of the EAPJ and EASJ acted as an important bridge that links the snow storm with anomalous cold and warm airmass activities over central and southern China. All these case studies show an apparent clue that the concurrent impacts of the two jets do exist. Since the EASJ and EAPJ still exist in summer, the concurrent impacts of the two jets may result in different weather or climate, compared to those affected by any single jet in summer. Consequently, in climatology, their concurrent impacts will be an important and specific issue to understand the summer rainfall variability over eastern China. This raises two questions in this study:

  1. What are the configurations of the EAPJ and EASJ and how do they affect summer rainfall anomalies in eastern China?
  2. What are the possible mechanisms maintaining the different configurations of the two jets?

This paper is organized as follows. Data and methods are described in section 2. Section 3 presents the configurations of the EAPJ and EASJ in summer and the associated summer rainfall anomalies over eastern China. The possible mechanisms maintaining the different configurations of the two jets are investigated in section 4. The conclusions and discussion are provided in section 5.

2 Data and analysis methods description

a. Data and statistics methods

The datasets used in this study include the following products that cover the period of 1960–2010:

  1. Daily precipitation in mainland China observed at 765 stations. The dataset is provided by the China Meteorological Administration (http://ncc.cma.gov.cn).
  2. The global SST dataset from the Hadley Centre Sea Ice and SST dataset (HadISST) with a spatial resolution of 1° × 1° (Rayner et al. 2003).
  3. The atmospheric variables (including sea level pressure, specific humidity, zonal winds, meridional winds, vertical winds, and geopotential height) extracted from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis Project (NNRP) with a horizontal resolution of 2.5° × 2.5° and a vertical resolution of 17 levels (Kalnay et al. 1996).
  4. The outgoing longwave radiation (OLR) dataset from National Oceanic and Atmospheric Administration (NOAA) satellite series with a horizontal resolution of 2.5° × 2.5° (Liebmann and Smith 1996). The time period of this dataset is from 1974 to 2010.
The composite, regression, and correlation analyses are used in this study. All the composite anomalies are compared to the climatology in 1960–2010. Following the work of Jiang and Perrie (2007) and Lu and Lu (2014), the t test is used for significance tests (Huang 2000).

b. Definition of jet active region

In this study, a jet core is identified if 1) the horizontal wind speed at 300 hPa is higher than 30 m s−1, which has been computed for each day and at each grid point; and 2) the wind speed at the central point is larger than that at its eight surrounding points (Ren et al. 2011).

To determine the active regions of the EASJ and EAPJ in summer (June–July–August), the jet core occurrence frequency is calculated, shown in Fig. 1. It is consistent with the result of Pena-Ortiz et al. (2013) who detects jet cores considering horizontal winds at different vertical levels from 400 to 100 hPa. Obviously, the EASJ occurs more frequently than the EAPJ. The dominant occurrence region of 57.5°–62.5°N, 70°–110°E (top blue dashed box) is chosen as the active region of the EAPJ. For the EASJ, there are two maximum jet core occurrence centers: the western center is located over the land along the northern flank of the Tibetan Plateau and the eastern center is located over East Sea off the coast of Japan. A close relationship between the western EASJ and the East Asian weather and climate has been revealed by many studies (e.g., Kuang and Zhang 2006; Ma et al. 2011; Xuan et al. 2011). Thus, in this study, we focus on the western branch of EASJ. Thus, region of 37.5°–42.5°N, 80°–100°E (bottom blue dashed box), where the western EASJ locates, is chosen as the active region of EASJ. The EAPJ and EASJ jet intensity is defined as averaged wind speed at 300 hPa over the active region of the EAPJ and EASJ, respectively.

Fig. 1.
Fig. 1.

Climatological occurrence frequency of summertime jet core at 300 hPa during 1960–2010. Shaded regions denote where numbers are more than 30. The blue dashed boxes indicate the active regions of the EAPJ and EASJ. The black bold solid line indicates the boundary of the Tibetan Plateau.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

c. Indices of two types of ENSO events

Recently, two types of El Niño–Southern Oscillation (ENSO) events have been recognized based on the location of peak SST anomalies: eastern Pacific (EP) ENSO (or traditional ENSO) (e.g., Ropelewski and Halpert 1987; Trenberth 1997) and central Pacific (CP) ENSO (e.g., Ashok et al. 2007; Kao and Yu 2009). The Niño-3.4 index is used to identify the traditional ENSO events (available online at http://www.cpc.noaa.gov/data/indices/). For the CP ENSO events, the El Niño Modoki index (EMI) is used (Ashok et al. 2007) in this study, which is defined as follows:
e1
The square bracket in Eq. (1) represents the SSTA averaged over a central region (A: 10°S–10°N, 165°E–140°W), an eastern region (B: 15°S–5°N, 110°–70°W), and a western region (C: 10°S–5°N, 125°–145°E), respectively. (The index is available online at http://www.jamstec.go.jp/frcgc/research/d1/iod/modoki_home.html.en.)

Since La Niña is the cold phase of ENSO, negative EMI and negative Niño-3.4 indices are used for the CP La Niña and EP La Niña, respectively. Here, “negative” means the reversed sign of the EMI or Niño-3.4 indices. Using the negative indices, it is more direct to understand the impact of the La Niña events on the East Asian jet streams by regression in sections 4a and 4b.

d. Definitions of the STEA-related variables

The intensity of STEA is usually expressed by the synoptic-scale transient eddy kinetic energy (Ke), which is calculated by
e2
where u and υ represent the zonal and meridional wind, the overbar represents the time-mean value in summer, and the primes denote perturbations within a period of 2.5–6 days according to the bandpass-filtered technique proposed by Murakami (1979).
Hoskins and Valdes (1990) pointed out that the STEA is largely attributed to the atmospheric baroclinicity, which can be measured by the maximum eddy growth rate (named as σ; Eady 1949) . The more σ, the larger Ke will be, indicating the intensified STEA. Here σ is defined as
e3
where f denotes the Coriolis parameter, N is the Brunt–Väisälä frequency, V is the horizontal wind velocity, and z is vertical height. Considering the fact that STEA is more sensitive to lower-level baroclinicity changes than to upper-level baroclinicity changes (e.g., Lunkeit et al. 1998), we calculate σ between 700 and 850 hPa in our study.
To reveal the STEA-induced mean flow changes, the Eliassen–Palm vector (E–P vector) (Hoskins et al. 1983) has been used, which is defined as
e4
The divergence of E–P vector corresponds to increase the mean westerly flow, vice versa.
Additionally, the mean flow intensity can be expressed by the time-mean kinetic energy (Km). The relationship between the STEA and the westerly can be illustrated by the conversion between the Km and Ke. The conversion is defined as
e5
The overbar and primes in Eqs. (4) and (5) have the same meanings as in Eq. (2). A positive C means the conversion from Km to Ke, corresponding to an intensified westerly mean flow, and vice versa (e.g., Lau 1979; Chang and Orlanski 1994).

3. The configurations of the summer EAPJ and EASJ and the associated rainfall anomalies over eastern China

The time series of normalized EAPJ and EASJ intensity are shown in Fig. 2a, both showing a significant interannual variability. To describe the configuration of the EAPJ and EASJ, the normalized difference between the normalized EAPJ and EASJ intensity is calculated as a new index, referred to as the normalized difference index (NDI). As shown in Fig. 2b, the NDI is dominated by interannual variation, and tends to flutuate more in recent years. To reveal the relationship between the jet configuration and summer rainfall over eastern China, the correlation between the NDI and rainfall is shown in Fig. 2c. It shows a good relationship with a positive (negative) correlation pattern over southern (northern) part of eastern China. This correlation pattern clearly suggests that different configurations of the two jets may lead to a south–north dipole pattern of summer rainfall anomalies over eastern China. Indeed, the same analysis is applied to reveal the relationship between the configuration of the EAPJ and the eastern EASJ (described in section 2b) and the summer rainfall. However, the correlation between the NDI and summer rainfall is not significant over eastern China (figure omitted).

Fig. 2.
Fig. 2.

Normalized time series of the (a) EAPJ and EASJ intensity, (b) the NDI, and (c) the correlation coefficient between the NDI and summer rainfall over eastern China during 1960–2010. Regions over the 95% significant level of the t test are shaded in (c).

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

To investigate how the configuration of the EASJ and EAPJ affect summer rainfall over eastern China, positive and negative configurations are selected based on the criterion of the NDI exceeding +1 and −1 standard deviations, respectively (Fig. 2b). Totally 19 typical years are selected for the two configurations as shown in Table 1. To examine the features of the two jet configurations, the composite wind speed at 300 hPa and their anomalies are shown in Fig. 3. In the left panels of Fig. 3, the strong westerly winds located along 40°N can be easily recognized as the EASJ in both configurations, while the intensity of the EAPJ is much weaker than that of the EASJ. It is obvious that the EASJ is much stronger in the negative configuration than that in the positive configuration, but it is not easy to distinguish the difference of the EAPJ intensity in climatology. Thus, the wind speed anomalies at 300 hPa in the two configurations are further analyzed. As shown in the right panels of Fig. 3, the EAPJ strengthens while the EASJ weakens in the positive configuration (Fig. 3b), and it is opposite in the negative configuration (Fig. 3d). The above results are further confirmed according to the intensity of EASJ and EAPJ in the two configurations (Table 2). Overall, the positive configuration indicates that the EAPJ strengthens while the EASJ weakens, and vice versa in the negative configuration.

Table 1.

Years in positive, negative, and neutral configurations in 1960–2010.

Table 1.
Fig. 3.
Fig. 3.

(left) Composite winds and (right) wind anomalies at 300 hPa in the (a),(b) positive and (c),(d) negative configuration (unit: m s−1). The black dashed boxes in (b) and (d) indicate the active regions of the EAPJ and EASJ.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

Table 2.

The mean intensity of the EAPJ and EASJ in the two configurations (unit: m s−1). (Numbers in parentheses are the intensity differences between each configuration and the climatologic mean of 1960–2010.)

Table 2.

To reveal the rainfall variation associated with the two jets configurations, the composite summer rainfall anomalies over eastern China are shown in Fig. 4. In the positive configuration (Fig. 4a), rainfall significantly decreases in the northern part of eastern China, while it increases in the southern part. In the negative configuration (Fig. 4b), rainfall anomalies are almost opposite to those in the positive configuration, except the southern regions near the coast. Two summer rainfall anomalies patterns are well known as dipole patterns, which are considered to be a dominant mode of rainfall pattern over eastern China and may significantly raise the possibility of floods or droughts (Ding et al. 2008). It is worth understanding the alternation of the dipole rainfall anomaly patterns in the two configurations. Consequently, we further investigate the regional atmospheric circulations and their linkage to different configurations of the EASJ and EAPJ.

Fig. 4.
Fig. 4.

Composite summer rainfall anomalies over eastern China in the (a) positive configuration and (b) negative configuration (unit: mm day−1). Regions over the 95% significant level of the t test are shaded.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

Water vapor content and vertical motion are two major factors directly affecting the summer rainfall over eastern China (e.g., Zhu et al. 2011). The anomalies of the vertically integrated water vapor flux from 1000 to 300 hPa (QVFLUX) in the two configurations are shown in Fig. 5. In the positive configuration (Fig. 5a), significant northeastern QVFLUX anomalies occur over eastern China, which may suppress the water vapor transport to northern China and reduce the rainfall. Meanwhile, a cyclone anomaly of QVFLUX may enhance precipitation in southern part of eastern China. In the negative configuration (Fig. 5b), significant southwestern QVFLUX anomalies, originated from the region of 25°–35°N, 130°–150°E, enhance water vapor transport to northern part of eastern China (40°–60°N, 120°–140°E). Meanwhile, northeastern anomalies dominate southern China, suppress water vapor transport, and lead to less precipitation over there.

Fig. 5.
Fig. 5.

Composite vertically integrated water vapor flux anomalies from (a),(b) 1000 to 300 hPa (unit: kg m−1 s−1) and latitude–height cross sections averaged over 110°–120°E of the (c),(d) vertical winds (vectors, unit: m s−1) and specific humidity anomalies (contours, unit: 10−3 g kg−1) in the (left) positive and (right) negative configuration. Regions over the 95% significant level of the t test are shaded in (a) and (b).

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

The vertical winds and water vapor anomalies are further examined to show the regional circulations in the two configurations. In the positive configuration (Fig. 5c), a descending flow with decreased air moisture occurs over the northern part of eastern China, while a significant anomalous ascending flow with increased air moisture exists over the southern part. The opposite is true in the negative configuration except for a small area over 15°–20°N (Fig. 5d). Precipitation is expected to be suppressed over the downward motion regions with less moisture, while more precipitation is expected to occur where there is strong upward motion with more moisture (Emori and Brown 2005). Thus, the two rainfall anomalies patterns correspond well to the regional circulation and moisture anomalies in two configurations of the EAPJ and EASJ.

4. The possible mechanisms maintaining the two configurations of the EAPJ and EASJ

Previous studies indicated that the variability of the EASJ and EAPJ in summer has been attributed to the external forcing and internal dynamical forcing (Ren et al. 2008; Lu et al. 2013). In this study, we focus on the SST as the external forcing and the STEA as the internal dynamical forcing to investigate the possible mechanisms maintaining the two configurations of the EAPJ and EASJ.

a. The positive configuration

As discussed in section 1, since SSTs in the previous seasons and in the current summer have different impacts on the summer jet streams, the previous winter (December–January–February), current spring (March–April–May), and current summer (June–July–August) SSTs are used to analyze the relationship between SST and the configurations of the two jets. As the season progresses, two apparent evolutions of SST anomalies can be recognized in the positive configuration (Fig. 6). One is the growing phase of meridional cold–warm SST anomalies over the eastern North Pacific. Another is the decaying phase of cold SST anomalies over central Pacific. The opposite evolution of the two SST anomalous patterns over different oceanic areas indicates that they may have a different relationship with the positive configuration of the EAPJ and EASJ in current summer.

Fig. 6.
Fig. 6.

Composite SST anomalies in the (a) previous winter, (b) current spring, and (c) current summer in the positive configuration (unit: °C). Regions over the 95% significant level of the t test are shaded.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

To analyze the relationship between the cold–warm SST anomalies over eastern North Pacific and the variation of the two jets, a SST variation index (SSTI) is used, defined as follows:
e6
The square brackets in Eq. (5) represent the regions of significant warm SST anomalies and cold SST anomalies in current summer, respectively (Fig. 6c). The SSTI is characterized by interannual variability and shows an increasing trend during the period of 1992–2010 (Fig. 7a). The anomalies of summer wind speed at 300 hPa obtained by regression with respect to the SSTI are shown in Fig. 7b. Significant positive wind anomalies at 300 hPa are located over the active EAPJ region, indicating the strengthened EAPJ in summer. On the contrast, we notice negative wind anomalies on the downstream of the active EASJ region, indicating the weakened EASJ. Furthermore, regional vertical atmospheric circulation and meridional temperature gradient regressed with SSTI are analyzed to understand the mechanisms maintaining strengthened EAPJ and weakened EASJ. In Fig. 7c, the vertical circulation shows distinct anomalous ascending motion at all levels around 60°N, which may intensify the EAPJ. Additionally, based on the principle of thermal wind, the intensity of the jet is directly proportional to the meridional temperature gradient (Wallace and Hobbs 2005; Zhang et al. 2006; Zhang and Huang 2011). It is known that the meridional temperature gradient in climatology is negative over East Asia due to the generally decreasing temperature from south to north. Thus, the negative temperature gradient anomalies from lower levels to higher levels around 60°N may also intensify the EAPJ. However, the positive temperature gradient anomalies around 40°N may weaken the EASJ. Overall, the cold–warm SST anomalies over the eastern North Pacific may lead to the positive configuration of the EAPJ and EASJ, via the regional circulation and meridional temperature gradient anomalies.
Fig. 7.
Fig. 7.

(a) Normalized time series of SSTI in 1960–2010, (b) the regression of summer winds at 300 hPa (unit: m s−1), and (c) the latitude–height cross sections averaged over 70°–110°E of the regression of summer vertical motion (unit: m s−1, vector) and meridional temperature gradient anomalies (unit:10−5, contour) with respect to the normalized SSTI for the period of 1960–2010. Regions over the 95% significant level of the t test are shaded in (b). The black boxes indicate the active region of the EAPJ and EASJ in (b). The red shaded area indicates the active regions of (left) EASJ and (right) EAPJ in (c). The black shaded area indicates the terrain.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

For the cold SST anomalies over the central Pacific, they can be recognized as the CP La Niña–like SST anomalies. Garfinkel et al. (2012) suggested that the CP La Niña tends to intensify the Arctic stratospheric vortex by using available observations. Baldwin and Dunkerton (2001) and Song and Robinson (2004) indicated that the Arctic vortex varies in strength and is disturbed by planetary-scale Rossby waves, which originate mainly in the troposphere and transport westward angular momentum upward. If the Arctic stratospheric vortex is strong, the tropospheric westerlies along 55°N are also anomalously strong, which is also proved by model experiments. In this study, by regression with the negative EMI, wind anomalies at 50 hPa show a significant Arctic vortex–like pattern (Fig. 8), which is consistent with the result of Garfinkel et al. (2012). Consequently, the CP La Niña–like SST anomalies in positive configuration may be teleconnectted with the intensified Arctic vortex, which may further lead to a strengthened EAPJ.

Fig. 8.
Fig. 8.

Regressions of summer geopotential height (contour, unit: gpm) and winds at 50 hPa (vector, unit: m s−1) with respect to normalized wintertime negative EMI for the period of 1960–2010. Regions over the 95% significant level of the t test are shaded.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

While SST is an important external factor affecting the configurations of the two jets, the internal dynamic processes of the atmosphere are also important for the midlatitude climate variations (Hoskins and Pearce 1983), including the jets. One of the strongest atmospheric baroclinic zones on the globe lies over the East Asian coast to midlatitude North Pacific region due to the East Asian jet streams. To reveal the relationship between STEA and jet stream variations, the structure of maximum eddy growth rate (σ) and transient eddy kinetic energy (Ke) have been examined. In the positive configuration, significant positive and negative σ anomalies are located over the active regions of the EAPJ and EASJ, respectively (Fig. 9a), similar for the Ke anomalies (Fig. 9b). Apparently, the structures of σ and Ke anomalies are similar to the mean flow anomalies in Fig. 3b, indicating a close relationship may exist between the STEA and the intensity of jet streams, especially for the EAPJ.

Fig. 9.
Fig. 9.

(a) Composite summer Eady growth rate anomalies between 700 and 850 hPa (unit: day−1), (b) the transient eddy kinetic energy anomalies (unit: m2 s−2), (c) the divergence of E–P vector anomalies (unit: 10−5 m s−2), and (d) the conversion anomalies between time-mean kinetic energy and transient eddy kinetic energy (unit: 10−5 m2 s−3) in the positive configuration. Regions over the 95% significant level of the t test are shaded.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

Then, how does the STEA affect the jet intensity in the positive configuration? Theoretically, the transient eddy-induced mean flow changes due to barotropic processes can be quantified by the divergence of E–P flux (e.g., Gong et al. 2011). A local convergence area is often characterized by anticyclonic vorticity forcing to the north of the convergence area and cyclonic vorticity forcing to the south. Therefore, a convergence area is usually associated with easterly acceleration of the mean flow and a divergence with westerly acceleration (James 1995). As shown in Fig. 9c, in the positive configuration, significant divergence anomalies of the E–P flux prevail over the active region of the EAPJ, while convergence anomalies prevail over the active region of the EASJ, which may strengthen the EAPJ and weaken the EASJ. Additionally, across the conversion point, the increased barotropic energy conversion from the eddies to the zonal mean flow may reduce eddy energy and enhance the EAPJ (Mahlman 1973; Lee 1997). As shown in Fig. 9d, the negative conversion anomalies covering the active EAPJ region are favorable for the conversion from Ke to Km, and subsequently strengthen the EAPJ, which benefits the positive configuration.

b. The negative configuration

Similar to the positive configuration, SST anomalies are first analyzed to investigate the external forcing. In the negative configuration, there also exist two regions of SST anomalies from previous winter to current summer: the WNP with a decaying phase of warm SST anomalies and the eastern Pacific with a developing phase of cold SST anomalies (Fig. 10, left panels). To examine how the warm SST anomalies affect the variation of jet streams via the convective activities over western North Pacific (Lu 2005), the corresponding composite OLR anomalies are shown in right panels of Fig. 10. The OLR anomalies generally exhibit a similar distribution to SST anomalies but with opposite sign, suggesting that the convective activities correspond well to the SST anomalies. The figures show that anomalous convective activities corresponding to OLR anomalies gradually develop from the previous winter to the current summer along the regions over 110°–160°E, particularly over the Philippine Sea. Actually, the convective activity over the WNP (Nitta 1987; Tsuyuki and Kurihara 1989; Huang and Sun 1992), combined with the feedback of the subtropical heat over East Asia (Lu and Lin 2009), may induce the meridional propagation of Rossby waves, which establish the meridional teleconnection between the WNP and East Asia in summer, including the EASJ variations. As Lu (2004) revealed, the enhanced convective activities over the WNP corresponded to westerly wind anomalies at 200 hPa in summer, indicating a strengthened EASJ.

Fig. 10.
Fig. 10.

(left) Composite SST anomalies (unit: °C) and (right) OLR anomalies (unit: W m−2) in the negative configuration in the (a),(b) previous winter; (c),(d) current spring; and (e),(f) current summer. Regions over the 95% significant level of the t test are shaded.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

Moreover, the tropical latent heating accompanied with convective activities is usually internal (with the maximum in the midtroposphere). However, the easterly vertical shear over the WNP permits the coupling of the heating-induced internal mode with the external mode, and triggers external barotropic-type responses (Lim and Chang 1986; Kasahara and Dias Silva 1986), which is necessary for the tropical–extratropical teleconnection mechanism (Lu 2004). Thus, the vertical shear of zonal wind can influence the linkage between the convective activities over the WNP and the EASJ (e.g., Tsuyuki and Kurihara 1989; Lu 2004). Consistent with previous studies, it is the easterly shear over the WNP in the climatology (Fig. 11a). In the negative configuration, the easterly shear enhances (Fig. 11b), which may strengthen the linkage between the convective activities and EASJ. Above all, in the negative configuration, associated with the warm SST anomalies over the WNP, the enhanced convective activities may lead to an intensified EASJ via the meridional teleconnection established by the meridional propagation of Rossby waves. The meridional teleconnection can be strengthened by the intensified easterly shear over the WNP.

Fig. 11.
Fig. 11.

(a) The vertical shear (300 hPa minus 850 hPa) of climatological summer winds during the period of 1960–2010 and (b) the composite vertical shear anomalies in the negative configuration (unit: m s−1). Regions over the 95% significant level of the t test are shaded in (b).

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

For the EP La Niña–like SST anomalies, the Walker circulation is investigated, which is one of the most prominent phenomena strongly linked to the SST anomalies with the ENSO phenomenon (Philander 1990). The Walker circulation weakens during El Niño years and strengthens during La Niña years, suggesting the enhanced downward motion in the eastern Pacific and upwelling in the west Pacific (Philander 1990). To understand the linkage between the EP La Niña event and the variations of two jets, regression analysis is performed in response to the negative Niño-3.4 index. Significant negative and positive geopotential height anomalies at 1000 hPa are zonally located over the Indian Ocean and Pacific Ocean, respectively, which may induce a Walker circulation anomaly with an ascending branch over the region of negative SST anomalies (Fig. 12a). The ascending motion can be proved in Fig. 12b, as the significant ascending anomalies over low-latitude regions, which may benefit the Hadley circulation. It is well known that the EASJ forms as a result of angular momentum transport by the thermally driven Hadley circulation (Held and Hou 1980), indicating that accompanied with enhanced Hadley circulation, the EASJ may be intensified. Consequently, in the negative configuration, EP La Niña–like SST anomalies may lead to a strengthened EASJ.

Fig. 12.
Fig. 12.

(a) Regressions of summer geopotential height (unit: gpm) at 1000 hPa and (b) latitude–height cross sections averaged over 60°–80°E of summer vertical motion (vector, unit: m s−1) with respect to normalized summer negative Niño-3.4 index for the period of 1960–2010. Regions over the 95% significant level of the t test are shaded in (a). The black shaded regions indicate the terrain in (b).

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

From the perspective of STEA, in the negative configuration, the structures of σ and Ke anomalies (Figs. 13a b) are also similar to that in mean flow anomalies in Fig. 3d. It further proves the close relationship between the STEA and the intensity of jet streams as indicated in the positive configuration, especially for the EAPJ. Additionally, the E–P vector and conversion from Ke to Km are also examined in the negative configuration. Significant convergence anomalies of the E–P vector are located over active region of the EAPJ, leading to a weakened EAPJ (Fig. 13c). Meanwhile, in Fig. 13d, the positive conversion anomalies over the active EAPJ region benefit the conversion from Km to Ke, also weakening the EAPJ (Fig. 13d). Additionally, both considering Figs. 9 and 13, the not-significant STEA-related anomalies over the active region of the EASJ indicate that the STEA may be less related to the EASJ, compared to the EAPJ.

Fig. 13.
Fig. 13.

As in Fig. 9, but for negative configuration.

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

5. Conclusions and discussion

In this study, we investigate the two configurations of the EASJ and EAPJ and the associated rainfall anomalies over eastern China in summer. One index, the NDI, is used to identify the configurations of EAPJ and EASJ. In the positive configuration, the EAPJ strengthens and the EASJ weakens, vice versa in the negative configuration. In the positive configuration, rainfall significantly decreases in the northern part of eastern China, due to the suppressed water vapor transportation. The enhanced rainfall occurs in the southern part of eastern China due to the increased ascending flow. However, fairly opposite features of rainfall anomalies and the associated regional circulations exist in the negative configuration.

The possible mechanisms maintaining the two configurations of the EAPJ and EASJ are further proposed. In the positive configuration, for the external forcing as SST, significant cold–warm SST anomalies are meridionally located over the eastern North Pacific. Associated with these, significant ascending motion anomalies with negative meridional temperature gradient anomalies are located over the active region of the EAPJ, while positive meridional temperature gradient anomalies cover the active region of the EASJ. These regional anomalies may favor the strengthened EAPJ and weakened EASJ. Additionally, a decaying phase of CP La Niña–like SST anomalies is indicated from the previous winter to the current summer. It is related to the stratospheric Arctic vortex–like anomalies, which may be associated with the strengthened EAPJ. For the internal forcing as STEA, the structures of maximum eddy growth rate and eddy kinetic energy anomalies are quite consistent with that in mean flow anomalies, indicating the close relationship between the STEA and the jet intensity. Significant divergence anomalies of the E–P flux and the conversion from Ke to Km prevail over the active region of the EAPJ, which may strengthen the EAPJ.

In the negative configuration, significant warm SST anomalies exist over the western North Pacific in the previous winter, current spring, and summer, corresponding to the prominent meridional convective anomalies located along the regions over 110°–160°E. Associated with the warm SST anomalies, the enhanced convective activities may lead to an intensified EASJ via the meridional teleconnection established by the meridional propagation of Rossby waves. The meridional teleconnection can be strengthened by the intensified easterly shear over the WNP. Additionally, a developing phase of EP La Niña–like SST anomalies may enhance the Walker circulation and hereafter strengthen the Hadley circulation along the longitude where the EASJ locates. Since the EASJ forms as a result of angular momentum transport by the thermally direct Hadley circulation, the enhanced Hadley circulation may strengthen the EASJ. For the internal forcing, the STEA-related anomalies, opposite to those in the positive configuration, may weaken the EAPJ. Moreover, the STEA-related anomalies are found to be more closely related with the EAPJ than with the EASJ.

In addition, during 1960–2010 (51 years), besides the configuration category of jet intensity significantly differing from the climatological mean intensity of EAPJ or EASJ (a total of 19 years), there is another configuration category of the near-climatology jet intensity, here named as neutral configuration. Based on the criterion of the NDI between ±0.3, 9 typical years are selected for the neutral configuration (Table 1). In Fig. 14a, wind anomaly centers shift from the active regions of two jets. It indicates that the neutral configuration may be related with the jet displacement, as the westward shift of the EAPJ and southward shift of the EASJ. In this configuration, the associated rainfall anomalies show an enhanced mei-yu rainfall pattern (Fig. 14b). Since mei-yu is a unique weather system with a quasi-stationary rainband over the Yangtze–Huaihe river basin (YHRB; 28°–32°N, 110°–122°E) in China and mei-yu rainfall anomalies often lead to drought or flood disasters over this densely populated region of YHRB (e.g., Ni and Zhou 2004; Ding et al. 2007; Huang et al. 2011; Zhu et al. 2013), the displacement configuration of the two jets and their climatic impacts may be an important issue for further investigation.

Fig. 14.
Fig. 14.

(a) Composite wind anomalies at 300 hPa (unit: m s−1) and (b) the summer rainfall anomalies (mm day−1) over eastern China in the neutral configuration. Regions over the 95% significant level of the t test are shaded in (b).

Citation: Journal of Climate 27, 21; 10.1175/JCLI-D-14-00067.1

Acknowledgments

This study is jointly sponsored by National Natural Science Foundation of China (Grants 41130963 and 51190091), the National Basic Research Program of China (973 Program; Grant 2011CB952002), and the National Natural Science Foundation of China (Grants 41105044 and 41205038). The numerical calculations in this paper have been done on the IBM Blade cluster system in the High Performance Computing Center (HPCC) of Nanjing University. This work is supported by the Jiangsu Collaborative Innovation Center for Climate Change. We thank the three anonymous reviewers and the editor for their insightful comments and scientific hints, which greatly improved the quality of the study.

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