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Characteristics of the Western Pacific Subtropical High and Summer Rainfall Anomalies
CHAPTER
CHARACTERISTICS OF THE WESTERN PACIFIC SUBTROPICAL HIGH AND SUMMER RAINFALL ANOMALIES
5
5.1 INTRODUCTION The western Pacific subtropical high (WPSH) is one of the most important atmospheric circulation subsystems in the East Asian summer monsoon (EASM) system. Changes in its location and intensity have a significant impact on the climatic anomaly in China and East Asia (Wu et al., 2002). In eastern China, the intraseasonal change of WPSH determines the northward shift of the rainfall belt during the flood season (Tao and Wei, 2006; Xu et al., 2001), while its interannual variation decides the spatial distribution of drought and flood (Tao and Zhu, 1964; Zhao, 1999; Liu et al., 2013). The interdecadal changes of WPSH and other climatic factors provide a favorable background for precipitation anomalies along the middle reaches of the Yangtze River during the summers of the 1990s (Hu, 1997; Xiong, 2001). In recent years, the extreme weather over the Yangtze River basin has appeared more frequently than before, for example, the summer of 1998’s catastrophic flood along the Yangtze River basin, the unusually long heatwaves affecting the south of the Yangtze River during the mid-to-late July of 2003, and the continued floods over southern China in the years between 2015 and 2017 (Tao et al., 1998, 2001; Zhou et al., 2004; Yuan et al., 2017; Gao et al., 2017). These abnormal weather and climatic events have a close association with the activities of the WPSH, which have always remained a point of interest for China’s meteorologists and weather forecasters. Hence the establishment of a set of monitoring indices which can truly and objectively reflect the changes of the WPSH status (including its size, intensity, eastwest and northsouth location), has become an imperative task. A set of the WPSH monitoring indices from the National Climate Center (NCC) will be introduced in this chapter, and its relationship with the climatic anomalies and possible mechanism over East Asia will be analyzed in Chapter 6, Tropospheric Biennial Oscillation of Western Pacific Subtropical High and Its Relationships With the Tropical Sea Surface Temperature and Atmospheric Circulation Anomalies. As the highest meteorological administration body in China, the NCC is responsible for monitoring and forecasting climate-influencing factors. Over the years, it has developed nearly 100 monitoring indices, including tropospheric atmosphere circulation, ocean temperature, sea ice, snow cover, and others. The set of WPSH monitoring indices
The Asian Summer Monsoon. DOI: https://doi.org/10.1016/B978-0-12-815881-4.00005-6 © 2019 Elsevier Inc. All rights reserved.
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introduced here contains area index, intensity index, ridgeline index, and the westernmost point of WPSH, which indicate the size, intensity, and location of the WPSH. They are obtained by calculating the NCEP/NCAR reanalysis datasets (Kalnay et al., 1996) which are now widely used in the climate monitoring operations, including 500 hPa geopotential height and zonal wind with a resolution of 2.5 3 2.5 . Different from the multiple sets of other WPSH indices used in meteorological research, this set of WPSH indices, not only reflect the changing characteristics of the WPSH itself in an objective manner, but can also be easily used in climatic operations, which explains why it has stood years of testing and gained popular applications in climatic monitoring services. This chapter is organized as follows. Section 5.2 introduces a definition of the WPSH indices, including the area index, intensity index, ridgeline index, and westernmost point. Section 5.3 shows the climatological characteristics of the WPSH and its correlation with the rainfall anomalies during JuneJulyAugust (JJA). Section 5.4 provides the nine different classifications to examine the guiding role of the WPSH position on the summer rainfall anomalies in China. Summary and discussions are provided in Section 5.5.
5.2 DEFINITION OF THE WESTERN PACIFIC SUBTROPICAL HIGH INDICES The WPSH is the subtropical high system that appears over the western North Pacific. It is represented by the area covered by the 5880 gpm contour lines within the range of 110 E180 on the 500 hPa geopotential height field (Fig. 5.1). The set of WPSH monitoring indices include area
FIGURE 5.1 Climatological 500 hPa geopotential height field for the northern hemisphere in summer for 19812010.
5.2 DEFINITION OF THE WESTERN PACIFIC SUBTROPICAL HIGH INDICES
87
index, intensity index, ridgeline index, and westernmost point, thereby pinpointing the size, intensity, and location features of the WPSH. The four indices of the WPSH are defined as: Area index(GM): The area index is characterized by the total area encircled by the 5880 gpm isolines within the range of 110 E180 and north of 10 N. GM 5 dx 3 dy 3 nij 5
XX ðnij 3 cosϕj Þ i
j
1; Hij $ 5880 0; Hij , 5880
In the formula, dx is the distance value of the latitudinal grid point and dy is the distance value of the meridional grid point; i is the ordinal number for the zonal grid point, i 5 1, 2, . . ., Nx , and Nx is the sum of grid points in the monitoring range, increasing from west to east; j 5 1, 2,. . ., Ny , and Ny is the sum of grid points in the monitoring range, increasing from south to north. Hij is the potential height value of the grid point on the 500 hPa height field, and ϕj is the latitude value of the grid point. Intensity index(GQ): The intensity index is characterized by the sum of the product of the total area encircled by the 5880 gpm isolines within the range of 110 E180 and north of 10 N, and the difference between the grid point’s height exceeding 5880 and 5870 gpm. GQ 5 dx 3 dy 3
XX ðnij 3 ðHij 2 5870Þ 3 cosϕj Þ i
j
Ridgeline index: The ridgeline index is characterized by the mean of the latitude location of the zonal wind shear line where the zonal wind u 5 0 and @u=@y . 0 encircled by the 5880 gpm isolines within the range of 110 150 E and north of 10 N; If there are no 5880 gpm isolines, the zonal wind shear line (u 5 0 and @u=@y . 0) encircled by the 5840 gpm isolines will be considered. If 5840 gpm isolines are absent, the historical minimum value of that month since 1951 will be taken as the substitute. Westernmost point: The westernmost point is characterized by the longitude where the westernmost 5880 gpm isolines within the range of 90 E180 and north of 10 N. If it lies to the west of 90 E, it will be denoted uniformly as 90 E; if there are no 5880 gpm isolines in a certain month, the historical maximum value of that month since 1951 will be taken as the substitute. When defining the ridgeline index, the following scenarios will not be considered: an isolated WPSH that contains only one 5880 gpm grid point within the range of 110 150 E; and the zonal wind shear line of the WPSH intersects with only one longitude. When defining the westernmost point, the following scenario will not be considered: an isolated WPSH contains only one 5880 gpm grid point within the range of 90 E180 . It can be seen from the definition that the WPSH area index is denoted by the “actual area” encircled by 5880 gpm isolines, which is independent from the data resolution. This renders comparability between the area indices obtained by calculation of data with different resolutions. The intensity index is more like a “volume” concept in which the area of each grid point acts as the base and the difference between the value of each grid point and the 5870 gpm acts as the height. In this way, the sum of the product of all the “bases” and “heights” is taken as the intensity
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CHAPTER 5 CHARACTERISTICS OF THE WESTERN PACIFIC SUBTROPICAL
index. It eliminates the index’s dependence on data resolution and is more vividly representative of the WPSH volume. The definition of the ridgeline index includes both the 500 hPa geopotential height and zonal wind, taking the zonal wind shear line (u 5 0 and the @u=@y . 0) as the WPSH ridgeline. If there are no 5880 gpm isolines, the zonal wind shear line within the 5840 gpm isolines will be defined as the ridgeline. In the climatic monitoring operation, it is indicated that despite the weak or nonexistence of the WPSH (5880 gpm isolines) on the 500 hPa height field, the atmospheric circulation system with a strong geopotential height (about 5840 gpm) still exists over the western Pacific subtropical region. Its northsouth location change exerts a pronounced influence on the rainfall distribution in eastern China (Li and Chou, 1998; Zhan et al., 2005). Hence it should be taken into consideration in climatic monitoring so as to better capture the WPSH’s impact on the location of the rainfall belt affecting China. The westernmost point illustrates the WPSH’s eastwest location, whose anomalous changes often incur a direct impact on the high temperature and rainstorm in the summer of southern China.
5.3 WESTERN PACIFIC SUBTROPICAL HIGH’S BASIC FEATURES AND CORRELATION WITH SUMMER RAINFALL ANOMALIES As the WPSH has the most pronounced influence on summer rainfall in the EASM region, the focus is placed on the summer season (averaged from JJA). The time sequences of the four WPSH indices are calculated in line with the definition in Section 5.2 (Fig. 5.2). It can be seen that the summer WPSH features salient interdecadal changes, with the area index, intensity index, and westernmost point showing transitions during the early 1980s. This indicates that the WPSH increased in area, intensity, and experienced a significant westward extension after the 1980s. The lack of evident interdecadal changes in the WPSH ridgeline index suggests that no major northsouth
FIGURE 5.2 The time series of the four WPSH indices from 1951 to 2017: (A) area index, (B) intensity index, (C) ridgeline index, and (D) westernmost point. WPSH, Western Pacific subtropical high.
5.3 WESTERN PACIFIC SUBTROPICAL HIGH’S BASIC FEATURES
89
change occurred. The climatological average of the four WPSH indices during 19812010 is: area index, 64.08 (105 km2); intensity index, 131.91 (106 km2 gpm); ridgeline index, 25.16 N; and the westernmost point, 127.5 E. The changes of the WPSH status (area, intensity, and eastwest and northsouth location) present a close correlation with the distribution of floods and drought over China in summer. In particular, the northsouth movement of the WPSH ridgeline index, often determines the location and intensity of Meiyu (or the plum rain, a special meteorological phenomenon over the middle and lower reaches of the Yangtze River basin during summer) (Ninomiya, 1984; Tao and Chen, 1987; Wang and Li, 2004). Fig. 5.3A is the correlation distribution of the WPSH ridgeline index with JJA rainfall anomalies in China. It shows a very good negative correlation along the Yangtze River basin and its southern region, reaching the 0.05 t-test significant level (the absolute value of correlation coefficient is $ 0.36). It indicates that when the WPSH is more southward, the rainfall belt will most likely stagnate over the Yangtze River basin, which is favorable for redundant rainfall there. When the WPSH moves further northward than normal, the rainfall belt tends to shift northward, resulting in deficient rainfall over the Yangtze River basin.
FIGURE 5.3 The correlation distribution between the WPSH ridgeline index and rainfall anomalies over China in (A) JJA, (B) June, (C) July, and (D) August. Significant correlations above the 0.05 confidence level are shaded. WPSH, Western Pacific subtropical high; JJA, JuneJulyAugust.
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CHAPTER 5 CHARACTERISTICS OF THE WESTERN PACIFIC SUBTROPICAL
The correlation distributions of the WPSH ridgeline and rainfall anomalies in China during JJA are shown in Fig. 5.3BD. It clearly reflects the impact of WPSH’s northsouth location on the rainfall belt over eastern China in different months. In June (Fig. 5.3B), the WPSH ridgeline presents a pronounced negative correlation with rainfall anomalies in southern and southeastern China, but a positive correlation with the YangtzeHuaihe basin region. That means that when the WPSH ridgeline is more southerly than normal in June, excessive rainfall is indicated in southern and southeastern China and deficient rainfall over the YangtzeHuaihe basin region. In July (Fig. 5.3C), the WPSH ridgeline “jumps northward” with the significantly negative correlation region moving to the Yangtze River basin and north of southeastern China, that is, the southerly WPSH ridgeline in July leads to excessive rainfall over the Yangtze River basin. In August (Fig. 5.3D), with the WPSH ridgeline “jumping northward for the second time,” its negative correlation region shifts slightly further northward than in July, but with reduced correlation. The WPSH ridgeline also presents a negative correlation with the Tibetan plateau and north of southwestern China. This is perhaps the indirect impact reflected from anomalies of other atmospheric circulation systems induced by the WPSH anomaly, such as the Indo-Burma trough.
5.4 INFLUENCE OF THE WESTERN PACIFIC SUBTROPICAL HIGH ON RAINFALL ANOMALIES IN CHINA The relationship among the four WPSH indices will be comprehensively considered in this section to attain different classifications according to their correlation coefficients and the corresponding distributions of the rainfall anomalies in JJA. The purpose of this endeavor is to analyze the influence of the WPSH on the rainfall anomalies in China.
5.4.1 CORRELATION OF THE WESTERN PACIFIC SUBTROPICAL HIGH INDICES Table 5.1 lists the correlation coefficients of the four monthly WPSH indices during the period of 19512017, which indicates that there is an almost identical change trend with the area index and intensity index, with their correlation coefficient reaching 0.97. While the westernmost point has a mostly opposite trend with the area index and intensity index, whose correlation coefficients stand at 0.75 and 0.67 respectively, exceeding the 0.01 t-test significant level. This indicates that when the WPSH gets larger and stronger, it will be a more westward extension, while it will be a Table 5.1 Correlation Coefficients Among the Reconstructed Monthly WPSH Indices Area Intensity Ridgeline Westernmost point
Area
Intensity
Ridgeline
Westernmost Point
1.00
0.97 1.00
0.38 0.39 1.00
2 0.73 2 0.65 2 0.10 1.00
WPSH, Western Pacific subtropical high.
5.4 INFLUENCE OF THE WESTERN PACIFIC SUBTROPICAL HIGH
91
more eastward withdrawal when it gets smaller and weaker. The two indices that represent the WPSH locations, namely the ridgeline index and westernmost point, show the least correlation (only 0.16). This signifies the distinct independence of these two indices and a comprehensive analysis of these two WPSH indices represents the WPSH climatological changes to the largest extent. The high correlation of the WPSH area index and westernmost point enables us to consider the eastwest location and the area and intensity change simultaneously. Thus to further study the relationship between the WPSH location and summer rainfall belt in China, these two independent indices are selected to construct nine classifications to analyze the guiding role of the position of the WPSH on the summer rainfall anomalies in China.
5.4.2 WESTERN PACIFIC SUBTROPICAL HIGH’S INFLUENCE ON RAINFALL ANOMALIES The nine classification charts, represented by two WPSH location indices from 1951 to 2017 are constructed, with the anomalies of WPSH westernmost point as the x-axis and the anomaly of ridgeline index as the y-axis (Fig. 5.4). The WPSH normal years are specified as those when the
FIGURE 5.4 The nine classifications chart according to the combination of the summer WPSH ridgeline and westernmost point indices from 1951 to 2017. (I) More northward and westward; (II) more northward; (III) more northward and eastward; (IV) more eastward; (V) more southward and eastward; (VI) more southward; (VII) more southward and westward; (VIII) more westward; and (IX) normal. WPSH, Western Pacific subtropical high.
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westernmost point index falls within 5 longitudes to the west and east relative to climatological average, and when ridgeline index falls within 0.5 latitude to the north and south. Other WPSH abnormal years go as follows: more northward and westward, more northward, more northward and eastward, more eastward, more southward and eastward, more southward, more southward and westward, and more westward. These nine WPSH classification charts reflect two basic features. (1) The WPSH experiences a pronounced interdecadal transition around the 1980s. The WPSH was always more eastward than normal before the 1980s, whereas the westernmost point extends westward significantly after the 1980s, which agrees with the features reflected in Fig. 5.2. (2) When the westernmost point is northward, there is a relatively high probability that it is also more eastward than normal. There were only 2 years of western anomalies of the WPSH over past 60 years, namely in 1990 and 2006. On the contrary, when the WPSH is southward, there is a relatively high probability of a more westward westernmost point. Only 5 years were observed with an eastward anomaly. The distribution of the JJA rainfall anomalies over China corresponding to the nine WPSH classifications are shown in Fig. 5.5. It can be seen that the rainfall belt in eastern China presents a corresponding distribution with the change of the WPSH’s location. During the years between 1951
FIGURE 5.5 Distributions of the summer rainfall anomalies corresponding with the nine classifications of the WPSH: (A) more northward and westward; (B) more northward; (C) more northward and eastward; (D) more eastward; (E) more southward and eastward; (F) more southward; (G) more southward and westward; (H) more westward; (I) normal. WPSH, Western Pacific subtropical high.
5.5 SUMMARY AND DISCUSSION
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and 2017, only in 1990 and 2006 did the WPSH location show the feature of being more northward and westward (Classification I). Under this classification, the JJA rainfall anomalies in eastern China presents two rainfall belts, which are located at the south of the Yangtze RiverSouth China and HuaiheYellow River basin, respectively, while there was deficient rainfall over the Yangtze River basin (Fig. 5.5A). When the WPSH is more northward (Classification II) in comparison with Classification I, the rainfall belt in northern China obviously lifts to the north of the Yellow River basin and connects with the rainfall belt in northeastern China, while the rainfall belt over the lower reaches of the Yangtze River severely weakened in scope and intensity (Fig. 5.5B). If the WPSH is not only northward but also westward (Classification III), the northern rainfall belt will further expand and become the main rainfall belt, while the rainfall belt in the south will weaken and almost disappear, which is the typical wet northdry south pattern of the JJA rainfall anomalies (Fig. 5.5C). When the WPSH displays a clear-cut westward anomaly (Classification IV), in comparison with Classification III, the rainfall belt in northern China shifts slightly toward the south, forming another new rainfall belt over the YangtzeHuaihe River region and the lower reaches of the Yangtze River basin, and features a twin rainfall belt distribution with an extended rainfall belt, but a drastic difference from the WPSH Classification I (Fig. 5.5D). When the WPSH falls southward and features more southward and eastward (Classification V), the rainfall belt in eastern China will shift south as well, while, at the same time, the rainfall belt in the north will weaken and largely vanish. However, a northeastsouthwest rainfall belt will appear in western China (Fig. 5.5E). If the WPSH presents a more southward feature (Classification VI), the JJA rainfall anomalies under this classification is demarcated with the Yangtze River basin, presenting the typical wet southdry north pattern (Fig. 5.5F). If the WPSH features not only southward but also westward (Classification VII), in comparison with Classification VI, the main rainfall belt will migrate northward, hovering over the Yangtze River and YangtzeHuaihe regions, resulting in scant rainfall in northern and southern China, showing a negativepositivenegative pattern over eastern China (from north to south) (Fig. 5.5G). When the WPSH shows a westward feature (Classification VIII), the rainfall belt will slightly shift northward in parallel, staying over the region between the Yangtze and Huaihe riversYellow River, with the emergence of a weak rainfall zone in the eastern part of southern China and south of the lower reaches of the Yangtze River basin (Fig. 5.5H). When the WPSH is at the climatic mean location (Classification IX), the JJA rainfall will concentrate in the lower reaches of the Yangtze River basin and southern China, with a significantly reduced rainfall belt compared to the one presented under Classification VI (Fig. 5.5I). Hence, it can be seen that there is a good relationship between the WPSH location and the JJA rainfall belt distribution in China. The northsouth location of the ridgeline directly determines the northsouth distribution of the main JJA rainfall belt, while the westernmost point’s eastwest matching feature will affect the range of the rainfall belt or determine whether a secondary rainfall belt will occur or not.
5.5 SUMMARY AND DISCUSSION This chapter introduced a set of WPSH monitoring indices used by the NCC, including area index, intensity index, ridgeline index, and westernmost point. The WPSH indices are calculated by 500 hPa geopotential height and zonal wind from the NCEP/NCAR reanalysis datasets from 1951
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to 2017. Through the analysis of the WPSH indices and their relationships with JJA rainfall anomalies in China, it was found that the WPSH indices has a number of advantages. The set of WPSH indices enable objectively-to describe the characteristics of the WPSH’s monthly change and overcome the defect of the excessive dependence on the data resolution. The ridgeline index has a significant correlation with the summer rainfall over the Yangtze River basin, which indicates that it takes full consideration of the impact of the WPSH system on the summer rainfall in eastern China. Moreover, two kinds of relatively independent WPSH indices, namely the ridgeline index and westernmost point, are selected to combine the nine classifications of anomalous WPSH features, which correspond with nine kinds of distributions of summer rainfall anomalies over eastern China. It provides a scientific basis to further understand the relation of the position anomalies of the WPSH and the summer main rainfall belts in eastern China. The analysis in this chapter shows that the WPSH’s northsouth and eastwest location anomalies have an important impact on the JJA rainfall anomalies in China. Through the nine classifications by the two WPSH location indices, it was possible to cover various distribution features of rainfall anomalies patterns over China in JJA to a great extent. It provides a scientific basis for our further understanding of the relationship between the anomalies of the WPSH’s location and the major rainfall belts in summer. The set of WPSH indices has been used by the NCC of the China Meteorological Administration in a real-time climate monitoring operation. Chapter 6, Tropospheric Biennial Oscillation of Western Pacific Subtropical High and Its Relationships With the Tropical Sea Surface Temperature and Atmospheric Circulation Anomalies and Chapter 7, Tropospheric Biennial Oscillation of Monsoon Rainfall and Its Association With El Nin˜o-Southern Oscillation, will discuss the interannual variability of the WPSH and its possible physical mechanisms.
REFERENCES Gao, H., Yuan, Y., Hong, J.L., et al., 2017. Overview of climate prediction of the summer 2016 and the precursory signals. Meteorol. Monthly 43, 486494 (in Chinese). Hu, Z.-Z., 1997. Interdecadal variability of summer climate over East Asia and its association with 500hPa height and global sea surface temperature. J. Geophys. Res. 102 (D16), 1940319412. Kalnay, E., Kanamitsu, M., Kistler, R., et al., 1996. The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteorol. Soc. 77, 437472. Li, J.P., Chou, J.F., 1998. Dynamic analysis of faults in the subtropical high zone—geotropic process. Chin. Sci. Bull. 43, 434437 (in Chinese). Liu, Y.Y., Ding, Y.H., Gao, H., et al., 2013. Tropospheric biennial oscillation of the western Pacific subtropical high and its relationships with the tropical SST and atmospheric circulation anomalies. Chin. Sci. Bull. 58, 36643672. Ninomiya, K., 1984. Characteristics of Baiu front as a predominant subtropical front in the summer Northern Hemisphere. J. Meteorol. Soc. Jpn 62, 880894. Tao, S.Y., Chen, L.X., 1987. A review of recent research on the East Asia summer monsoon over China. In: Chang, C.P., Krishnamurti, T.N. (Eds.), Monsoon Meteorology. Oxford University Press, New York, NY, pp. 5092. Tao, S.Y., Wei, J., 2006. The westward and northward advance of the subtropical high over the western Pacific in Summer. J. Appl. Meteorol. Sci. 17, 513525 (in Chinese).
FURTHER READING
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Tao, S.Y., Zhu, F.K., 1964. The variation of 100-mb flow patterns in southern Asia in summer and its relationship to the movement of western Pacific subtropical high. Acta Meteorol. Sin. 34, 385390 (in Chinese). Tao, S.Y., Zhang, Q.Y., Zhang, S.L., 1998. The great floods in the Yangtze River valley in 1998. Clim. Environ. Res. 3, 290299 (in Chinese). Tao, S.Y., Ni, Y.Q., Zhao, S.X., et al., 2001. Study on the Formation Mechanism and Forecast of Rainstorm in Summer of 1998 in China. Meteor. Press, Beijing, pp. 1931 (in Chinese). Wang, B., Li, T., 2004. East Asian monsoon-ENSO interactions. In: Chang, C.P. (Ed.), East Asian Monsoon. World Scientific Publishing, Singapore, pp. 177212. Wu, G.X., Chou, J.F., Liu, Y.M., 2002. Dynamics of Formation and Variation of Subtropical High. Science Press, Beijing, pp. 120 (in Chinese). Xiong, A.Y., 2001. Analysis on background of climatic variation for extremely rainy over the middle reaches of the Yangtze River in the 1990s. J. Appl. Meteorol. Sci. 12, 113117 (in Chinese). Xu, H.M., He, J.H., Zhou, B., 2001. The features of atmospheric circulation during Meiyu onset and possible mechanisms for westward extension and northward shift of western Pacific subtropical high. J. Appl. Meteorol. Sci. 12, 150158 (in Chinese). Yuan, Y., Gao, H., Liu, Y.J., 2017. Analysis of the characteristics and causes of precipitation anomalies over eastern China in the summer of 2016. Meteorol. Monthly 43, 115121 (in Chinese). Zhan, R.F., Li, J.P., He, J.H., 2005. Statistical characteristics of the double ridgelines of the subtropical high in northern hemisphere. Chin. Sci. Bull. 50, 20222026 (in Chinese). Zhao, Z.G., 1999. Summer Drought and Flood and Environmental Field in China. Meteor. Press, Beijing, pp. 4546 (in Chinese). Zhou, B., He, J.H., Xu, H.M., 2004. Effect of rainstorm process on variability of subtropical high. J. Appl. Meteorol. Sci. 15, 394406 (in Chinese).
FURTHER READING Tao, S.Y., 1963. Study on Several Problems of Subtropical Weather System in Summer over China. Science Press, Beijing, 146 pp. (in Chinese).
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5.1 INTRODUCTION The western Pacific subtropical high (WPSH) is one of the most important atmospheric circulation subsystems in the East Asian summer monsoon (EASM) system. Changes in its location and intensity have a significant impact on the climatic anomaly in China and East Asia (Wu et al., 2002). In eastern China, the intraseasonal change of WPSH determines the northward shift of the rainfall belt during the flood season (Tao and Wei, 2006; Xu et al., 2001), while its interannual variation decides the spatial distribution of drought and flood (Tao and Zhu, 1964; Zhao, 1999; Liu et al., 2013). The interdecadal changes of WPSH and other climatic factors provide a favorable background for precipitation anomalies along the middle reaches of the Yangtze River during the summers of the 1990s (Hu, 1997; Xiong, 2001). In recent years, the extreme weather over the Yangtze River basin has appeared more frequently than before, for example, the summer of 1998’s catastrophic flood along the Yangtze River basin, the unusually long heatwaves affecting the south of the Yangtze River during the mid-to-late July of 2003, and the continued floods over southern China in the years between 2015 and 2017 (Tao et al., 1998, 2001; Zhou et al., 2004; Yuan et al., 2017; Gao et al., 2017). These abnormal weather and climatic events have a close association with the activities of the WPSH, which have always remained a point of interest for China’s meteorologists and weather forecasters. Hence the establishment of a set of monitoring indices which can truly and objectively reflect the changes of the WPSH status (including its size, intensity, eastwest and northsouth location), has become an imperative task. A set of the WPSH monitoring indices from the National Climate Center (NCC) will be introduced in this chapter, and its relationship with the climatic anomalies and possible mechanism over East Asia will be analyzed in Chapter 6, Tropospheric Biennial Oscillation of Western Pacific Subtropical High and Its Relationships With the Tropical Sea Surface Temperature and Atmospheric Circulation Anomalies. As the highest meteorological administration body in China, the NCC is responsible for monitoring and forecasting climate-influencing factors. Over the years, it has developed nearly 100 monitoring indices, including tropospheric atmosphere circulation, ocean temperature, sea ice, snow cover, and others. The set of WPSH monitoring indices
The Asian Summer Monsoon. DOI: https://doi.org/10.1016/B978-0-12-815881-4.00005-6 © 2019 Elsevier Inc. All rights reserved.
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introduced here contains area index, intensity index, ridgeline index, and the westernmost point of WPSH, which indicate the size, intensity, and location of the WPSH. They are obtained by calculating the NCEP/NCAR reanalysis datasets (Kalnay et al., 1996) which are now widely used in the climate monitoring operations, including 500 hPa geopotential height and zonal wind with a resolution of 2.5 3 2.5 . Different from the multiple sets of other WPSH indices used in meteorological research, this set of WPSH indices, not only reflect the changing characteristics of the WPSH itself in an objective manner, but can also be easily used in climatic operations, which explains why it has stood years of testing and gained popular applications in climatic monitoring services. This chapter is organized as follows. Section 5.2 introduces a definition of the WPSH indices, including the area index, intensity index, ridgeline index, and westernmost point. Section 5.3 shows the climatological characteristics of the WPSH and its correlation with the rainfall anomalies during JuneJulyAugust (JJA). Section 5.4 provides the nine different classifications to examine the guiding role of the WPSH position on the summer rainfall anomalies in China. Summary and discussions are provided in Section 5.5.
5.2 DEFINITION OF THE WESTERN PACIFIC SUBTROPICAL HIGH INDICES The WPSH is the subtropical high system that appears over the western North Pacific. It is represented by the area covered by the 5880 gpm contour lines within the range of 110 E180 on the 500 hPa geopotential height field (Fig. 5.1). The set of WPSH monitoring indices include area
FIGURE 5.1 Climatological 500 hPa geopotential height field for the northern hemisphere in summer for 19812010.
5.2 DEFINITION OF THE WESTERN PACIFIC SUBTROPICAL HIGH INDICES
87
index, intensity index, ridgeline index, and westernmost point, thereby pinpointing the size, intensity, and location features of the WPSH. The four indices of the WPSH are defined as: Area index(GM): The area index is characterized by the total area encircled by the 5880 gpm isolines within the range of 110 E180 and north of 10 N. GM 5 dx 3 dy 3 nij 5
XX ðnij 3 cosϕj Þ i
j
1; Hij $ 5880 0; Hij , 5880
In the formula, dx is the distance value of the latitudinal grid point and dy is the distance value of the meridional grid point; i is the ordinal number for the zonal grid point, i 5 1, 2, . . ., Nx , and Nx is the sum of grid points in the monitoring range, increasing from west to east; j 5 1, 2,. . ., Ny , and Ny is the sum of grid points in the monitoring range, increasing from south to north. Hij is the potential height value of the grid point on the 500 hPa height field, and ϕj is the latitude value of the grid point. Intensity index(GQ): The intensity index is characterized by the sum of the product of the total area encircled by the 5880 gpm isolines within the range of 110 E180 and north of 10 N, and the difference between the grid point’s height exceeding 5880 and 5870 gpm. GQ 5 dx 3 dy 3
XX ðnij 3 ðHij 2 5870Þ 3 cosϕj Þ i
j
Ridgeline index: The ridgeline index is characterized by the mean of the latitude location of the zonal wind shear line where the zonal wind u 5 0 and @u=@y . 0 encircled by the 5880 gpm isolines within the range of 110 150 E and north of 10 N; If there are no 5880 gpm isolines, the zonal wind shear line (u 5 0 and @u=@y . 0) encircled by the 5840 gpm isolines will be considered. If 5840 gpm isolines are absent, the historical minimum value of that month since 1951 will be taken as the substitute. Westernmost point: The westernmost point is characterized by the longitude where the westernmost 5880 gpm isolines within the range of 90 E180 and north of 10 N. If it lies to the west of 90 E, it will be denoted uniformly as 90 E; if there are no 5880 gpm isolines in a certain month, the historical maximum value of that month since 1951 will be taken as the substitute. When defining the ridgeline index, the following scenarios will not be considered: an isolated WPSH that contains only one 5880 gpm grid point within the range of 110 150 E; and the zonal wind shear line of the WPSH intersects with only one longitude. When defining the westernmost point, the following scenario will not be considered: an isolated WPSH contains only one 5880 gpm grid point within the range of 90 E180 . It can be seen from the definition that the WPSH area index is denoted by the “actual area” encircled by 5880 gpm isolines, which is independent from the data resolution. This renders comparability between the area indices obtained by calculation of data with different resolutions. The intensity index is more like a “volume” concept in which the area of each grid point acts as the base and the difference between the value of each grid point and the 5870 gpm acts as the height. In this way, the sum of the product of all the “bases” and “heights” is taken as the intensity
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index. It eliminates the index’s dependence on data resolution and is more vividly representative of the WPSH volume. The definition of the ridgeline index includes both the 500 hPa geopotential height and zonal wind, taking the zonal wind shear line (u 5 0 and the @u=@y . 0) as the WPSH ridgeline. If there are no 5880 gpm isolines, the zonal wind shear line within the 5840 gpm isolines will be defined as the ridgeline. In the climatic monitoring operation, it is indicated that despite the weak or nonexistence of the WPSH (5880 gpm isolines) on the 500 hPa height field, the atmospheric circulation system with a strong geopotential height (about 5840 gpm) still exists over the western Pacific subtropical region. Its northsouth location change exerts a pronounced influence on the rainfall distribution in eastern China (Li and Chou, 1998; Zhan et al., 2005). Hence it should be taken into consideration in climatic monitoring so as to better capture the WPSH’s impact on the location of the rainfall belt affecting China. The westernmost point illustrates the WPSH’s eastwest location, whose anomalous changes often incur a direct impact on the high temperature and rainstorm in the summer of southern China.
5.3 WESTERN PACIFIC SUBTROPICAL HIGH’S BASIC FEATURES AND CORRELATION WITH SUMMER RAINFALL ANOMALIES As the WPSH has the most pronounced influence on summer rainfall in the EASM region, the focus is placed on the summer season (averaged from JJA). The time sequences of the four WPSH indices are calculated in line with the definition in Section 5.2 (Fig. 5.2). It can be seen that the summer WPSH features salient interdecadal changes, with the area index, intensity index, and westernmost point showing transitions during the early 1980s. This indicates that the WPSH increased in area, intensity, and experienced a significant westward extension after the 1980s. The lack of evident interdecadal changes in the WPSH ridgeline index suggests that no major northsouth
FIGURE 5.2 The time series of the four WPSH indices from 1951 to 2017: (A) area index, (B) intensity index, (C) ridgeline index, and (D) westernmost point. WPSH, Western Pacific subtropical high.
5.3 WESTERN PACIFIC SUBTROPICAL HIGH’S BASIC FEATURES
89
change occurred. The climatological average of the four WPSH indices during 19812010 is: area index, 64.08 (105 km2); intensity index, 131.91 (106 km2 gpm); ridgeline index, 25.16 N; and the westernmost point, 127.5 E. The changes of the WPSH status (area, intensity, and eastwest and northsouth location) present a close correlation with the distribution of floods and drought over China in summer. In particular, the northsouth movement of the WPSH ridgeline index, often determines the location and intensity of Meiyu (or the plum rain, a special meteorological phenomenon over the middle and lower reaches of the Yangtze River basin during summer) (Ninomiya, 1984; Tao and Chen, 1987; Wang and Li, 2004). Fig. 5.3A is the correlation distribution of the WPSH ridgeline index with JJA rainfall anomalies in China. It shows a very good negative correlation along the Yangtze River basin and its southern region, reaching the 0.05 t-test significant level (the absolute value of correlation coefficient is $ 0.36). It indicates that when the WPSH is more southward, the rainfall belt will most likely stagnate over the Yangtze River basin, which is favorable for redundant rainfall there. When the WPSH moves further northward than normal, the rainfall belt tends to shift northward, resulting in deficient rainfall over the Yangtze River basin.
FIGURE 5.3 The correlation distribution between the WPSH ridgeline index and rainfall anomalies over China in (A) JJA, (B) June, (C) July, and (D) August. Significant correlations above the 0.05 confidence level are shaded. WPSH, Western Pacific subtropical high; JJA, JuneJulyAugust.
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CHAPTER 5 CHARACTERISTICS OF THE WESTERN PACIFIC SUBTROPICAL
The correlation distributions of the WPSH ridgeline and rainfall anomalies in China during JJA are shown in Fig. 5.3BD. It clearly reflects the impact of WPSH’s northsouth location on the rainfall belt over eastern China in different months. In June (Fig. 5.3B), the WPSH ridgeline presents a pronounced negative correlation with rainfall anomalies in southern and southeastern China, but a positive correlation with the YangtzeHuaihe basin region. That means that when the WPSH ridgeline is more southerly than normal in June, excessive rainfall is indicated in southern and southeastern China and deficient rainfall over the YangtzeHuaihe basin region. In July (Fig. 5.3C), the WPSH ridgeline “jumps northward” with the significantly negative correlation region moving to the Yangtze River basin and north of southeastern China, that is, the southerly WPSH ridgeline in July leads to excessive rainfall over the Yangtze River basin. In August (Fig. 5.3D), with the WPSH ridgeline “jumping northward for the second time,” its negative correlation region shifts slightly further northward than in July, but with reduced correlation. The WPSH ridgeline also presents a negative correlation with the Tibetan plateau and north of southwestern China. This is perhaps the indirect impact reflected from anomalies of other atmospheric circulation systems induced by the WPSH anomaly, such as the Indo-Burma trough.
5.4 INFLUENCE OF THE WESTERN PACIFIC SUBTROPICAL HIGH ON RAINFALL ANOMALIES IN CHINA The relationship among the four WPSH indices will be comprehensively considered in this section to attain different classifications according to their correlation coefficients and the corresponding distributions of the rainfall anomalies in JJA. The purpose of this endeavor is to analyze the influence of the WPSH on the rainfall anomalies in China.
5.4.1 CORRELATION OF THE WESTERN PACIFIC SUBTROPICAL HIGH INDICES Table 5.1 lists the correlation coefficients of the four monthly WPSH indices during the period of 19512017, which indicates that there is an almost identical change trend with the area index and intensity index, with their correlation coefficient reaching 0.97. While the westernmost point has a mostly opposite trend with the area index and intensity index, whose correlation coefficients stand at 0.75 and 0.67 respectively, exceeding the 0.01 t-test significant level. This indicates that when the WPSH gets larger and stronger, it will be a more westward extension, while it will be a Table 5.1 Correlation Coefficients Among the Reconstructed Monthly WPSH Indices Area Intensity Ridgeline Westernmost point
Area
Intensity
Ridgeline
Westernmost Point
1.00
0.97 1.00
0.38 0.39 1.00
2 0.73 2 0.65 2 0.10 1.00
WPSH, Western Pacific subtropical high.
5.4 INFLUENCE OF THE WESTERN PACIFIC SUBTROPICAL HIGH
91
more eastward withdrawal when it gets smaller and weaker. The two indices that represent the WPSH locations, namely the ridgeline index and westernmost point, show the least correlation (only 0.16). This signifies the distinct independence of these two indices and a comprehensive analysis of these two WPSH indices represents the WPSH climatological changes to the largest extent. The high correlation of the WPSH area index and westernmost point enables us to consider the eastwest location and the area and intensity change simultaneously. Thus to further study the relationship between the WPSH location and summer rainfall belt in China, these two independent indices are selected to construct nine classifications to analyze the guiding role of the position of the WPSH on the summer rainfall anomalies in China.
5.4.2 WESTERN PACIFIC SUBTROPICAL HIGH’S INFLUENCE ON RAINFALL ANOMALIES The nine classification charts, represented by two WPSH location indices from 1951 to 2017 are constructed, with the anomalies of WPSH westernmost point as the x-axis and the anomaly of ridgeline index as the y-axis (Fig. 5.4). The WPSH normal years are specified as those when the
FIGURE 5.4 The nine classifications chart according to the combination of the summer WPSH ridgeline and westernmost point indices from 1951 to 2017. (I) More northward and westward; (II) more northward; (III) more northward and eastward; (IV) more eastward; (V) more southward and eastward; (VI) more southward; (VII) more southward and westward; (VIII) more westward; and (IX) normal. WPSH, Western Pacific subtropical high.
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westernmost point index falls within 5 longitudes to the west and east relative to climatological average, and when ridgeline index falls within 0.5 latitude to the north and south. Other WPSH abnormal years go as follows: more northward and westward, more northward, more northward and eastward, more eastward, more southward and eastward, more southward, more southward and westward, and more westward. These nine WPSH classification charts reflect two basic features. (1) The WPSH experiences a pronounced interdecadal transition around the 1980s. The WPSH was always more eastward than normal before the 1980s, whereas the westernmost point extends westward significantly after the 1980s, which agrees with the features reflected in Fig. 5.2. (2) When the westernmost point is northward, there is a relatively high probability that it is also more eastward than normal. There were only 2 years of western anomalies of the WPSH over past 60 years, namely in 1990 and 2006. On the contrary, when the WPSH is southward, there is a relatively high probability of a more westward westernmost point. Only 5 years were observed with an eastward anomaly. The distribution of the JJA rainfall anomalies over China corresponding to the nine WPSH classifications are shown in Fig. 5.5. It can be seen that the rainfall belt in eastern China presents a corresponding distribution with the change of the WPSH’s location. During the years between 1951
FIGURE 5.5 Distributions of the summer rainfall anomalies corresponding with the nine classifications of the WPSH: (A) more northward and westward; (B) more northward; (C) more northward and eastward; (D) more eastward; (E) more southward and eastward; (F) more southward; (G) more southward and westward; (H) more westward; (I) normal. WPSH, Western Pacific subtropical high.
5.5 SUMMARY AND DISCUSSION
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and 2017, only in 1990 and 2006 did the WPSH location show the feature of being more northward and westward (Classification I). Under this classification, the JJA rainfall anomalies in eastern China presents two rainfall belts, which are located at the south of the Yangtze RiverSouth China and HuaiheYellow River basin, respectively, while there was deficient rainfall over the Yangtze River basin (Fig. 5.5A). When the WPSH is more northward (Classification II) in comparison with Classification I, the rainfall belt in northern China obviously lifts to the north of the Yellow River basin and connects with the rainfall belt in northeastern China, while the rainfall belt over the lower reaches of the Yangtze River severely weakened in scope and intensity (Fig. 5.5B). If the WPSH is not only northward but also westward (Classification III), the northern rainfall belt will further expand and become the main rainfall belt, while the rainfall belt in the south will weaken and almost disappear, which is the typical wet northdry south pattern of the JJA rainfall anomalies (Fig. 5.5C). When the WPSH displays a clear-cut westward anomaly (Classification IV), in comparison with Classification III, the rainfall belt in northern China shifts slightly toward the south, forming another new rainfall belt over the YangtzeHuaihe River region and the lower reaches of the Yangtze River basin, and features a twin rainfall belt distribution with an extended rainfall belt, but a drastic difference from the WPSH Classification I (Fig. 5.5D). When the WPSH falls southward and features more southward and eastward (Classification V), the rainfall belt in eastern China will shift south as well, while, at the same time, the rainfall belt in the north will weaken and largely vanish. However, a northeastsouthwest rainfall belt will appear in western China (Fig. 5.5E). If the WPSH presents a more southward feature (Classification VI), the JJA rainfall anomalies under this classification is demarcated with the Yangtze River basin, presenting the typical wet southdry north pattern (Fig. 5.5F). If the WPSH features not only southward but also westward (Classification VII), in comparison with Classification VI, the main rainfall belt will migrate northward, hovering over the Yangtze River and YangtzeHuaihe regions, resulting in scant rainfall in northern and southern China, showing a negativepositivenegative pattern over eastern China (from north to south) (Fig. 5.5G). When the WPSH shows a westward feature (Classification VIII), the rainfall belt will slightly shift northward in parallel, staying over the region between the Yangtze and Huaihe riversYellow River, with the emergence of a weak rainfall zone in the eastern part of southern China and south of the lower reaches of the Yangtze River basin (Fig. 5.5H). When the WPSH is at the climatic mean location (Classification IX), the JJA rainfall will concentrate in the lower reaches of the Yangtze River basin and southern China, with a significantly reduced rainfall belt compared to the one presented under Classification VI (Fig. 5.5I). Hence, it can be seen that there is a good relationship between the WPSH location and the JJA rainfall belt distribution in China. The northsouth location of the ridgeline directly determines the northsouth distribution of the main JJA rainfall belt, while the westernmost point’s eastwest matching feature will affect the range of the rainfall belt or determine whether a secondary rainfall belt will occur or not.
5.5 SUMMARY AND DISCUSSION This chapter introduced a set of WPSH monitoring indices used by the NCC, including area index, intensity index, ridgeline index, and westernmost point. The WPSH indices are calculated by 500 hPa geopotential height and zonal wind from the NCEP/NCAR reanalysis datasets from 1951
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to 2017. Through the analysis of the WPSH indices and their relationships with JJA rainfall anomalies in China, it was found that the WPSH indices has a number of advantages. The set of WPSH indices enable objectively-to describe the characteristics of the WPSH’s monthly change and overcome the defect of the excessive dependence on the data resolution. The ridgeline index has a significant correlation with the summer rainfall over the Yangtze River basin, which indicates that it takes full consideration of the impact of the WPSH system on the summer rainfall in eastern China. Moreover, two kinds of relatively independent WPSH indices, namely the ridgeline index and westernmost point, are selected to combine the nine classifications of anomalous WPSH features, which correspond with nine kinds of distributions of summer rainfall anomalies over eastern China. It provides a scientific basis to further understand the relation of the position anomalies of the WPSH and the summer main rainfall belts in eastern China. The analysis in this chapter shows that the WPSH’s northsouth and eastwest location anomalies have an important impact on the JJA rainfall anomalies in China. Through the nine classifications by the two WPSH location indices, it was possible to cover various distribution features of rainfall anomalies patterns over China in JJA to a great extent. It provides a scientific basis for our further understanding of the relationship between the anomalies of the WPSH’s location and the major rainfall belts in summer. The set of WPSH indices has been used by the NCC of the China Meteorological Administration in a real-time climate monitoring operation. Chapter 6, Tropospheric Biennial Oscillation of Western Pacific Subtropical High and Its Relationships With the Tropical Sea Surface Temperature and Atmospheric Circulation Anomalies and Chapter 7, Tropospheric Biennial Oscillation of Monsoon Rainfall and Its Association With El Nin˜o-Southern Oscillation, will discuss the interannual variability of the WPSH and its possible physical mechanisms.
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Tao, S.Y., Zhu, F.K., 1964. The variation of 100-mb flow patterns in southern Asia in summer and its relationship to the movement of western Pacific subtropical high. Acta Meteorol. Sin. 34, 385390 (in Chinese). Tao, S.Y., Zhang, Q.Y., Zhang, S.L., 1998. The great floods in the Yangtze River valley in 1998. Clim. Environ. Res. 3, 290299 (in Chinese). Tao, S.Y., Ni, Y.Q., Zhao, S.X., et al., 2001. Study on the Formation Mechanism and Forecast of Rainstorm in Summer of 1998 in China. Meteor. Press, Beijing, pp. 1931 (in Chinese). Wang, B., Li, T., 2004. East Asian monsoon-ENSO interactions. In: Chang, C.P. (Ed.), East Asian Monsoon. World Scientific Publishing, Singapore, pp. 177212. Wu, G.X., Chou, J.F., Liu, Y.M., 2002. Dynamics of Formation and Variation of Subtropical High. Science Press, Beijing, pp. 120 (in Chinese). Xiong, A.Y., 2001. Analysis on background of climatic variation for extremely rainy over the middle reaches of the Yangtze River in the 1990s. J. Appl. Meteorol. Sci. 12, 113117 (in Chinese). Xu, H.M., He, J.H., Zhou, B., 2001. The features of atmospheric circulation during Meiyu onset and possible mechanisms for westward extension and northward shift of western Pacific subtropical high. J. Appl. Meteorol. Sci. 12, 150158 (in Chinese). Yuan, Y., Gao, H., Liu, Y.J., 2017. Analysis of the characteristics and causes of precipitation anomalies over eastern China in the summer of 2016. Meteorol. Monthly 43, 115121 (in Chinese). Zhan, R.F., Li, J.P., He, J.H., 2005. Statistical characteristics of the double ridgelines of the subtropical high in northern hemisphere. Chin. Sci. Bull. 50, 20222026 (in Chinese). Zhao, Z.G., 1999. Summer Drought and Flood and Environmental Field in China. Meteor. Press, Beijing, pp. 4546 (in Chinese). Zhou, B., He, J.H., Xu, H.M., 2004. Effect of rainstorm process on variability of subtropical high. J. Appl. Meteorol. Sci. 15, 394406 (in Chinese).
FURTHER READING Tao, S.Y., 1963. Study on Several Problems of Subtropical Weather System in Summer over China. Science Press, Beijing, 146 pp. (in Chinese).