Recent Change of the South Asian High

YANG Kai-Qingand JIANG Da-Bang

1Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

2Climate Change Research Center, Chinese Academy of Sciences, Beijing 100029, China

3Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Recent Change of the South Asian High

YANG Kai-Qing1,3and JIANG Da-Bang1,2

1Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

2Climate Change Research Center, Chinese Academy of Sciences, Beijing 100029, China

3Graduate University of Chinese Academy of Sciences, Beijing 100049, China

This paper investigates the variability of the summer (May-September) South Asian High (SAH) for the period 1979-2012. Results show that the intensity and the area of the summer SAH decreased around 2002 at the decadal scale; and the East Asian westerly jet suppressed at the north edge of the SAH, which is consistent with the SAH variation. The precipitation pattern over eastern China also shifted during the same periods, with increased rainfall in the Huang-Huai River region and South China and decreased rainfall in the Yangtze River region. The relationship between the two variations is evidently strengthened via changes in moisture flux.

South Asian High, decadal variation

1 Introduction

In boreal summer, the upper-tropospheric South Asian High (SAH) is the strongest, largest, and most stable anticyclone near the 100-hPa level (Mason and Anderson, 1963; Zhu et al., 1980). It is one of the leading components of the Asian summer monsoon system and significantly affects the Asian climate (Zhu et al., 1980). In the early 1960s, Mason and Anderson (1963) first discovered the SAH, and since then much work has been done to investigate its impact upon Asian weather and climate. For example, Zhu et al. (1980) revealed a relationship between the eastwest oscillation of the 100-hPa SAH and the summer weather in China. Zhang and Wu (2001) further pointed out that its intensity is positively correlated with the precipitation anomalies in the Yangtze River Valley.

A prominent decadal climate transition has been reported to have occurred in East Asia in the late 1970s (Wang, 2001, 2002; Jiang and Wang, 2005; Han and Wang, 2007), which was accompanied by an increase in the area and intensity of the SAH (Zhang et al., 2000); and since then, there has been a change in the precipitation pattern in eastern China (Hu et al., 2003; Ding et al., 2008). Recently, a new decadal global climate change has been noticed. Tsonis et al. (2007) showed a foreshadowing of dynamically driven climate shifts suggested by the intrinsic mechanism of the climate system. Swanson and Tsonis (2009) further updated this work and identified a shifting point in 2002. Meanwhile, Zhu et al. (2011) pointed out a decadalsummer precipitation pattern shift in East China occurred in about 1999, induced by the lower level atmospheric circulations. However, little focus has been given to the upper tropospheric circulation. Therefore, the present study aims to address the decadal variation at this level.

2 Data and methods

The atmospheric circulation data applied from the European Centre for Medium-Range Weather Forecasts (ECMWF) re-analysis (Simmons et al., 2007). The variables analyzed include the geopotential heights, winds, and the vertical integral of water vapor flux (calculated from the surface to the 0.1-hPa level) at a horizontal grid resolution of 1.5° × 1.5°. The sea surface temperature data are provided by the National Oceanic and Atmospheric Administration (NOAA) (specifically, the NOAA Extended Reconstructed Sea Surface Temperature V3b dataset), which have a horizontal resolution of 2° × 2° (Smith et al., 2008). The precipitation analysis employs a high-resolution (0.25° × 0.25°) gridded daily dataset, CN05.1, developed by Wu and Gao (2013), which is derived based on interpolation from 2416 observation stations across China.

To remove the long-term global warming trend, the zonal mean geopotential height is subtracted from the height fields at each grid point (H＇=H-Hzonalmean). Based on the features of the new height fields, we refer to the definition methods used by Zhang et al. (2000) and define four indices to characterize the area and intensity of the SAH in the region between 15-45°N and 45-105°E: (1) the number of grids withH＇ greater than 1000 gpm, denoted asA; (2) the maximumH＇ value, denoted asI1; (3) the sum of (H＇-1000 gpm), on the premise that theH＇ is greater than 1000 gpm, denoted asI2; and (4)I2divided byA, denoted asI3. All the analyses are performed for the period 1979-2012, except for the precipitation and the water vapor flux, as the CN05.1 data are only available up to 2010.

3 Results

Previous studies have found that the SAH is most outstanding in May to September (MJJAS) over the Tibetan Plateau, and its decadal-scale variations are also significant in MJJAS (Zhu et al., 1980; Zhang et al., 2000). Hence, the analyses presented below focus mainly on the MJJAS mean conditions.

3.1 Recent changes of the SAH

We begin our study on when the decadal change occurred for the MJJAS meanH＇ fields at the 100-hPa level, by testing the abrupt transition at each grid point using the 10-year movingt-test. The distributions of the dominant change regions in all the available years show that 2002 is the most remarkable. Thus, we take 2002 as the decadal shift point.

The MJJASH＇ fields at the 100-hPa level during the two periods show that the decadal weakening is located over the Tibetan Plateau and the Iranian Plateau (Fig. 1a), with a reduction in the area of the SAH and a weakening in the intensity of the SAH. To quantitatively show the weakening of the SAH, we further calculate the four normalized indices defined in section 2 and their low-pass filtering trends. Figures 1b-e show that all the indices have a decreasing trend, and their decadal changes are appreciable. Since the linear trends do not statistically reveal the decadal scale shifts, we perform significance tests for all the indices. By using 10-year movingt-tests, it is evident that all the indices significantly decrease in 2002 at a 95% confidence level, especially for the indexI1, which represents the maximum of theH＇. Therefore, it can be concluded that the intensity and area of the SAH decreased after 2002 at the decadal scale, and the trend is contrary to that before subtracting the zonal mean height.

Besides the SAH, the East Asian Westerly Jet stream (EAWJ) is another major feature in the upper troposphere. Similar to the SAH, the EAWJ is also considered as a thermo-driven system centered at 200 hPa that plays an important role in the East Asian climate (Dong et al., 1999; Zhang et al., 2006). The meridional location and the intensity of the EAWJ are closely associated with the rain band in the East Asian monsoon region (Lu, 2004; Zhou and Wang, 2006; Zhou, 2011), and the SAH and the EAWJ have been shown to be related via two different SST patterns (Liao et al., 2004). Given these foundations, we perform an analysis of the EAWJ during the same period to further confirm the decadal transition of the SAH.

At the 200-hPa level, the MJJAS EAWJ climatology has two main cores near (40°N, 100°E) and (40°N, 150°E) respectively (Fig. 2a), and the SAH lies to the southwest of the west one. In the difference fields between the periods 2002-2012 and 1979-2001 (Fig. 2b), two reduction centers stand out. One is located along the north of the jet axis at the entrance region and the other along the south of the jet axis at the exit region. The decrease center at the entrance region lies beneath the north edge of the SAH, the weakened zonal wind here is consistent with the intensity decrease of the SAH, providing evidence for the SAH variation. In addition, the decrease center at the exit region is located at the south of the EAWJ core, suggesting a northward displacement of the EAWJ. Taken together, the variations of the SAH and the EAWJ are consistent with each other, and both illustrate a significant decadal weakening of the SAH.

3.2 Relationship between the SAH and precipitation

Figure 1 (a) May to September (MJJAS) contour lines of 1000 and 1200 gpm forH＇(H＇=H-Hzonalmean) at the 100-hPa level for the periods 1979-2001 (black) and 2002-2012 (blue). Shading indicates the significantly changed regions exceeding the 95% confidence level for 2002. (b-e) Normalized time series of the MJJAS South Asian High (SAH) indices: (b) area indexA; (c) intensity indexI1; (d) intensity indexI2; and (e) intensity indexI3.The solid lines are the 10-year low-pass filterings.

Figure 2 MJJAS zonal winds at the 200-hPa level for (a) 1979-2012 climatology and (b) the difference between 2002-2012 and 1979-2001. Shading in (b) indicates the significantly changed regions that exceed the 95% confidence level.

Since the relationship between the SAH and precipitation is evident, it is possible that the decadal variations of the SAH and that of the precipitation are related. To determine whether or not this link exists, we investigate the precipitation difference and the regressed vertical integral of water vapor flux, a dominant factor for the local precipitation formation.

Figure 3 Difference of MJJAS precipitation between 2002-2010 and 1979-2001. Shading indicates significant values that exceed the 95% confidence level.

Figure 3 shows that the precipitation pattern shifts at the decadal scale, with increased rainfall over the Huang-Huai River and South China but decreased rainfall over the Yangtze River during 2002-2010 compared to 1979-2001, and the variation over the Huang-Huai River is significant at the 95% confidence level. Situations for the regression patterns of the vertical integral of water vapor flux for the two periods show that in 1979-2001 (Fig. 4a) the relation between the area of the SAH and the moisture flux is not outstanding, while for the period 2002-2010 (Fig. 4b) the water vapor flux over the Huang-Huai River and South China are presented to be associated more with the area of the SAH. Significant anomalous westerly moisture flux transports from the Philippines to the subtropical western North Pacific, then turns easterly near 170°E, and finally arrives at South China. A part of the anomalous water vapor converges in South China, and the rest moves northward and becomes anomalous westerly winds. The anomalous westerly vapor converges again in the Huang-Huai River, resulting in more precipitation in South China and the Huang-Huai River. This anomalous transportation corresponds to a positive indexAand indicates a strengthened connection between the SAH and the precipitation after 2002.

4 Summary and discussion

The variability of the zonal-mean subtracted SAH was investigated in this study. The results show an abrupt decadal decrease in the MJJAS SAH intensity and area in 2002. The EAWJ also shows a decrease at the entrance region along the north of the axis, located just at the north edge of the SAH, and further confirms the summer SAH decadal variation. The precipitation pattern over eastern China exhibits a decadal change during the same period. The link between the two transitions is strengthened after 2002 via the anomalous vertical integral of water vapor flux.

Figure 4 The regressed MJJAS vertical integral of water vapor flux during (a) 1979-2001 and (b) 2002-2010, both against indexA, and (c) the difference between the two periods against indexA. Shading indicates significant areas that are above the 95% confidence level.

It should be noted that the limitations caused by the different time lengths between the circulation data and the precipitation data generate much uncertainty about the results, so more in-depth analyses are needed. Additionally, many studies have pointed out that the SST anomalies in the Indian Ocean and eastern equatorial Pacific are highly correlated with the SAH (Zhang et al., 2000; Yang et al., 2007). Meanwhile, the circum-global teleconnection revealed by Ding and Wang (2005) shows that it is possible for the signal derived from the northern Atlantic to transport to East Asia. These studies provide the basis for a hypothesis that the SST anomalies may be a possible underlying factor for this decadal transition, which needs further study.

Acknowledgments. This research was supported by the National Natural Science Foundation of China (Grants Nos. 41210007 and 41130103).

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Received 13 January 2014; revised 21 February 2014; accepted 24 February 2014; published 16 July 2014

YANG Kai-Qing, yangkq@mail.iap.ac.cn