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INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 35: 1172–1179 (2015) Published online 26 May 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.4047

Interdecadal variability of winter precipitation in Northwest China and its association with the North Atlantic SST change Lian-Tong Zhoua* and Renguang Wub b

a Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China Institute of Space and Earth Information Science, Department of Geography and Resource Management, The Chinese University of Hong Kong, Hong Kong, China

ABSTRACT: Winter precipitation in Northwest China experienced an obvious interdecadal increase around 1987. Consistent increase in winter precipitation occurred in Middle Asia. This study investigates associated changes in atmospheric circulation and sea surface temperature (SST). Analyses show that winter water vapour flux and atmospheric circulation over the North Atlantic Ocean and Eurasia and SST in the North Atlantic Ocean were very different before and after 1987. During 1987–2008, a significant enhancement of tropospheric moisture convergence and ascending motion was observed over Northwest China and Middle Asia. This contributed to the increase of winter precipitation in Northwest China and Middle Asia. The wind difference field before and after 1987 features cyclones over Middle Asia and North Atlantic Ocean and anticyclones over East Asia and southern Europe-northern Africa, signifying an obvious change in the Eurasian (EU) teleconnection pattern over middle latitudes of Eurasia. The results indicate that the Middle Asia and Northwest China were under the influence of enhanced westerlies from the North Atlantic Ocean that strengthened the water vapour transport to Middle Asia and Northwest China after 1987. Moreover, the interdecadal variability in the EU pattern is associated with the SST increase in the North Atlantic Ocean. Thus, the North Atlantic SST change is likely an important reason for the winter precipitation increase in Middle Asia and Northwest China. KEY WORDS

winter precipitation; Middle Asia and Northwest China; interdecadal variability; EU pattern; North Atlantic SST

Received 21 October 2013; Revised 25 March 2014; Accepted 22 April 2014

1.

Introduction

With the frequent occurrence of winter snow and ice storms, the variability of winter precipitation has been concerned increasingly (Xu and Chan, 2002; Wu et al., 2003; Gu et al., 2008; Zhou and Wu, 2010; Zhou et al., 2009; Wang and Feng, 2011; Zhou, 2011a). The winter precipitation anomalies have seriously affected the industrial and agricultural production as well as lives of people. Thus, the winter precipitation variability in China is an issue of particular importance and considerable urgency that needs investigation. However, previous studies mostly focused on the eastern China. In spite of relatively small annual precipitation in Northwest China, the same amount of precipitation anomalies can cause a greater impact in Northwest China relative to eastern China. Therefore, it is very important to understand the variability of winter precipitation in Northwest China and plausible reasons. Northwest China is located in arid and semi-arid regions to the north of 35∘ N and west of 110∘ E. The average annual total precipitation in Northwest China is 300 mm, and evaporation is greater than precipitation in this region. Besides, the Northwest China and Middle Asia are located * Correspondence to: L.-T. Zhou, Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, P.O. Box 2718, Beijing 100190, China. E-mail: [email protected]

© 2014 Royal Meteorological Society

over the westerlies. Thus, precipitation mainly occurs in spring and winter (Xie, 2000; Song and Zhang, 2003). The interdecadal variability in winter precipitation in Northwest China has been studied recently (e.g. Wei et al., 2000; Shi et al., 2002; Li et al., 2003; Song and Zhang, 2003; Yu et al., 2003; Zhou and Huang, 2010; Wang and Feng, 2011). Shi et al. (2002) pointed out a climatic shift from warm-dry to warm-humid in Northwest China around the mid-1980s, especially, west of Tien Mountain. The interdecadal variability in winter precipitation has been further examined by other researchers (Li et al., 2003; Song and Zhang, 2003; Yu et al., 2003; Zhou, 2011b). Li et al. (2003) indicated that the interdecadal variability in winter precipitation was associated with 500 hPa circulation change. Some studies indicated that the climate variability in middle latitudes of Asia was associated with the North Atlantic Oscillation (NAO), westerly jet (Rogers and Van Loon, 1979; Aizen et al., 2001). As it is well known, there was a Eurasian (EU) teleconnection pattern in atmospheric circulation during the Northern Hemisphere winter (Wallace and Gutzler, 1981). Previous studies showed that the North Atlantic sea surface temperature (SST) anomalies could excite the EU pattern, which was verified by numerical model experiments (Gambo and Lu, 1983). It has been indicated that the northwestern Atlantic SST warming plays an important role in the formation of the Ural Blocking High (Li, 2004; Fu et al., 2008; Li and Gu, 2010).

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Moreover, Chinese researchers indicated that the winter precipitation was associated with the activity of the Ural Blocking High (e.g. Li and Gu, 2010; Wang et al., 2010). However, it is not clear what are the reasons for the interdecadal variability in winter precipitation in Northwest China. Therefore, in this study, atmospheric circulation and SST changes associated with the interdecadal variability in winter precipitation in Northwest China will be analysed by using reanalysis and observational station data from 1960 to 2008. The organization of the rest of the text is as follows. The datasets used in this study are described in Section 2. The interdecadal variability of winter precipitation in Northwest China is analysed in Section 3. Section 4 presents interdecadal variability of water vapour flux anomalies. Interdecadal variability of circulation anomalies is examined in Section 5. Section 6 discusses the possible cause of interdecadal variability in winter precipitation. Summary and discussions are provided in Section 7.

Figure 1. Map of location of 16 stations in Northwest China. (a) DJF Rainfall in NWC

2. Datasets We use monthly precipitation data at 16 stations in Northwest China for the period 1960–2008, which are provided by the Chinese Meteorological Data Center. These station observations are operated by the Chinese Meteorological Administration consistently through the observation period, meeting the standards of the World Meteorological Organization. In this study, Northwest China refers to the region to the north of 35∘ N and west of 110∘ E, including western Inner Mongolia, Ningxia, Gansu, Qinghai, and Xinjiang provinces or autonomous districts. The global land precipitation data which are derived from the Precipitation Reconstruction (PREC/L) for the period 1960–2008, are provide by the Climate Prediction Center (CPC). The resolution of the PREC/L is 1.0∘ × 1.0∘ (Chen et al., 2002). The monthly mean wind, omega, geopotential height, and special humidity were acquired from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis for the period 1960–2008 (Kalnay et al., 1996; Kistler et al., 2001). The resolution of the reanalysis variables is 2.5∘ × 2.5∘ . The monthly mean SST data were obtained from the Met Office Hadley Center for the period 1960–2008 (Rayner et al., 2003). The resolution of the SST is 1.0∘ × 1.0∘ . 3. Interdecadal variability of winter precipitation in Northwest China The interdecadal variability of winter (December–January –February or DJF for short) precipitation in Northwest China is analysed by using observations at 16 stations selected from 160 stations. These 16 stations are located to north of 35∘ N and west of 110∘ E of China (Figure 1). Figure 2(a) shows the time series of winter precipitation anomaly averaged for 16 stations in Northwest China. The © 2014 Royal Meteorological Society

(b) M–K Test

Figure 2. Time series of winter (DJF) precipitation anomaly (unit: mm) (dashed curves) and the corresponding 9-year running mean (solid curves) in Northwest China (a). The climatological monthly mean is based on the period 1971–2000. The forward and backward statistic rank series (dashed curves) and the corresponding 3-year running means (solid curves) in the Mann–Kendall test of the summer rainfall in Northwest China (b). The dashed straight lines indicate the 5% significant level of the Mann–Kendall test.

winter precipitation displays an interdecadal shift around the middle and late of 1980s. Winter precipitation in Northwest China was large with persisting positive precipitation anomalies after the mid-1980s, whereas below normal precipitation was often seen in the period before the mid-1980s (Figure 2(a)). Before the mid-1980s, there are several interdecadal variabilities with precipitation larger Int. J. Climatol. 35: 1172–1179 (2015)

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during 1970s than during 1960s and early-mid-1980s. As such, Northwest China precipitation displays both interannual and interdecadal variability before the late 1980s, whereas it mainly features interannual variability after that. This indicates that the interdecadal component of Northwest China precipitation variability is strong before but weak after the late 1980s. In order to examine the regime shift of winter precipitation in Northwest China, the Mann–Kendall (M–K) test (Mann, 1945; Kendall, 1975) is applied. Figure 2(b) shows the two statistical rank series (dashed curves) and the corresponding 3-year running means (solid curves) of the M–K test. From Figure 2(b), we can see the crossing of the two curves at the year 1987 at the 95% significant level. The result verifies the interdecadal shift around 1987 with the winter precipitation in the period 1987–2008 more than that in the period 1960–1986. The precipitation change in Northwest China appears to be consistent with that in Middle Asian region. Chen et al. (2011) pointed out that the precipitation increased in the mid-1980s in Middle Asia. Therefore, precipitation in Northwest China and Middle Asia experienced a similar interdecadal variability. Figure 3 shows the winter precipitation anomalies averaged for 1960–1986, 1987–2008, and the difference between 1987–2008 and 1960–1986. From Figure 3(a), in the period 1960–1986, winter precipitation was below normal over Northwest

China and Middle Asia and above normal in Mongolia. In contrast, in the period 1987–2008 (Figure 3(b)), the precipitation anomalies tend to be opposite to those in the period 1960–1986 with above-normal winter precipitation in Northwest China and Middle Asia. The difference of winter precipitation between 1987–2008 and 1960–1986 shows an increase of precipitation in a large region including Middle Asia and Northwest China (Figure 3(c)). The results suggest that the variability over Northwest China could be part of a large-scale climate change. The interdecadal variability of winter precipitation in Northwest China is consistent with previous studies (e.g. Shi et al., 2002; Li et al., 2003; Song and Zhang, 2003; Yu et al., 2003; Zhou, 2011b). 4. Interdecadal variability of water vapour flux anomalies The precipitation change is closely associated with water vapour flux anomalies (e.g. Li and Zhou, 2012). Here, we examine water vapour flux anomalies averaged for 1960–1986, 1987–2008, and the difference between 1987–2008 and 1960–1986 (Figure 4). During the period (a)

(a)

(b)

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(c) (c)

Figure 3. Distributions of the winter (DJF) precipitation anomalies (unit: mm day−1 ) averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The solid and dashed contours indicate positive and negative values, respectively. Shaded regions denote positive anomalies in (a) and (b), and the 5% significant level of the difference according to the Student’s t-test in (c). The contour interval is 0.05 mm day−1 in (a) and (b) and 0.1 mm day−1 in (c).

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Figure 4. Distributions of the winter (DJF) water vapour flux (integration of 1000–100 hPa) anomalies (unit: kg m−1 second−1 ) averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. Shaded regions in (c) indicate the 5% significant level of the difference of the u-component of the water vapour flux according to the Student’s t-test in (c). The scale for the vector is shown at the bottom of the respective panel. Int. J. Climatol. 35: 1172–1179 (2015)

WINTER PRECIPITATION IN NORTHWEST CHINA

1960–1986 (Figure 4(a)), southwestward water vapour flux anomalies occurred over Middle Asia and Northwest China, and eastward water vapour flux anomalies occurred over eastern China and western North Pacific, respectively. The results indicate a decrease in the amount of water vapour transported to Northwest China and Middle Asia. In comparison, during the period 1987–2008 (Figure 4(b)), an opposite spatial distribution is seen in the water vapour flux anomalies. The results indicate an increase in the water vapour flux to Northwest China and Middle Asia. The difference between 1987–2008 and 1960–1986 displays a significant interdecadal variability in the water vapour flux (Figure 4(c)). Although the water vapour flux provides the source for precipitation, it is the water vapour flux convergence that directly leads to precipitation. Therefore, the water vapour flux convergence and vertical motion are examined to understand the interdecadal variability in winter precipitation on Northwest China and Middle Asia. Figures 5 and 6 show column-integrated water vapour flux convergence and 850 hPa vertical velocity (omega) anomalies averaged for 1960–1986, 1987–2008, and the difference between 1987–2008 and 1960–1986. During the period 1960–1986 (Figure 5(a) and 6(a)), there was anomalous

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(a)

(b)

(c)

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Figure 6. Distributions of the winter (DJF) omega anomalies (unit: Pa second−1 ) at 850 hPa averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The solid and dashed lines indicate positive (descent motion) and negative (ascent motion) values, respectively. Shaded regions denote ascent motion in (a) and (b), and the 5% significant level of the difference according to the Student’s t-test in (c). The contour interval is 0.005 Pa second−1 in (a) and (b) and 0.01 Pa second−1 in (c).

tropospheric moisture divergence and descent motion over most parts of Northwest China and Middle Asia. In contrast, during the period 1987–2008 (Figures 5(b) and 6(b)), a significant enhancement of tropospheric moisture convergence and ascent motion is seen over Northwest China and Middle Asia. The difference in winter water vapour flux convergence and omega between 1987–2008 and 1960–1986 show enhancement of tropospheric moisture convergence and ascending motion over Northwest China and Middle Asia (Figures 5(c) and 6(c)), which is consistent with the increase in winter precipitation.

5. Figure 5. Distributions of the winter (DJF) water vapour flux (integration 1000–100 hPa) convergence anomalies (unit: g m−2 second−1 ) averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The solid and dashed lines indicate positive (divergence) and negative (convergence) values, respectively. Shaded regions denote water vapour flux convergence in (a) and (b), and the 5% significance level of the difference according to the Student’s t-test in (c). The contour interval is 0.005 g m−2 second−1 in (a) and (b) and 0.01 g m−2 second−1 in (c).

© 2014 Royal Meteorological Society

Interdecadal variability of circulation anomalies

The precipitation variability is closely related to atmospheric circulation changes. To understand the interdecadal variability in winter precipitation, we contrast the circulation anomalies between the two periods, 1987– 2008 and 1960–1986. Figure 7 shows the winter wind anomalies at 850 hPa averaged for 1960–1986, 1987–2008, and the difference between 1987–2008 and 1960–1986. During the period 1960–1986 (Figure 7(a)), Int. J. Climatol. 35: 1172–1179 (2015)

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(a)

(b)

(b)

(c)

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Figure 8. Distributions of the winter (DJF) wind anomalies (unit: m second−1 ) at 200 hPa averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The shaded regions in (c) indicate the 5% significant level of the u-wind difference according to the Student’s t-test. The scale for the vector is shown at the bottom of the respective panel. Figure 7. Distributions of the winter (DJF) wind anomalies (unit: m second−1 ) at 850 hPa averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The shaded regions in (c) indicate the 5% significant level of the u-wind difference according to the Student’s t-test. The scale for the vector is shown at the bottom of the respective panel.

cyclonic wind anomalies appeared over East Asia and southern Europe-northern Africa, whereas anticyclonic wind anomalies appeared over Middle Asia and North Atlantic. Opposite wind anomalies were seen during the period 1987–2008 (Figure 7(b)). The difference between 1987–2008 and 1960–1986 clearly shows enhanced westerlies spanning the North Atlantic through Europe to Northwest China (Figure 7(c)). This contributes to enhanced water vapour transport from the North Atlantic and strengthened water vapour convergence and ascending motion, and increase of precipitation in Middle Asia and Northwest China. The 850 hPa wind distribution is similar to the column-integrated water vapour flux. This indicates a dominant contribution of water vapour transport by low-level winds. Yang et al. (2002) pointed out that the Asian Jet stream may influence the Asian winter climate anomalies. Figure 8 shows mean and difference of wind anomalies at 200 hPa. During the period 1960–1986 (Figure 8(a)), easterly anomalies dominated the middle latitudes. During period 1987–2008 (Figure 8(b)), anomalous winds over the middle latitudes were westerlies with smaller © 2014 Royal Meteorological Society

magnitude compared with 1960–1986. In particular, wind anomalies were opposite over Middle Asia between the two periods. The difference between 1987–2008 and 1960–1986 clearly shows enhanced westerlies over Middle Asia with cyclone and anticyclone to the north and south, respectively (Figure 8(c)). The results indicate that the westerly jet had a significant change around 1986/1987, and it becomes stronger over middle latitudes after 1987.

6. Possible cause of interdecadal variability in winter precipitation One factor often considered for atmospheric circulation change is anomalous ocean state. Figure 9 shows SST anomalies averaged for 1960–1986, 1987–2008, and the difference between 1987–2008 and 1960–1986. During the period 1960–1986 (Figure 9(a)), there were negative SST anomalies in most regions of global oceans. In contrast, during the period 1987–2008 (Figure 9(b)), the SST anomalies were mostly positive. The difference between 1987–2008 and 1960–1986 shows clearly SST increase in global oceans (Figure 9(c)) with the largest increase of 0.5 ∘ C or more in North Atlantic Ocean and northwestern Pacific. Figure 10 displays SST anomalies averaged in the regions of 50∘ W–10∘ W, 35∘ –45∘ N and 115∘ –140∘ E, 20∘ –40∘ N where large SST increases were observed. Both the North Atlantic and northwestern Pacific SST experienced an obvious increase in the late 1980s (Figure 10(a) Int. J. Climatol. 35: 1172–1179 (2015)

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(b)

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Figure 10. The normalized time series of winter (DJF) sea surface temperature in North Atlantic (a) averaged for 50∘ W–10∘ W, 35∘ –45∘ N, and northwestern Pacific (b) averaged for 115∘ –140∘ E, 20∘ –40∘ N. The solid lines indicated the 11-year running mean. Figure 9. Distributions of the winter (DJF) sea surface temperature anomalies (unit: ∘ C) averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The solid and dashed lines indicate positive and negative values, respectively. Shaded regions denote positive anomalies in (a) and (b), and the 1% significant level of the difference according to the Student’s t-test in (c). The contour interval is 0.2 ∘ C in (a) and (b) and 0.3 ∘ C in (c).

and (b)). In the recent decade, the SST anomalies reached 1 ∘ C or more, whereas the SST anomalies of −0.5 ∘ C or more were often seen during 1970s. The averaged value of SST anomalies in the region of 50∘ W–10∘ W, 35∘ –45∘ N is −0.23 for the period 1960–1986, and 0.92 for the period 1987–2008. This result verified that the SST in North Atlantic illustrated an apparent interdecadal increase after 1987. Northwest China climate variability is under the influence of westerly winds over the North Atlantic and Eurasia (Rogers and van Loon, 1979; Aizen et al., 2001). Because Northwest China is located downstream of the North Atlantic Ocean, the SST change in the North Atlantic Ocean may induce atmospheric circulation changes over Eurasia and thus may contribute to Northwest China precipitation change. Previous studies indicated that the North Atlantic SST anomalies can excite the EU pattern, influencing the circulation anomalies in Ural and the formation of Ural Blocking High (Li, 2004; Fu et al., 2008; Li and Gu, 2010). The EU teleconnection pattern controls largely the Northern Hemisphere winter (Wallace and Gutzler, 1981). The formation of the EU teleconnection pattern is related to the Rossby wave train (Hoskins and Karoly, 1981). Figure 11 shows the winter geopotential height anomalies at 500 hPa averaged for 1960–1986, © 2014 Royal Meteorological Society

1987–2008, and the difference between 1987–2008 and 1960–1986. In the period 1960–1986 (Figure 11(a)), positive anomalies were seen over North Atlantic and eastern Europe, while negative anomalies were seen over Northern Africa and East Asia. The distribution features the EU teleconnection pattern. In the period 1987–2008 (Figure 11(b)), opposite EU pattern was observed. The Middle Asia and Northwest China were located to the east of low system where convergence and ascending motion are favoured. The difference between 1987–2008 and 1960–1986 features an EU pattern over middle latitudes with the difference of geopotential height reaching 30 m or more in the centers (Figure 11(c)). The results indicated that the EU pattern play a role in carrying the influence of the North Atlantic SST anomalies to the Middle Asia and Northwest China.

7.

Summary and discussions

This analysis investigates interdecadal variability of winter precipitation in Northwest China and plausible reasons. The winter precipitation in Northwest China experienced an obvious increase around 1987. This increase is part of the change in a large domain of Middle Asia. The interdecadal variability of winter water vapour flux, circulation, and SST are analysed in this study to understand the cause for the interdecadal variability of winter precipitation in Northwest China using station observations, PREC/L land precipitation data, the NCEP/NCAR Int. J. Climatol. 35: 1172–1179 (2015)

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(b)

It is possible that the SST increase in these regions may also have contributed to the interdecadal variability in winter precipitation in Northwest China and Middle Asia. However, it remains to be investigated what are the plausible processes for connecting the SST change in tropical Indo-Pacific Ocean and precipitation change in Northwest China. Due to the limited length of record, it cannot be determined whether the identified interdecadal variability in winter precipitation in Middle Asia is a permanent one or the change will reverse in the future. In view of the impacts of global warming on both tropical SST and land surface temperature, it is likely that the winter precipitation over Northwest China will subject to changes in the future due to global warming. Acknowledgement

(c)

This paper was supported by the National Nature Science Foundation of China (grant nos. 41175055 and 41230527).

References

Figure 11. Distributions of the winter (DJF) geopotential height anomalies (unit: gpm) at 500 hPa averaged for 1960–1986 (a), 1987–2008 (b), and the difference between 1987–2008 and 1960–1986 (c). The climatological monthly mean is based on the period 1971–2000. The solid and dashed lines indicate positive and negative values, respectively. The shaded regions in (c) indicate the 5% significant level of the difference according to the Student’s t-test. The contour interval is 10 gpm in (a) and (c) and 5 gpm in (c).

reanalysis, and the Met Office Hadley Center’s SST data for the period 1960–2008. Analyses show very different winter water vapour fluxes and atmospheric winds over Middle Asia and Northwest China before and after 1987. During the period 1987–2008, a significant enhancement of tropospheric moisture convergence and ascending motion is seen over Northwest China and Middle Asia. This contributes to the increase of winter precipitation in Northwest China and Middle Asia. The circulation change around 1987 features cyclonic anomalies over Middle Asia and North Atlantic, and anticyclonic anomalies over East Asia and southern Europe-northern Africa. Moreover, the EU pattern experienced a significant change. The enhanced westerlies extending from North Atlantic to Middle Asia contribute to the increase of water vapour transport to Middle Asia and Northwest China. The interdecadal variability in winter precipitation in Middle Asia and Northwest China is associated with SST anomalies in North Atlantic Ocean via the EU pattern. In addition to the role of the North Atlantic SST change, the SST in the tropical Indian Ocean and northwestern Pacific experienced an increase around late 1980s as well. © 2014 Royal Meteorological Society

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