The Interdecadal Variation of the Western Pacific Subtropical High as Measured by 500 hPa Eddy Geopotential Height

HUANG Yan-Yan and LI Xiao-Fan

School of Earth Science, Zhejiang University, Hangzhou 310027, China

The Interdecadal Variation of the Western Pacific Subtropical High as Measured by 500 hPa Eddy Geopotential Height

HUANG Yan-Yan and LI Xiao-Fan

School of Earth Science, Zhejiang University, Hangzhou 310027, China

The interdecadal variation of the summer western Pacific subtropical high (WPSH) during 1948-2009 is investigated in this study. Compared with most previous works, which focused on the 500 hPa geopotential height, the interdecadal variation of the horizontal winds, relative vorticity, and eddy geopotential height over the western Pacific are all analyzed. The weakened anticyclone and decreased negative relative vorticity at middle-low levels over the western Pacific suggest that the WPSH weakened during 1979-2009 relative to 1948-78. After subtracting the zonal belt mean height between 0° and 40°N, the 500 hPa eddy geopotential height with significant negative anomalies over the western Pacific can correctly depict this weakened interdecadal variation of the WPSH. The illusory westward extension signal reflected by the 500 hPa geopotential height may derive from the significant increment of the geopotential height at middle and lower latitudes in the late 1970s under global warming.

western Pacific subtropical high, interdecadal variation, east Asian summer climate

1　Introduction

Owing to its great influence on the east Asian summer climate and its complicated structure, the western Pacific subtropical high (WPSH) has received much attention from East Asian scientists. The seasonal movement and interannual variability of the WPSH and its relationship with the East Asian summer monsoon (EASM) (Ye and Zhu, 1958; Tao and Chen, 1987; Wang and Chen, 2012; Wang et al., 2013) and precipitation over China (Huang and Yu, 1962; Chang et al., 2000; Wang et al., 2001; Zhu et al., 2010) have been intensively studied. In recent decades, the interdecadal variation of the WPSH has been investigated to explain the interdecadal change of the East Asian summer climate. Comparing the location of the 5880/5870 gpm contour line in the 500 hPa geopotential height field before the 1970s with that after the 1970s, Nitta and Hu (1996), Hu (1997), and Gong and Ho (2002) found a westward extended interdecadal variation of the WPSH in the late 1970s. The abrupt changes of summertemperature and rainfall over the subtropical regions of East Asia (Nitta and Hu, 1996; Hu, 1997), the increased (decreased) rainfall over the middle and lower reaches of the Yangtze River valley (North China) (Gong and Ho, 2002), and interdecadal variation of the typhoon tracks in the western North Pacific (Gong and He, 2002; Ho et al., 2004) after the late 1970s are attributed to the westward extension of the WPSH. Based upon statistical analysis (Hu, 1997; Gong and Ho, 2002; He and Gong, 2002) and numerical modeling (Zhou et al., 2009), the Indian Ocean-western Pacific warming is proposed as having been responsible for this westward extended WPSH.

Of note is that previous research focused only on measuring the interdecadal variation of the WPSH in the 500 hPa geopotential height field; the interdecadal changes of other atmospheric variables associated with the WPSH have not yet been investigated. Meanwhile, some researchers have also argued that direct use of the geopotential height field itself to examine the interdecadal variation of the WPSH may be inappropriate, due to the artificial trends of the lifted isobaric surface at middle and lower latitudes, caused by global warming (Yang and Sun, 2003; Lu et al., 2008). Huang et al. (2015) reinvestigated the interdecadal variation of the WPSH by using multiple WPSH-associated atmospheric variables at 850 hPa. According to the consistent interdecadal signals, their results suggested that the WPSH at 850 hPa recessed eastward during 1979-2009 relative to 1948-78. However, the interdecadal variation of the WPSH at 500 hPa has not yet been reexamined. This will be addressed in the present paper.

The remainder of the paper is organized as follows: Section 2 describes the data and method used in this study. The interdecadal variation of the WPSH in the fields of horizontal winds and relative vorticity is examined in section 3. A comparative study between 500 hPa geopotential height and eddy geopotential height in reflecting the interdecadal variation of the WPSH is presented in section 4. Section 5 summarizes the key findings of the study.

2　Data and method

The interdecadal variation of the summer (June-July-August, JJA) WPSH during 1979-2009 relative to 1948-78 is analyzed in this study. The geopotential height and winds data derive from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis 1 (Kalnay et al.,1996) and the precipitation from the Climate Research Unit (Mitchell and Jones, 2005). The interdecadal variation is characterized by the difference between the means for the period 1979-2009 and 1948-78. The statistical significance is assessed using the Student's t-test.

3　The interdecadal signals in horizontal winds and relative vorticity

Figure 1a shows the difference in summer middlelower tropospheric (850-500 hPa) layer-averaged winds between 1979-2009 and 1948-78. A significant anomalous cyclone is observed over the western Pacific, suggesting a weakened subtropical anticyclone since the late 1970s. In addition, anomalous northeasterly winds prevail from North China to South China, indicating a weakened EASM after the 1970s (Wang, 2001).

The difference in the summer relative vorticity at middle and lower levels (1000-500 hPa) along 15-35°N between 1979-2009 and 1948-78 is illustrated in Fig. 1b. Remarkable positive anomalies appear over the western Pacific. According to a stronger WPSH corresponding with a higher negative relative vorticity over the western Pacific, these positive anomalies of relative vorticity also suggest a weakened WPSH during 1979-2009 relative to 1948-78. More interdecadal variations of other WPSH-related atmospheric variables can be seen in Huang et al. (2015). Overall, the anomalous cyclone and positive anomalies of relative vorticity at middle and lower levels over the western Pacific suggest that the WPSH weakened during 1979-2009 relative to 1948-78.

4　The interdecadal signals at 500 hPa geopotential height and eddy geopotential height

In most previous studies, the interdecadal variation of the WPSH was mainly measured by the interdecadal change in the location of the 5860/5870 gpm contour line in the 500 hPa geopotential height field. Considering the contrary results in previous work, the shortcomings of the traditional measurement based upon 500 hPa geopotential height are analyzed in this section. Meanwhile, a new measurement is also proposed.

The climatological summer 500 hPa geopotential heights during 1948-78 and 1979-2009 are shown in Figs. 2a and 2b, respectively. Compared with 1948-78, the 5860 gpm contour covers a larger area and the west end of this contour shifts remarkably westward during 1979-2009. It seems to indicate a westward extended WPSH in the late 1970s, which has been documented in many previous studies. However, on the difference map of 500 hPa geopotential height between 1979-2009 and 1948-78, significant increased geopotential height is observed not only over the western Pacific but also over other south of 40°N (Fig. 2c). Actually, the value of the zonal belt mean height at 500 hPa between 0° and 40°N significantly increases after the late 1970s, with averaged values of 5853 gpm in 1948-78 and 5867 gpm in 1979-2009 (Fig. 2d). This implies that the 500 hPa isobaric surface between 0° and 40°N lifted after 1979, which may be attributable to global warming. In addition, the increments in the averaged values of the zonal belt mean height between 0° and 40°N during 1979-2009 relative to 1948-78 can be observed at each level from 1000 hPa to 500 hPa (figure not shown). The European Center for Medium-Range Weather Forecasts 40-yr Reanalysis dataset (ERA-40; Uppala et al., 2005) also illustrates the increments of the geopotential height at middle and lower latitudes in the late 1970s, but with slightly smaller magnitudes (figure not shown). Therefore, the westward extension signal of the WPSH reflected by the 500 hPa geopotential may derive from the lifted isobaric surface, which is not the objective interdecadal variation of the WPSH.

In order to avoid the above-mentioned artificial trends caused by global warming, we propose to use the 500 hPa eddy geopotential height to examine the interdecadal variation here. The eddy geopotential height is calculated by subtracting the zonal belt mean height between 0° and 40°N from the geopotential height (i.e., the values in Fig. 2d). Figures 2e and 2f display the climatological summer 500 hPa eddy geopotential height during 1948-78 and 1979-2009, respectively. During 1948-78, the 10 gpm contour in Fig. 2e corresponds well with the 5860 gpm contour in Fig. 2a, which hints that there may be consistency between the 10 gpm contour in eddy geopotential height and the 5860 gpm contour in geopotential height in representing the WPSH. To further verify this consistency, we calculate the western ridge point index (WRPI) of the WPSH: the longitudinal position of the western ridge of the WPSH over (0°-45°N, 90-180°E) based on the 10 gpm contour in eddy geopotential height (WRPI-10) and the 5860 gpm contour in geopotential height (WRPI-5860), separately. The correlation coefficient between WRPI-10 and WRPI-5860 is 0.42 during 1948-2009, significant at the 99.9% confidence level. This suggests consistency between the two contours in describing the interannual variability of the WPSH. Moreover, compared with the correlation coefficient of -0.33 between WRPI-5860 and the regional-averaged summer precipitation over the middle and lower Yangtze River valley (28-34°N, 106-122°E) (Yangtze River rainfall index, YRI), the correlation coefficient between WRPI-10 and YRI is -0.40 during 1948-2009, significant at the 99% confidence level. This further implies consistency between the two contours in reflecting the influence of the WPSH on theEast Asian summer climate. Therefore, we choose the 10 gpm contour in eddy geopotential height to identity the WPSH in the following part. Compared with 1948-78, the 10 gpm contour does not extend westward during 1979-2009. In Fig. 2f, the coverage area surrounded by the 20 gpm contour is even smaller during 1979-2009 compared with 1948-78. On the difference map in 500 hPa eddy geopotential height, significantly decreased eddy geopotential height is observed over the western Pacific (Fig. 2g), which is consistent with the interdecadal signals in the fields of horizontal winds and relative vorticity.

Moreover, the intensity index of the WPSH is further calculated following the method of the National Climate Center of China. For comparative purposes, the WPSH intensity indices based upon 500 hPa geopotential height and eddy geopotential height are both analyzed, which we refer to as H500 index and H'500 index, respectively. The traditional H500 index (H'500 index) is defined as the accumulated values with a 500 hPa geopotential height greater than 5860 gpm (500 hPa eddy geopotential height greater than 10 gpm) within the domain (0°-40°N, 110-180°E). In Fig. 2h, the magnitudes of the two indices are close during 1948-78. However, after 1979 the magnitude for the H500 index increases remarkably, which may be attributable to the lifted isobaric surface caused by global warming, as mentioned above. Because of this increased magnitude, the H500 index displays an illusory strengthening trend of 53.65 during 1948-2009, significant at the 99.9% confidence level. After subtracting the zonal belt mean height between 0° and 40°N, the H'500 index does not show any remarkable magnitude change around 1978. In contrast with the H500 index, a significant weakening trend of -14.86 can be observed in the H'500 index (significant at the 99.9% confidence level) during 1948-2009. The different interdecadal signals in the WPSH area index between 500 hPa geopotential height and eddy geopotential height can also be observed (figures not shown), in which the WPSH area index is defined as the number of grid points whose height/eddy height is greater than 5860 gpm/10 gpm in the same domain. In addition, the correlation coefficient between the detrended H500 index and the detrended H'500 index is 0.51 during 1948-2009, significant at the 99.9% confidence level. This implies consistency between these two indices in representing the interannual variability of the WPSH. Overall, compared with the traditional measurement based upon 500 hPa geopotential height, the 500 hPa eddy geopotential height is more adequate to investigate the interdecadal variation of the WPSH.

5　Summary

The interdecadal variation of the WPSH during 1979-2009 relative to 1948-78 is analyzed using NCEP/NCAR reanalysis data in this study. The anomalous cyclone in the middle-lower tropospheric (850-500 hPa) layer-averaged winds and significant positive anomalies of relative vorticity at middle and lower levels (1000-500 hPa) along 15-35°N over the western Pacific both suggest that the WPSH weakened during 1979-2009 relative to 1948-78. The illusory westward extended signals reflected by 500 hPa geopotential height may derive from the contribution of the lifted isobaric surface between 0° and 40°N caused by global warming. After subtracting the zonal belt mean height between 0° and 40°N, the 500 hPa eddy geopotential height can accurately reflect the interdecadal weakened signal of the WPSH since the late 1970s. This feature of the eddy geopotential height may be attributable to the fact that the eddy geopotential height field can, at least partly, avoid the influence of the artificial trends caused by global warming.

Huang et al. (2015) suggested some reasonable factors that may have been responsible for the eastward recessed WPSH in the late 1970s, such as an interdecadal recharging phase of the Indian Ocean (Li et al., 2008) and western North Pacific warming, tropospheric cooling over East Asia (Yu et al., 2004), decadal-weakened Asia-Pacific Oscillation (Zhao et al., 2007), and a weakening of the Tibetan Plateau thermal forcing (Liu et al., 2012). However, the specific mechanism for the interdecadal weakening of the WPSH in the late 1970s is still unknown. Further quantitative diagnostic analyses, as well as state-of-the art climate modeling, is needed in further work.

Acknowledgments. This study was supported by the National Natural Science Foundation of China (Grant No. 41475039), the National Key Basic Research Program of China (Grant No. 2015CB-953601), and a China Postdoctoral Science Foundation-funded project (Grant No. 2015M570500).

References

Chang, C. P., Y. Zhang, and T. Li, 2000: Interannual and interdecadal variations of the East Asian summer monsoon and tropical Pacific SSTs. Part I: Roles of the subtropical ridge, J. Climate, 13, 4310-4325.

Gong, D. Y., and X. Z. He, 2002: Interdecadal change in western Pacific subtropical high and climatic effects, Acta Geogr. Sinica (in Chinese), 57, 185-193.

Gong, D. Y., and C. H. Ho, 2002: Shift in the summer rainfall over the Yangtze River valley in the late 1970s, Geophys. Res. Lett., 29(10), doi:10.1029/2001GL014523.

He, X. Z., and D. Y. Gong, 2002: Interdecadal change in western pacific subtropical high and climatic effects, J. Geogr. Sci., 12, 202-209.

Ho, C. H., J. J. Baik, J. H. Kim, et al., 2004: Interdecadal changes in summertime typhoon tracks, J. Climate, 17, 1767-1776.

Hu, Z. Z., 1997: Interdecadal variability of summer climate over East Asia and its association with 500 hPa height and global sea surface temperature, J. Geophys. Res., 102(D16), 19403-19412, doi:10.1029/97JD01052.

Huang, S. S., and Z. H. Yu, 1962: On the structure of the subtropical highs and some associated aspects of the general circulation of atmosphere, Acta Meteor. Sinica (in Chinese), 31, 339-359.

Huang, Y., H. Wang, K. Fan, et al., 2015: The western Pacific subtropical high after the 1970s: Westward or eastward shift? Climate Dyn., 44, 2035-2047.

Kalnay, E., M. Kanamitsu, R. Kistler, et al., 1996: The NCEP/ NCAR 40-year reanalysis project, Bull. Amer. Meteor. Soc., 77, 437-471.

Li, S., J. Lu, G. Huang, et al., 2008: Tropical Indian Ocean basin warming and East Asian Summer Monsoon: A multiple AGCM study, J. Climate, 21, 6080-6088.

Liu, Y., G. Wu, J. Hong, et al., 2012: Revisiting Asian monsoonformation and change associated with Tibetan Plateau forcing: II. Change, Climate Dyn., 39, 1183-1195.

Lu, R., Y. Li, and C. S. Ryu, 2008: Relationship between the zonal displacement of the western Pacific subtropical high and the dominant modes of low-tropospheric circulation in summer, Prog. Nat. Sci., 18, 161-165.

Mitchell, T. D., and P. D. Jones, 2005: An improved method of constructing a database of monthly climate observations and associated high-resolution grids, Int. J. Climatol., 25, 693-712.

Nitta, T., and Z.-Z. Hu, 1996: Summer climate variability in China and its association with 500 hPa height an tropical convection, J. Meteor. Soc. Japan, 74, 425-445.

Tao, S. Y., and L. X. Chen, 1987: A review of recent research on the East Asian summer monsoon in China, in: Monsoon Meteorology, vol. 7, Oxford University Press, Oxford, 60-92.

Uppala, S. M., P. W. KÅllberg, A. J. Simmons, et al., 2005: The ERA-40 re-analysis, Quart. J. Roy. Meteor. Soc., 131, 2961-3012.

Wang, B., R. Wu, and K. M. Lau, 2001: Interannual variability of the Asian Summer Monsoon: Contrasts between the Indian and the Western North Pacific-East Asian Monsoons, J. Climate, 14, 4073-4090.

Wang, B., B. Xiang, and J. Y. Lee, 2013: Subtropical High predictability establishes a promising way for monsoon and tropical storm predictions, Proc. Natl. Acad. Sci. USA, 110, 2718-2722.

Wang, H., and H. Chen, 2012: Climate control for southeastern China moisture and precipitation: Indian or East Asian monsoon? J. Geophys. Res., 117, doi:10.1029/2012JD017734.

Wang, H. J., 2001: The weakening of the Asian monsoon circulation after the end of 1970's, Adv. Atmos. Sci., 18, 376-386.

Yang, H., and S. Sun, 2003: Longitudinal displacement of the subtropical high in the western Pacific in summer and its influence, Adv. Atmos. Sci., 20, 921-933.

Ye, D. Z., and B. Zhu, 1958: Some Fundamental Problems of the General Circulation of the Atmosphere (in Chinese), Science Press, Beijing, 159pp.

Yu, R., B. Wang, and T. Zhou, 2004: Tropospheric cooling and summer monsoon weakening trend over East Asia, Geophys. Res. Lett., 31, doi:10.1029/2004g1021270.

Zhao, P., Y. Zhu, and R. Zhang, 2007: An Asian-Pacific teleconnection in summer tropospheric temperature and associated Asian climate variability, Climate Dyn., 29, 293-303.

Zhou, T., R. Yu, T. Zhang, et al., 2009: Why the Western Pacific Subtropical High has extended westward since the late 1970s, J. Climate, 22, 2199-2215.

Zhu, Y., H. Wang, W. Zhou, et al., 2010: Recent changes in the summer precipitation pattern in East China and the background circulation, Climate Dyn., 36, 1463-1473.

Huang, Y.-Y., and X.-F. Li, 2015: The interdecadal variation of the western Pacific subtropical high as measured by 500 hPa eddy geopotential height, Atmos. Oceanic Sci. Lett., 8, 371-375,

10.3878/ AOSL20150038.

28 April 2015; revised 20 June 2015; accepted 30 June 2015; published 16 November 2015

HUANG Yan-Yan, yanyanhuang@zju.edu.cn