Linkage Between the Northeast Mongolian Precipitation and the Northern Hemisphere Zonal Circulation

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1 DVNCES IN TMOSPHERIC SCIENCES, VOL. 23, NO. 5, 2006, Linkage Between the Northeast Mongolian Precipitation and the Northern Hemisphere Zonal Circulation WNG Huijun ( ) Institute of tmospheric Physics (IP), Chinese cademy of Sciences, Beijing (Received 6 pril 2006; revised 19 May 2006) BSTRCT The long-term relationship between the tree-ring-reconstructed annual precipitation in northeastern Mongolia (PRM) and the Northern Hemisphere Zonal Circulation (NHZC) defined as the normalized zonal mean sea-level pressure at 60 N in May-June-July, is examined in this study. significant correlation coefficient (0.31) was found between the NHZC indices and PRM based on the dataset for the period of The mechanisms responsible for the relationship are discussed through analyses of the atmospheric general circulation variability associated with NHZC. It follows that NHZC-related atmospheric circulation variability provides an anomalous southeast flow from the ocean to Northeast Mongolia (northwest flow from Northeast Mongolia to the ocean) in the middle and low troposphere in positive (negative) phase of NHZC, resulting in more (less) water vapor transport to the target region and more (less) precipitation in Northeast Mongolia. Key words: Tree-ring-reconstructed precipitation, Northern Hemisphere zonal circulation, atmospheric general circulation doi: /s Introduction Instrumental records of precipitation are scarce in Mongolia, thus the extension of the series beyond the period with instrumental observations is usually carried out with the help of proxy data, such as tree-rings, ice cores, loess, and lake sediments. The (prior ugust to July) annual precipitation for 345 years (D ) was reconstructed for northeastern Mongolia based on tree-ring width data (Pederson et al., 2001). Verifications show that variations in instrumental precipitation are within the range of those reconstructed over the length of the tree-ring record (Pederson et al., 2001). Previous research shows that Northern Hemisphere (NH) nnular Mode (NM), or, rctic Oscillation (O), has apparent influences on the climate in many regions in the NH. Thompson and Wallace (1998) showed the influence of rctic Oscillation on the wintertime geopotential height and temperature fields in the NH. Wettstein and Mearns (2002) studied the influences of the North tlantic Oscillation on the mean, variance, and extremes of temperature in the northern United States and Canada. Gong and Ho (2003) indicated the negative correlation between O (or, NM) and the summer monsoon rainfall over the Yangtze * wanghj@mail.iap.ac.cn River Valley and the southern Japan region. Mongolia is located in the semi-arid or arid region between 40 N and 50 N and the meteorological stations are spatially sparse and the instrumental records are temporally limited. Therefore, little is known about the interannual variability of the Mongolian precipitation. Our work was motivated by the interannual variability of the precipitation as revealed from the long-term tree-ring-reconstructed data and by examining the signals of rctic and high-latitude zonal circulation in the northeastern Mongolian precipitation and the possible underlying mechanisms. In this paper, the Northern Hemisphere zonal circulation (NHZC) is defined by the zonal mean normalized sea-level pressure (SLP) averaged at 60 N. The NHZC is closely correlated with the rctic Oscillation Index defined by the zonal mean SLP difference between 40 N and 60 N (the correlation coefficient is 0.72 for the May-June-July (MJJ) mean in the interannual variability for the 127 years of ). The index of O is also closely related to the North tlantic Oscillation (NO) index documented by Hurrell (1995) and Jones et al. (1997) with a correlation of 0.45 for MJJ in the 127-year period. Thus the NHZC can represent O (and NO as well) to a great extent. Regarding the definition of O, more information may

2 660 MONGOLIN PRECIPITTION ND THE NORTHERN HEMISPHERE ZONL CIRCULTION VOL. 23 be found in Thompson and Wallace (1998; 2000), Gong and Ho (2003). Meanwhile, the NHZC has an apparent implication for the zonal mean air mass in middle and high latitudes where frequent blocking highs occur in MJJ, and the blocking highs over the Ural Mountains, Lake Baikal, and Sea of Okhotsk regions have substantial impacts on the weather and climate over North sia and East sia (Tao and Xu, 1962; Zhou et al., 1995; Huang and Jiang, 2002). The middle latitudes in Eurasia are also the regions where the Eurasia-sia teleconnection patterns are located (Wakabayashi and Kawamura, 2004). The Eurasia teleconnection patterns are also very crucial for the climate of sia. nother issue is related to the target season for analysis the running of the atmospheric mean time circulation series. associated with the PRM. Since the main rain season for the northeastern Mongolia precipitation for a hydrological year (defined as the 12 months from the prior ugust to July) (PRM) is May-June-July (MJJ), we focus on MJJ when considering the atmospheric circulation related to PRM. 2. Datasets Several datasets are employed in this study. The annual (prior ugust to July) precipitation for 345 years (D ) was reconstructed by Pederson et al. (2001) for northeastern Mongolia based on tree-ring width data in Urgan Nars (UN) at N, E, with elevation 1070 m. The final UN chronology extends from 1651 to 1996 and is composed of data from 16 trees and 33 cores. The quality of the dataset was verified by comparing with the instrumental record and a correlation coefficient of was obtained between the station-observed precipitation and Figure 1 The internnual variation (thin lines) and 5-year running means (thick lines) of NHZC in May-June-July (upper) and PRM (lower). Bars indicate the tree-ring-recorded the presence precipitation of local maxima in the years (minima) 1942 in The sea-level pressure (SLP) data for was reanalyzed and compiled by the Hadley Centre for Climate Prediction and Research of the UK (Basnett and Parker, 1997). The monthly mean SLP has a 5 5 horizontal resolution. The NO dataset ( ) is taken from results of Hurrell (1995) via the Internet. In addition, the US National Centers for Fig. 1. The interannual variation (thin lines) and 5-year running means (thick lines) of normalized NHZC in May-June-July (upper) and PRM (lower). Bars indicate the presence of local maxima and minima in the running mean time series.

3 Figure 2 The NH geographical distribution for the correlation coefficients of NHZC-SLP (a) and PRM-SLP (b) for MJJ during Shaded areas indicate significant correlation at 95% level, estimated by a local student t-test. NO. 5 WNG 661 (a) (b) Fig. 2. The NH geographical distribution for the correlation coefficients of (a) NHZC-SLP and (b) PRM-SLP for MJJ during Shaded areas indicate significant correlation at the 95% level, estimated by a local Student s t-test. Environment Prediction/National Center for tmospheric Research (NCEP/NCR) atmospheric monthly reanalysis dataset (Kalnay et al., 1996) at 17 vertical pressure levels with a horizontal resolution of ( ) is used in the analyses. 3. Results The NHZC index for MJJ and the tree-ringreconstructed precipitation series in northeastern Mongolia (PRM) for the period are plotted in Fig. 1. ll the time series have been normalized by removing the temporal averages and then by dividing by the standard deviations before plotting. The correlation coefficient of 0.31 is significant at the 99% confidence level, and it provides an estimation of the signal of the NHZC index in the variations of PRM. ll the correlation and composite analyses in this paper are based on the interannual variability. In section 1, we mentioned that NHZC is highly correlated to O. Therefore, we further computed the correlation coefficient between O (as defined by the zonal mean SLP difference between 40 N and 60 N) and PRM here, and the result is 0.21 (significant at the 95% level). Thus, NHZC is more closely correlated to PRM than O. The 5-year running mean time series shown by the thick lines in Fig. 1 also display a close positive relationship, which is clearly indicated by the shaded bars. fairly good correspondence is seen for many episodes in the past 100 years or more like the late 1870s, late 1880s, late 1910s, around 1930, mid-1940s, around 1960, and the early 1990s. The NHZC-PRM relationship can be further demonstrated by analyzing the long-term Hadley Centre SLP field. The global distribution for the correlation coefficient of NHZC-SLP and PRM-SLP in is depicted in Fig. 2. It follows that the MJJ NHZC is closely correlated with the Northern Hemisphere SLP field, with a significant positive correlation coefficient over latitudes between 50 N to 70 N. Evidently, this pattern well displays the NH nnular Mode. Furthermore, the PRM-related SLP pattern is similar over the Eurasian continent, with a positive correlation coefficient in 50 N 70 N. It is also noted that the Eurasian continent is the most pronounced region with significant SLP consequences for the interannual variability of PRM. Thus the SLP patterns shown in Fig. 2, to some extent, illustrated the linkage between NHZC and PRM. 15 Now we discuss the atmospheric general circulation changes underlying the NHZC-PRM relationship. s mentioned above, the MJJ precipitation is the main part of PRM (which is the annual precipitation). Thus we analyze the simultaneous atmospheric circulation related to the NHZC averaged for MJJ. The composite analysis in this paper is carried out for the period of The years with positive NHZC anomalies

4 Figure 3 The geographical distribution of NHZC-based composite differences of vorticity (in 10 s -1 ) at 300 hpa for over the Eurasian area. Shaded areas indicate significant changes at 95% level estimated by a student t-test. E and M denote the regions over the Western Europe and North Mongolia respectively, and the thick line conceptually indicates the teleconnection from region E to region M. 662 MONGOLIN PRECIPITTION ND THE NORTHERN HEMISPHERE ZONL CIRCULTION VOL. 23 E E M Fig. 3. The geographical distribution of NHZC-based composite differences of vorticity (in 10 6 s 1 ) at 300 hpa for over the Eurasian area. Shaded areas indicate significant changes at the 95% level estimated by a local Student s t-test. E and M denote the regions over the Western Europe and North Mongolia respectively, and the thick line conceptually indicates the teleconnection from region E to region M. and negative anomalies are divided into two groups. Figure 3 clearly displays a teleconnection wave train from West Europe ( E ) to North Mongolia ( M ), with alternative positive and negative values of the composite vorticity differences (differences between averages of positive and negative NHZC anomaly composites) along the thick line plotted in the figure. This teleconnection pattern is similar to that found by Wakabayash and Kawamura (2004), when the teleconnection patterns possibly associated with the anomalous summer climate in Japan were studied. Through this Europe-North Mongolia (ENM) teleconnection pattern, a tropospheric cyclonic anomalous circulation is introduced to the north of Mongolia for the positive phase of NHZC as compared to the negative phase of NHZC. Moving on, we analyze the velocity variability associated with NHZC (as well as PRM), toward understanding the linkage between NHZC and PRM. Figure 4 depicts the correlation coefficients between the velocity and NHZC (as well as PRM), with zonal and meridional components, of the correlation coefficients between zonal (meridional) winds and NHZC (as well as PRM), combined in the arrows. Figure 4a displays two anomalous anti-cyclonic systems located respectively in Europe and Northeast sia at 500 hpa in latitudes around 60 N that are directly associated with the positive phase of NHZC. Meanwhile, the NHZCassociated anomalous flow in Northeast China and Mongolia is southeastward, from the ocean to the inland areas, thus favorable to the water vapor transport to our target region northeast Mongolia for the positive phase of NHZC. The pattern of wind-prm correlation coefficient shown in Fig. 4b well resembles that in Fig. 4a, with the two anomalous anti-cyclonic systems respectively in Europe and Northeast sia. The key point here, however, is that the anomalous southeast flow appears in Northeast China and Northeast Mongolia in both Figs. 4a and b, transporting more water vapor from the ocean to the northeast Mongolia region and hence more precipitation, for the positive phase of NHZC. We note that such anomalous southeast flow exists at 850 hpa as well (figures not shown) for both cases, but the two anti-cyclonic systems are less pronounced than those at 500 hpa. The favorable circulation conditions could be further illustrated in the NHZC-based composite geopotential height changes in the middle troposphere represented by 300 hpa (see Fig. 5), which show positive anomalies in Western Europe and the Okhotsk Sea region but negative anomalies in Siberia (to the north of Mongolia). This feature of the circulation changes resembles a very typical double-blocking pattern in Eurasia, noted by many studies, which has substantial impacts on the climate of sia (Tao and Xu, 1962; Huang and Jiang, 2002), and is similar to the pattern that can be seen in Fig. 4 as well. Climatologically, there is a major trough in the East sia and North sia, which provides basic circulation conditions for the rain pattern in the region. Under the blocking circulation pattern, the major trough is deepened and the low-pressure systems and synoptic precipitation processes frequently occur in North sia, resulting in ac- 16 cumulated precipitation, with a typical example of the heavy rainfall over northeastern China in the summer of 1998 (Zhang et al., 2001). Therefore, the negative anomalies of geopotential height in Siberia, which is related to the pattern shown in Fig. 4, are favorable to the intensification of the major trough in sia, thus

5 Figure 4 The NH geographical distribution for the correlation coefficients between MJJ velocity at 500 hpa and NHZC (a) as well as PRM (b) during The zonal (meridional) component of the arrow is the correlation coefficient between zonal (meridional) wind and NHZC (a) as well as PRM (b). in the figure denotes the anti-cyclonic wind system and the long thick arrow indicates the anomalous southeast flow from the ocean to the northeast Mongolia. NO. 5 WNG 663 (a) (b) Figure 5 Fig. The 4. geographical The NH geographical distribution distribution of NHZC-based for the correlation composite coefficients differences between of geopotential height (in MJJ gpm) velocity at 300 athpa 500for hpa and (a) NHZC Shaded as well areas as (b) indicate PRMsignificant during changes The at 95% level estimated zonal by a (meridional) student t-test. component of the arrow is the correlation coefficient between zonal (meridional) wind and (a) NHZC as well as (b) PRM. in the figure denotes the anti-cyclonic wind system and the long thick arrow indicates the anomalous southeast flow from the ocean to Northeast Mongolia. Fig. 5. The geographical distribution of NHZC-based composite differences of geopotential height 17(in gpm) at 300 hpa for Shaded areas indicate significant changes at the 95% level estimated by a local Student s t-test. providing circulation conditions for more precipitation over North sia, including northeastern Mongolia for the positive phase of NHZC. 4. Concluding remarks The zonal mean circulation in high latitudes of the NH can be well represented by the zonal averaged SLP at 60 N (NHZC) and it is associated with the large-scale atmospheric circulation in the NH. It is an indication of the rctic Oscillation (or the NH nnular Mode) as well, with a correlation coefficient of 0.72 for the interannual variability in the 127-year

6 664 MONGOLIN PRECIPITTION ND THE NORTHERN HEMISPHERE ZONL CIRCULTION VOL. 23 period. The conclusion derived from this study is that the NHZC in May-June-July and the tree-ringreconstructed northeastern Mongolia annual precipitation are closely connected to each other. The circulation background behind this relationship can be explained by the NHZC-related NH atmospheric circulation changes, especially the Eurasian circulation changes. It follows that the NHZC-related wind variability yields an anomalous southeast flow in the middle and lower troposphere from the ocean to Northeast Mongolia, transporting more water vapor to Northeast Mongolia, reinforcing the precipitation there for the positive phase of NHZC. The apparent teleconnection pattern is displayed from Western Europe to North Mongolia (ENM), and the positive (negative) phase of NHZC is accompanied by a doubleblocking pattern and a deepening (weakening) of the major trough in North sia, which is favorable (unfavorable) to the precipitation in the target region as well for the positive (negative) phase of NHZC. cknowledgments. This research was jointly supported by the Chinese cademy of Sciences key program under Grant KZCX3-SW-221, and the National Natural Science Foundation of China under Grant Nos and Thanks are due to Prof. Helge Drange at the Nansen Environment and Remote Sensing Center for his valuable comments on the manuscript. REFERENCES Basnett, T.., and D. E. Parker, 1997: Development of the Global Mean Sea Level Pressure Data Set GMSLP2. Hadley Centre Climate Research Technical Note CRTN 79, Hadley Centre, Meteorological Office, Bracknell, 16pp. Gong, D. Y., and C. H. Ho, 2003: rctic oscillation signals in the East sian summer monsoon. J. Geophys. Res., 108(D2), 4066, doi: /2002jd Huang Fei, and Jiang Zhina, 2002: Study on the Statistical Characteristics of atmospheric locking in the Eurasia and its relationship with the summer rainfall over the East of China. J. Ocean University of Qingdao, 32, (in Chinese) Hurrell, J. W., 1995: Decadal trends in the North tlantic Oscillation and relationships to regional temperature and precipitation. Science, 269, Jones, P. D., T. Jonsson, and D. Wheeler, 1997: Extension to the North tlantic Oscillation using early instrumental pressure observations from Gibraltar and South-West Iceland. Int. J. Climatol., 17, Kalnay, E., and Coauthors, 1996: The NCEP/NCR 40- year reanalysis project. Bull. mer. Meteor. Soc., 77, Pederson, N., G. C. Jacoby, R. D rrigo, B. Buckley, C. Dugarjav, and R. Mijiddorj, 2001: Hydrometeorological reconstructions for northeastern Mongolia derived from tree rings: D J. Climate, 14, Tao Shiyan, and Xu Shuying, 1962: Some aspects of the circulation during the periods of the persistent drought and flood in the Yangtze River Valley in summer. cta Meteorologica Sinica, 32, (in Chinese) Thompson, D. W. J., and J. M. Wallace, 1998: The rctic Oscillation signature in the wintertime geopotential height and temperature fields. Geophys. Res. Lett. Vol. 25, No. 9, p (98GL00950) Thompson, D. W. J., and J. M. Wallace, 2000: nnular modes in the extratropical circulation: Part I: monthto-month variability. J. Climate, 13, Wakabayashi, S., and R. Kawamura, 2004: Extraction of major teleconnection patterns possibly associated with the anomalous summer climate in Japan. J. Meteor. Soc. Japan, 82, Wettstein, J. J., and L. O. Mearns, 2002: The Influence of the North tlantic-rctic Oscillation on Mean, Variance, and Extremes of Temperature in the Northeastern United States and Canada. J. Climate, 15, Zhang Qingyun, Tao Shiyan, and Zhang Shunli, 2001: study of excessive heavy rainfall in the Songhuajiang- Nenjiang River Valley in Chinese J. tmos. Sci., 25, (in Chinese) Zhou Xueming, He Jinhai, and Rongsheng Ye, 1995: The numerical experiments on the impacts of the blocking high over Ural on East China precipitation. Journal of Nanjing Institute of Meteorology, 18, (in Chinese)