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Teleconnection between Sea Ice in the Barents Sea in June and the Silk Road, Pacific-Japan and East Asian Rainfall Patterns in August


doi: 10.1007/s00376-017-7029-y

  • In contrast to previous studies that have tended to focus on the influence of the total Arctic sea-ice cover on the East Asian summer tripole rainfall pattern, the present study identifies the Barents Sea as the key region where the June sea-ice variability exerts the most significant impacts on the East Asian August tripole rainfall pattern, and explores the teleconnection mechanisms involved. The results reveal that a reduction in June sea ice excites anomalous upward air motion due to strong near-surface thermal forcing, which further triggers a meridional overturning wave-like pattern extending to midlatitudes. Anomalous downward motion therefore forms over the Caspian Sea, which in turn induces zonally oriented overturning circulation along the subtropical jet stream, exhibiting the east-west Rossby wave train known as the Silk Road pattern. It is suggested that the Bonin high, a subtropical anticyclone predominant near South Korea, shows a significant anomaly due to the eastward extension of the Silk Road pattern to East Asia. As a possible descending branch of the Hadley cell, the Bonin high anomaly ultimately triggers a meridional overturning, establishing the Pacific-Japan pattern. This in turn induces an anomalous anticyclone and cyclone pair over East Asia, and a tripole vertical convection anomaly meridionally oriented over East Asia. Consequently, a tripole rainfall anomaly pattern is observed over East Asia. Results from numerical experiments using version 5 of the Community Atmosphere Model support the interpretation of this chain of events.
    摘要: 以往研究主要关注整个北极区域海冰变化对东亚夏季三极子型降水的影响, 本文则从遥相关角度揭示了6月巴伦支海海冰变率对8月东亚三极子型降水的作用. 结果表明, 6月巴伦支海海冰减少, 通过近地表较强的热力作用引起局地大气的上升运动异常, 进一步激发向中纬度延伸的经向翻转波列, 在里海形成大气下沉运动异常. 通过沿着副热带急流的纬向翻转环流, 该下层运动异常会激发一个东西向的罗斯贝波列, 类似于丝绸之路型. 丝绸之路型向东亚的延伸, 对位于韩国附近的副热带反气旋环流-小笠原高压产生显著影响. 作为哈德莱环流圈的一个下沉支, 异常小笠原高压引起的经向翻转环流形成了太平洋-日本遥相关型, 进一步促进了东亚地区一对异常反气旋和气旋环流、异常的经向三极子垂直对流的产生. 最终, 东亚出现三极子型降水异常. CAM5的数值模拟结果也支持本文的观点.
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  • Chen M. Y., P. P. Xie, J. E. Janowiak, and P. A. Arkin, 2002: Global land precipitation: A 50-yr monthly analysis based on gauge observations. Journal of Hydrometeorology, 3, 249-266, .https://doi.org/10.1175/1525-7541(2002)003<0249:GLPAYM>2.0,CO;210.1175/1525-7541(2002)003<0249:GLPAYM>2.0.CO;202ed5cbd2cfb77e05229e28c9cb641cahttp%3A%2F%2Fjpe.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2F1525-7541%282002%290032.0.CO%3B2%26amp%3Blink_type%3DDOIhttp://journals.ametsoc.org/doi/abs/10.1175/1525-7541%282002%29003%3C0249%3AGLPAYM%3E2.0.CO%3B2
    Ding Q. H., B. Wang, 2005: Circumglobal teleconnection in the northern hemisphere summer.J. Climate,18,3483-3505, https://doi.org/10.1175/JCLI3473.1.10.1175/JCLI3473.1e18c4652063ff9837f9b242cae362904http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2005AGUSM.A33B..01Dhttp://journals.ametsoc.org/doi/abs/10.1175/JCLI3473.1Analysis of the 56-yr NCEP09“NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the upper-tropospheric circulation. The CGT, having a zonal wavenumber-5 structure, is primarily positioned within a waveguide that is associated with the westerly jet stream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August, and September, but the pattern in July shows shorter wavelengths in the North Pacific09“North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western Europe, European Russia, India, east Asia, and North America. This implies that the CGT may be a source of climate variability and predictability in the above-mentioned midlatitude regions. The CGT has significant correlations with the Indian summer monsoon (ISM) and El Ni01±o09“Southern Oscillation (ENSO). However, in normal ISM years the CGT09“ENSO correlation disappears; on the other hand, in the absence of El Ni01±o or La Ni01±a, the CGT09“ISM correlation remains significant. It is suggested that the ISM acts as a 0904conductor0909 connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the western North Pacific summer monsoon. The circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of India, where a baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite an anomalous west-central Asian high and downstream Rossby wave train extending to the North Pacific and North America. On the other hand, a wave train that is excited in the jet exit region of the North Atlantic may affect the west-central Asian high and, thus, the intensity of the ISM. It is hypothesized that the interaction between the global wave train and the ISM heat source may be instrumental in maintaining the boreal summer CGT.
    Enomoto T., 2004: Interannual variability of the bonin high associated with the propagation of Rossby waves along the Asian jet.J. Meteor. Soc. Japan,82,1019-1034, 2004. 1019.https://doi.org/10.2151/jmsj.10.2151/jmsj.2004.1019ecf4888ad33f75476c33e2fdd06d4e8ahttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F110001803127http://joi.jlc.jst.go.jp/JST.JSTAGE/jmsj/2004.1019?from=CrossRefInterannual variability of the Ogasawara (Bonin) high in August is examined in relation to propagation of stationary Rossby waves along the Asian jet using monthly averages from the NCEP/NCAR reanalysis dataset for 52 years. The perturbation kinetic energy at 200 hPa is used as a measure of the activity of stationary Rossby waves along the Asian jet. Composite maps of five relatively wavy-jet years with close phases show an enhanced anticyclone over Japan. This anomalous ridge has a maximum amplitude at 250 hPa and extends throughout the troposphere with little zonal and slight northward tilts. Wave-activity and isentropic potential vorticity analyses clearly show that the ridge is created by the propagation of stationary Rossby waves to Japan. The anomalous ridge accompanies a positive temperature anomaly over Japan in the entire troposphere. A negative temperature anomaly to the east of Japan is also created in the lower troposphere by the northerly flow between the anomalous ridge and trough. By contrast, the equivalent-barotropic ridge over Japan is very weak in the zonal-jet years. Although Rossby waves are as strong as those in the wavy-jet years near the source, they are found to converge to the southeast of its source with little further downstream propagation. This contrast in the behaviour of Rossby waves is consistent with the intensity of the Asian jet to the east of 90ºE. The composite analysis suggests that the enhancement of a deep ridge near Japan is regulated by the intensity of the Asian jet. The composite analysis study conducted here emphasizes the importance of the propagation of stationary Rossby waves along the Asian jet for the late summer climate in northeastern Asia.
    Enomoto T., B. J. Hoskins, and Y. Matsuda, 2003: The formation mechanism of the Bonin high in August.Quart. J. Roy. Meteor. Soc.,129,157-178, https://doi.org/10.1256/qj.01.211.10.1256/qj.01.2111c51821855266189851ad2c1a89d7910http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1256%2Fqj.01.211%2Ffullhttp://doi.wiley.com/10.1256/qj.01.211Abstract The Bonin high is a subtropical anticyclone that is predominant near Japan in the summer. This anticyclone is associated with an equivalent-barotropic structure, often extending throughout the entire troposphere. Although the equivalent-barotropic structure of the Bonin high has been known for years among synopticians because of its importance to the summer climate in east Asia, there are few dynamical explanations for such a structure. The present paper attempts to provide a formation mechanism for the deep ridge near Japan. We propose a new hypothesis that this equivalent-barotropic ridge near Japan is formed as a result of the propagation of stationary Rossby waves along the Asian jet in the upper troposphere ( he Silk Road pattern). First, the monthly mean climatology is examined in order to demonstrate this hypothesis. It is shown that the enhanced Asian jet in August is favourable for the propagation of stationary Rossby waves and that the regions of descent over the eastern Mediterranean Sea and the Aral Sea act as two major wave sources. Second, a primitive-equation model is used to simulate the climatology of August. The model successfully simulates the Bonin high with an equivalent-barotropic structure. The upper-tropospheric ridge is found to be enhanced by a height anomaly of more than 80 m at 200 hPa, when a wave packet arrives. Sensitivity experiments are conducted to show that the removal of the diabatic cooling over the Asian jet suppresses the Silk Road pattern and formation of an equivalent-barotropic ridge near Japan, while the removal of the diabatic heating in the western Pacific does not. Copyright 2003 Royal Meteorological Society
    Grunseich G., B. Wang, 2016: Arctic sea ice patterns driven by the Asian summer monsoon.J. Climate,29,9097-9112, https://doi.org/10.1175/JCLI-D-16-0207.1.10.1175/JCLI-D-16-0207.1fbbb894ab3951381fd43fdf05f710b4bhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F308186763_Arctic_sea_ice_patterns_driven_by_the_Asian_Summer_Monsoonhttp://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0207.1react-text: 395 Century-long efforts have been devoted to seasonal forecast of Indian summer monsoon rainfall (ISMR). Most studies of seasonal forecast so far have focused on predicting the total amount of summer rainfall averaged over the entire India (i.e., all Indian rainfall index-AIRI). However, it is practically more useful to forecast anomalous seasonal rainfall distribution (anomaly pattern) across... /react-text react-text: 396 /react-text [Show full abstract]
    Guan Z. Y., T. Yamagata, 2003: The unusual summer of 1994 in East Asia: IOD teleconnections,Geophys. Res. Lett.,30,1544, https://doi.org/10.1029/2002GL016831.10.1029/2002GL016831d9a0f1a631c4563882e61b1edb5496ebhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2002GL016831%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2002GL016831/pdfAn extremely hot and dry summer of 1994 was reported in East Asian countries. Using observational data, we have demonstrated that the Indian Ocean Dipole (IOD) is at least one possible cause of the abnormal East Asian summer climate. An anomalous cyclonic circulation over the western Pacific and the southern China weakened the monsoonal northward flow in the lower troposphere. An anomalous anticyclonic circulation with the equivalent barotropic structure around Japan, Korea and the northeastern part of China caused the hot and dry summer of 1994. This accumulation of the lower potential vorticity in the Far East is related to the wave activity from the Mediterranean/Sahara region. The monsoon-desert mechanism connects a Rossby wave source with the IOD-induced diabatic heating around the Bay of Bengal. Another Rossby wave-train pattern was generated in the upper troposphere and propagates northeastward from the southern China. Both the Rossby wave patterns influenced the circulation changes over East Asia.
    Guo D., Y. Q. Gao, I. Bethke, D. Y. Gong, O. M. Johannessen, and H. J. Wang, 2014: Mechanism on how the spring Arctic sea ice impacts the East Asian summer monsoon.Theor. Appl. Climatol.,115,107-119, https://doi.org/10.1007/s00704-013-0872-6.10.1007/s00704-013-0872-6e974c7d3c1cbd752fc0be37fda1807fehttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00704-013-0872-6http://link.springer.com/10.1007/s00704-013-0872-6Observational analysis and purposely designed coupled atmosphere cean (AOGCM) and atmosphere-only (AGCM) model simulations are used together to investigate a new mechanism describing how spring Arctic sea ice impacts the East Asian summer monsoon (EASM). Consistent with previous studies, analysis of observational data from 1979 to 2009 show that spring Arctic sea ice is significantly linked to the EASM on inter-annual timescales. Results of a multivariate Empirical Orthogonal Function analysis reveal that sea surface temperature (SST) changes in the North Pacific play a mediating role for the inter-seasonal connection between spring Arctic sea ice and the EASM. Large-scale atmospheric circulation and precipitation changes are consistent with the SST changes. The mechanism found in the observational data is confirmed by the numerical experiments and can be described as follows: spring Arctic sea ice anomalies cause atmospheric circulation anomalies, which, in turn, cause SST anomalies in the North Pacific. The SST anomalies can persist into summer and then impact the summer monsoon circulation and precipitation over East Asia. The mediating role of SST changes is highlighted by the result that only the AOGCM, but not the AGCM, reproduces the observed sea ice-EASM linkage.
    He S.-P., 2015: Potential connection between the Australian summer monsoon circulation and summer precipitation over central China.Atmospheric and Oceanic Science Letters,8,120-126, https://doi.org/10.3878/AOSL20140091.10.3878/AOSL2014009147ce8a5fcbd3381ef146913fa5506b8ahttp%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fabs%2F10.3878%2FAOSL20140091http://www.tandfonline.com/doi/full/10.1080/16742834.2015.11447248这研究在华中上调查了在澳大利亚的夏天季风(ASM ) 和夏天降水之间的连接。它被发现那,跟随 weaker-than-normal ASM,东方亚洲夏天季风和西方的向北和平的副热带的高度趋于更强壮,让步异常向北方从西方的太平洋要搬运到华中的潮湿。而且,异常 upwelling 运动超过 3037.5 出现
    Hong X. W., R. Y. Lu, 2016: The meridional displacement of the summer Asian jet,silk road pattern,and tropical SST anomalies. J. Climate,29, 3753-3766,.https://doi.org/10.1175/JCLI-D-15-0541.1
    Hsu H.-H., X. Liu, 2003: Relationship between the Tibetan Plateau heating and East Asian summer monsoon rainfall,Geophys. Res. Lett.,30,2066, https://doi.org/10.1029/2003GL017909.10.1029/2003GL0179098fea788712753f702d00abd5311183edhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2003GL017909%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2003GL017909/fullThis study reveals the close correspondence between the interannual variability of the dominant East Asian summer rainfall pattern and the diabatic heating over the Tibetan Plateau in both spring and summer. The heating fluctuation over the Tibetan Plateau is associated with two wave-like circulation patterns, which bear the characteristics of forced Rossby wave emanating away from the deep heating. The wave-like pattern in turn affects the East Asian summer rainfall. Because of the persistent heating over the Tibetan Plateau from spring to summer and its possible effect on the surrounding areas, the heating index defined in this study can be used as a good predictor for the JJA heating and precipitation distributions. Evidences shown suggest that external conditions other than the SST anomaly must be considered to understand the interannual variability of the East Asian summer rainfall.
    Hsu H.-H., S.-M. Lin, 2007: Asymmetry of the tripole rainfall pattern during the East Asian summer.J. Climate,20,4443-4458, https://doi.org/10.1175/JCLI4246.1.10.1175/JCLI4246.156eb76d8f87ccb1f21008b0d16c95594http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2007jcli...20.4443hhttp://journals.ametsoc.org/doi/abs/10.1175/JCLI4246.1This study investigates the tripole rainfall pattern in East Asia during the northern summer. The tripole pattern is characterized by a zonally elongated and meridionally banded structure with signs changing alternatively from 2000° to 5000°N along the East Asian coast. The positive (negative) phase of the pattern is characterized by more (less) rainfall in central-eastern China, Japan, and South Korea, and less (more) rainfall in northern and southern China. Asymmetry between the positive and negative phases is one of the key findings of this study. The tripole pattern is closely associated with two wavelike patterns: the Pacific09“Japan pattern and the Silk Road pattern. The former, which emanates from the tropical western Pacific to extratropical East Asia, is more evident in the positive phase, while the latter, emanating across the Eurasian continent, is more evident in the negative phase. The positive phase appears to have a stronger tropical connection, while the negative phase has a stronger extratropical connection. The positive and negative phases are associated with the positive and negative SSTA in the equatorial eastern Pacific, respectively. It is suggested that in the positive phase the zonally oriented overturning circulation driven by the positive SSTA in the equatorial eastern Pacific induces heating anomalies in the tropical western Pacific, which in turn triggers a wavelike pattern emanating northward toward extratropical East Asia. This indirect SSTA effect is not evident in the negative phase, which is predominantly affected by the extratropical Eurasian wavelike perturbations. On the other hand, anomalous heating over the eastern Tibetan Plateau seems to induce the eastward-propagating wavelike structure in both phases. It is suggested that the tripole pattern is a result of the amplification of an intrinsic dynamic mode that can be triggered by various factors despite their different origins.
    Huang R. H., F. Y. Sun, 1992: Impacts of the tropical western Pacific on the East Asian summer monsoon.J. Meteor. Soc. Japan,70,243-256, https://doi.org/10.2151/jmsj1965.70.1B_243.10.1175/1520-0469(1992)049<0256:PAPIAP>2.0.CO;21aaabaa5e72c71c10dbbd6497a52bf19http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F40000634875https://www.jstage.jst.go.jp/article/jmsj1965/70/1B/70_1B_243/_articleIn this paper, the impacts of the convective activities in the western Pacific warm pool on the interannual and intraseasonal variations of the summer monsoon in East Asia are analyzed by using the observed data for 12 summers from 1978 to 1989. The analyzed results show that both interannual and intraseasonal variabilities of the East Asian summer monsoon are greatly influenced by the convective activities in the warm pool. Generally, the monsoon rainfall is below normal in East Asia and the abrupt change of the monsoon circulation is obvious in the summer of strong convective activities around the Philippines. The impacts of the convective activities in the warm pool on the summer monsoon in East Asia and the East Asia/Pacific teleconnection pattern of summer circulation anomalies due to the convection are discussed by using the theory of planetary wave propagation and the numerical modelling by the IAP-GCM, respectively.
    Ju L.-X., Z.-W. Han, 2013: Impact of different East Asian summer monsoon circulations on aerosol-induced climatic effects.Atmospheric and Oceanic Science Letters,6,227-232, https://doi.org/10.3878/j.issn.1674-2834.13.0018.10.3878/j.issn.1674-2834.13.00182f14eb8fe207f1fc94bd2fc29e16ddbchttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqhhykxkb201305002.aspxhttp://www.tandfonline.com/doi/full/10.1080/16742834.2013.11447086weak (2003) and a strong (2006) East Asian summer monsoon (EASM) circulation were simulated using a high-resolution regional climate model (RegCM3). Results showed that the atmospheric circulations of summer monsoon have direct relations with transport of aerosols and their climatic effects. Both the top-of-the-atmosphere (TOA) and the surface-negative radiative forcing of aerosols were stronger in weak EASM circulations. The main difference in aerosol-induced negative forcing in two summers varied between 2 and 14 W m from the Sichuan Basin to North China, where a maximum in aerosol-induced negative forcing was also noticed in the EASM-dominated areas. The spatial difference in the simulated aerosol optical depth (AOD) in two summers generally showed the similar pictures. Surface cooling effects induced by aerosols were spatially more uniform in weak EASM circulations and cooler by about 1-4.5ºC. A preliminary analysis here indicated that a weaker low-level wind speed not conducive to the transport and diffusion of aerosols could make more contributions to the differences in the two circulations.
    Kalnay, E., Coauthors, 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc., 77, 437-471, https://doi.org/10.1175/1520-0477(1996)077<0437:TNYRP>2.0,CO;2.10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;29bfeacc7ab553b364e43408563ad850bhttp%3A%2F%2Fintl-icb.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2F1520-0477%281996%290772.0.CO%3B2%26amp%3Blink_type%3DDOIhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0477%281996%29077%3C0437%3ATNYRP%3E2.0.CO%3B2
    Kosaka Y., H. Nakamura, 2006: Structure and dynamics of the summertime Pacific-Japan teleconnection pattern.Quart. J. Roy. Meteor. Soc.,132,2009-2030, https://doi.org/10.1256/qj.05.204.10.1256/qj.05.2042dc5d851f039ea3a6f8e45fc9b82fbd2http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1256%2Fqj.05.204%2Ffullhttp://doi.wiley.com/10.1256/qj.05.204Abstract Convective activity over the tropical western Pacific is known to influence the extratropical circulation over East Asia in the boreal summer in the form of teleconnection, called the ‘Pacific–Japan (PJ) pattern’, but its structure and dynamics have not yet been studied in depth. In this study, a composite analysis is performed for 32 monthly events of enhanced convection observed to the east of the Philippines. The composited monthly mean vorticity anomalies associated with the PJ pattern are elongated zonally with a distinct poleward tilt with height. This structure differs fundamentally from a combination of the first baroclinic mode in the tropics and the barotropic structure in midlatitudes, as has widely been accepted as a conceptual model of the PJ pattern. A wave-activity flux points polewards only in the lower troposphere, indicating that Rossby wave teleconnection occurs primarily through a low-level south-westerly jet. Those tilted anomalies over the western Pacific can effectively gain kinetic energy in the exits of the mean jet streams in the upper and lower troposphere and available potential energy (APE) in the presence of the vertically sheared jets. The enhanced convection can generate APE effectively, and the associated low-level anomalous circulation acts to increase moisture supply into the convective region while enhancing evaporation from the pre-warmed ocean surface. It is thus hypothesized that the PJ pattern may be regarded as a dynamical mode that can be effectively excited in the zonally asymmetric baroclinic mean flow associated with the Asian summer monsoon with an efficient self-sustaining mechanism through moist processes. Copyright 08 2006 Royal Meteorological Society
    Lau K.-M., H. Y. Weng, 2001: Coherent modes of global SST and summer rainfall over China: An assessment of the regional impacts of the 1997-98 El Niño. J. Climate, 14, 1294-1308, https://doi.org/10.1175/1520-0442(2001)014<1294:CMOGSA>2.0,CO;2.10.1175/1520-0442(2001)0142.0.CO;2b4336635ea269347aeea28e54d5108c4http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127219%2Fhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282001%29014%3C1294%3ACMOGSA%3E2.0.CO%3B2In this paper, the authors have identified three coherent modes of summertime rainfall variability over China and global sea surface temperature (SST) for the period of 1955-98 by Singular Value Decomposition. Based on these modes, the impacts of the 1997-98 El Ni09o on major drought and flood occurrences over China have been assessed. The first mode can be identified with the growing phase of El Ni09o superimposed on a warming trend since the mid-1950s. This mode strongly influences rainfall over northern China. The second mode comprises a quasi-biennial (QB) variability manifested in alternate wet and dry years over the Yangtze River Valley (YRV) of central China. The third mode is dominated by a quasi-decadal oscillation in eastern China between the Yangtze River and the Yellow River, with an opposite tendency in southern China.Using a mode-by-mode reconstruction, the contributions of these leading modes to the 1997 and 1998 observed rainfall anomalies are evaluated. It is found that the severe drought in northern China, and to a lesser degree the flood in southern China, in 1997 is likely a result of the influence of anomalous SST forcing during the growing phase of the 1997-98 El Ni09o. The severe flood over YRV in 1998 is associated with the biennial tendency of basin-scale SST anomaly during the transition from El Ni09o to La Ni09a in 1997-98. In addition, the prolonged dry tendency over northern China and wet tendency over YRV since the 1970s may be associated with a long-term warming trend in the tropical Indian Ocean and western Pacific. The long-term dry background exacerbated the drought situation over northern China in 1997, and the wet background exacerbated the flood situation over YRV in 1998, under the impacts of the 1997-98 El Ni09o. In contrast, the rainfall variability in southern China is most chaotic, with no clear dominance of either El Ni09o or QB signals. The significance, reliability, and stability of these results are also discussed.
    Lu R.-Y., J.-H. Oh, and B.-J. Kim, 2002: A teleconnection pattern in upper-level meridional wind over the North African and Eurasian continent in summer. Tellus A 54, 44-55. http://dx.doi.org/10.3402/tellusa.v54i1.12122.10.1034/j.1600-0870.2002.00248.xca72f33b50a1704343239c73d29e6b23http%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fabs%2F10.3402%2Ftellusa.v54i1.12122http://onlinelibrary.wiley.com/doi/10.1034/j.1600-0870.2002.00248.x/citedbyOne-point correlation analysis on upper-level meridional wind identified the existence of a teleconnection pattern in July, which emerges from North Africa to East Asia along the westerly jet in the middle latitudes. We examined the spatial and temporal structures of this teleconnection pattern, and found the unique characteristics rather different from the patterns in other elements such as geopotential height, streamfunction and vorticity. We also investigated the relationship between this teleconnection and precipitation, and suggested that the teleconnection is a possible linkage of the EASM to the Indian monsoon, and even to subtropical heating anomalies over Atlantic.
    Matsumura S., K. Yamazaki, 2012: Eurasian subarctic summer climate in response to anomalous snow cover.J. Climate,25,1305-1317, https://doi.org/10.1175/2011JCLI4116.1.10.1175/2011JCLI4116.1fcdc5ac21691461825fafd08de0ac944http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012JCli...25.1305Mhttp://journals.ametsoc.org/doi/abs/10.1175/2011JCLI4116.1The summer climate in northern Eurasia is examined as a function of anomalous snow cover and processes associated with land-atmosphere coupling, based on a composite analysis using observational and reanalysis data. The analysis confirms that the snow-hydrological effect, which is enhanced soil moisture persisting later into the summer and contributing to cooling and precipitation recycling, is active in eastern Siberia and contributes to the formation of the subpolar jet through the thermal wind relationship in early snowmelt years. Strong anticyclonic differences (early - late snowmelt years) with a baroclinic structure form over eastern Siberia as a result of surface heating through the snow-hydrological effect in early snowmelt years. Surface heating contributes to the development of thermally generated stationary Rossby waves that propagate eastward to eastern Siberia. Rossby wave activity is maintained into early autumn, together with cyclonic differences over far eastern Siberia, in conjunction with the early appearance of snow cover in this region. The anticyclonic differences over eastern Siberia act like a blocking anticyclone, thereby strengthening upstream storm track activity. Furthermore, it is possible that surface anticyclonic differences over the Arctic contribute to year-to-year variability of summer Arctic sea ice concentration along the Siberian coast. The results suggest that variations in northern Eurasian snow cover and associated land-atmosphere coupling processes have important implications for the predictability of Eurasian subarctic summer climate.
    Matsumura S., X. D. Zhang, and K. Yamazaki, 2014: Summer Arctic atmospheric circulation response to spring Eurasian snow cover and its possible linkage to accelerated sea ice decrease.J. Climate,27,6551-6558, https://doi.org/10.1175/JCLI-D-13-00549.1.10.1175/JCLI-D-13-00549.1eb65879038b9a251573f335ebbaccbc7http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F120005549583%2Fja%2Fhttp://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00549.1Anticyclonic circulation has intensified over the Arctic Ocean in summer during recent decades. However, the underlying mechanism is, as yet, not well understood. Here, it is shown that earlier spring Eurasian snowmelt leads to anomalously negative sea level pressure (SLP) over Eurasia and positive SLP over the Arctic, which has strong projection on the negative phase of the northern annular mode (NAM) in summer through the wave-mean flow interaction. Specifically, earlier spring snowmelt over Eurasia leads to a warmer land surface, because of reduced surface albedo. The warmed surface amplifies stationary Rossby waves, leading to a deceleration of the subpolar jet. As a consequence, rising motion is enhanced over the land, and compensating subsidence and adiabatic heating occur in the Arctic troposphere, forming the negative NAM. The intensified anticyclonic circulation has played a contributing role in accelerating the sea ice decline observed during the last two decades. The results here provide important information for improving seasonal prediction of summer sea ice cover.
    Nitta T., 1987: Convective activities in the tropical western Pacific and their impact on the Northern Hemisphere summer circulation.J. Meteor. Soc. Japan,65,373-390, https://doi.org/10.2151/jmsj1965.65.3_373.10.1175/1520-0469(1987)044<1554:TAOPVT>2.0.CO;2e2ef16e7dae890eb1752e3cd22affb10http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013126166%2Fhttps://www.jstage.jst.go.jp/article/jmsj1965/65/3/65_3_373/_articleInterannual and intraseasonal variations of convective activities in the tropical western Pacific during summer and their impact on the Northern Hemisphere circulation are investigated by using satellite cloud amount, sea surface temperature (SST) and geopotential data for 7 years (1978-1984). During summers when SST in the tropical western Pacific is about 1.0ºC warmer than normal, active convection regions consisting of a number of typhoons and tropical depressions are shifted northeastward from the normal position near Philippines to the subtropical western Pacific around 20ºN and cloud amounts both in the middle latitudes and in the equatorial regions are greatly suppressed. A high pressure anomaly with little vertical tilt predominates in middle latitudes extending from East China, through Japan Islands to North Pacific during these summers. Analyses of 5-day mean cloud amount reveal that the convective activity is largely modulated by the intraseasonal variations (ISV). The amplitude of ISV of convective activity in the Philippine Sea around 15ºN-20ºN is more intensified in warm SST summers than in cold SST summers resulting in stronger season mean convective activities in the former than in the latter. Correlation computations between 5-day mean tropical cloud amount and 500mb geopotential height show that there exist wave trains of geopotential height emanating from the heat source region near Philippines to North America. Daily analyses of geopotential height indicate that these wave trains appear to be generated when convective activities in the Philippine Sea become intense and that the amplification occurs downstream from the western Pacific to the west coast of North America taking about 5 days. It is concluded that Rossby waves are generated by the tropical heat source associated with ISV, and high pressure anomalies over East Asia and Northwest Pacific during warm SST summers can be understood as the results of frequent occurrence of Rossby wave generation.
    Nitta T., Z.-Z. Hu, 1996: Summer climate variability in China and its association with 500 hPa height and tropical convection.J. Meteor. Soc. Japan,74,425-445, https://doi.org/10.2151/jmsj1965.74.4_425.10.1175/1520-0469(1996)053<2283:OTFOLI>2.0.CO;289b69c2c64c12a09c2d165ea0548b543http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127230%2Fhttps://www.jstage.jst.go.jp/article/jmsj1965/74/4/74_4_425/_articleThis paper is concernd with interannual and interdecadal variabilities of summer rainfall and temperature patterns in China and their association with 500 hPa height in the Northern Hemisphere (NH), tropical convective activities and global sea surface temperature anomaly (SSTA). The temporal evolutions and spatial structures of interannual variation of summer (JJA) rainfall and temperature from 1951 to 1994 over China are revealed through EOF analysis. The spatial pattern of EOF1 for rainfall (EOF1.R) is dominated by a maximum over the middle-lower reaches of the Yangtze River, and a large negative value region in the middle reach of the Yellow River is also obvious. The spatial pattern of EOF1 for temperature (EOF1.T) reflects coherent variations over most regions of China, and it is dominated by a maximum over the middle-lower reaches of the Yangtze River. Linear increase and decrease trends are found in the time coefficients of EOF1.R and EOF1.T, respectively. The quasi-biennial oscillation (QBO) signal is also strong after the middle of the 1970's in repect of their time coefficients. The coupled patterns of rainfall and temperature are picked up through the singular value decomposition (SVD) analysis. The spatial patterns and their temporal evolutions of SVD1 for rainfall (SVD1.R) and SVD1 for temperature (SVD1.T) are quite similar to those of EOF1.R and EOF1.T. There is an abrupt change in the middle 1970's in the time coefficients of SVD2.R and SVD2.T. The variations of summer rainfall and temperature coupled patterns in China are closely connected with the 500 hPa height anomaly over the Northern Hemisphere (NH). The Pacific-Japan (PJ) and Eurasia (EU) teleconnection patterns play a very important role in the spatial patterns of SVD1.R and SVD1.T, especially in the East Asia monsoon region along the middle-lower reaches of the Yangtze River. The abrupt change of China summer climate in the middle 1970's is related with the intensification and southerly location of the western Pacific subtropical high and also the geopotential height changes over Eurasia and in the regions to the north of the Japan Sea in 1977 or 1978. Correlations between the summer rainfall and temperature coupled patterns and monthly-averaged outgoing longwave radiation (OLR) and high-cloud amount (HCA) data are significant with the PJ teleconnection pattern. There exist positive correlations between the coupled patterns and sea surface temperature anomaly (SSTA) in the North Pacific and the tropical western Pacific. A comparison study shows that there are coherent variations between summer rainfall in the middle-lower reaches of the Yangtze River and in the western part of Japan. It is also demonstrated that there are close correlations between the summer temperature variations in China and in Japan.
    Rayner N. A., D. E. Parker, E. B. Horton, C. K. Folland , L. V. Alexand er, D. P. Rowell, E. C. Kent, and A. Kaplan, 2003: Global analyses of sea surface temperature,sea ice, and night marine air temperature since the late Nineteenth Century.J. Geophys. Res.,108,4407, https://doi.org/10.1029/2002JD002670.10.1029/2002JD0026700831f099871c89699f00bb6e2586346bhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2002JD002670%2Ffullhttp://doi.wiley.com/10.1029/2002JD002670We present the Met Office Hadley Centre's sea ice and sea surface temperature (SST) data set, HadISST1, and the nighttime marine air temperature (NMAT) data set, HadMAT1. HadISST1 replaces the global sea ice and sea surface temperature (GISST) data sets and is a unique combination of monthly globally complete fields of SST and sea ice concentration on a 1º latitude-longitude grid from 1871. The companion HadMAT1 runs monthly from 1856 on a 5º latitude-longitude grid and incorporates new corrections for the effect on NMAT of increasing deck (and hence measurement) heights. HadISST1 and HadMAT1 temperatures are reconstructed using a two-stage reduced-space optimal interpolation procedure, followed by superposition of quality-improved gridded observations onto the reconstructions to restore local detail. The sea ice fields are made more homogeneous by compensating satellite microwave-based sea ice concentrations for the impact of surface melt effects on retrievals in the Arctic and for algorithm deficiencies in the Antarctic and by making the historical in situ concentrations consistent with the satellite data. SSTs near sea ice are estimated using statistical relationships between SST and sea ice concentration. HadISST1 compares well with other published analyses, capturing trends in global, hemispheric, and regional SST well, containing SST fields with more uniform variance through time and better month-to-month persistence than those in GISST. HadMAT1 is more consistent with SST and with collocated land surface air temperatures than previous NMAT data sets.
    Sardeshmukh P. D., B. J. Hoskins, 1988: The generation of global rotational flow by steady idealized tropical divergence. J. Atmos. Sci., 45, 1228-1251, https://doi.org/10.1175/1520-0469(1988)045<1228:TGOGRF>2.0,CO;2.10.1175/1520-0469(1988)0452.0.CO;2b2412b73da38526cb734520e966688b9http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FADS%3Fid%3D1988JAtS...45.1228Shttp://journals.ametsoc.org/doi/abs/10.1175/1520-0469%281988%29045%3C1228%3ATGOGRF%3E2.0.CO%3B2Abstract Tropical convective heating is balanced on the large scale by the adiabatic cooling of ascent. The horizontal divergence of the wind above this heating may be viewed as driving the upper tropospheric rotational wind field. A vorticity equation model is used to diagnose this relationship. It is shown that because of the advection of vorticity by the divergent component of the flow, the Rossby wave source can be very different from the simple 抐D source often used. In particular, an equatorial region of divergence situated in easterly winds can lead to a Rossby wave source in the subtropical westerlies where it is extremely effective. This part of the source can be relatively insensitive to the longitudinal position of the equatorial divergence. A divergence field which is asymmetric about the equator can lead to a quite symmetric Rossby wave source. For a steady frictionless flow the Rossby wave source averaged over regions within closed streamfunction or absolute vorticity contours is, under cert...
    Screen J. A., 2013: Influence of Arctic sea ice on European summer precipitation,Environmental Research Letters,8,044015, https://doi.org/10.1088/1748-9326/8/4/044015.10.1088/1748-9326/8/4/0440158c1f32f7ca34bd27f63b35abea55aec9http%3A%2F%2Fwww.ingentaconnect.com%2Fsearch%2Farticle%3Foption1%3Dtka%26amp%3Bvalue1%3DEuropean%2Bseas%26amp%3BpageSize%3D10%26amp%3Bindex%3D4http://stacks.iop.org/1748-9326/8/i=4/a=044015?key=crossref.42089eb0cee2159ec299ee960d1840a4
    Takaya K., H. Nakamura, 2001: A formulation of a phase-independent wave-activity flux for stationary and migratory quasigeostrophic eddies on a zonally varying basic flow. J. Atmos. Sci., 58, 608-627, https://doi.org/10.1175/1520-0469(2001)058<0608:AFOAPI>2.0,CO;2.10.1175/1520-0469(2001)058<0608:AFOAPI>2.0.CO;2cd8c40c8181e2ef17726a6d7ec840f85http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2001JAtS...58..608Thttp://adsabs.harvard.edu/abs/2001JAtS...58..608TA new formulation of an approximate conservation relation of wave-activity pseudomomentum is derived, which is applicable for either stationary or migratory quasigeostrophic (QG) eddies on a zonally varying basic flow. The authors utilize a combination of a quantity A that is proportional to wave enstrophy and another quantity that is proportional to wave energy. Both A and are approximately related to the wave-activity pseudomomentum. It is shown for QG eddies on a slowly varying, unforced nonzonal flow that a particular linear combination of A and , namely, M (A + )/2, is independent of the wave phase, even if unaveraged, in the limit of a small-amplitude plane wave. In the same limit, a flux of M is also free from an oscillatory component on a scale of one-half wavelength even without any averaging. It is shown that M is conserved under steady, unforced, and nondissipative conditions and the flux of M is parallel to the local three-dimensional group velocity in the WKB limit. The authors' conservation relation based on a straightforward derivation is a generalization of that for stationary Rossby waves on a zonally uniform basic flow as derived by Plumb and others.A dynamical interpretation is presented for each term of such a phase-independent flux of the authors or Plumb. Terms that consist of eddy heat and momentum fluxes are shown to represent systematic upstream transport of the mean-flow westerly momentum by a propagating wave packet, whereas other terms proportional to eddy streamfunction anomalies are shown to represent an ageostrophic flux of geopotential in the direction of the local group velocity. In such a flux, these two dynamical processes acting most strongly on the node lines and ridge/trough lines of the eddy streamfunction field, respectively, are appropriately combined to eliminate its phase dependency. The authors also derive generalized three-dimensional transformed Eulerian-mean equations with the residual circulation and eddy forcing both expressed in phase-independent forms.The flux may not be particularly suited for evaluating the exact local budget of M, because of several assumptions imposed in the derivation. Nevertheless, these assumptions seem qualitatively valid in the assessment based on observed and simulated data. The wave-activity flux is a useful diagnostic tool for illustrating a`snapshot' of a propagating packet of stationary or migratory QG wave disturbances and thereby for inferring where the packet is emitted and absorbed, as verified in several applications to the data. It may also be useful for routine climate diagnoses in an operational center.
    Tian S.-F., T. Yasunari, 1992: Time and space structure of interannual variations in summer rainfall over China.J. Meteor. Soc. Japan,70,585-596, https://doi.org/10.2151/jmsj1965.70.1B_585.10.2151/jmsj1965.70.1B_5855899a2dd11d2b0e561da3d3978e6d962http%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F40000634892https://www.jstage.jst.go.jp/article/jmsj1965/70/1B/70_1B_585/_articlereact-text: 526 Numerical weather prediction is considered as an initial/boundary value problem: given an estimate of the present state of the atmosphere (initial conditions), and appropriate surface and lateral boundary conditions, an atmospheric model is able to simulate the future state of the atmosphere. Therefore, the more accurate the estimate of the initial conditions, the better the quality of the... /react-text react-text: 527 /react-text [Show full abstract]
    Wang H. J., S. P. He, 2015: The North China/Northeastern Asia severe summer drought in 2014.J. Climate,28,6667-6681, https://doi.org/10.1175/JCLI-D-15-0202.1.10.1175/JCLI-D-15-0202.1b073ea7a25f753b91ff51ee7281f9c48http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.6667Whttp://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0202.1ABSTRACT In summer 2014, north China and large areas of northeastern Asia (NCNEA) suffered from the most severe drought of the past 60 years. This study indicates that the East Asian summer precipitation in 2014 exhibited a tripole anomaly, with severe negative anomalies in NCNEA, strong positive anomalies in south China, South Korea, and Japan, and intense negative anomalies in the western North Pacific. Along with the severe tripole precipitation anomalies, there were strong intensities of the Silk Road pattern, the Pacific-Japan pattern, and the Eurasian teleconnection pattern, which were responsible for the strong precipitation anomaly in 2014 through changes to the western Pacific subtropical high (WPSH) and the East Asian trough. Further analysis indicates that the sea surface temperature (SST) in the North Pacific was nearly the warmest in the past 60 years and, together with the strong SST warming in the warm pool region, thus caused the strong Pacific-Japan teleconnection pattern, southward positioning of the WPSH, and weakened East Asian summer monsoon. Additionally, the summertime sea ice cover in the Arctic Ocean was anomalous, resulting in high SST in the Laptev-Kara Sea and, hence, triggering a strong Eurasian teleconnection pattern and contributing to the severe drought of NCNEA. Furthermore, the intense warming over the European Continent and Caspian Sea favored the Silk Road pattern, also contributing to the southward positioning of the WPSH and the NCNEA drought. The NCNEA severe drought was therefore the joint result of Pacific SST anomalies, Arctic sea ice anomalies, and warming over the European Continent and Caspian Sea.
    Wang H. J., H. P. Chen, 2012: Climate control for southeastern China moisture and precipitation: Indian or East Asian monsoon? J,Geophys. Res.,117,D12109, https://doi.org/10.1029/2012JD017734.10.1029/2012JD01773400f6fdf0394cbe73e16f92c88c161251http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2012JD017734%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2012JD017734/fullIn this study, the water vapor sources for the precipitation processes in southeastern China (SECN) during 1981-2010 were investigated using atmospheric reanalysis data. We also studied the factors influencing the summer atmospheric moisture over SECN. These two issues are all closely related to the climate signals recorded in stalagmites recovered from caves in SECN. Result supports that the atmospheric water vapor over SECN during the whole summer time is primarily transported from the Indian Ocean. However, the vertically integrated water vapor content throughout the year in SECN has two main sources: the Indian Ocean and the tropical western Pacific. In addition, the water vapor transport for the precipitation processes in SECN has complex vertical structure. At approximately 700 hPa to 500 hPa, part of the water vapor for the precipitation in SECN comes from the Arab-Caspian region. Finally, the water vapor content over SECN is regulated primarily by both the Indian and East Asian monsoons. Further analysis indicated that the variability of the East Asian summer monsoon is substantially regulated by the western Pacific subtropical high, the Eurasia-Atlantic thermal conditions, as well as the large-scale Eurasia-Atlantic atmospheric circulation. Therefore, the SECN Cave proxies can record the signals from faraway middle and high latitude Eurasia-Atlantic climate, besides the regional East Asian monsoon and remote Indian monsoon.
    Wang T., H. J. Wang, O. H. Otter濮橈拷, Y. Q. Gao, L. L. Suo, T. Furevik, and L. Yu, 2013: Anthropogenic agent implicated as a prime driver of shift in precipitation in eastern China in the late 1970s.Atmospheric Chemistry and Physics,13,12433-12450, .http://doi.org/10.5194/acp-13-12433-201310.5194/acp-13-12433-2013211d47d45a58674be11484110cfae655http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013acp....1312433whttp://www.atmos-chem-phys.net/13/12433/2013/Observation shows that eastern China experienced an interdecadal shift in the summer precipitation during the second half of the 20th century. The summer precipitation increased in the middle and lower reaches of the Yangtze River valley, whereas it decreased in northern China. Here we use a coupled ocean-atmosphere general circulation model and multi-ensemble simulations to show that the interdecadal shift is mainly caused by the anthropogenic forcing. The rapidly increasing greenhouse gases induce a notable Indian Ocean warming, causing a westward shift of the western Pacific subtropical high (WPSH) and a southward displacement of the East Asia westerly jet (EAJ) on an interdecadal timescale, leading to more precipitation in Yangtze River valley. At the same time the surface cooling effects from the stronger convection, higher precipitation and rapidly increasing anthropogenic aerosols contribute to a reduced summer land-sea thermal contrast. Due to the changes in the WPSH, the EAJ and the land-sea thermal contrast, the East Asian summer monsoon weakened resulting in drought in northern China. Consequently, an anomalous precipitation pattern started to emerge over eastern China in the late 1970s. According to the model, the natural forcing played an opposite role in regulating the changes in WPSH and EAJ, and postponed the anthropogenically forced climate changes in eastern China. The Indian Ocean sea surface temperature is crucial to the response, and acts as a bridge to link the external forcings and East Asian summer climate together on a decadal and longer timescales. Our results further highlight the dominant roles of anthropogenic forcing agents in shaping interdecadal changes of the East Asian climate during the second half of the 20th century.
    Weng H. Y., K.-M. Lau, and Y. K. Xue, 1999: Multi-scale summer rainfall variability over China and its long-term link to global sea surface temperature variability.J. Meteor. Soc. Japan,77,845-857, https://doi.org/10.2151/jmsj1965.77.4_845.10.1175/1520-0469(1999)056<2728:OTFSIT>2.0.CO;2dbace0ebcb5bb0802aaf81e6adde35cahttp%3A%2F%2Fci.nii.ac.jp%2Fnaid%2F10013127193https://www.jstage.jst.go.jp/article/jmsj1965/77/4/77_4_845/_articleMulti-scale summer (Jun-Aug) rainfall variability over China and its long-term link to global sea surface temperature (SST) variability are studied for the period of 1955-1997. First, the dominant spatial and temporal patterns of the observed rainfall anomaly are studied by empirical orthogonal function (EOF) analysis. By a wavelet transform, interannual and decadal-interdecadal variabilities as well as a trend are found, with different dominance, in the first two EOF modes. EOF1 shows a sudden shift in rainfall anomaly over China in the late 1970s, representing overall wetter conditions in central China and drier conditions in northern and southern China in the 1980s than the conditions in the 1960s. This sudden shift is associated with a quasi-in-phase reinforcement between bidecadal and quadridecadal variabilities. EOF2 represents an increasing trend in the rainfall anomaly in broad central and southern China, especially in the Yangtze River valley, without an apparent shift in the late 1970s. The lack of such a shift is associated with an out-of-phase partial cancellation between a bidecadal cycle and the trend around that time. Second, to understand the long-term rainfall variability that is linked to global SST variability, the singular value decomposition (SVD) analysis for the two fields is carried out. SVD1 links drought conditions in northern China and flood conditions in central China to an El Niño-like SST anomaly distribution. This mode shows both an apparent trend and a regime shift in the late 1970s, which do not coexist in the rainfall EOF modes. SVD2 links the rainfall anomaly in the area between the Yangtze River and the Yellow River and the opposite anomaly in southern China to a wave-like SST anomaly distribution in the eastern Pacific from tropics to extratropics. SVD3 links the rainfall anomaly in the Yangtze River valley to the SST anomaly in the western Pacific centered near 20ºN 140ºE. The rainfall variability in different areas of China that can be attributed to SST effects results from the interplay of the SVD modes. The most significant links found from SVD analysis are verified by cross-correlation functions. A scenario for a long-term link on the trend scale between the rainfall over China and global SST variabilities, through the associated large-scale circulation, is presented.
    Wu B. Y., R. H. Zhang, B. Wang, and R. D'Arrigo, 2009: On the association between spring Arctic sea ice concentration and Chinese summer rainfall,Geophys. Res. Lett.,36,L09501, https://doi.org/10.1029/2009GL037299.10.1029/2009GL037299942eda0778ebc3ab2750515e08040092http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00376-009-9009-3http://link.springer.com/article/10.1007/s00376-009-9009-3In our previous study, a statistical linkage between the spring Arctic sea ice concentration (SIC) and the succeeding Chinese summer rainfall during the period 19682005 was identified. This linkage is demonstrated by the leading singular value decomposition (SVD) that accounts for 19% of the co-variance. Both spring SIC and Chinese summer rainfall exhibit a coherent interannual variability and two apparent interdecadal variations that occurred in the late 1970s and the early 1990s. The combined impacts of both spring Arctic SIC and Eurasian snow cover on the summer Eurasian wave train may explain their statistical linkage. In this study, we show that evolution of atmospheric circulation anomalies from spring to summer, to a great extent, may explain the spatial distribution of spring and summer Arctic SIC anomalies, and is dynamically consistent with Chinese summer rainfall anomalies in recent decades. The association between spring Arctic SIC and Chinese summer rainfall on interannual time scales is more important relative to interdecadal time scales. The summer Arctic dipole anomaly may serve as the bridge linking the spring Arctic SIC and Chinese summer rainfall, and their coherent interdecadal variations may reflect the feedback of spring SIC variability on the atmosphere. The summer Arctic dipole anomaly shows a closer relationship with the Chinese summer rainfall relative to the Arctic Oscillation.
    Xie S.-P., K. M. Hu, J. Hafner, H. Tokinaga, Y. Du, G. Huang, and T. Sampe, 2009: Indian Ocean capacitor effect on Indo-western Pacific climate during the summer following El Niño.J. Climate,22,730-747, .http://doi.org/10.1175/2008JCLI2544.110.1175/2008JCLI2544.15ca7332e21ffff9143909ee76fe9bab3http%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20093117314.htmlhttp://journals.ametsoc.org/doi/abs/10.1175/2008JCLI2544.1Significant climate anomalies persist through the summer (June-August) after El Nino dissipates in spring over the equatorial Pacific. They include the tropical Indian Ocean (TIO) sea surface temperature (SST) warming, increased tropical tropospheric temperature, an anomalous anticyclone over the subtropical northwest Pacific, and increased mei-yu-baiu rainfall over East Asia. The cause of these lingering El Nino effects during summer is investigated using observations and an atmospheric general circulation model (GCM). The results herein indicate that the TIO warming acts like a capacitor anchoring atmospheric anomalies over the Indo-western Pacific Oceans. It causes tropospheric temperature to increase by a moist-adiabatic adjustment in deep convection, emanating a baroclinic Kelvin wave into the Pacific. In the northwest Pacific, this equatorial Kelvin wave induces northeasterly surface wind anomalies, and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone. The GCM results support this Kelvin wave-induced Ekman divergence mechanism. In response to a prescribed SST increase over the TIO, the model simulates the Kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest Pacific. An additional experiment further indicates that the north Indian Ocean warming is most important for the Kelvin wave and northwest Pacific anticyclone, a result corroborated by observations. These results have important implications for the predictability of Indo-western Pacific summer climate: the spatial distribution and magnitude of the TIO warming, rather than simply whether there is an El Nino in the preceding winter, affect summer climate anomalies over the Indo-western Pacific and East Asia.
    Xu Z. Q., K. Fan, and H. J. Wang, 2016: Role of sea surface temperature anomalies in the tropical Indo-Pacific region in the northeast Asia severe drought in summer 2014: Month-to-month perspective. Climate Dyn., https://doi.org/10.1007/s00382-016-3406-y.10.1007/s00382-016-3406-yf514ada3f3d59407ad4e8024a91ee600http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-016-3406-yhttp://link.springer.com/10.1007/s00382-016-3406-yAbstract The severe drought over northeast Asia in summer 2014 and the contribution to it by sea surface temperature (SST) anomalies in the tropical Indo-Pacific region were investigated from the month-to-month perspective. The severe drought was accompanied by weak lower-level summer monsoon flow and featured an obvious northward movement during summer. The mid-latitude Asian summer (MAS) pattern and East Asia/Pacific teleconnection (EAP) pattern, induced by the Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) rainfall anomalies respectively, were two main bridges between the SST anomalies in the tropical Indo-Pacific region and the severe drought. Warming in the Arabian Sea induced reduced rainfall over northeast India and then triggered a negative MAS pattern favoring the severe drought in June 2014. In July 2014, warming in the tropical western North Pacific led to a strong WNPSM and increased rainfall over the Philippine Sea, triggering a positive EAP pattern. The equatorial eastern Pacific and local warming resulted in increased rainfall over the off-equatorial western Pacific and triggered an EAP-like pattern. The EAP pattern and EAP-like pattern contributed to the severe drought in July 2014. A negative Indian Ocean dipole induced an anomalous meridional circulation, and warming in the equatorial eastern Pacific induced an anomalous zonal circulation, in August 2014. The two anomalous cells led to a weak ISM and WNPSM, triggering the negative MAS and EAP patterns responsible for the severe drought. Two possible reasons for the northward movement of the drought were also proposed.
    Xue F., J.-J. Zhao, 2017: Intraseasonal variation of the East Asian summer monsoon in La Niña years.Atmospheric and Oceanic Science Letters,10,156-161, 2016. 1254008.https://doi.org/10.1080/16742834.10.1080/16742834.2016.125400879bb64b46b3f8138050fa942054bb3f8http%3A%2F%2Fkns.cnki.net%2FKCMS%2Fdetail%2Fdetail.aspx%3Ffilename%3Daosl201702007%26dbname%3DCJFD%26dbcode%3DCJFQhttps://www.tandfonline.com/doi/full/10.1080/16742834.2016.1254008
    Yu L., 2013: Potential correlation between the decadal East Asian summer monsoon variability and the Pacific decadal oscillation.Atmospheric and Oceanic Science Letters,6,394-397, https://doi.org/10.3878/j.issn.1674-2834.13.0040.10.1080/16742834.2013.11447114bb0135e065d65455d98515035a97d6d7http%3A%2F%2Fkns.cnki.net%2FKCMS%2Fdetail%2Fdetail.aspx%3Ffilename%3Daosl201305030%26dbname%3DCJFD%26dbcode%3DCJFQhttp://www.tandfonline.com/doi/full/10.1080/16742834.2013.11447114
    Yu L., T. Furevik, O. H. Otter濮橈拷, and Y. Q. Gao, 2015: Modulation of the Pacific Decadal Oscillation on the summer precipitation over East China: A comparison of observations to 600-years control run of Bergen Climate Model.Climate Dyn.,44,475-494, https://doi.org/10.1007/s00382-014-2141-5.10.1007/s00382-014-2141-594610e85c29b460a2c6696a0555cc478http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-014-2141-5http://link.springer.com/10.1007/s00382-014-2141-5Observations show that the summer precipitation over East China often goes through decadal variations of opposite sign over North China and the Yangtze River valley (YRV), such as the “southern flood and northern drought” pattern that occurred during the late 1970s–1990s. In this study it is shown that a modulation of the Pacific Decadal Oscillation (PDO) on the summer precipitation pattern over East China during the last century is partly responsible for this characteristic precipitation pattern. During positive PDO phases, the warm winter sea surface temperatures (SSTs) in the eastern subtropical Pacific along the western coast of North American propagate to the tropics in the following summer due to weakened oceanic meridional circulation and the existence of a coupled wind–evaporation–SST feedback mechanism, resulting in a warming in the eastern tropical Pacific Ocean (5°N–20°N, 160°W–120°W) in summer. This in turn causes a zonal anomalous circulation over the subtropical–tropical Pacific Ocean that induces a strengthened western Pacific subtropical high (WPSH) and thus more moisture over the YRV region. The end result of these events is that the summer precipitation is increased over the YRV region while it is decreased over North China. The suggested mechanism is found both in the observations and in a 600-years fully coupled pre-industrial multi-century control simulations with Bergen Climate Model. The intensification of the WPSH due to the warming in the eastern tropical Pacific Ocean was also examined in idealized SSTA-forced AGCM experiments.
    Yu R.C., T. J. Zhou, 2007: Seasonality and three-dimensional structure of interdecadal change in the East Asian monsoon.J. Climate,20,5344-5355, http://doi.org/10.1175/2007JCLI1559.1.10.1175/2007JCLI1559.14c86b70a5798b4316f51386783f01c76http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2007JCli...20.5344Yhttp://journals.ametsoc.org/doi/abs/10.1175/2007JCLI1559.1A significant interdecadal cooling with vivid seasonality and three-dimensional (3D) structure is first revealed in the upper troposphere and lower stratosphere over East Asia. A robust upper-tropospheric cooling appears in March and has two peaks in April and August, but in June, a moderate uppertropospheric warming interrupts the cooling, while strong cooling occurs in the lower stratosphere. The seasonally dependent upper-tropospheric cooling leads to a clear seasonality of interdecadal changes in the atmospheric general circulation and precipitation against their normal seasonal cycle over East Asia. In March, precipitation over southern China (south of 26°N) has increased in accordance with the strong upper-tropospheric cooling occurring in northeast Asia. In April and May, following the southward extension and intensification of the upper-tropospheric cooling, the normal seasonal march of the monsoon rainband has been interrupted, resulting in a drying band to the south of the Yangtze River valley in late spring. In June, the moderate upper-tropospheric warming and strong lower-stratospheric cooling over northeast Asia has suddenly enhanced the northward migration of the rainband and resulted in an increase of precipitation in the mid–lower reaches of the Yangtze River and farther north. During July and August, the return of upper-tropospheric cooling has weakened the northward progression of southerly monsoon winds, resulting in a mid–lower Yellow River valley (34°–40°N) drought and excessive rain in the Yangtze River valley. The change of surface temperature is well correlated with the change in precipitation, especially in the spring. The surface cooling is generally collocated with excessive rain, while the warming is generally collocated with droughts. Possible causes for the robust interdecadal change are discussed, and stratosphere–troposphere interaction is suggested to play a crucial role in seasonally dependent 3D atmospheric cooling over East Asia.
    Zhao P., X. D. Zhang, X. J. Zhou, M. Ikeda, and Y. H. Yin, 2004: The sea ice extent anomaly in the North Pacific and its impact on the East Asian summer monsoon rainfall. J. Climate, 17, 3434-3447, https://doi.org/10.1175/1520-0442(2004)017<3434:TSIEAI>2.0,CO;2.10.1175/1520-0442(2004)0172.0.CO;2b61b47b1a73a75d0e1f59d515bc723fehttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2004jcli...17.3434zhttp://journals.ametsoc.org/doi/abs/10.1175/1520-0442%282004%29017%3C3434%3ATSIEAI%3E2.0.CO%3B2The relationship between extreme anomalies of the spring sea ice extent over the Bering Sea and the Sea of Okhotsk and rainfall variability in the east Asian summer monsoon was examined through an analysis of observed data and modeling experiments. The results show that reduced sea ice extent leads to an enhanced summer monsoon rainfall in southeastern China. This relationship is well supported by the background atmospheric circulation changes and the stationary wave dynamics. A difference in the 500-hPa geopotential height composed from the NCEP-NCAR reanalysis data and model output between the light and heavy sea ice cases shows an anomalous high in the east Asian summer, which favors the invasion of a cold air mass into southern China and prevents the east Asian summer monsoon from advancing northward. Hence, the mei-yu front and its associated rainfall intensify and stay in southeastern China. The generation of the summer anomalous high and its interseasonal link to the spring sea ice extent anomalies can be accounted for by the stationary wave dynamics and the land surface process. In spring, the decrease in sea ice extent forces eastward-propagating wave activity flux and causes an anomalous high in Europe along with a decrease in precipitation. The decreased soil water content results in a higher land surface temperature and more sensible heat flux in summer, and this strengthens summer stationary wave activities in Europe. The eastward propagation of the wave energy and its intensification in east Asia are responsible for the anomalous high in the east Asian summer. In this process, the European land surface acts as a bridge linking the spring sea ice extent anomalies with the east Asian summer monsoon.
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Manuscript History

Manuscript received: 01 February 2017
Manuscript revised: 31 May 2017
Manuscript accepted: 22 June 2017
通讯作者: 陈斌, bchen63@163.com
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Teleconnection between Sea Ice in the Barents Sea in June and the Silk Road, Pacific-Japan and East Asian Rainfall Patterns in August

  • 1. Geophysical Institute, University of Bergen and Bjerknes Centre for Climate Research, Bergen 5007, Norway
  • 2. Nansen-Zhu International Research Centre, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 3. Nansen Environmental and Remote Sensing Center, Bergen 5006, Norway
  • 4. Climate Change Research Center, Chinese Academy of Sciences, Beijing 100029, China
  • 5. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters/Key Laboratory of Meteorological Disaster, Ministry of Education, Nanjing University of Information Science and Technology, Nanjing 210044, China
  • 6. NILU —— Norwegian Institute for Air Research, Kjeller 2007, Norway

Abstract: In contrast to previous studies that have tended to focus on the influence of the total Arctic sea-ice cover on the East Asian summer tripole rainfall pattern, the present study identifies the Barents Sea as the key region where the June sea-ice variability exerts the most significant impacts on the East Asian August tripole rainfall pattern, and explores the teleconnection mechanisms involved. The results reveal that a reduction in June sea ice excites anomalous upward air motion due to strong near-surface thermal forcing, which further triggers a meridional overturning wave-like pattern extending to midlatitudes. Anomalous downward motion therefore forms over the Caspian Sea, which in turn induces zonally oriented overturning circulation along the subtropical jet stream, exhibiting the east-west Rossby wave train known as the Silk Road pattern. It is suggested that the Bonin high, a subtropical anticyclone predominant near South Korea, shows a significant anomaly due to the eastward extension of the Silk Road pattern to East Asia. As a possible descending branch of the Hadley cell, the Bonin high anomaly ultimately triggers a meridional overturning, establishing the Pacific-Japan pattern. This in turn induces an anomalous anticyclone and cyclone pair over East Asia, and a tripole vertical convection anomaly meridionally oriented over East Asia. Consequently, a tripole rainfall anomaly pattern is observed over East Asia. Results from numerical experiments using version 5 of the Community Atmosphere Model support the interpretation of this chain of events.

摘要: 以往研究主要关注整个北极区域海冰变化对东亚夏季三极子型降水的影响, 本文则从遥相关角度揭示了6月巴伦支海海冰变率对8月东亚三极子型降水的作用. 结果表明, 6月巴伦支海海冰减少, 通过近地表较强的热力作用引起局地大气的上升运动异常, 进一步激发向中纬度延伸的经向翻转波列, 在里海形成大气下沉运动异常. 通过沿着副热带急流的纬向翻转环流, 该下层运动异常会激发一个东西向的罗斯贝波列, 类似于丝绸之路型. 丝绸之路型向东亚的延伸, 对位于韩国附近的副热带反气旋环流-小笠原高压产生显著影响. 作为哈德莱环流圈的一个下沉支, 异常小笠原高压引起的经向翻转环流形成了太平洋-日本遥相关型, 进一步促进了东亚地区一对异常反气旋和气旋环流、异常的经向三极子垂直对流的产生. 最终, 东亚出现三极子型降水异常. CAM5的数值模拟结果也支持本文的观点.

1. Introduction
  • Significant interannual variability is observed in East Asian summer rainfall. A dominant spatial pattern of the East Asian summer rainfall anomaly, known as the tripole pattern (similar to the "northern drought-southern flooding" pattern in China, but with an additional anomaly center in the western North Pacific), has been documented by many previous studies (Enomoto, 2004; Hsu and Lin, 2007; Wang and He, 2015). That is, when the rainfall anomaly in central-eastern China and Japan is negative (positive), significant positive (negative) rainfall anomalies emerge in northern and southern China, as well as the subtropical western North Pacific (Tian and Yasunari, 1992; Hsu and Liu, 2003; Wang and He, 2015). The tripole rainfall pattern is often related to flooding and droughts in East Asia, causing profound economic and social impacts on the region. For example, during summer 2014, northern China suffered its severest drought since 1979, and this event had clear links to the tripole pattern in the rainfall anomaly over East Asia and the western Pacific (Wang and He, 2015; Xu et al., 2016).

    Several mechanisms have been proposed to be responsible for the tripole rainfall anomaly. Some studies have suggested that the drought/flooding conditions in northern/central China during summer are closely linked to an El Niño-like sea surface temperature (SST) anomaly (Weng et al., 1999; Lau and Weng, 2001; Xue and Zhao, 2017), stratosphere-troposphere interactions over East Asia (Yu and Zhou, 2007) and, on longer timescales, to the Pacific Decadal Oscillation (Yu, 2013; Yu et al., 2015) and to changes in the distribution of aerosols (Ju and Han, 2013; Wang et al., 2013). As an existing teleconnection pattern in atmospheric circulation, the tripole rainfall pattern over East Asia also shows close linkages with other remote forcings. By stimulating a wave-like pattern, variations in heating over the Tibetan Plateau are also thought to be an important contributor to the tripole summer rainfall pattern over East Asia (Hsu and Liu, 2003).

    Regarding atmospheric teleconnection, the Pacific-Japan (PJ) pattern is an important mechanism to explain the existence of the tripole rainfall pattern (Nitta, 1987; Huang and Sun, 1992). Therefore, much effort has been devoted to understanding the mechanisms responsible for the formation of the PJ pattern. A general understanding is that the PJ pattern is forced by anomalous heating associated with anomalous SST in the Philippine Sea (Nitta, 1987; Nitta and Hu, 1996), as well as convective heating over the tropical western North Pacific, which could excite Rossby waves propagating northwards to midlatitudes (Kosaka and Nakamura, 2006). A recent study by (Xie et al., 2009) suggested that a positive SST anomaly in the tropical Indian Ocean associated with an El Niño could also trigger the PJ pattern by exciting a warm equatorial tropospheric Kelvin wave that could induce surface Ekman divergence over the tropical western North Pacific. Besides, it has also been suggested that the anomalous thermal conditions associated with Indian Ocean SST could initiate zonal wave activity that propagates from the Mediterranean eastwards to East Asia and results in an anomalous anticyclone over the Okhotsk Sea (Guan and Yamagata, 2003).

    The above-cited studies mainly focused on the connection of the PJ pattern or the tripole rainfall pattern over East Asia with lower latitudes (i.e., the Indian Ocean, Philippine Sea, western North Pacific, and tropical eastern Pacific). Some studies have pointed out that there is another forcing from mid- and high latitudes for the PJ pattern and the related rainfall anomaly. Based on both observational and numerical simulation results, (Enomoto et al., 2003) suggested that the Eurasian continent diabatic heating anomaly could stimulate an eastward-propagating Rossby wave, named the Silk Road pattern (Lu et al., 2002), which propagates along the waveguide of the subtropical jet stream. The Silk Road pattern further influences the variability of the Bonin high, which results in the formation of the PJ pattern (Hsu and Lin, 2007).

    These studies imply a potential teleconnection between the East Asian summer tripole rainfall pattern and upstream forcing. A much-explored upstream forcing is the variations in Arctic sea-ice cover. (Screen, 2013) focused on the influence of the entire Arctic sea ice on European summer precipitation and found an apparent annular large-scale Rossby wave at midlatitudes, suggesting a potential teleconnection between the Arctic sea ice and East Asian summer rainfall. Some early studies documented that the variability of spring (generally March, April and May) Arctic sea-ice area is significantly related to the summer rainfall anomaly over East Asia via an atmospheric pathway (Zhao et al., 2004; Wu et al., 2009). In contrast, (Guo et al., 2014) suggested that the formation of the East Asian summer tripole rainfall pattern might be attributable to anomalous spring Arctic sea ice via an oceanic pathway. Despite numerous interesting and meaningful results having been revealed in previous studies based on the summer seasonal mean (generally June, July and August), several questions remain unanswered owing to the tripole rainfall pattern showing significant sub-seasonal variability. For example, central China, South Korea and southern Japan suffered severe drought, while northern China experienced above-normal rainfall, in June 2014, and the conditions reversed in August 2014 (Wang and He, 2015). Additionally, it has been revealed that the mechanisms responsible for the rainfall anomaly in June 2014 were different from those in August 2014 (Xu et al., 2016). The fact that the atmospheric circulation and rainfall anomalies during summer differ from month to month (Wang and He, 2015; Xu et al., 2016) raises a new question: is the teleconnection of the monthly rainfall anomaly over East Asia with the Arctic sea ice different from what has been found in previous studies that focused on the summer seasonal mean? This is the main motivation of the present study.

2. Data and method
  • The datasets used in this study are the reanalysis products derived from the National Centers for Environmental Prediction-National Center for Atmospheric Research, with a horizontal resolution of 2.5°× 2.5° (Kalnay et al., 1996); the monthly sea-ice concentration from the Met Office Hadley Centre (Rayner et al., 2003); and the monthly precipitation anomaly, at a horizontal resolution of 2.5°× 2.5° (Chen et al., 2002), obtained from the National Oceanic and Atmospheric Administration.

    Since this study mainly focuses on the interannual variability, linear trends have been removed from the datasets prior to analysis. To illustrate the teleconnection between the Arctic sea ice and the tripole rainfall pattern, we apply the 3D wave activity flux (WAF; Takaya and Nakamura, 2001). The WAFs are calculated in the quasi-geostrophic framework, which can identify the origin and propagation of the energy of the Rossby wave-like perturbation (Hsu and Lin, 2007). The 3D WAF is defined as follows: \begin{eqnarray} F_x&=&\dfrac{p\cos\varPhi}{2|U|}\Bigg(\dfrac{u}{a^2\cos^2\varPhi}\left[\left(\dfrac{\partial\varPsi'}{\partial\lambda}\right)^2- \varPsi'\dfrac{\partial^2\varPsi'}{\partial\lambda^2}\right]\nonumber\\[-0.5mm] &&+\dfrac{v}{a^2\cos\varPhi}\left[\dfrac{\partial\varPsi'}{\partial\lambda}\dfrac{\partial\varPsi'}{\partial\varPhi}- \varPsi'\dfrac{\partial^2\varPsi'}{\partial\lambda\partial\varPhi}\right]\Bigg) ;\ \ (1)\\[-0.5mm] F_y&=&\dfrac{p\cos\varPhi}{2|U|}\Bigg(\dfrac{u}{a^2\cos^2\varPhi}\left[\dfrac{\partial\varPsi'}{\partial\lambda} \dfrac{\partial\varPsi'}{\partial\varPhi}-\varPsi'\dfrac{\partial^2\varPsi'}{\partial\lambda\partial\varPhi}\right]\nonumber\\[-0.5mm] &&+\dfrac{v}{a^2\cos\varPhi}\left[\left(\dfrac{\partial\varPsi'}{\partial\varPhi}\right)^2- \varPsi'\dfrac{\partial^2\varPsi'}{\partial\varPhi^2}\right]\Bigg) ;\ \ (2)\\[-0.5mm] F_z&=&\dfrac{pf_0^2\cos\varPhi}{2N^2|U|}\Bigg(\dfrac{u}{a^2\cos^2\varPhi}\left[\dfrac{\partial\varPsi'}{\partial\lambda} \dfrac{\partial\varPsi'}{\partial\varPhi}-\varPsi'\dfrac{\partial^2\varPsi'}{\partial\lambda\partial\varPhi}\right]\nonumber\\[-0.5mm] &&+\dfrac{v}{a^2\cos\varPhi}\left[\left(\dfrac{\partial\varPsi'}{\partial\varPhi}\right)^2- \varPsi'\dfrac{\partial^2\varPsi'}{\partial\varPhi^2}\right]\Bigg) .\ \ (3) \end{eqnarray} Here, p is pressure, \(U(=\sqrt{u^2+v^2})\) is the climatological wind speed during 1981-2010; u and v are the zonal and meridional wind components, respectively; \(\varPhi\) and Λ are the latitude and longitude, respectively; a is the Earth's radius; \(\varPsi'\) is the quasi-geostrophic streamfunction, defined as gz/f0, where z is geopotential height, g is gravitational acceleration and f0 is the Coriolis parameter, defined as \(2\Omega\sin\varPhi\), where Ω is the speed of Earth's rotation; and N2 is the Brunt-Vaisala frequency. The Rossby wave source can be defined as χ·∇ζ; that is, -∇·Νχ (ζ+f) (Sardeshmukh and Hoskins, 1988). Here, Νχ is the divergence wind component, ζ is the absolute vorticity, and f is the Coriolis parameter. The PJ pattern index is defined as the difference in 850-hPa geopotential height between the domains of (30°-40°N, 120°-140°E) and (15°-25°N, 110°-130°E). Based on a previous study (Enomoto et al., 2003), which suggested a potential relationship between the PJ pattern and upstream Rossby waves in August, the present study mainly focuses on the tripole rainfall pattern and atmospheric circulation in August over the period 1980-2014.

3. Relationships between June Arctic sea ice, the East Asian rainfall pattern, and atmospheric circulation
  • To depict the tripole pattern of East Asian August rainfall, we apply empirical orthogonal function (EOF) analysis to the rainfall in this region. It is clear that the spatial distribution of the first EOF of August rainfall displays a tripole structure, with a negative (positive) anomaly in northern China, southern China and the subtropical western North Pacific, and a positive (negative) anomaly in Japan, South Korea and central China (Fig. 1a). The pattern is consistent with that revealed from the summer seasonal mean by (Hsu and Lin, 2007). The first EOF explains about 38.1% of the total variance of August rainfall, which means that the variance of East Asian August rainfall is dominated by the tripole anomaly pattern. The corresponding time series (principal component) of the first EOF describes the fluctuations of the tripole rainfall anomaly, referred to as Precip-EOF-PC1.

    Figure 1.  (a, b) Correlation of Precip-EOF-PC1 with (a) August precipitation and (b) June Arctic sea-ice area. (c) Detrended and normalized Precip-EOF-PC1 and SIAI during 1980-2014. (d-f) Heterogeneous correlation map of the first SVD mode for the detrended and normalized (d) August precipitation and (e) June Arctic sea-ice area during 1980-2014, in which stippled values are significant at the 90% confidence level, and the (f) corresponding SVD time series.

    To investigate the possible relationships between Arctic sea ice and the tripole rainfall pattern, we first calculate the correlation of Arctic sea ice in March, April and May with respect to the August Precip-EOF-PC1 (figures not shown). It is found that the relationship between the spring (March, April and May) monthly Arctic sea ice and August Precip-EOF-PC1 is very weak (barely statistically significant). There is a statistically significant correlation over the Barents Sea, but the correlation coefficients between the spring monthly area-averaged sea-ice area in the Barents Sea (70°-90°N, 0°-60°E) and August Precip-EOF-PC1 are only 0.32, 0.23 and 0.33, respectively. Since the variations in June are much stronger, and the effects of a varying sea-ice cover much larger due to the high level of incoming shortwave radiation, we expect the effects of the varying Barents Sea ice to be largest in early summer (Matsumura and Yamazaki, 2012; Matsumura et al., 2014). We therefore examine the correlation of Arctic sea ice in June with the August Precip-EOF-PC1, as shown in Fig. 1b. It is apparent that significant positive correlations can be found in the Barents Sea. Corresponding to a decrease in August Precip-EOF-PC1 of one standard deviation, the magnitude of reduced sea-ice area with a statistically significant anomaly is much larger in June (≈ 2.6× 104 km2) than that in March (≈9.5 × 103 km2), April (≈ 1.5 × 103 km2) and May (≈ 1.2× 104 km2). Thus, to represent the reduction in June sea ice, the area-averaged sea-ice area in June over the Barents Sea (70°-90°N, 0°-60°N) is referred to as the sea-ice area index (SIAI) and is depicted in Fig. 1c (solid curve). The correlation coefficient between June SIAI and August Precip-EOF-PC1 is 0.48, significant at the 99% confidence level and substantially higher than those in spring months. It is therefore suggested that there is a potential teleconnection between June Arctic sea ice and the East Asian tripole rainfall pattern in August. To find more evidence for this, we apply singular value decomposition (SVD), which can depict the covariability between two variables, to the standardized June Arctic sea-ice area and August precipitation over East Asia. The results reveal that, when the heterogeneous correlations of August rainfall over East Asia show a tripole pattern (Fig. 1d), significant correlations of June sea-ice area are located in the Barents Sea (Fig. 1e). The corresponding time coefficients of the first SVD component (explains about 29.1% variance) are correlated at 0.62 (Fig. 1f). It should be noted that the results show barely any difference when the signals of El Niño-Southern Oscillation (ENSO) in the previous winter are removed (figures not shown). The SVD results further confirm the speculation that the region where the Arctic sea-ice anomaly exerts its most significant influence on the August tripole rainfall pattern is the Barents Sea in June. The following analysis focuses on the June SIAI-related atmospheric anomaly, to explain how the June sea ice in the Barents Sea influences the August rainfall pattern over East Asia.

    Figure 2.  (a, b) Climatology of the (a) 850-hPa wind and (b) vertically (surface to 300 hPa) integrated water vapor transport vector (units: kg m-1 s-1) during August 1980-2014. (c, d) Regression maps of the (c) 850-hPa wind and (d) vertically integrated water vapor transport vector (kg m-1 s-1) anomalies during August 1980-2014 with respect to the preceding June inverted SIAI. Shaded/stippled values in (c, d) are significant at the 90% confidence level based on the Student’s t-test. The black shading indicate the regions where the topographical height is higher than 3500 meters.

    Generally, southerly (including southeasterly or southwesterly) wind prevails over East Asia and the western North Pacific in August (Fig. 2a). Correspondingly, the vertically integrated water vapor content in this region is transported mainly from the tropical western Pacific and Indian Ocean (Fig. 2b), with larger quantities of water vapor being transported from the Indian Ocean (Wang and Chen, 2012; He, 2015). The 850-hPa wind and vertically integrated water vapor content anomalies in August regressed onto the June inverted SIAI are presented in Figs. 2c and d, respectively. The regression analysis indicates that, corresponding to a reduction in June sea ice, represented by the June inverted SIAI, an anomalous cyclone is present in the following August over the western North Pacific between 10°N and 30°N, accompanied by an anticyclonic anomaly located over South Korea (Fig. 2c), resembling the PJ pattern. It is apparent that a significant northeasterly wind anomaly appears over central eastern China and Japan. Additionally, significant anomalous southwesterly wind emerges over northern China and the western North Pacific. This means that the southerly wind prevailing in August is significantly weakened over central China and Japan, and strengthened over northern China and the western North Pacific, when the June sea-ice area in the Barents Sea is less than normal. Such anomalous atmospheric circulation suggests that the quantity of moisture transported to northern China and the western North Pacific is more than normal, while it is less than normal for central China, South Korea and Japan (Fig. 2d). It is apparent that the moisture conditions in August following a reduction in June sea ice in the Barents Sea is favorable (unfavorable) for rainfall over northern China and the western North Pacific (central China, South Korea and Japan).

    In addition to the horizontal moisture conditions, upward motion is essential for the formation of rainfall. Correspondingly, Fig. 3 presents the regression of the vertical wind anomaly in August related to the June inverted SIAI. Since the tripole rainfall pattern extends meridionally, we show a cross section along 115°-135°E, which represents the west-to-east conditions of the tripole rainfall pattern. It is found that significant anomalous ascending, descending and ascending motion occurs at low (south of 20°N), middle (center at 30°N) and high (center at 40°N) latitudes, respectively, along 115°E (Fig. 3a). The anomalous vertical motion in August related to the previous June inverted SIAI also exhibits a meridional tripole structure along 115°E, which is consistent with the rainfall anomaly in this region. The corresponding vertical motion anomaly along 135°E displays a meridional dipole structure, with anomalous ascending motion south of 30°N and anomalous descending motion north of 40°N (Fig. 3b). It is clear that the anomalous descending motion along 135°E is located more northwards than that along 115°E, corresponding to the northeast-southwest elongation of the dipole rainfall pattern (Fig. 1a).

    To provide more detail on the anomalous vertical motion related to the June SIAI anomaly, the outgoing longwave radiation (OLR) and total cloud-cover anomalies are shown in Figs. 4a and b. Following a reduction in June sea ice in the Barents Sea, significant positive OLR anomalies extend from Japan southwestwards along the Yangtze River basin in August. Besides, significant negative OLR anomalies appear over the subtropical western North Pacific, as well as southern and northeastern China (Fig. 4a). The positive (negative) OLR anomalies correspond to an anomalous descending (ascending) air mass. Correspondingly, the spatial distribution of the total cloud-cover anomaly also shows a tripole pattern, which is highly similar to that of OLR but with opposite sign (Fig. 4b). Additionally, the 200-hPa zonal wind anomaly displays a tripole pattern, with a statistically significant negative anomaly over (30°-40°N, 60°-150°E) and positive anomaly over (40°-55°N, 110°-150°E) and (15°-25°N, 60°-130°E) (Fig. 4c; shaded). This seesaw pattern between the south and north of the Asian westerly jet axis is closely associated with the Silk Road pattern (Hong and Lu, 2016), implying that the Silk Road pattern plays a key role in connecting the Arctic sea ice with East Asian precipitation.

    In summary, the reduction in June sea ice in the Barents Sea can cause an anomalous anticyclone in South Korea and an anomalous cyclone over the western North Pacific in the following August (Fig. 2c). This leads to less moisture over central China, Japan and South Korea, and more moisture over the subtropical western North Pacific, as well as northern and southern China (Fig. 2d). Additionally, significant anomalous ascending, descending and ascending motion emerges at low (around 15°N), middle (around 30°N) and high (around 40°N) latitudes over the East Asian region, respectively, exhibiting a meridional overturning wave pattern (Fig. 3). The corresponding OLR, total cloud cover and 200-hPa zonal wind anomalies show a tripole pattern (Fig. 4). It is apparent that the atmospheric circulation anomalies exhibit a meridionally oriented PJ pattern, and are favorable for the formation of a tripole pattern in East Asian rainfall. The following analysis focuses on the mechanisms bridging a reduction in June sea ice in the Barents Sea with the atmospheric anomalies and the East Asian tripole rainfall pattern in the following August.

    Figure 3.  Regression of the vertical-horizontal cross section for August vertical wind (vectors; units: m s-1) and omega (contours, units: × 10-3 Pa s-1) anomalies along (a) 115°E and (b) 135°E during 1980-2014 onto the preceding June inverted SIAI. Stippled regions indicate omega anomalies significant at the 90% confidence level based on the Student’s t-test.

    Figure 4.  Regression maps of August (a) OLR (shaded; units: W m-2; data available to December 2013 only), (b) total cloud cover (shaded; units: %), and (c) 200-hPa zonal wind (shaded; units: m s-1) onto the June inverted SIAI during 1980-2014. Gridded or dotted regions indicate statistical significance at the 90% confidence level based on the Student’s t-test. Contours in (c) indicate the climatological 200-hPa zonal wind during August 1980-2014.

4. Wave activity associated with reduced sea-ice cover in June
  • The conclusions made by (Hong and Lu, 2016) that the meridional displacement of the Asian jet (as shown in Fig. 4c) is associated with the Silk Road pattern, and the conclusions by (Hsu and Lin, 2007) that the zonally Silk Road pattern could favor the formation of a meridional PJ pattern, motivate us to examine the propagation of the Rossby wave activity and the potential associated contribution from the reduction in sea-ice cover in June.

    Concurrent with the statistically significant reduction in sea-ice area over the Barents Sea in June (Fig. 5a), there are significant negative (maximum of about -5 W m-2) turbulent heat flux anomalies in situ (Fig. 5c), meaning the ocean obtains net heat fluxes in June that might be caused by a lower albedo and more open water associated with the reduction in sea-ice area. As the sea ice continues to melt, the sea-ice area in the following months becomes even smaller, with a large area of open water (Fig. 5b) and greater absorption of solar radiation. In August, the warmer open water induced by the reduction in sea-ice area starts to influence the atmosphere, which is supported by the inverse sign of turbulent heat flux anomalies, from negative to positive, especially in northern regions (Fig. 5d). This means that the net total turbulent heat flux is upwards, implying that the ocean releases heat to the atmosphere in August. Because of the change in thermal conditions, anomalous upward motion emerges over the Barents Sea, which further triggers meridionally anomalous downward, upward and downward motion, which might be the effect of meridional circulation (e.g., the Ferrel cell and Polar cell) (Fig. 5e). At midlatitudes, significant anomalous upward and downward motion appears alternately along the subtropical jet stream (along 40°N; green line in Fig. 5e). As anomalous vertical motion is generally associated with anomalous divergence, the reduction in June sea-ice area over the Barents Sea may dynamically induce a Rossby wave, because the advection of vorticity by divergence wind can be regarded as a wave source (Sardeshmukh and Hoskins, 1988). As shown in Fig. 5f (shaded), statistically significant anomalous Rossby wave sources are centered near the Mediterranean and Caspian seas, where anomalous upward motion is located (Fig. 5e). Consequently, an apparent Rossby wave, which is evident by inspecting the WAF related to the June SIAI (Fig. 5f; vectors), propagates eastwards along the subtropical jet stream to East Asia (30°-55°N; green frame in Fig. 5f).

    Figure 5.  (a, b) Regression maps of sea-ice area (units: km2) in (a) June and (b) August onto the June inverted SIAI during 1980-2014. (c, d) As in (a, b) but for the total turbulent heat flux (sensible plus latent heat flux; units: W m-2). (e, f) Regression maps of 200-hPa (e) vertical-component wind (units: × 10-3 Pa s-1) and (f) Rossby wave source (shaded; units: × 10-11 s-2) and WAF (vectors; units: m2 s-2) in August onto June inverted SIAI during 1980-2014. Stippled regions indicate values significant at the 90% confidence level based on a two-tailed Student’s t-test. The blue frames in (a-d) indicate where the Barents Sea is located. The blue frame in (e) indicate pathway of the meridional overturning wave pattern related to the SIAI. The blue and green frames in (f) indicate the zonally wave patterns related to the SIAI.

    Figure 6.  (a, d, e) Regression maps of (a) 200-hPa meridional wind (shaded; units: m s-1), (d) vertical-horizontal cross section (outer is 40°N) meridional wind anomalies (shaded; units: m s-1) and WAF (vectors; units: m2 s-2), and (e) 500-hPa geopotential height (contours; units: gpm) and WAF (vectors; units: m2 s-2) in August associated with the preceding June inverted SIAI during 1980-2014. (b, c) Regression maps of August 200-hPa meridional wind (shaded; units: m s-1) onto the time series of the (b) first and (c) second leading EOF mode for the August meridional wind in the domain (30°-60°N, 30°-130°E). (f) 500-hPa geopotential height (contours; units: gpm) and WAF (vectors; units: m2 s-2) in August regressed on the simultaneous PJ index. Regions enclosed by contours in (a-d) and shaded in (e, f) denote anomalies significant at the 90% confidence level based on a two-tailed Student’s t-test.

    Figure 7.  Vertical-horizontal cross section for vertical wind (vectors; units: m s-1) and omega (contours; units: × 10-3 Pa s-1) anomalies along (a) 50°E and (b) 40°N uring August 1980-2014 regressed onto the preceding June inverted SIAI. Dotted regions indicate omega anomalies significant at the 90% confidence level based on the Student’s t-test.

    We further examine the structure of WAF in August related to the reduction in June sea ice in the Barents Sea. Figure 6a shows the meridional wind anomaly in August related to the June inverted SIAI. An apparent east-west wave disturbance is observed at midlatitudes. Significant positive, negative, positive and negative meridional wind anomalies are found over the Caspian Sea, Balkhash, eastern China and Okhotsk Sea, respectively (Fig. 6a). This anomalous wave-like pattern closely resembles the first leading mode of EOF (EOF1) for the August 200-hPa meridional wind (Fig. 6b; explains about 32.4% of the total variance), which is used to describe the Silk Road pattern (Lu et al., 2002; Ding and Wang, 2005). Additionally, the August wave-like pattern related to the June SIAI shows many similarities with the second leading mode of EOF (EOF2) for the August 200-hPa meridional wind (Fig. 6c; explains about 21.9% of the total variance), especially over East Asia. The corresponding time series of EOF1/EOF2 is correlated with the inverted June SIAI at 0.33/0.33, statistically significant at the 95% confidence level. This means that the zonal Rossby wave at midlatitudes in August is closely related to the June sea-ice variability over the Barents Sea. To better depict the propagation of this east-west wave-like pattern, a vertical cross section of the meridional wind anomaly and associated WAF in August along 40°N is further examined (Fig. 6d). The vertical cross section of meridional wind anomaly clearly exhibits a wave-like pattern, with significant positive (centered at 50°E), negative (90°E), positive (105°E) and negative (150°E) anomalies along the entire meridional domain (Fig. 6d; shaded). The WAF indicates that the propagation of this wave-like pattern is mainly confined to the middle and upper troposphere (Fig. 6d; vectors). Close examination of Fig. 6d reveals that there is a wave source around 50°E (judging from the divergence of WAF), consistent with the results in Fig. 5f. Meanwhile, a significant negative center emerges to the north of the Caspian Sea (Fig. 6a). Such a north-south dipole (along 50°E) supports the speculation that the east-west wave-like pattern at midlatitudes (along 40°N) might originate from high latitudes (e.g., the Arctic). The anomalous vertical motion in August further supports this view (Fig. 7). Corresponding to a significant reduction in sea ice in the Barents Sea in June, significant anomalous upward motion, possibly due to strong near-surface thermal forcing, appears over the Arctic region (70°-80°N; Fig. 7a) in August, where the anomalous sea ice is observed. The anomalous motion ascends upwards into the upper troposphere (at around 200 hPa), then bends equatorwards to midlatitudes (40°-50°N), and finally descends downwards. Such atmospheric perturbation further propagates eastwards along the midlatitudes. As shown in Fig. 7b, apparent anomalous descending motion appears at around 45°E, consistent with Fig. 7a. At the same time, anomalous ascending and descending motion appear alternately at 60°E, 90°E, 120°E, 140°E and 160°E. The results revealed by Fig. 7 are consistent with that in Fig. 5e. It is therefore suggested that anomalous thermal conditions related to the reduction in sea ice over the Barents Sea could induce anomalous vertical motion in situ from June to August, driving the meridional overturning and resulting in anomalous vertical motion at midlatitudes in its southern branch, which further induces an eastward propagation of a Rossby wave along the subtropical westerly jet. Consequently, the east-west wave-like atmospheric perturbation at midlatitudes, which resembles the Silk Road pattern, is established. The propagation of the Silk Road pattern is favorable for the variation in the Bonin high (Fig. 2a), which is a subtropical anticyclone observed over Japan (Enomoto, 2004). As suggested earlier, the Bonin high might be related to the descending branch of the Hadley cell. Here, we show that the anomalous August Bonin high associated with the reduction in June sea ice in the Barents Sea could trigger a meridional overturning, leading to the formation of the PJ pattern (Enomoto et al., 2003; Hsu and Lin, 2007). As shown in Fig. 6e, the spatial distribution of 500-hPa geopotential height anomalies and WAF in August related to the June inverted SIAI closely resembles its counterpart related to the PJ pattern index (Fig. 6f). The correlation coefficient between the June inverted SIAI and August PJ pattern index is 0.55, statistically significant at the 99% confidence level. Such an interpretation is illustrated schematically in Fig. 8.

    Figure 8.  Schematic diagrams summarizing the dynamical linkage between the reduction in June sea ice in the Barents Sea with the Silk Road pattern as well as the Pacific-Japan pattern. The cross sections for (a) and (b) are taken from Figs. 7a and b, respectively. Blue and red arrows denote zonal and meridional overturning, respectively.

    Finally, to provide more robust evidence for the interpretation of the teleconnection between the reduction in June sea ice in the Barents Sea and the downstream atmospheric perturbation in August, we conduct two numerical experiments——a control experiment and a sensitivity experiment——to examine the atmospheric response to the June sea-ice variability in the Barents Sea. We use version 5 of the Community Atmosphere Model (CAM5), with a 1.9° × 2.5° finite volume grid, and with 26 hybrid sigma pressure levels. The sea-ice concentration and SST are prescribed as boundary conditions in the model; all other external variables are fixed. First, we run a control experiment for 25 years forced by seasonal-varying climatology (1979-2000) of sea ice. Then, we perform a pair of 12-month sensitivity experiments from January to December. The sea ice is reduced in June over the Barents Sea (70°-90°N, 0°-60°E) (as shown in Fig. 5a), while other months are prescribed by climatological sea ice. The experiment is repeated 20 times, with different initial conditions on 1 January, taken from the 6th to 25th model year. The SSTs in both experiments are set to their climatological monthly values. We focus on the difference between the sensitivity and control experiments in August to reveal any lagged response of the atmosphere to the reduction in June sea ice.

    Model-simulated differences in 850-hPa wind and 200-hPa zonal wind between the sensitivity and control experiments are displayed in Fig. 9. The 850-hPa wind anomaly is clearly characterized by a significant anomalous anticyclone over South Korea and a dominant anomalous cyclone over the subtropical western North Pacific (Fig. 9a), which is highly consistent with the observational results (Fig. 2c). The response of August 200-hPa zonal wind shows apparent change in the meridional shear of zonal wind, with significant positive and negative anomalies over (45°-50°N, 110°-150°E) and (25°-35°N, 110°-150°E), respectively (Fig. 9b), resembling its observational counterpart (Fig. 4c). Such a distribution of 200-hPa zonal wind over East Asia will lead to a south-north tripole rainfall anomaly (Guo et al., 2014), as shown in Fig. 1a.

    Based on the observational results, we suggest that the reduction in June sea ice in the Barents Sea may first trigger an east-west wave-like pattern at midlatitudes (i.e., the Silk Road pattern), and then induce a meridional overturning wave-like pattern over the East Asia-Pacific region (i.e., PJ pattern). Figure 10a shows the model-simulated difference in 200-hPa meridional winds in August. Clear negative and positive anomaly centers appear alternately at 60°E, 90°E, 120°E and 150°E, roughly along 40°N, which displays an apparent wave-like pattern (Figs. 10a and b). Note that there is a dipole pattern over (30°-60°N, 30°-90°E; Fig. 10a), supporting the existence of the southward-propagating wave pattern described by observations (Figs. 6a and 7a). When the east-west wave pattern is established, a south-north wave pattern emerges over East Asia and the western North Pacific, as estimated from the 500-hPa geopotential difference (Fig. 10c; contours). The WAF clearly suggests that this wave-like pattern propagates from the subtropical western North Pacific northwards to East Asia (Fig. 10c; vectors), resembling its observational counterpart well (Fig. 6e).

    Figure 9.  The difference in (a) 850-hPa wind (units: m s-1) and (b) 200-hPa zonal wind (m s-1) anomalies in August between the sensitivity and control experiment in CAM5. Values shaded or enclosed by grids are significant at the 90% confidence level based on the Student’s t-test.

    Figure 10.  The difference in the (a) 200-hPa meridional wind (units: m s-1), (b) vertical-horizontal cross section (outer is 40°N) of meridional wind anomalies (shaded; units: m s-1) and WAF (vectors; units: m2 s-2), and (c) 500-hPa geopotential height (contours; units: gpm) and WAF (vectors; units: m2 s-2) in August between the sensitivity and control experiment in AM5. Regions enclosed by white contours in (a, b) and shaded in (c) denote anomalies significant at the 90% confidence level based on a two-tailed Student’s t-test.

5. Conclusion and discussion
  • While most previous studies on the East Asian summer climate have focused on the summer seasonal mean, several recent studies have pointed out that the variability of East Asian summer rainfall shows distinct features on the monthly timescale (Wang and He, 2015; Xu et al., 2016). This implies that, for different months, the monthly rainfall over East Asia can relate to different external factors. The present study has a special focus on August, and on the potential teleconnection between the East Asian tripole rainfall pattern and June Arctic sea ice. While others have looked at how variations in the total Arctic sea-ice variability influence East Asian summer rainfall (Wu et al., 2009; Guo et al., 2014), our study shows that it is the sea ice in the Barents Sea in June that has the most significant relationship with the East Asian tripole rainfall pattern in August. The main finding of the present study is the dominant teleconnection of June Barents sea-ice reduction with the Silk Road pattern, Pacific-Japan pattern, and East Asian tripole rainfall pattern in August.

    It is found that a reduction in June sea ice over the Barents Sea is accompanied by an anomalous anticyclone over South Korea and anomalous cyclone over the subtropical western North Pacific in the following August, which results in a decrease in moisture transport to central China, South Korea and Japan, and an increase in moisture to northern and southern China as well as the subtropical western North Pacific. The anomalous moisture conditions favorable for the formation of the tripole rainfall pattern are therefore provided. Additionally, corresponding to a reduction in June sea ice in the Barents Sea, the vertical convection over East Asia and western North Pacific in August also shows significant anomalies. Along the section of 115°E, significant anomalous ascending, descending and ascending motion is observed around 20°N, 30°N and 40°N, respectively. Such anomalous vertical motion is further supported by the tripole pattern of the OLR (total cloud cover) anomaly, with s significant negative (positive), positive (negative) and negative (positive) anomaly in northern China, central China to Japan, and southern China to the western North Pacific, respectively. It is thus suggested that the anomalous dynamical conditions related to the reduction in sea ice in the Barents Sea are favorable for the East Asian tripole rainfall pattern.

    Further analyses indicate that the near-surface anomalous thermal conditions associated with a reduction in sea ice tends to excite anomalous upward motion due to strong near-surface thermal forcing over the Barents Sea region, which triggers a meridional overturning extending to midlatitudes in the form of a wave train. The wave train propagates southwards and forms anomalous downward motion over the Caspian Sea, which further induces a zonally oriented overturning circulation, exhibiting an east-west wave train (i.e., Silk Road pattern). The mechanism of southward propagation, which might be related to the effect of meridional circulation (e.g., the Ferrel cell and Polar cell), is still unclear. The Silk Road pattern propagates eastwards along the subtropical jet stream to East Asia and causes an anomalous anticyclone over South Korea (i.e., the Bonin high). As the Bonin high might be related to the descending branch of the Hadley cell (Enomoto et al., 2003), the anomalous Bonin high ultimately triggers a meridional overturning and a south-north wave-like pattern is formed——the so-called PJ pattern. Consequently, the tripole rainfall pattern emerges over East Asia.

    To further explore the observational findings, numerical experiments using CAM5 are conducted. The results support the conclusion that the atmospheric anomalies over East Asia are related to the reduction in June sea ice in the Barents Sea. The perturbed experiment integrated with prescribed sea-ice loss in June over the Barents Sea successfully reproduces a respective pair of anticyclonic and cyclonic anomalies over East Asia. Meanwhile, significant meridional shear of 200-hPa zonal wind that favors the tripole rainfall pattern over East Asia, with significant positive and negative anomalies over (45°-50°N, 110°-150°E) and (25°-35°N, 110°-150°E), respectively, is also produced by the model. More importantly, the Silk Road pattern and PJ pattern are both reproduced in our experiment. Therefore, analysis based on both observations and numerical simulations suggests that the reduction in June sea ice in the Barents Sea tends to induce a tripole rainfall pattern over East Asia in August by modulating the atmospheric anomaly via a triggering of the Silk Road pattern and PJ pattern.

    The variability of Arctic sea ice could explain 25%-36% of the total variance in East Asian precipitation (Figs. 1c and f), showing that there are also other types of forcing for the climate variability in this region. Most previous studies have linked the tripole rainfall pattern over East Asia to tropical forcing, such as ENSO. The 2015/16 winter featured a super El Niño event in the eastern tropical Pacific of almost the same strength as the one that occurred in winter 1997/98. However, the rainfall anomaly in August 2016 was nearly opposite to that in August 1998, suggesting a complex set of climatic systems and the need to comprehensively consider the effects of different forcings. Our results might shed more light on understanding the diversity of East Asian precipitation. The effect of sea ice over the Barents Sea should be taken into account in ENSO-based predictions of East Asian summer rainfall anomalies. Additionally, statistically significant correlations between the East Asian tripole rainfall pattern and Arctic sea-ice area are found over the Chukchi Sea (Figs. 1c and f), which might be a result of the teleconnection related to East Asian monsoonal rainfall (Grunseich and Wang, 2016). Besides, although the model results in the present study are largely consistent with the observations, they are obtained from only one climate model; further results from multiple models are therefore needed. This is especially so given that, despite the model's simulation supporting the chain of events indicated by the observations, several differences are still apparent. For example, the observed PJ pattern related to the reduction in June sea-ice area over the Barents Sea originates from the subtropical western North Pacific, propagates northwards, and turns eastwards over Japan (Fig. 6e). By contrast, the simulated PJ pattern is dominated by northward propagation (Fig. 10c). This might be induced by the stronger northward shift of the Asian jet stream simulated by the model (Fig. 9b versus Fig. 4c). Meanwhile, the observational 200-hPa zonal wind anomaly shows a comparable dipole pattern in the west (60°-90°E) and east (120°-150°E) of the Eurasian continent (Fig. 4c; shaded), which is closely related to the Silk Road pattern (Hong and Lu, 2016). However, only one dipole anomaly of 200-hPa zonal wind appears over East Asia in the model simulation (Fig. 9b). The inconsistency between the model and reanalysis data might be attributable to topographical effects, as shown in Fig. 9a. Unfortunately, a deeper dynamical and physical explanation regarding the model's performance remains unclear.

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