Southern Hemisphere mean zonal wind in upper troposphere and East Asian summer monsoon circulation

Chinese Science Bulletin 2006 Vol. 51 No. 12 1508 1514 DOI: 10.1007/s11434-006-2009-0 Southern Hemisphere mean zonal wind in upper troposphere and East Asian summer monsoon circulation WANG Huijun 1 & FAN Ke 2 1. Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; 2. Department of Atmospheric Science, Yunnan University, Kunming 650091, China Correspondence should be addressed to Wang Huijun (wanghj@mail. iap.ac.cn) Received March 15, 2006; accepted April 27, 2006 Abstract Rlationship between the Southern Hemisphere upper troposphere zonal wind variability and the East Asian summer monsoon was studied. The mean zonal wind difference in 150 hpa between 60 S and 30 S is defined as an index (ISH). The research reveals that ISH is negatively correlated with the East Asian summer monsoon (EASM) circulation in the interannual scale. It is also suggested that the meridional teleconnection in the zonal wind field, with the main part in Eastern Hemisphere, from mid-andhigh latitudes in the Southern Hemisphere to the tropical region is responsible for the ISH-EASM linkage. The teleconnection may result in changes of the wind fields and pressure system changes causing variability of the East Asian monsoon. In addition, the anomalous teleconnection pattern in Eurasia is likely to be another mechanism. ISH and EASM circulation are found to have both simultaneous and lag correlations, thus, the relationship has implications for seasonal climate prediction over East Asia. Keywords: Southern Hemisphere, upper troposphere, mean zonal wind, East Asian summer monsoon. 1 Introduction Variability of the East Asian summer monsoon (EASM) has been detected by considering roles of El Nino and Southern Oscillation (ENSO) cycle, snow cover over Eurasia and Tibetan Plateau, and signals from the soil (namely, the soil temperature and soil moisture). Relationship of EAM and ENSO was found to be highly complicated, although there are some connections [1-6]. Wang et al. [2] showed that Central Pacific and East Asian teleconnection exist in the lower troposphere in which the bridge for the weak (strong) East Asian monsoon and the warm (cold) event is the anomalous lower-troposphere anticyclone (cyclone) in the Western North Pacific. Studies by Yang and Lau [7] showed that land surface soil moisture anomalies may also plays the role as a link of the connection between Asian summer monsoon and sea surface temperature (SST), so that SST event may have direct (by changes of winterspring westerly over subtropical Asia) and indirect effects (through soil moisture changes) on Asian monsoon. Researches to seek other factors that affect the variability of EASM have been continuously a hot topic in recent years. Xue et al. [8, 9] reported the significant correlation between subtropical highs (namely, the Mascarene high and Australian high) in Southern Hemisphere (SH) during the March to May and June to August (JJA) rainfall in East Asia through changes of western Pacific subtropical high. Wang and Xue [10] revealed the relationship between Somali jet in the atmosphere and East Asian monsoon flow. Fan and Wang [11] also indicated the linkage between Antarctic Oscillation and East Asian winter-spring climate as well as the dust weather frequency. However, AAO major represents the low-level atmospheric variability in the SH middle and high latitudes. Although AAO has quasi-barotropic structure, the high-level troposphere has some different characteristics. For example, next section will display the oscillation feature between the middle and high latitudes zonal mean zonal winds in high troposphere. Does such oscillation have the relation with the East Asian summer monsoon circulation? What is the possible mechanism? These questions remain unknown so far. This paper will focus on the connection between the SH high troposphere zonal wind and the East Asian summer monsoon circulation. The long-term trends of the SH annular modes (SAM) have been addressed by several works [12-17]. Marshall [15] indicated that SAM is likely due to combined effects of anthropogenic and natural forcing. This research, based on the previous work, will focus on the linkage of SH mean zonal wind flow in upper troposphere to East Asian summer monsoon, with the emphasis on the interannual variability. The atmospheric data employed in the analysis are the monthly reanalysis data set with 2.5 2.5 horizontal resolution 17 pressure levels for 1949-2002 by NCAR/NCEP. 1508 Chinese Science Bulletin Vol. 51 No. 12 June 2006

2 Results Time series for the June to August mean (JJA) zonal averaged westerly wind anomalies at 60 S and 30 S for 150 hpa shown in Fig. 1 displays negative correlation between each other in both interannual scale and decadal scale. They correlated at 0.52 with the detrended series in 1949-2002. The empirical orthogonal function (EOF) analysis for the JJA zonal wind in the region (40 N-90 S,110 o W-180 o E) is conducted and the first two EOFs (29% and 17% of the total variance respectively) are plotted in Fig. 2. The EOF1 shows clearly a meridional teleconnection pattern between roughly 30 S and 60 S. The pattern has a zonally symmetric structure, with the major part in the Eastern Hemisphere. In EOF2 diagram, the meridional mode exists in the Pacific sector, from the middle and high latitudes of SH to the subtropics of the Northern Hemisphere (NH). Thus we may define an index for the meridional teleconnection as the difference of the mean zonal winds between 60 S and 30 S and then discuss its relationship to the East Asian summer monsoon. The interannual and intraseasonal variability and the long-term trends of the zonal winds in SH were addressed in refs. [18-20]. They also indicated the barotropic structure of this zonal mode in SH. However, its connection to EASM has not yet been studied and this is the motivation for this work. Following ref. [4], the East Asian monsoon index is defined as the area mean (20-40 N, 110-125 E) wind speed (multi-year average is removed) at 850 hpa in this paper. Table 1 shows that correlation coefficient between ISH and EASM index (EASMI) is the largest for the simultaneous JJA season ( 0.78 and 0.40 for the non-detrended and detrended results respectively; significance level is 99%). It is interesting that lag correlation (spring-summer, summer-spring) is also significant at 90% level for the detrended results, suggesting the possible two-ways influences between ISH and the East Asian monsoon. The ISH is negatively correlated with the middle and lower reaches of the Yangtze-Huai River Valley precipitation as well, supporting the negative correlation between ISH and the East Asian summer monsoon (figure not shown). An apparent decadal scale transitions occurred at the end of 1970s for both the ISH and EASMI in summer, but with reverse trends (Fig. 3). Correlation coefficient between ISH and the zonal mean westerly wind plotted in Fig. 4 clearly shows a meridional teleconnection from the Southern Hemisphere to the Northern Hemisphere, through which the tropical zonal wind (0-20 N) in the middle-level troposphere is linked to the high-level troposphere zonal wind represented by ISH. The tropical zonal wind anomalies (0-20 N) in the middle-level troposphere over South China Sea region indicate apparently the anomalies of the East Asian monsoon. While the tropical zonal wind anomalies (0-20 N) in the middlelevel troposphere over the Central America imply that there is possible relationship between ISH and the American monsoon. At 30 N and 50 N there are negative and positive correlations, respectively, in low-level troposphere, which has implications for the monsoon circu- Fig. 1. Temporal series for JJA normalized mean zonal wind anomalies at 60 S (solid) and 30 S (hollow) at 150 hpa for 1949-2002. www.scichina.com www.springerlink.com 1509

Fig. 2. The first 2 EOFs for the JJA zonal winds in 150 hpa (standardized), analyzed in the region (40 N 90 S, 110 W 180 E) for 1949 2002. Table 1 Simultaneous and lag correlation coefficients between ISH and East Asian monsoon index a) Correlation coefficient Correlation coefficient (for non-detrended series) (for detrended series) ISH-JJA vs EASMI-JJA 0.78 (significant at 99%) 0.40 (significant at 99%) ISH-MAM vs EASMI-JJA 0.71 (significant at 99%) 0.24 (significant at 90%) ISH-JJA vs EASMI-MAM 0.69 (significant at 99%) 0.23 (significant at 90%) a) JJA and MAM stand for June-July-August mean and March-April-May mean respectively. lation change as well. The composite analysis of velocity at 100 hpa between positive and negative ISH anomalies during 1971 2000 is conducted (Fig. 5). There is an apparent meridional teleconnection of zonal winds from middle and 1510 high latitudes of SH to the tropical region. The long arrow-headed lines indicate the characteristic wind anomalies in various regions. In positive phase of ISH, the mean zonal winds anomalies are positive in high latitudes around 60 S and negative in middle latitudes Chinese Science Bulletin Vol. 51 No. 12 June 2006

Fig. 3. Temporal series for the JJA normalized ISH index (solid) and EASMI (hollow) for 1949 2002. Fig. 4. The latitude-height cross sections for the correlation coefficients between JJA ISH and mean zonal winds for 1949 2002. (a) Whole zonal mean; (b) 0 180 E zonal mean. The Ls represent the standard pressure levels of NCAR/NCEP reanalysis data (1000, 925, 850, 700, 600, 500, 400, 300, 200, 250, 200, 150, 100, 70, 50, 30, 20, 10 hpa from the bottom to the top levels). Shaded areas indicate significant changes at 95% level estimated by a local student t-test. www.scichina.com www.springerlink.com 1511

Fig. 5. The composite difference of JJA velocity (m/s) at 100 hpa between years with positive and negative ISH anomalies during 1971 2000. Shaded area indicates significant changes at 95% level estimated by a local student t-test. The arrows indicate schematically the systematic zonal wind responses to the ISH in the Southern Hemisphere. A indicates anticylone, and C cyclone. around 30 S in SH. In addition, the zonal winds anomalies are positive in tropical regions at African, Indian, Asian, and mid-american areas, indicating the weakening of South Asian and East Asian summer monsoon circulation, and vice versa. In addition, there is an anomalous teleconnection wave train in Eurasia associated with ISH, which displays an anomalous cyclonic circulation in Europe and North Asia respectively and an anomalous anticyclonic circulation in-between for the positive phase of ISH. Consequently, anomalous south wind is derived from East Asia, reducing the East Asian monsoon flow, and vice versa. However, the ISH-related pattern of the velocity variation in 850 hpa is different from that at 100 hpa. The zonal sysmetric meridional teleconnection in zonal wind field does not clearly exist at 850 hpa (Fig. 6). Instead, the meridional teleconnection in zonal wind field is mainly exhibited in Pacific sector, causing an anomalous easterly (westerly) winds in tropical western Pacific in positive (negative) phase of ISH, which is favorable to the weakening of EASM circulation. The velocity changes in the region to the east of Philippines cause the anomalous convergence, resulting in the intensifying of the convective activities, then introducing an additional reduction of the East Asian monsoon via the well-known East Asian teleconnection pattern (or Pacific-Japan teleconnection pattern). Besides, the ISH-associated circulation changes in high latitudes of Eurasia bring about anomalous north wind in East Asia 1512 weakening the monsoon flow. Though the ISH-EASM is closely connected, as noted above, the significance level for the ISH-precipitation correlation is less than 95%. This illustrates that the precipitation variability is more complicated. The EASM circulation is just one of the factors that have influences on the precipitation. That is why the precipitation variability is very difficult to predict. The AAO, by its definition, is describing the oscillation of the air masses between the middle and high latitudes in SH, thus representing the mean zonal wind in-between. However, the ISH is defined by the mean zonal wind differences between the middle and high latitudes in SH, thus it could stand for the mean zonal wind shear. Therefore, the AAO and ISH have the different physical meanings, thus they have the different linkages with the precipitation. AAO has closer connection with the precipitation than ISH, while ISH more relates with the East Asian monsoon circulation, especially the high-troposphere circulation in East Asia. 3 Summary This research reveals the relationship between East Asian summer monsoon and the Southern Hemisphere zonal wind variability in upper troposphere based on studies of the NCAR/NCEP reanalysis. It is shown that SH meridional structure of the mean zonal wind in upper troposphere may well be expressed by the mean zonal wind difference at 60 S and 30 S at 150 hpa (ISH Chinese Science Bulletin Vol. 51 No. 12 June 2006

Fig. 6. The composite difference of JJA velocity (m/s) at 850 hpa between years with positive and negative ISH anomalies during 1971 2000. Shaded area indicates significant changes at 95% level estimated by a local student t-test. The arrows indicate schematically the systematic zonal wind responses to the ISH in the Southern Hemisphere. C indicates cyclone. index). ISH is related to global mean zonal wind in both upper troposphere and lower troposphere, especially in SH. It is indicated that East Asian summer monsoon and ISH are significantly linked together in JJA season. They are negatively correlated each other in the interannual scale. They also well correspond to each other in the decadal scale variation at the end of the 1970s. Linkage between EASM and ISH may be explained by the meridional teleconnection from SH to NH. Through high troposphere meridional teleconnection, enhancement (weakening) of ISH will result in zonal winds weakening in Southern Hemisphere middle latitudes (enhancement) and tropical zonal winds enhancement (weakening) in high troposphere (as indicated by the thick arrow-headed lines in Fig. 5). The zonal wind enhancement (weakening) in tropical Asia indicates the weakening (enhancement) of the monsoon system. The anomalous teleconnection wave train in Eurasia plays a key role in changing the intensity of EASM as well. Such meridional teleconnection was discussed in ref. [11], and studied through the numerical experiments by use of the AGCM 1). In 850 hpa, positive phase of ISH will result in the westerly winds enhancement over the western Pacific and an anomalous north-to-south winds in the East Asia (as indicated by the long and thick arrow-headed lines in Fig. 6), causing the weakening of the East Asian summer monsoon circulation. In addition, the intensifying of the convective activities to the east of Philippines, as a result of the anomalous convergence, could introduce an additional reduction of the East Asian monsoon via the well-known East Asian teleconnection pattern (or Pacific-Japan teleconnection pattern). Acknowledgements This work was jointly supported by the Chinese Academy of Sciences (Grant No. KZCX3-SW221), the National Natural Science Foundation of China (Grant No. 40620130113) and International Partnership Project of the Chinese Academy of Sciences. References 1 Ju J, Slingo J M. The Asian summer monsoon and ENSO. Quart J Roy Meteor Soc, 1995, 121(525): 1133 1168 2 Wang B, Wu R, Fu X H. Pacific East Asian teleconnection: How does ENSO affect East Asian climate? J Climate, 2000, 13(9): 1517 1536 3 Wang H J. The weakening of the Asian Monsoon Circulation after the end of 1970 s. Adv Atmos Sci, 2001(3), 18: 376 386 4 Wang H J. Instibility of the East Asian Summer monsoon-enso 1) Fan K, Wang H J. Numerical experiment study on the anomalies of AAO and their impacts on the Northern Hemisphere Circulation. Chinese Journal of Geophysics (in press). www.scichina.com www.springerlink.com 1513

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