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Link between the Barents Oscillation and Recent Boreal Winter Cooling over the Asian Midlatitudes


doi: 10.1007/s00376-017-7021-6

  • The link between boreal winter cooling over the midlatitudes of Asia and the Barents Oscillation (BO) since the late 1980s is discussed in this study, based on five datasets. Results indicate that there is a large-scale boreal winter cooling during 1990-2015 over the Asian midlatitudes, and that it is a part of the decadal oscillations of long-term surface air temperature (SAT) anomalies. The SAT anomalies over the Asian midlatitudes are significantly correlated with the BO in boreal winter. When the BO is in its positive phase, anomalously high sea level pressure over the Barents region, with a clockwise wind anomaly, causes cold air from the high latitudes to move over the midlatitudes of Asia, resulting in anomalous cold conditions in that region. Therefore, the recent increasing trend of the BO has contributed to recent winter cooling over the Asian midlatitudes.
    摘要: 本文基于5种数据集, 分析研究了20世纪80年代末以来欧亚大陆中纬度地区冬季变冷与巴伦支震荡的关系. 分析结果显示, 1990–2015年间欧亚大陆中纬度地区的冬季表层气温存在大范围的变冷趋势, 这种变冷趋势属于该地区表层气温年代际震荡的一部分, 进一步研究表明该地区冬季表层气温异常与巴伦支震荡存在显著的相关关系, 当巴伦支震荡处于正位相时, 在巴伦支地区高压异常, 存在顺时针方向的风场异常, 异常风场会给欧亚大陆中纬度地区带来异常的冷空气, 使该地区降温. 近期巴伦支震荡存在增强的趋势, 这会导致欧亚大陆中纬度地区的冬季变冷.
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  • Berrisford, P., Coauthors, 2011: The ERA-interim archive,version 2.0. ERA Report Series, ECMWF, 23 pp.
    Chen H. W., Q. Zhang, H. Körnich, and D. Chen, 2013: A robust mode of climate variability in the arctic: The Barents oscillation.Geophys. Res. Lett.,40(11),2856-2861, https://doi.org/10.1002/grl.50551.10.1002/grl.50551cae18578b78b99650ebf430c37a96395http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fgrl.50551%2Ffullhttp://doi.wiley.com/10.1002/grl.50551The Barents Oscillation (BO) is an anomalous wintertime atmospheric circulation pattern in the Northern Hemisphere that has been linked to the meridional flow over the Nordic Seas. There are speculations that the BO has important implications for the Arctic climate; however, it has also been suggested that the pattern is an artifact of Empirical Orthogonal Function (EOF) analysis due to an eastward shift of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In this study, EOF analyses are performed to show that a robust pattern resembling the BO can be found during different time periods, even when the AO/NAO is relatively stationary. This BO has a high and stable temporal correlation with the geostrophic zonal wind over the Barents Sea, while the contribution from the AO/NAO is small. The surface air temperature anomalies over the Barents Sea are closely associated with this mode of climate variability.
    Cohen J. L., J. C. Furtado, M. A. Barlow, V. A. Alexeev, and J. E. Cherry, 2012: Arctic warming,increasing snow cover and widespread boreal winter cooling.Environmental Research Letters,7,014007, https://doi.org/10.1088/1748-9326/7/1/014007.10.1088/1748-9326/7/1/014007577d901c7bfb946e00b6b6dd7d8543fbhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2012ERL.....7a4007Chttp://stacks.iop.org/1748-9326/7/i=1/a=014007?key=crossref.b774aafc53389f43a9e3492e8a36e8b1The most up to date consensus from global climate models predicts warming in the Northern Hemisphere (NH) high latitudes to middle latitudes during boreal winter. However, recent trends in observed NH winter surface temperatures diverge from these projections. For the last two decades, large-scale cooling trends have existed instead across large stretches of eastern North America and northern Eurasia. We argue that this unforeseen trend is probably not due to internal variability alone. Instead, evidence suggests that summer and autumn warming trends are concurrent with increases in high-latitude moisture and an increase in Eurasian snow cover, which dynamically induces large-scale wintertime cooling. Understanding this counterintuitive response to radiative warming of the climate system has the potential for improving climate predictions at seasonal and longer timescales.
    Comiso J. C., C. L. Parkinson, R. Gersten, and L. Stock, 2008: Accelerated decline in the Arctic sea ice cover. Geophys. Res. Lett. 35, https://doi.org/10.1029/2007GL031972.10.1029/2007GL031972343feb606e7415f45d25b40e917085b6http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2007GL031972%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2007GL031972/pdfSatellite data reveal unusually low Arctic sea ice coverage during the summer of 2007, caused in part by anomalously high temperatures and southerly winds. The extent and area of the ice cover reached minima on 14 September 2007 at 4.1 10kmand 3.6 10km, respectively. These are 24% and 27% lower than the previous record lows, both reached on 21 September 2005, and 37% and 38% less than the climatological averages. Acceleration in the decline is evident as the extent and area trends of the entire ice cover (seasonal and perennial ice) have shifted from about -2.2 and -3.0% per decade in 1979-1996 to about -10.1 and -10.7% per decade in the last 10 years. The latter trends are now comparable to the high negative trends of -10.2 and -11.4% per decade for the perennial ice extent and area, 1979-2007.
    Compo, G. P., Coauthors, 2011: The twentieth century reanalysis project.Quart. J. Roy. Meteor. Soc.,137,1-28, https://doi.org/10.1002/qj.776.10.1002/qj.77604625428c9538cbba9d0077dab5bb471http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.776%2Fpdfhttp://adsabs.harvard.edu/abs/2009EGUGA..1111820CA potential consequence of climate variability and change is an altered likelihood of weather extremes. To estimate the fidelity of regional projections of these altered risks in the Twenty-first century, daily data is needed to assess the simulations of weather and climate throughout the Twentieth century. Such daily data must have quantified estimates of uncertainty in Twentieth century weather to allow quantitative comparison with simulations. To this end, we have begun the Twentieth Century Reanalysis Project. This Project is an effort to produce a reanalysis dataset spanning the 20th Century assimilating only surface observations of synoptic pressure, monthly sea surface temperature and sea ice distribution. The project uses the recently developed Ensemble Filter data assimilation system which allows direct computation of both the analysis and the uncertainty in that analysis. The dataset will provide the first estimate of global tropospheric and stratospheric variability spanning more than 100 years with 6 hourly resolution. The first version has global coverage spanning 1908-1958 and 2 degree longitude-latitude horizontal resolution. Comparison with independent radiosonde data indicates that the analyses have a high quality, with correlations higher than 0.94 throughout the troposphere. Overall, the quality is similar to that of current 3-day operational numerical weather prediction forecasts, as anticipated from previous studies.
    Dee, D. P., Coauthors, 2011: The ERA-Interim reanalysis: Configuration and performance of the data assimilation system.Quart. J. Roy. Meteor. Soc.,137,553-597, https://doi.org/10.1002/qj.828.10.1002/qj.8285b3115ec8b338ee97111270a1831c4b2http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fqj.828%2Fpdfhttp://doi.wiley.com/10.1002/qj.v137.656ERA-Interim is the latest global atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). The ERA-Interim project was conducted in part to prepare for a new atmospheric reanalysis to replace ERA-40, which will extend back to the early part of the twentieth century. This article describes the forecast model, data assimilation method, and input datasets used to produce ERA-Interim, and discusses the performance of the system. Special emphasis is placed on various difficulties encountered in the production of ERA-40, including the representation of the hydrological cycle, the quality of the stratospheric circulation, and the consistency in time of the reanalysed fields. We provide evidence for substantial improvements in each of these aspects. We also identify areas where further work is needed and describe opportunities and objectives for future reanalysis projects at ECMWF. Copyright 2011 Royal Meteorological Society
    Honda M., J. Inoue, and S. Yamane, 2009: Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett. 36, https://doi.org/10.1029/2008GL037079.10.1029/2008GL03707911ff7459b32da24cee92554351efd9cbhttp%3A%2F%2Fwww.cabdirect.org%2Fabstracts%2F20103055291.htmlhttp://www.cabdirect.org/abstracts/20103055291.htmlInfluence of low Arctic sea-ice minima in early autumn on the wintertime climate over Eurasia is investigated. Observational evidence shows that significant cold anomalies over the Far East in early winter and zonally elongated cold anomalies from Europe to Far East in late winter are associated with the decrease of the Arctic sea-ice cover in the preceding summer-to-autumn seasons. Results fro...
    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
    Kryzhov V. N., O. V. Gorelits, 2015: The Arctic Oscillation and its impact on temperature and precipitation in Northern Eurasia in the 20th Century.Russian Meteorology and Hydrology,40,711-721, 3103/ S1068373915110011.https://doi.org/10.10.3103/S1068373915110011211ae5db2e77a65d7f7ee18f3eeaa083http%3A%2F%2Flink.springer.com%2Farticle%2F10.3103%2FS1068373915110011http://link.springer.com/10.3103/S1068373915110011Presented is the review of the modern knowledge of the Arctic Oscillation (AO). Demonstrated is the relation of air temperature and precipitation in Northern Eurasia to this dominant type of wintertime atmospheric variability at northern extratropical latitudes. It is demonstrated that AO is a result of the coupling between the troposphere and stratosphere. The attention is paid to the long-range forecasting of AO index and to the factors complicating the forecasting. Given are the new results of the authors research. Used is the wintertime AO index computed by the authors from the 20th Century Reanalysis dataset. The high- and low-frequency components of AO index variability and the periods of statistically significant trends are analyzed using the 112-year series (1901-2012). Demonstrated is the key impact of wintertime AO phase on the anomalies of air temperature and precipitation in Northern Eurasia at the time scale of years and decades. This impact is manifested in the northern part of Northern Eurasia in the prevalence of warmer and wetter winters at the positive AO phase and of colder and drier winters at the negative AO phase. The precipitation anomalies of opposite sign prevail in the southern part of Northern Eurasia. It is demonstrated that the winter AO phase affects the terms of the springtime air temperature transition to positive values.
    Morice C. P., J. J. Kennedy, N. A. Rayner, and P. D. Jones, 2012: Quantifying uncertainties in global and regional temperature change using an ensemble of observational estimates: The HadCRUT4 data set. J. Geophys. Res. 117, https://doi.org/10.1029/2011JD017187.10.1029/2011JD0171878b1cc10538405cb9260ddc3ff5fdae8bhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2011JD017187%2Fabstracthttp://onlinelibrary.wiley.com/doi/10.1029/2011JD017187/abstractRecent developments in observational near-surface air temperature and sea-surface temperature analyses are combined to produce HadCRUT4, a new data set of global and regional temperature evolution from 1850 to the present. This includes the addition of newly digitized measurement data, both over land and sea, new sea-surface temperature bias adjustments and a more comprehensive error model for describing uncertainties in sea-surface temperature measurements. An ensemble approach has been adopted to better describe complex temporal and spatial interdependencies of measurement and bias uncertainties and to allow these correlated uncertainties to be taken into account in studies that are based upon HadCRUT4. Climate diagnostics computed from the gridded data set broadly agree with those of other global near-surface temperature analyses. Fitted linear trends in temperature anomalies are approximately 0.07ºC/decade from 1901 to 2010 and 0.17ºC/decade from 1979 to 2010 globally. Northern/southern hemispheric trends are 0.08/0.07ºC/decade over 1901 to 2010 and 0.24/0.10ºC/decade over 1979 to 2010. Linear trends in other prominent near-surface temperature analyses agree well with the range of trends computed from the HadCRUT4 ensemble members.
    Outten S. D., I. Esau, 2012: A link between Arctic sea ice and recent cooling trends over Eurasia.Climatic Change,110,1069-1075, https://doi.org/10.1007/s10584-011-0334-z.10.1007/s10584-011-0334-z9fbb230fa04380926a7eed28b2c0f6bbhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs10584-011-0334-zhttp://link.springer.com/10.1007/s10584-011-0334-zA band of cooling that extends across mid-latitude Eurasia is identified in the wintertime surface air temperatures of the latest ECMWF reanalysis. This cooling is related to extreme warming around the Kara Sea through changes in the meridional temperature gradient. Surface temperatures in the Arctic have risen faster than those at lower latitudes, and as the Arctic warming increases, this north outh temperature gradient is weakened. This change in the meridional temperature gradient causes a decrease in the westerly winds that help maintain the mild European climate by transporting heat from the Atlantic. Since decreasing sea ice concentrations have been shown to be a driving factor in Arctic amplification, a singular value decomposition analysis is used to confirm the co-variability of the Arctic sea ice, including the Kara Sea, and the temperatures over the mid-latitude Eurasia. These findings suggest that decreasing sea ice concentrations can change the meridional temperature gradient and hence the large-scale atmospheric flow of the Northern Hemisphere.
    Outten S., R. Davy, and I. Esau, 2013: Eurasian winter cooling: Intercomparison of reanalyses and CMIP5 Data Sets.Atmospheric and Oceanic Science Letters,6,324-331, https://doi.org/10.3878/j.issn.1674-2834.12.0112.10.3878/j.issn.1674-2834.12.01126f754e444b8d4c1aa339921ab60b455dhttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqhhykxkb201305018.aspxhttp://www.tandfonline.com/doi/full/10.1080/16742834.2013.11447102A cooling trend in wintertime surface air temperature over continental Eurasia has been identified in reanalysis and the Coupled Model Inter-comparison Project phase 5 (CMIP5) ‘historical’ simulations over the period 1989-2009. Here the authors have shown that this cooling trend is related to changes in Arctic sea-ice around the Barents-Kara seas. This study illustrates a consistent spatial and temporal structure of the wintertime temperature variability centered over Asia using state-of-the-art reanalyses and global climate model datasets. Our findings indicate that there is a physical basis for seasonal predictions of near-surface temperatures over continental Asia based on changes to the ice-cover in the Barents-Kara seas.
    Overland, J. E., M. Y. Wang, 2010: Large-scale atmospheric circulation changes are associated with the recent loss of Arctic sea ice,Tellus A,62,1-9, http://dx.doi.org/10.1111/j.1600-0870.2009.00421.x.10.1111/j.1600-0870.2009.00421.xb0ed8ee2685a605c97c5df18b284831chttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1111%2Fj.1600-0870.2009.00421.x%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1111/j.1600-0870.2009.00421.x/citedbyRecent loss of summer sea ice in the Arctic is directly connected to shifts in northern wind patterns in the following autumn, which has the potential of altering the heat budget at the cold end of the global heat engine. With continuing loss of summer sea ice to less than 20% of its climatological mean over the next decades, we anticipate increased modification of atmospheric circulation patterns. While a shift to a more meridional atmospheric climate pattern, the Arctic Dipole (AD), over the last decade contributed to recent reductions in summer Arctic sea ice extent, the increase in late summer open water area is, in turn, directly contributing to a modification of large scale atmospheric circulation patterns through the additional heat stored in the Arctic Ocean and released to the atmosphere during the autumn season. Extensive regions in the Arctic during late autumn beginning in 2002 have surface air temperature anomalies of greater than 3 ºC and temperature anomalies above 850 hPa of 1 ºC. These temperatures contribute to an increase in the 1000�500 hPa thickness field in every recent year with reduced sea ice cover. While gradients in this thickness field can be considered a baroclinic contribution to the flow field from loss of sea ice, atmospheric circulation also has a more variable barotropic contribution. Thus, reduction in sea ice has a direct connection to increased thickness fields in every year, but not necessarily to the sea level pressure (SLP) fields. Compositing wind fields for late autumn 2002�2008 helps to highlight the baroclinic contribution; for the years with diminished sea ice cover there were composite anomalous tropospheric easterly winds of 651.4 m s�1, relative to climatological easterly winds near the surface and upper tropospheric westerlies of 653 m s�1. Loss of summer sea ice is supported by decadal shifts in atmospheric climate patterns. A persistent positive Arctic Oscillation pattern in late autumn (OND) during 1988�1994 and in winter (JFM) during 1989�1997 shifted to more interannual variability in the following years. An anomalous meridional wind pattern with high SLP on the North American side of the Arctiche AD pattern, shifted from primarily small interannual variability to a persistent phase during spring (AMJ) beginning in 1997 (except for 2006) and extending to summer (JAS) beginning in 2005.
    Petoukhov V., V. A. Semenov, 2010: A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents. J. Geophys. Res. 115, https://doi.org/10.1029/2009JD013568.10.1029/2009JD013568dc1ac9e62c94b87f316ae99122829c96http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2009JD013568%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2009JD013568/pdfThe recent overall Northern Hemisphere warming was accompanied by several severe northern continental winters, as for example, extremely cold winter 2005-2006 in Europe and northern Asia. Here we show that anomalous decrease of wintertime sea ice concentration in the Barents-Kara (B-K) seas could bring about extreme cold events like winter 2005-2006. Our simulations with the ECHAM5 general circulation model demonstrate that lower-troposphere heating over the B-K seas in the Eastern Arctic caused by the sea ice reduction may result in strong anticyclonic anomaly over the Polar Ocean and anomalous easterly advection over northern continents. This causes a continental-scale winter cooling reaching -1.5ºC, with more than 3 times increased probability of cold winter extremes over large areas including Europe. Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it, being in qualitative agreement with the simulated large-scale atmospheric circulation realignment. Furthermore, our results suggest that high-latitude atmospheric circulation response to the B-K sea ice decrease is highly nonlinear and characterized by transition from anomalous cyclonic circulation to anticyclonic one and then back again to cyclonic type of circulation as the B-K sea ice concentration gradually reduces from 100% to ice free conditions. We present a conceptual model that may explain the nonlinear local atmospheric response in the B-K seas region by counter play between convection over the surface heat source and baroclinic effect due to modified temperature gradients in the vicinity of the heating area.
    Poli, P., Coauthors, 2013: The data assimilation system and initial performance evaluation of the ECMWF pilot reanalysis of the 20th-century assimilating surface observations only (ERA-20C). ERA Report Series,59 pp.ffb0e0849f3f8e6fdf25cf7bb40e8622http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F306168220_The_data_assimilation_system_and_initial_performance_evaluation_of_the_ECMWF_pilot_reanalysis_of_the_20th-century_assimilating_surface_observations_only_ERA-20Chttp://www.researchgate.net/publication/306168220_The_data_assimilation_system_and_initial_performance_evaluation_of_the_ECMWF_pilot_reanalysis_of_the_20th-century_assimilating_surface_observations_only_ERA-20Creact-text: 522 Passive microwave and infrared nadir sounders such as the Advanced Microwave Sounding Unit A (AMSU-A) and the Atmospheric InfraRed Sounder (AIRS), both flying on NASA s EOS Aqua satellite, provide information about vertical temperature and humidity structure that is used in data assimilation systems for numerical weather prediction and climate applications. These instruments scan cross track... /react-text react-text: 523 /react-text [Show full abstract]http://www.researchgate.net/publication/306168220_The_data_assimilation_system_and_initial_performance_evaluation_of_the_ECMWF_pilot_reanalysis_of_the_20th-century_assimilating_surface_observations_only_ERA-20C
    Skeie P., 2000: Meridional flow variability over the Nordic Seas in the Arctic oscillation framework.Geophys. Res. Lett.,27(16),2569-2572, https://doi.org/10.1029/2000GL011529.10.1029/2000GL011529505295e71d1919af468de20348135fc9http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2000GL011529%2Ffullhttp://doi.wiley.com/10.1029/2000GL011529An anomalous recurring atmospheric circulation pattern of high relevance for the climate of the Nordic Seas and Siberia is identified. It is found as the second Empirical Orthogonal Function (EOF) of monthly winter sea level pressure (SLP) anomalies poleward of 30°N where the leading EOF is the Arctic Oscillation (AO). The most prominent centre of action of the circulation pattern is located over the Barents Region. This “Barents Oscillation” (BO) is shown to have a high temporal correlation with the sensible heat loss of the Nordic Seas (r=0.76). The BO also correlates to Eurasian surface air temperature (SAT) anomalies with r=0.72 after the AO related SAT variations are removed by means of a linear regression. Two sets of SLP composites are constructed where one is based on low and high Nordic Seas heat loss months and the other is based on warm and cold Eurasian months. Patterns reminiscent of the BO emerge in the two composites when AO related variability is removed.
    Thompson D. W. J., J. M. Wallace, 1998: The Arctic oscillation signature in the wintertime geopotential height and temperature fields.Geophys. Res. Lett.,25,1297-1300, https://doi.org/10.1029/98GL00950.10.1029/98GL00950adf244c1165dc0c5b3e8ecc1d4c5e7fehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F98GL00950%2Fpdfhttp://doi.wiley.com/10.1029/98GL00950The leading empirical orthogonal function of the wintertime sea-level pressure field is more strongly coupled to surface air temperature fluctuations over the Eurasian continent than the North Atlantic Oscillation (NAO). It resembles the NAO in many respects; but its primary center of action covers more of the Arctic, giving it a more zonally symmetric appearance. Coupled to strong fluctuations at the 50-hPa level on the intraseasonal, interannual, and interdecadal time scales, this rctic Oscillation (AO) can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft. It is proposed that the zonally asymmetric surface air temperature and mid-tropospheric circulation anomalies observed in association with the AO may be secondary baroclinic features induced by the land-sea contrasts. The same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature, sea-level pressure, and 50-hPa height over the past 30 years: parts of Eurasia have warmed by as much as several K, sea-level pressure over parts of the Arctic has fallen by 4 hPa, and the core of the lower stratospheric polar vortex has cooled by several K. These trends can be interpreted as the development of a systematic bias in one of the atmosphere's dominant, naturally occurring modes of variability.
    Wu B. Y., D. Hand orf, K. Dethloff, A. Rinke, and A. X. Hu, 2013: Winter weather patterns over Northern Eurasia and Arctic sea ice loss.Mon. Wea. Rev.,141,3786-3800, https://doi.org/10.1175/MWR-D-13-00046.1.10.1175/MWR-D-13-00046.18d1b497a4f4ba7b05b32ce57fb1fbb60http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2013MWRv..141.3786Whttp://journals.ametsoc.org/doi/abs/10.1175/MWR-D-13-00046.1Using NCEP-NCAR reanalysis and Japanese 25-yr Reanalysis (JRA-25) winter daily (1 December-28 February) data for the period 1979-2012, this paper reveals the leading pattern of winter daily 850-hPa wind variability over northern Eurasia from a dynamic perspective. The results show that the leading pattern accounts for 18% of the total anomalous kinetic energy and consists of two subpatterns: the dipole and the tripole wind patterns. The dipole wind pattern does not exhibit any apparent trend. The tripole wind pattern, however, has displayed significant trends since the late 1980s. The negative phase of the tripole wind pattern corresponds to an anomalous anticyclone over northern Eurasia during winter, as well as two anomalous cyclones occurring over southern Europe and in the mid- to high latitudes of East Asia. These anomalous cyclones in turn lead to enhanced winter precipitation in these two regions, as well as negative surface temperature anomalies over the mid- to high latitudes of Asia. The intensity of the tripole wind pattern and the frequency of its extreme negative phase are significantly correlated with autumn Arctic sea ice anomalies. Simulation experiments further demonstrate that the winter atmospheric response to Arctic sea ice decrease is dynamically consistent with the observed trend in the tripole wind pattern over the past 24 winters, which is one of the causes of the observed declining winter surface air temperature trend over Central and East Asia. The results of this study also imply that East Asia may experience more frequent and/or intense winter extreme weather events in association with the loss of Arctic sea ice.
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Manuscript History

Manuscript received: 10 January 2017
Manuscript revised: 17 May 2017
Manuscript accepted: 16 June 2017
通讯作者: 陈斌, bchen63@163.com
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Link between the Barents Oscillation and Recent Boreal Winter Cooling over the Asian Midlatitudes

  • 1. First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China
  • 2. Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China
  • 3. Key Laboratory of Marine Science and Numerical Modeling, State Oceanic Administration, Qingdao 266061, China

Abstract: The link between boreal winter cooling over the midlatitudes of Asia and the Barents Oscillation (BO) since the late 1980s is discussed in this study, based on five datasets. Results indicate that there is a large-scale boreal winter cooling during 1990-2015 over the Asian midlatitudes, and that it is a part of the decadal oscillations of long-term surface air temperature (SAT) anomalies. The SAT anomalies over the Asian midlatitudes are significantly correlated with the BO in boreal winter. When the BO is in its positive phase, anomalously high sea level pressure over the Barents region, with a clockwise wind anomaly, causes cold air from the high latitudes to move over the midlatitudes of Asia, resulting in anomalous cold conditions in that region. Therefore, the recent increasing trend of the BO has contributed to recent winter cooling over the Asian midlatitudes.

摘要: 本文基于5种数据集, 分析研究了20世纪80年代末以来欧亚大陆中纬度地区冬季变冷与巴伦支震荡的关系. 分析结果显示, 1990–2015年间欧亚大陆中纬度地区的冬季表层气温存在大范围的变冷趋势, 这种变冷趋势属于该地区表层气温年代际震荡的一部分, 进一步研究表明该地区冬季表层气温异常与巴伦支震荡存在显著的相关关系, 当巴伦支震荡处于正位相时, 在巴伦支地区高压异常, 存在顺时针方向的风场异常, 异常风场会给欧亚大陆中纬度地区带来异常的冷空气, 使该地区降温. 近期巴伦支震荡存在增强的趋势, 这会导致欧亚大陆中纬度地区的冬季变冷.

1. Introduction
  • Observations and reanalysis data have shown that the Asian midlatitudes have experienced a large-scale cooling trend in boreal winter since the late 1980s (Figs. 1a-c) (Honda et al., 2009; Cohen et al., 2012; Outten and Esau, 2012; Outten et al., 2013; Wu et al., 2013). This cooling trend is considered to be related to the loss of Arctic sea ice (Petoukhov and Semenov, 2010; Outten and Esau, 2012; Outten et al., 2013; Wu et al., 2013), increases in both high-latitude moisture and Eurasian snow cover (Cohen et al., 2012), and the Arctic Oscillation (AO) (Thompson and Wallace, 1998; Kryzhov and Gorelits, 2015).

    Large-scale atmospheric circulations could change with a continuing loss of Arctic sea ice (Overland and Wang, 2010). (Outten and Esau, 2012) suggested that the recent reduction in Arctic sea-ice concentrations could change the meridional temperature gradient, and hence the large-scale atmospheric flow of the Northern Hemisphere. (Wu et al., 2013) found a tripole wind pattern over northern Eurasia, which could lead to winter precipitation and surface temperature anomalies over the mid-to-high latitudes of Asia.

    Furthermore, the trend in this tripole wind pattern since the late 1980s correlates significantly with autumn Arctic sea-ice anomalies. (Cohen et al., 2012) suggested that summer and autumn warming trends in the Northern Hemisphere over the last two decades have coincided with increases in both high-latitude moisture and Eurasian snow cover, which dynamically induce large-scale wintertime cooling. Eurasian mean surface air temperature (SAT) anomalies are also highly correlated with the AO index (Thompson and Wallace, 1998). Usually, winters in northern parts of northern Eurasia are warmer and wetter during positive AO phases, and colder and drier during negative AO phases (Kryzhov and Gorelits, 2015). The AO index has exhibited a decreasing trend since the late 1980s, and therefore the recent boreal winter cooling over the midlatitudes of Asia might be related to this decrease in the AO index. However, (Cohen et al., 2012) questioned whether the boreal winter cooling trend is a consequence of internal variability or a response to changes in boundary forcing —— an issue that remains open to debate.

    The Barents Oscillation (BO) was first proposed by (Skeie, 2000). It is the second leading mode of the empirical orthogonal function (EOF) of sea level pressure (SLP) anomalies polewards of 30°N, and its importance is connected with the climate of the Eurasia, the Nordic seas and the Barents Sea (Skeie, 2000; Chen et al., 2013). In this study, we emphasize that the BO is also important for winter SAT anomalies in the Asian midlatitudes, and that the BO could also contribute to the recent boreal winter cooling in this region. During our EOF analysis, we expand the study area from north of 30°N to north of 20°N.

2. Data and method
  • To obtain reliable results, five different datasets are used in this study. The primary variables used are the monthly mean surface or near-surface air temperature, surface wind, SLP, and geopotential height from the HadCRUT4 (Morice et al., 2012), National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) Reanalysis (Kalnay et al., 1996), Twentieth Century Reanalysis V2 (20CR) (Compo et al., 2011), ERA-Interim (Berrisford et al., 2011; Dee et al., 2011), and ERA-20C (Poli et al., 2013) datasets. For the data of HadCRUT4 and ERA-Interim, the period we use is from 1979 to 2015. For the NCEP-NCAR Reanalysis, the data cover from 1948 to 2015. The 20CR and ERA-20C datasets provide more than 100 years of reanalysis data. The horizontal resolutions of HadCRUT4, NCEP-NCAR Reanalysis, 20CR, ERA-Interim, and ERA-20C are 5°× 5°, 2.5°× 2.5°, 1°× 1°, 0.75°× 0.75°, and 1°× 1°, respectively.

    We focus mainly on the trends in boreal winter (December-February) SAT anomalies over the midlatitudes of Asia (40°-65°N, 60°-140°E). EOF analysis is applied to the monthly mean SLP north of 20°N to obtain the second leading mode (i.e., the BO) of the variation in SLP, and the methods of linear trend analysis, correlation analysis, and regression analysis are also employed.

3. Results
  • Unlike the global mean surface temperature, which exhibits an increasing trend since 1850, the HadCRUT4, NCEP-NCAR Reanalysis, and ERA-Interim datasets all show a large-scale cooling trend in boreal winter for the past 25 years over the Asian midlatitudes (Figs. 1a-c). These three independent datasets share similar spatial patterns, indicating a strong cooling trend located over northern China, Mongolia, Kazakhstan, and southern Russia (red box in Fig. 1). Previous studies have indicated that boreal winter cooling in Eurasia is related to Arctic sea-ice loss in autumn and winter, and to Arctic warming (Petoukhov and Semenov, 2010; Cohen et al., 2012; Outten and Esau, 2012; Wu et al., 2013). Here, we also examine the linear trend of boreal winter SAT during 1979-1990. Different to the cooling trend of 1990-2015, Figs. 1d-f show a significant warming trend in the area of the red box during 1979-90. Satellite observations have indicated that Arctic sea ice has retreated continuously since October 1978 (Comiso et al., 2008); however, the strong cooling trend has only been observed since the late 1980s. Therefore, this might indicate that other factors, such as the AO and BO, in addition to Arctic sea-ice loss, might also affect the winter climate in this region. To study the boreal winter SAT changes from a long-term perspective, SAT anomalies in the NCEP-NCAR Reanalysis (1948-2015), ERA-Interim (1979-2015) and 20CR (1871-2011) datasets over the Asian midlatitudes are illustrated in Figs. 1g and h. The smoothed lines show that boreal winter SAT in this region contains decadal oscillations. The cooling trend since the late 1980s reflects part of these decadal oscillations, and the warming trend during 1979-90 reflects another part of them. In addition to the recent cooling trend, there is also a cooling trend during 1890-1930 (Fig. 1h), for which the spatial pattern of cooling (not shown) is similar to that shown in Figs. 1a-c.

    Figure 1.  Linear trend [°C (10 yr)-1] of winter surface or near-surface air temperature during (a-c) 1990-2015 and (d-f) 1979-90, and (g, h) winter SAT anomalies (°C) over the midlatitudes of Asia [red box in (a)]: (a, d) HadCRUT4; (b, e) NCEP-NCAR Reanalysis; (c, f) ERA-Interim. The blue and red lines in (g) are for NCEP-NCAR Reanalysis and ERA-Interim, respectively; (h) is for 20CR. The thick lines in (g, h) are 10-year smoothed running means.

  • The first two leading EOF modes of the winter mean SLP are the AO and BO, respectively (Skeie, 2000). The BO has a primary center of action located over the Barents region, and is related to the meridional flow over the Nordic seas and sensible-heat loss in the same region (Skeie, 2000). The BO also correlates with Eurasian SAT anomalies after the AO-related SAT variations are removed. Here, we find that the BO has also contributed to recent winter cooling over the Asian midlatitudes.

    Figure 2.  The (a, d) first (AO) and (b, e) second (BO) leading EOF modes of winter monthly mean SLP north of 20°N, and the (c, f) correlation between the winter SAT anomalies over the midlatitudes of Asia and the BO time-varying index: (a-c) 20CR; (d-f) ERA-20C. The red boxes in (b) and (e) are the same as that in Fig. 1.

    Figure 3.  Regression map of boreal winter SAT (shading; °C) and surface wind (vectors; m s-1) on normalized BO time-varying index. The red line is the BO pattern. The period for (a) is 1871-2010, and the data are from 20CR. The period for (b) is 1900-2010, and the data are from the ERA-20C.

    The first two leading EOF modes of boreal winter mean SLP north of 20°N from the 20CR and ERA-20C datasets are shown in Fig. 2. The first leading mode (Figs. 2a and d), which is the AO, accounts for 42% and 44% of the variance for the 20CR and ERA-20C datasets, respectively. The AO has a strong annular structure, and when it is in its positive phase, a ring of strong winds circulates around the North Pole. The second leading mode (Figs. 2b and e), which is the BO, accounts for 10% and 11% of the variance for the 20CR and ERA-20C datasets, respectively. There are three main centers of action for EOF2 (Figs. 2b and e): the Barents center, the western Arctic/Atlantic center, and the Pacific center (Skeie, 2000). The BO has a remarkable meridional structure across the Arctic and the Nordic seas, and anomalous northerly flow over the Nordic seas associated with the BO is concurrent with anomalous southwesterly flow over central Siberia, meaning the BO also correlates with Eurasian SAT anomalies (Skeie, 2000).

    Figures 2b and e show that the anomalous SLP center (the Barents center), which is located over the Barents region, has strong meridionality over the high latitudes of Asia. This strong meridionality could also regulate the SAT anomalies in the Asian midlatitudes. The correlation coefficient between the BO time-varying index and boreal winter SAT anomalies in the Asian midlatitudes (red box in Figs. 2b and e) is -0.66 for 20CR and -0.46 for ERA-20C (Figs. 2c and f), and both exceed the 0.01 significance level. The dynamic link between the BO and SAT anomalies over the midlatitudes of Asia is illustrated in Fig. 3. This figure shows that when the BO is in its positive phase, there is an anomalously high SLP center located over the Barents region. The surface wind pattern related to this anomalously high SLP center is clockwise, and the wind direction to the east of the center is southerly. This anomalous southerly wind can transport cold air from the high latitudes to the midlatitudes, and therefore the Asian midlatitudes become cooler. This cooling pattern can reach subtropical regions along eastern parts of China and central and southern Asia. It is worth noting that both independent long-term datasets indicate a weak low SLP center at (34°N, 100°E). This low SLP center can restrict the cooling to northern parts of Asia, and thus prevent cooling in subtropical regions of China between 80°E and 100°E. Therefore, the geopotential height difference between the anomalously high SLP center and weak low SLP center should directly determine the strength of the anomalous clockwise wind and the magnitude of cooling over the Asian midlatitudes. The correlation coefficient between the boreal winter SAT anomalies over the midlatitudes of Asia and the anomalies of the 1000-hPa geopotential height difference between (65°N, 60°E) and (34°N, 100°E) is -0.83 during 1971-2011 in 20CR, and -0.88 during 1948-2015 in the NCEP-NCAR Reanalysis (Fig. 4). The BO time-varying index in Fig. 2 and the anomalies of the 1000-hPa geopotential height difference between (65°N, 60°E) and (34°N, 100°E) in Fig. 4 show that they also contain decadal oscillations. The increasing trends of the BO time-varying index and the 1000-hPa geopotential height difference during 1990-2015, which have contributed to the recent boreal winter cooling over the Asian midlatitudes, are also part of the decadal oscillations.

    Figure 4.  Correlation between boreal winter SAT anomalies (°C) and the anomalies of 1000-hPa geopotential height difference (m) between (65°N, 60°E) and (34°N, 100°E). Data in (a) are from 20CR. Data in (b) are from NCEP-NCAR Reanalysis.

4. Conclusions and discussion
  • In this study, the recent boreal winter cooling over the midlatitudes of Asia and its link with the BO are investigated based on observations and reanalysis datasets. The results indicate that the recent large-scale cooling trend since the late 1980s over the Asian midlatitudes is part of the decadal oscillations of long-term SAT anomalies, and that the BO and SAT anomalies in that region correlate significantly in boreal winter. When the BO is in its positive phase, the anomalously high SLP center over the Barents region can force a clockwise wind anomaly able to advect cold air from the high latitudes to the midlatitudes of Asia, resulting in anomalous cold conditions in that region. The recent increase in the BO time-varying index means that the BO has a increasing trend. The increasing BO has also contributed to winter cooling over the Asian midlatitudes during 1990-2015.

    The winter cooling trend over the midlatitudes of Asia is related to several other factors, such as a decreasing AO, Arctic sea-ice loss, and increases in both high-latitude moisture and Eurasian snow cover. The determination of the most dominant factor for the recent winter cooling over the midlatitudes should be the subject of further study.

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