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  • View in gallery

    Average distribution of a precipitation process (mm) in China. The number in the top-left corner of each panel refers to the date of the precipitation occurrence.

  • View in gallery

    Composited temporal evolution of precipitation (mm) in China. The number in the top-left corner of each panel refers to the dth day prior to (negative) and after (positive) the occurrence of precipitation.

  • View in gallery

    Composited 850-hPa wind (vectors; m s−1) and normalized water vapor anomaly (shaded; every 0.5σ). The number above each panel has the same meaning as in Fig. 2.

  • View in gallery

    Composited 500-hPa geopotential height (contours; every 5 dagpm) and normalized height anomalies (shaded; every 0.5σ). The vectors indicate wave activity flux (m2 s−2). The number above each panel has the same meaning as in Fig. 2.

  • View in gallery

    Composited 500-hPa geopotential height (contours; every 5 dagpm) and normalized height anomaly (shaded; every 0.5σ) latitude–time evolution along 110°–130°E associated with the EAP cases of SC persistent rain. The vectors indicate wave activity flux (m2 s−2).

  • View in gallery

    Distribution of the WPSH in SC EAP cases (black dotted line represents the average of all cases).

  • View in gallery

    Composited OLR anomaly (W m−2). The number above each panel has the same meaning as in Fig. 2.

  • View in gallery

    Latitude–time evolution of OLR anomalies along 110°–120°E (W m−2).

  • View in gallery

    Distribution of lead–lag correlation coefficients between composited EAP index and OLR (shaded). The negative (positive) number in the top-left corner refers to the OLR leading (lagging) the SC EAP index dth day. The correlations significant at the 0.05 level are shown as black lines.

  • View in gallery

    Composites of the SSTA (shaded), the 850-hPa wind anomaly (vector), and the 588- and 586-gpdam contour lines (red contours) of the A-SC case. The black line represents the SSTA that is significant at the 0.05 level. The number above each panel has the same meaning as in Fig. 8. The black boxes represent the key area of SSTAs.

  • View in gallery

    Lead–lag correlation between the composite of the EAP index and the SSTA. The number above each panel has the same meaning as in Fig. 8, and the black contours indicate that the values are significant at the ≥0.05 significance level. The black boxes represent the key area of SSTAs.

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Potential Influence of the East Asia–Pacific Teleconnection Pattern on Persistent Precipitation in South China: Implications of Atypical Yangtze River Valley Cases

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  • 1 State Key Laboratory of Severe Weather, China Academy of Meteorological Sciences, Beijing, and Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environment Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China
  • | 2 State Key Laboratory of Severe Weather, China Academy of Meteorological Sciences, Beijing, China
  • | 3 Key Laboratory of Meteorological Disaster, Ministry of Education (KLME)/Joint International Research Laboratory of Climate and Environment Change (ILCEC)/Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science and Technology, Nanjing, China
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Abstract

In this study, cases of the East Asia–Pacific (EAP) teleconnection pattern not responsible for persistent precipitation processes in the Yangtze River valley (YRV) have been investigated. The results suggest that such a type of EAP pattern has some linkage with persistent precipitation processes in south China (SC) with the following properties: 1) in response to the negative SSTAs and anticyclone near the Philippines, the meridional energy propagates from the low latitudes over the north of the Philippines; 2) the western Pacific subtropical high (WPSH) then intensifies and extends westward; 3) a meridional triple structure of the EAP teleconnection pattern is established; 4) at the same time, the cyclonic circulation over northeastern China introduces cold and dry air to the lower latitudes, merging with the water vapor into SC and leading to heavy precipitation from the fringe of the WPSH, the South China Sea, and the Bay of Bengal and the combination of systems persists for at least 3 days, leading to the persistent precipitation processes in SC; and 5) compared with the EAP teleconnection responsible for the precipitation in YRV, the positions of the three centers in the mid- and low latitudes are more southerly located than the YRV EAP centers. Further study indicates that the ocean surface heat conditions in the areas near the Philippines seem to be important in affecting the EAP teleconnection pattern for persistent precipitation processes in SC. Finally, all of the cases with persistent precipitation in SC during 1961–2010 linked with the EAP pattern have been investigated; the results are consistent with the above conclusions.

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

Corresponding author: Panmao Zhai, pmzhai@cma.gov.cn

Abstract

In this study, cases of the East Asia–Pacific (EAP) teleconnection pattern not responsible for persistent precipitation processes in the Yangtze River valley (YRV) have been investigated. The results suggest that such a type of EAP pattern has some linkage with persistent precipitation processes in south China (SC) with the following properties: 1) in response to the negative SSTAs and anticyclone near the Philippines, the meridional energy propagates from the low latitudes over the north of the Philippines; 2) the western Pacific subtropical high (WPSH) then intensifies and extends westward; 3) a meridional triple structure of the EAP teleconnection pattern is established; 4) at the same time, the cyclonic circulation over northeastern China introduces cold and dry air to the lower latitudes, merging with the water vapor into SC and leading to heavy precipitation from the fringe of the WPSH, the South China Sea, and the Bay of Bengal and the combination of systems persists for at least 3 days, leading to the persistent precipitation processes in SC; and 5) compared with the EAP teleconnection responsible for the precipitation in YRV, the positions of the three centers in the mid- and low latitudes are more southerly located than the YRV EAP centers. Further study indicates that the ocean surface heat conditions in the areas near the Philippines seem to be important in affecting the EAP teleconnection pattern for persistent precipitation processes in SC. Finally, all of the cases with persistent precipitation in SC during 1961–2010 linked with the EAP pattern have been investigated; the results are consistent with the above conclusions.

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

Corresponding author: Panmao Zhai, pmzhai@cma.gov.cn

1. Introduction

Precipitation, as a key component of the hydrological cycle, is vital in determining the distribution of water resources and is associated with natural disasters such as droughts and floods. Studying the features of precipitation and revealing the main mechanisms accounting for the variation of precipitation are very important in increasing our understanding of the formation of precipitation processes and in improving weather and climate predictions.

In summer, variations in atmospheric circulation play important roles in the distribution and maintenance of the monsoon rain belt in China. The study of teleconnection patterns in summer began with Nitta (1987) and Huang and Li (1987), who proposed the concepts of the East Asia–Pacific (EAP) teleconnection and the Pacific–Japan (P–J) teleconnection—two patterns that are essentially the same. They both pointed out that the relationship between East Asian climate and northwestern Pacific climate is established by teleconnection, and the formation of this teleconnection pattern is related to convective heating anomalies near the western Pacific and the Philippines. Lau and Weng (2002) demonstrated that this teleconnection pattern exists during the Northern Hemisphere summer and that the effect of this teleconnection pattern is the most significant over East Asia. The EAP pattern reflects variations in three critical circulation systems: the Okhotsk high, the mei-yu trough, and the western Pacific subtropical high (WPSH). Anomalies in precipitation in the Yangtze River valley (YRV) are often linked closely with these three anomalous systems. Based on previous studies, it seems that the EAP (P–J) teleconnection is primarily triggered by anomalous convective activity near the western Pacific warm pool (Huang and Li 1987; Nitta 1987). More recently, Kosaka and Nakamura (2006, 2010) proposed that the EAP teleconnection pattern is maintained by obtaining energy from the base flow.

Most previous studies about the EAP pattern have focused on the climatological time scale rather than the synoptic scale. However, some recent studies also investigated intraseasonal EAP variations (e.g., Bueh et al. 2008). Furthermore, Chen and Zhai (2015) identified that the typical EAP teleconnection pattern is responsible for precipitation processes in the YRV on the synoptic scale. The synoptic-scale EAP is seemingly affected by Pacific equatorial wave activity, such as equatorial Rossby waves as indicated by Kiladis and Wheeler (1995) or tropical depression (TD)-type waves as revealed by Wu et al. (2015a,b). The equatorial Rossby (ER) waves propagate to the west (Kiladis and Wheeler 1995). In addition, the mixed Rossby–gravity (MRG) waves and TD-type disturbances are also kinds of westward-propagating tropical waves over the Pacific (e.g., Matsuno 1966; Wheeler and Kiladis 1999). However, the teleconnection pattern is mainly meridional. Further, this pattern also includes a third center (Sea of Okhotsk) in the higher latitudes. The pattern structure reflected at 500 hPa is more close to that of the EAP teleconnection. Thus, we deem this pattern as the synoptic-scale EAP.

According to Chen and Zhai (2015), the identified EAP teleconnection pattern only explains 64.5% of cases of precipitation processes in the YRV. So, why do all the identified EAP teleconnection patterns not have impacts on the persistent precipitation in the YRV? What are the implications of the other EAP cases for persistent precipitation in other regions? Further, why do the persistent precipitation processes occur in other regions? To answer these questions, an investigation was conducted from the starting point of the 11 EAP cases (as shown in Table 1) that are not directly linked to persistent precipitation in the YRV for the study period of 1961–2010 (Chen and Zhai 2015).

Table 1.

The 11 atypical EAP teleconnection pattern cases not responsible for persistent precipitation cases in the YRV during 1961–2010. Seven of the 11 cases are A-SC EAP cases with persistent precipitation in SC and 4 of them are dry SC EAP cases without persistent precipitation in SC. The last four columns show the average index values of the EAP and the three centers constituting the EAP pattern, namely, WP (20°N, 120°E), EA (37.5°N, 120°E), and OK (60°N, 130°E).

Table 1.

2. Data and methods

a. Data

In this study, the following data are used:

  1. Daily rain gauge precipitation observation data during 1961–2010 were collected from 756 stations in China, provided by the China Meteorological Information Center (CMA).
  2. The NCEP–NCAR daily reanalysis dataset during 1961–2010 with a horizontal resolution of 2.5° × 2.5° and 17 vertical levels, including geopotential height (gpm), horizontal wind (m s−1), and specific humidity (kg kg−1) (Kalnay et al. 1996) was accessed.
  3. The daily outgoing longwave radiation (OLR) dataset during 1979–2010, on a global 2.5° × 2.5°grid, is from NOAA.
  4. The daily sea surface temperature (SST) dataset during 1981–2010, on a global 0.25° × 0.25°grid, is from NOAA.

b. Methods

A precipitation process that persisted for at least three consecutive days is referred to as a persistent precipitation process.

The EAP index IEAP was normally defined based on three key anomaly center points, namely, the Sea of Okhotsk (OK), the midlatitudes of East Asia (EA), and the western Pacific (WP), during the EAP regimes (Nitta 1987; Bueh et al. 2008; Hirota and Takahashi 2012; Chen and Zhai 2015):
e1
where , , and represent the normalized 500-hPa geopotential height anomalies of OK, EA, and WP, respectively. This study is based on the typical EAP teleconnection pattern responsible for no persistent heavy precipitation cases in the YRV in June and July, as identified by Chen and Zhai (2015) for the period 1961–2010. The cases are selected by requiring that the normalized domain-averaged daily precipitation in the YRV should be smaller than −1 standard deviation (denoted by σ hereafter), and are referred to as dry YRV EAP cases. At the same time, the three key centers of the EAP teleconnection pattern are required to be greater than 0.75σ. All of the above criteria have to be satisfied for longer than 3 days.

In the study period of 1961–2010, there are 11 EAP cases identified without persistent precipitation in YRV (Table 1). As for the 11 EAP cases, the persistent precipitation processes are found in south China (SC) during the summers of 1966, 1973, 1997, 1998, 2002, 2003, and 2004. Such a combination of the EAP teleconnection pattern and concurrent persistent precipitation processes is referred to as an A-SC EAP pattern hereafter. At the same time, the other four cases that have not been observed to have persistent precipitation processes in the SC region are defined as being dry SC EAP patterns.

In the discussion section below, some additional cases are used to further illuminate the relationship between SC persistent precipitation processes and the EAP teleconnection. The selection criteria are similar to those outlined above. In particular, 1) the three key centers of the EAP teleconnection pattern were required to be greater than 0.75σ and to persist for at least 3 days and 2) normalized domain-averaged precipitation in SC must have persisted for at least 3 days. In total, 13 cases are selected from the period 1961–2010, and these cases are referred to as SC EAP.

This study uses a composite analysis, which is a simple and effective method for identifying and classifying synoptic-scale circulation patterns and their precursors associated with extreme cases (Sisson and Gyakum 2004; Grotjahn and Faure 2008). Following the method described by Hart and Grumm (2001), the climate state is averaged from 1979 to 2010 and then calculated using the 21-day moving average.

Three-dimensional wave action flux, including zonal nonuniform elementary flow, is used. This can describe the energy propagation characteristics of quasi-stationary waves and transient fluctuations (Takaya and Nakamura 2001). Moreover, this flux is independent of the wave phase under the assumption of the Wentzel–Kramers–Brillouin (WKB) approximation and is consistent with the local group velocity direction of the regular Rossby wave series. Therefore, the propagation direction of the wave energy can be well described as
e2
where p = pressure/1000 hPa, is the basic flow horizontal wind, is the zonal wind component, is the meridional wind component, and is the streamfunction for quasigeostrophic flow. The basic flow field of this flux formula contains zonal and meridional wind fields with zonal nonuniformity, so this formula is suitable for complex mid- to high-latitude circulation patterns during the summer. The region where the wave flux diverges can be regarded as the wave source, and the convergence region is regarded as the energy intersection.

3. Precipitation patterns as reflected by 11 atypical YRV EAP cases

Chen and Zhai (2015) investigated the influences of the EAP teleconnection pattern on precipitation processes in the YRV. In their study, there are 11 cases (Table 1) of EAP teleconnection patterns that are not responsible for precipitation processes in the YRV. They are atypical YRV EAP cases.

The precipitation processes of these 11 cases are analyzed separately (as shown in Fig. 1). During these EAP teleconnection periods, persistent precipitation processes are obvious in SC during 1966, 1973, 1997, 1998, 2002, 2003, and 2004, accounting for 64% of the total precipitation in the 11 cases. In addition, such precipitation processes in SC persist for at least three consecutive days. Thus, it can be seen that the seven cases of EAP teleconnection patterns that are not responsible for precipitation processes in the YRV are directly linked to persistent precipitation processes in SC. Nevertheless, the precipitation processes of the other four cases are mainly found in the YRV, but the intensity does not meet the criteria defined by Chen and Zhai (2015).

Fig. 1.
Fig. 1.

Average distribution of a precipitation process (mm) in China. The number in the top-left corner of each panel refers to the date of the precipitation occurrence.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

To further understand the general properties of these seven persistent precipitation cases under the influences of the EAP, persistent precipitation processes that occur during the summers of 1966, 1973, 1997, 1998, 2002, 2003, and 2004 (A-SC EAP cases) are composited. Day 0 denotes the onset (start date in Table 1) of an EAP teleconnection pattern related to the precipitation process in SC, and day d refers to the dth day prior to (negative) or after (positive) onset. By definition, EAP onset occurs when the EAP daily index starts to reach the anomalous level with height anomalies of the three relevant centers over the thresholds as given in Eq. (1). It can be seen from Fig. 2 that the precipitation occurred in the YRV, parts of southwestern China, and SC on day −5. After this, the precipitation in SC begins to increase on day −3, and then the range of precipitation expands and the intensity increases further on day 0, persisting until day 1. Previous studies have found that the EAP pattern can lead to persistent precipitation processes in June in SC (Chen et al. 2012). From Fig. 2, it can be seen that persistent precipitation processes can occur both in June and July when the A-SC EAP pattern appears.

Fig. 2.
Fig. 2.

Composited temporal evolution of precipitation (mm) in China. The number in the top-left corner of each panel refers to the dth day prior to (negative) and after (positive) the occurrence of precipitation.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

Because of the influence of the East Asian monsoon, the Indian monsoon, the WPSH, and the dynamic/thermodynamic effects of the Tibetan Plateau, SC receives the greatest annual precipitation in China (Tao 1980, 45–46; Hong and Ren 2013). Precipitation is mainly concentrated during the months from April to September, and it is likely to lead to large-scale flooding. Persistent rain is one of the main causes of large-scale flooding. Wu and Zhai (2013) further stressed the importance in studies where the persistent precipitation increased at a rate of 5.4 mm decade−1 from 1961 to 2010.

The causes of rainfall over SC have been found to be related to midlatitude westerly troughs, the intertropical convergence zone (Huang et al. 2005), and tropical cyclone activity (Ren et al. 2006; Chen and Huang 2012). Tao (1980, 45–46) indicated that heavy rain in SC is closely linked with low-latitude weather systems, and He et al. (2010) suggested the existence of complex multiscale interactions. Zhu et al. (1986) pointed out that persistent, heavy rainstorms in SC are related to the large-scale circulation and the East Asian summer monsoon anomaly. However, the relationship between the EAP teleconnection and persistent precipitation processes in SC on synoptic time scales has been less thoroughly explored. In the following section, the relationship between the seven EAP teleconnection cases and persistent precipitation processes in SC will be addressed. The differences from the other four cases will also be briefly discussed.

4. Persistent precipitation processes in South China and the EAP pattern

a. Link to wind and water vapor transport at 850 hPa

Since the occurrence of persistent precipitation processes is related to the water vapor transport in the lower troposphere, it is necessary to analyze the anomalies of wind speed in the lower troposphere, as well as water vapor transport. In Fig. 3, it can be seen that an anticyclone exists over the northwestern Pacific on day −7, while a cyclonic circulation exists in northeastern China. In the lower latitudes, the northwesterly winds from the Bay of Bengal are more vigorous, but there is no convergence of water vapor over SC and thus no precipitation at this time. On day −3, the convergence of water vapor occurs near the Philippines, and the intensity of the convergence then increases and the range expands, moving northwestward until it arrives in the southern area of SC. The anomaly of the water vapor intensity exceeds 3.5σ and reaches the extreme level. During day −1, the intensity of the water vapor convergence weakens. The anticyclone related to the WPSH moves westward, and then the water vapor from the WPSH merges with that from the South China Sea and the Bay of Bengal. From day 0 to day 1, the convergence of water vapor between the anomalous cyclone and anticyclone persists over SC. Such persistence leads to the precipitation processes lasting for more than 3 days.

Fig. 3.
Fig. 3.

Composited 850-hPa wind (vectors; m s−1) and normalized water vapor anomaly (shaded; every 0.5σ). The number above each panel has the same meaning as in Fig. 2.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

For the features of water vapor transport of the EAP teleconnection cases with persistent precipitation processes in the YRV [defined as YRV EAP cases by Chen and Zhai (2015)], the location of the moisture flux and the water vapor convergence are more northward than those in A-SC EAP cases. Specifically, an anticyclone also exists over the northwest Pacific on day −7, but the position is more westward and northward, which is more favorable for the moisture flux reaching the YRV. In the lower latitudes, the northwesterly moisture flux transported from the Bay of Bengal is more intense, but there is no convergence of water vapor over the YRV at that time (not shown). On day −5, convergence of water vapor occurs near SC and to the west of SC; the intensity then increases and the range expands, moving westward until it arrives at the east of YRV. The anomaly in the water vapor intensity exceeds 2.5σ. During day −1, the range of water vapor expands and covers the YRV. Also, water vapor from the WPSH merges with that from the South China Sea and the Bay of Bengal. During days 0 and 1, the convergence of water vapor continues to be maintained, leading to the YRV precipitation processes lasting for more than 3 days.

b. Link to the EAP pattern

The evolution of persistent precipitation processes in SC can be clearly seen from the features of the wind field and water vapor transport at 850 hPa. To further study the influence of the EAP pattern on persistent precipitation processes in SC, the evolution of the EAP pattern is analyzed, as described in this section.

Kosaka and Nakamura (2006, 2010) indicated that the EAP pattern is mainly observed in the middle and lower troposphere, but with a quasi-positive pressure structure in the mid- to high-latitude system at the upper levels. Therefore, the characteristics of wave activities and the evolution of the EAP pattern in the middle and lower troposphere are mainly analyzed in the following text.

At 500 hPa, on day −7, an eastward wave flux exists near the Ural Mountains in the high latitudes (Fig. 4). At the same time, two ridges with positive height anomalies (1σ above normal) exist near the Sea of Okhotsk and to the west of the Ural Mountains (near 90°E), in the mid- to high latitudes. A broad and shallow trough is sandwiched between them, providing a zonally elongated westerly waveguide for the Rossby wave energy dispersion (Hoskins and Ambrizzi 1993). Therefore, energy propagation along this westerly waveguide from west of the Ural Mountains to East Asia can be identified. This is consistent with the conclusions drawn by Takaya and Nakamura (2001). Noticeably, there is already meridional energy propagation near the north of the Philippines in the low latitudes. On day −5, two negative anomalies of 1σ below normal appear in northeastern China and the southeastern part of SC, respectively. These two systems combine into a low pressure system near 30°–45°N on day −1. The intensity of the low pressure system of −3σ reaches its strongest value in the midlatitudes. At the same time, the WPSH begins to strengthen and extends westward, and the meridional energy propagation is enhanced near the north of the Philippines in the low latitudes. As a result, a complete positive–negative–positive (+ − +) configuration of the EAP pattern forms over the coast of East Asia. On day 0, the westward-extending WPSH reaches 110°E, and the intensity reaches its strongest level. The strong convergence of the zonal wave fluxes from upstream and the meridonal wave fluxes from the lower latitudes near the north of the Philippines to the higher latitudes results in a further development and enhancement of the Okhotsk blocking high and creates the low pressure system that deepens in the low latitudes. The intensity of the low pressure system reaches −3σ. The intensity of the EAP pattern during that period reaches its strongest level and lasts for over 3 days. The long-lived Okhotsk blocking high acts to maintain a robust meridional circulation, providing beneficial conditions for the development of the EAP pattern and related persistent precipitation processes. On day −1, the strengthening of the Okhotsk blocking high is favorable for forcing the westerly flow to move equatorward. This indicates prevailing northwesterly flow upstream, leading to the arrival of cold/dry air in SC, where it converges with the warm/wet air from the edge of the WPSH. Consequently, a persistent precipitation process occurs across SC.

Fig. 4.
Fig. 4.

Composited 500-hPa geopotential height (contours; every 5 dagpm) and normalized height anomalies (shaded; every 0.5σ). The vectors indicate wave activity flux (m2 s−2). The number above each panel has the same meaning as in Fig. 2.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

The formation time of the three systems and the intensity of the SC EAP pattern can be deduced from Fig. 5. The blocking high in the high latitudes appears on day −3 and intensifies thereafter, with the intensity exceeding 1.5σ on day 0. The WPSH is located at about 10°N from day −7 to day −2. The WPSH suddenly moves northward on day −1 to 20°–25°N on day 0. In addition, it should be noted that the low pressure system in the midlatitudes is formed by the combination of two low pressure systems from the high latitudes and the low latitudes, respectively, with the intensity reaching −2.5σ on day 0.

Fig. 5.
Fig. 5.

Composited 500-hPa geopotential height (contours; every 5 dagpm) and normalized height anomaly (shaded; every 0.5σ) latitude–time evolution along 110°–130°E associated with the EAP cases of SC persistent rain. The vectors indicate wave activity flux (m2 s−2).

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

In summary, the main features of the EAP pattern associated with precipitation processes in SC during A-SC EAP cases are the positive anomalies of 1σ above normal in the northwest Pacific and East Asia in the high latitudes, with regions between them covered by negative anomalies of 3σ below normal. The EAP pattern characterizes the westward extension and strengthening of the WPSH and the establishment and maintenance of the Okhotsk blocking high. A low-latitude system is favorable for the southward movement of cold air to SC. This circulation anomaly tends to last for more than 3 days, resulting in persistent precipitation processes in SC. Figure 6 shows the positions of the WPSH in all A-SC EAP cases. The average westward-extending ridge point of the WPSH is located around 110°E, and the ridge line is near 15°N. Noticeably, it is more westward and southward than the WPSH position of the YRV EAP pattern cases, with the average westward-extending ridge point of the WPSH located around 120°E and the ridge line near 22°N, as indicated by Chen and Zhai (2015). It seems that the shift in location of the WPSH more to the west and south is the key factor in the location of the persistent rain belt in SC during such circumstances.

Fig. 6.
Fig. 6.

Distribution of the WPSH in SC EAP cases (black dotted line represents the average of all cases).

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

The above discussion explains the impact of the location of the WPSH on persistent precipitation processes in SC during A-SC EAP cases. In the following section, the differences between three key systems for these two types of EAP cases are studied further.

In addition, the circulations of the EAP teleconnection for the cases with persistent precipitation processes in the YRV are composited (not shown). From the distribution and intensity of the three key centers of the EAP teleconnection, it is clear that the intensity and position are different for the different types of cases, as shown in Table 2. The intensities of the three key centers for the A-SC EAP cases (Fig. 5) are stronger than those for the YRV EAP cases. For the A-SC cases, the intensity of the OK center is stronger than 2σ, the EA center is deeper than −3σ, and the WP center is larger than 3σ. For the YRV EAP case, the intensity of the OK center is greater than 1.5σ, the EA center is below 3σ, and the WP center is greater than 1.5σ.

Table 2.

Intensities and positions of the three key centers of the A-SC EAP and YRV EAP cases.

Table 2.

Furthermore, the positions of the three key centers in the EAP pattern in those two cases also exhibit differences. The position of the OK center in the A-SC EAP cases is at about 65°N, 135°E, the EA center is at about 35°N, 118°E, and the WP center is at about 15°N, 145°E. For the YRV EAP cases, the position of the OK center is at about 60°N, 125°E, the EA center is at about 37.5°N, 122.5°E, and the WP center is at about 15°N, 122.5°E.

In general, the centers of SC EAP are stronger than those of YRV EAP. The positions of the A-SC EAP centers in the mid- and low latitudes were more southerly than the YRV EAP centers. In this situation, the rain belt is located in SC.

c. Link to tropical convective activity and SST anomalies

The above analysis demonstrates the formation of the EAP pattern and its influence on persistent precipitation processes over SC on the synoptic scale. At the climatological scale, Huang and Li (1987) pointed out that the formation of the EAP pattern is related to convective heating anomalies near the western Pacific and the Philippines. That is, the heat source forcing forms the quasi-stationary wave, which propagates from the Philippines to the North American coast through East Asia. But does this mean that the occurrence of the synoptic-scale EAP pattern is also associated with convective activity near the Philippines? By compositing the OLR corresponding to the A-SC EAP pattern cases (Fig. 7), it is found that the convective activity anomalies appear in the Philippines on day −7. The convective activity is suppressed over SC, the western Pacific, and the Maritime Continent. The distributions of convective activity form a meridional triple pattern. Then, this pattern moves northward and intensifies until day −1, the convective activity arrives in SC, and the suppressed convection reaches the YRV region and the Philippines. Such a suppression of convection in the warm pool region favors the formation of a WPSH (Mao et al. 2010). On day 0, the intensity of the convective activity reaches its strongest level. At the same time, the Philippines are completely under the influence of the suppressed convection, and SC is influenced by strong convection. Subsequently, persistent precipitation processes occur in SC. It is also evident from the latitudinal–temporal evolution of the composited OLR anomalies (Fig. 8) that there is a clear northward + − + triple convection pattern from the tropical western Pacific to SC on day −7. The triple convection pattern arrives in SC on day 0. The triple convection pattern in the low latitudes corresponds to the EAP pattern. The enhancement of the suppressed convection system provides the energy for the formation and maintenance of the EAP pattern.

Fig. 7.
Fig. 7.

Composited OLR anomaly (W m−2). The number above each panel has the same meaning as in Fig. 2.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

Fig. 8.
Fig. 8.

Latitude–time evolution of OLR anomalies along 110°–120°E (W m−2).

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

To analyze the influence of OLR anomalies on A-SC EAP cases, a lead–lag correlation is used. The IEAP indices in 1966, 1973, 1997, 1998, 2002, 2003, and 2004 are composited, and a 31-day window is used to build the time series, including 15 days precase, the case day, and 15 days postcase. This provides enough of the early signal and time sequence for correlation analysis. It is found that there is a significant negative correlation between the convective activity in the Philippines and the IEAP, as shown in Fig. 9. As day 0 approaches, the negative correlation diminishes. This indicates that a weakening of the convection in the Philippines is favorable for the formation of the A-SC EAP pattern. In addition, the evolution of OLR in Fig. 9 also illustrates this point. It can be seen that the synoptic-scale EAP patterns are also inextricably linked to the convective activity in the Philippines. This suggests that the convection is favorable for the formation of the SC EAP pattern. It may affect the convection and EAP pattern activities through SST variations.

Fig. 9.
Fig. 9.

Distribution of lead–lag correlation coefficients between composited EAP index and OLR (shaded). The negative (positive) number in the top-left corner refers to the OLR leading (lagging) the SC EAP index dth day. The correlations significant at the 0.05 level are shown as black lines.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

The ocean and atmosphere are coupled, and the impact of the sea surface temperature anomalies (SSTAs) on the atmosphere is complex. In addition, it is related to time scales. For the climate scale, it is possible that the ocean has a strong effect on the EAP teleconnection (Lu 2001; Lu and Dong 2001; Wu et al. 2010). However, on the synoptic scale, the SSTAs and the EAP pattern are likely to affect each other. Ren et al. (2013) revealed that a feedback process exists between the WPSH and the SSTAs beneath it. As the WPSH is a key center and initiation point of the EAP pattern (Chen and Zhai 2015), the relationship between the WPSH and SSTAs will be analyzed.

Compositing the SSTAs of the A-SC EAP cases as shown in Fig. 10, it can be found that the westward-extending ridge point of the WPSH is located around 140°E, and the ridge line is near 25°N on day −7 when the A-SC EAP case occurs. At the same time, the SSTAs near the Philippines are significantly below normal. The anomalous cyclone north of the Philippines is likely linked to cloudy weather and results in a decrease in the solar radiation reaching the ocean surface. The SSTAs near the Philippines further decrease on day −3. The much cooler than normal SSTAs in the region are favorable for downward motion and anomalous anticyclonic circulation conditions, which leads to the intensification and westward extension of the WPSH. The WPSH moves westward and southward and arrives at about 15°N, 115°E. However, on day 0, the WPSH extends to about 110°E with the areal coverage of the WPSH being the strongest while the intensity of the negative SSTAs is reduced near the Philippines. This is directly linked to the intensity of the EAP center of WP, and further, the two other centers (EA and OK) are also enhanced as a result of the influence of the meridional energy that propagates from the area north of the Philippines (Huang and Li 1989). The two other centers of EAP reach their strongest levels, and the EAP pattern is clearly established.

Fig. 10.
Fig. 10.

Composites of the SSTA (shaded), the 850-hPa wind anomaly (vector), and the 588- and 586-gpdam contour lines (red contours) of the A-SC case. The black line represents the SSTA that is significant at the 0.05 level. The number above each panel has the same meaning as in Fig. 8. The black boxes represent the key area of SSTAs.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

In addition, Fig. 11 shows the possible link between the IEAP and the SSTAs. The SSTAs near the Philippines are significantly correlated with the EAP index 7 days earlier. After that point, the negative correlation increases, indicating that negative SSTAs in this area are favorable for the development of the A-SC EAP pattern. Compared with the convective activity reflected by OLR over the Philippines in Fig. 7, the OLR anomalies switch from negative to positive between days −7 and −1. The study by Hong and Ren (2013) indicated that, before rainfall in SC, the local ocean receives less radiation under strong-convection and more-cloud conditions, and this suppresses the release of latent heat over the local ocean. Thus, the convective activity turns out to be suppressed over the Philippines. The colder than normal SST conditions suppress the development of convective activity (from days −7 to 0), resulting in the contraction of the air column over these regions, the formation of a cold high pressure, divergent airflow, and an abnormal anticyclone in the lower troposphere. Such conditions are conducive to the westward extension of the WPSH. As for the maintenance of the WPSH, the increased solar radiation and suppressed heat exchange lead to a rapid increase in the SST anomalies. At the same time, the range of the cold SST anomalies is reduced significantly. Compared with the composited SSTAs for the cases with persistent precipitation processes in the YRV (not shown), it is shown that the significant signals are the warm SSTAs near the Maritime Continent and to its west and the cold SSTAs near the South China Sea.

Fig. 11.
Fig. 11.

Lead–lag correlation between the composite of the EAP index and the SSTA. The number above each panel has the same meaning as in Fig. 8, and the black contours indicate that the values are significant at the ≥0.05 significance level. The black boxes represent the key area of SSTAs.

Citation: Weather and Forecasting 33, 1; 10.1175/WAF-D-17-0011.1

In general, for the two types of cases, the distributions of the SSTA are different. For A-SC EAP cases, the negative SSTAs in the Philippines are the significant signal. However, for YRV EAP cases, the positive SSTAs across the Maritime Continent and an area of colder than normal SSTAs extending from the South China Sea to the western Pacific are the significant signals. Therefore, the positions of the rain belts of the two cases are located in different regions. The persistent precipitation processes of the A-SC EAP cases are located in SC, and those of YRV EAP cases are located in the YRV.

As mentioned previously, there are four other cases of the EAP teleconnection pattern not responsible for precipitation processes in the SC. These four cases (dry SC EAP cases) are composited (not shown). The comparison reveals that the most obvious differences during the dry SC EAP cases involve the meridional location of the WPSH, the strength of the blocking high, and the mei-yu trough. Though the WPSH also extends to the west of 120°E during the dry SC EAP cases, it is moved more to the north in comparison to the A-SC EAP cases, with its ridge anchored near 22.5°N. The moist air is therefore less likely to be conveyed to SC, leading to less precipitation. In the midlatitudes, the mei-yu trough is weaker, making it hard for the cold air to reach SC. Moreover, anomalous water vapor can be transported to the more northward region. Moisture convergence fails to form in SC. As a result, precipitation cannot occur in SC without cold air and warm, moist air.

5. Conclusions and discussion

In this study, 11 cases of the EAP teleconnection pattern that are not responsible for persistent extreme precipitation processes in the YRV are studied as a starting point to see if the EAP teleconnection pattern has any link to persistent precipitation processes in SC. The main conclusions can be summarized as follows:

  1. When an atypical YRV EAP pattern appears, it is likely that a persistent precipitation process will occur in SC. In such cases, the intensities of A-SC EAP centers are stronger, and the positions of A-SC EAP centers in the mid- and low latitudes are more southerly than the YRV EAP centers. In this situation, the cold air can reach a more southerly region and converge with warm air in SC. This means that persistent precipitation processes will likely occur in SC.
  2. For A-SC EAP cases, on day −5, the meridional energy propagates from the low latitudes over the Philippines. The WPSH begins to extend westward, and the intensity increases 3 days earlier. On day −1, the A-SC EAP pattern is established, with its intensity reaching its strongest level. The cyclonic circulation over northeastern China introduces cold and dry air to the lower latitudes and merges with the water vapor transferred from the western edge of the WPSH, the South China Sea, and the Bay of Bengal, thus leading to precipitation processes in SC. Compared with the EAP teleconnection responsible for precipitation in YRV, the position of the A-SC EAP centers in the mid- and low latitudes is more southerly than is the case for the YRV EAP centers. Such a combination of systems persists for at least 3 days, meaning persistent precipitation processes occur in SC.
  3. A triple convective pattern is closely related to the formation of the A-SC EAP. Further investigation suggests that the key area where SST affects the A-SC EAP is near the Philippines. The colder than normal SSTs near the Philippines suppress the development of convective activities, and the anomalous anticyclone promotes the westward extension of the WPSH. Compared with SSTAs for the cases with persistent precipitation processes in the YRV, the key area of SSTAs is different. Therefore, the locations of the rain belts are different.

The above analyses mainly illustrate that atypical YRV EAP teleconnection patterns such as those identified by Chen and Zhai (2015) are likely to affect precipitation in SC. However, a question remains: What of the cases with starting points in SC? To address this concern, 13 cases of persistent precipitation processes in SC accompanied by an obvious EAP teleconnection pattern are further investigated. These cases are hereafter referred to as SC EAP cases. After analyzing their composited 500-hPa geopotential fields, 850-hPa wind fields, and water vapor flux fields (not shown), it is found that the distribution of circulation systems influencing these SC EAP cases is similar to those of A-SC EAP cases, but some aspects are slightly different: the intensities of the three key centers of the EAP are weaker, but the water flux is stronger, as compared to the seven A-SC EAP cases. Five of the seven A-SC EAP cases are included in the SC EAP cases. A possible cause of the differences is that the 1966 and 1997 cases are not included in the SC EAP cases.

In the above discussion, although similarities and differences between the SC EAP cases and the A-SC EAP cases are highlighted, the conclusions seem obvious and need to be further studied. Also, the mechanisms by which persistent precipitation processes in SC are linked to the EAP teleconnection patterns, which are clearly complex, need to be further investigated in the future.

Acknowledgments

This study was jointly supported by the National Natural Science Foundation of China (41605029, 41575094 and 41565005) and the China Special Fund for Meteorological Research in the Public Interest (GYHY201506001). The authors declare no competing financial interests regarding the publication of this paper.

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