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Using NWP to Assess the Influence of the Arctic Atmosphere on Midlatitude Weather and Climate

doi: 10.1007/s00376-017-6290-4

  • The influence of the Arctic atmosphere on Northern Hemisphere midlatitude tropospheric weather and climate is explored by comparing the skill of two sets of 14-day weather forecast experiments using the ECMWF model with and without relaxation of the Arctic atmosphere towards ERA-Interim reanalysis data during the integration. Two pathways are identified along which the Arctic influences midlatitude weather: a pronounced one over Asia and Eastern Europe, and a secondary one over North America. In general, linkages are found to be strongest (weakest) during boreal winter (summer) when the amplitude of stationary planetary waves over the Northern Hemisphere is strongest (weakest). No discernible Arctic impact is found over the North Atlantic and North Pacific region, which is consistent with predominantly southwesterly flow. An analysis of the flow-dependence of the linkages shows that anomalous northerly flow conditions increase the Arctic influence on midlatitude weather over the continents. Specifically, an anomalous northerly flow from the Kara Sea towards West Asia leads to cold surface temperature anomalies not only over West Asia but also over Eastern and Central Europe. Finally, the results of this study are discussed in the light of potential midlatitude benefits of improved Arctic prediction capabilities.
    摘要: 本文采用ECMWF模式进行了两组14天天气预报试验, 两组试验的区别是积分过程中是否采用趋向ERA再分析数据的北极大气松弛系数, 通过比较来分析北极大气对北半球中纬度对流层天气和气候的影响, 并确定了两条北极影响中纬度天气的路径: 主要路径在亚洲和东欧, 次要路径在北美. 总的来说, 北半球冬季(夏季)静止行星波幅度最大(最弱)的情况下, 关联最强(最弱);北大西洋和北太平洋地区的北极影响很弱, 这与西南气流是一致的. 对气流依赖性的分析表明, 北极异常气流加强了北极对中纬度大陆地区的影响, 具体来说, 喀拉海向西亚的偏北异常气流导致了西亚及东欧和中欧地区的异常低温. 最后, 从改进北极预测能力以利于中纬度的角度讨论了本文的研究结果. (翻译: 宋米荣)
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    Cohen J., K. Saito, and D. Entekhabi, 2001: The role of the Siberian high in Northern Hemisphere climate variability. Geophys. Res. Lett,, 28( 2), 299- 302.10.1029/ dominant mode of sea level pressure (SLP) variability during the winter months in the Northern Hemisphere (NH) is characterized by a dipole with one anomaly center covering the Arctic with the opposite sign anomaly stretched across the mid-latitudes. Associated with the SLP anomaly, is a surface temperature anomaly induced by the anomalous circulation. We will show that this anomaly pattern originates in the early fall, on a much more regional scale, in Siberia. As the season progresses this anomaly pattern propagates and amplifies to dominate much of the extratropical NH, making the Siberian high a dominant force in NH climate variability in winter. Also since the SLP and surface temperature anomalies originate in a region of maximum fall snow cover variability, we argue that snow cover partially forces the phase of winter variability and can potentially be used for the skillful prediction of winter climate.
    Cohen J. L., M. A. Barlow, V. A. Alexeev, J. E. Cherry, et al., 2012: Arctic warming, increasing snow cover and widespread boreal winter cooling. Environmental Research Letters, 7( 1), 14 007- 14 014.10.1088/1748-9326/7/1/ 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.
    Francis J. A., W. Chan, D. J. Leathers, J. R. Miller, and D. E. Veron, 2009: Winter Northern Hemisphere weather patterns remember summer Arctic sea-ice extent. Geophys. Res. Lett,, 36( 7), L07503.10.1029/ dramatic decline in Arctic summer sea-ice cover is a compelling indicator of change in the global climate system and has been attributed to a combination of natural and anthropogenic effects. Through its role in regulating the exchange of energy between the ocean and atmosphere, ice loss is anticipated to influence atmospheric circulation and weather patterns. By combining satellite measurements of sea-ice extent and conventional atmospheric observations, we find that varying summer ice conditions are associated with large-scale atmospheric features during the following autumn and winter well beyond the Arctic's boundary. Mechanisms by which the atmosphere 渞emembers� a reduction in summer ice cover include warming and destabilization of the lower troposphere, increased cloudiness, and slackening of the poleward thickness gradient that weakens the polar jet stream. This ice-atmosphere relationship suggests a potential long-range outlook for weather patterns in the northern hemisphere.
    Francis, J. A. and S. J. Vavrus, 2012: Evidence linking Arctic amplification to extreme weather in mid-latitudes. Geophys. Res. Lett,, 39( 6), L06801.10.1029/ in high northern latitudes relative to the northern hemisphere 閳� is evident in lower-tropospheric temperatures and in 1000-to-500 hPa thicknesses. Daily fields of 500 hPa heights from the National Centers for Environmental Prediction Reanalysis are analyzed over N. America and the N. Atlantic to assess changes in north-south (Rossby) wave characteristics associated with AA and the relaxation of poleward thickness gradients. Two effects are identified that each contribute to a slower eastward progression of Rossby waves in the upper-level flow: 1) weakened zonal winds, and 2) increased wave amplitude. These effects are particularly evident in autumn and winter consistent with sea-ice loss, but are also apparent in summer, possibly related to earlier snow melt on high-latitude land. Slower progression of upper-level waves would cause associated weather patterns in mid-latitudes to be more persistent, which may lead to an increased probability of extreme weather events that result from prolonged conditions, such as drought, flooding, cold spells, and heat waves. Citation: Francis, J. A., and S. J. Vavrus (2012), Evidence linking Arctic amplification to extreme
    Gao Y., et al., 2015: Arctic Sea Ice and Eurasian Climate: A Review. Adv. Atmos. Sci,, 32, 92- 114.10.1007/鍖楁瀬鍦ㄦ皵鍊欑郴缁熻捣涓�涓熀鏈綔鐢紝鍖呮嫭娓╂殩鐨勫寳鏋佸拰鍖楁瀬娴峰啺绋嬪害鍜屽帤搴︾殑琛拌惤骞朵笖鍦ㄦ渶杩戠殑鍗佸勾鏄剧ず鍑洪噸瑕佹皵鍊欏彉鍖栥�備笌娓╂殩鐨勫寳鏋佸拰鍖楁瀬娴峰啺鐨勫噺灏忕浉瀵圭収锛屾娲诧紝涓滀簹鍜屽寳缇庢床缁忓巻浜嗗弽甯稿湴鍐风殑鏉′欢锛屼笌鍦ㄦ渶杩戠殑骞存湡闂寸殑璁板綍闄嶉洩銆傚湪杩欑瘒璁烘枃锛屾垜浠湪娆т簹鐨勬皵鍊欎笂鑰冨療娴峰啺褰卞搷鐨勫綋鍓嶇殑鐞嗚В銆侾aleo锛岃瀵熷苟涓斿缓妯$爺绌惰鐩栦綇鎬荤粨鍑犱釜涓昏涓婚锛屽寘鎷細鍖楁瀬娴峰啺鍜屽畠鐨勬帶鍒剁殑鍙彉鎬э紱鍙兘鐨勫師鍥犲拰鍖楁瀬娴峰啺鐨勬槑鏄剧殑褰卞搷鍦ㄥ崼鏄熸椂浠o紝浠ュ強杩囧幓鍜屾姇灏勬湭鏉ュ奖鍝嶅拰瓒嬪娍鏈熼棿琛伴��锛涘湪鍖楁瀬娴峰啺鍜屽寳鏋佹憜鍔� / 鍖楁柟澶цタ娲嬫憜鍔ㄤ箣闂寸殑杩炴帴鍜屽弽棣堟満鍒讹紝鏈�杩戠殑娆т簹鐨勫喎鍗达紝澶ф皵鐨勫惊鐜紝鍦ㄤ笢浜氱殑澶忓ぉ闄嶆按锛屽湪娆т簹澶ч檰涓婄殑鏄ュぉ闄嶉洩锛屼笢鏂逛簹娲插啲瀛e椋庯紝鍜� midlatitude 鏋佺鎹辫繃鐨勫啲瀛o紱骞朵笖閬ヨ繙鐨勬皵鍊欏弽搴�(渚嬪锛屽ぇ姘旂殑寰幆锛岀┖姘旀俯搴�) 鍒板湪鍖楁瀬娴峰啺鐨勫彉鍖栥�傛垜浠负鏈潵鐮旂┒涓庝竴绡囩畝鐭拰寤鸿寰楀嚭缁撹銆�
    Honda M., J. Inoue, and S. Yamane, 2009: Influence of low Arctic sea-ice minima on anomalously cold Eurasian winters. Geophys. Res. Lett,, 36( 8), L08707.10.1029/ 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...
    Jung T., M. A. Kasper, T. Semmler, and S. Serrar, 2014: Arctic influence on medium-range and extended-range prediction in mid-latitudes. Geophys. Res. Lett 41,
    Jung T., M. Miller, and T. Palmer, 2010a: Diagnosing the origin of extended-range forecast errors. Mon. Wea. Rev,, 138( 6), 2434- 2446.10.1175/ with the ECMWF model are carried out to study the influence that a correct representation of the lower boundary conditions, the tropical atmosphere, and the Northern Hemisphere stratosphere would have on extended-range forecast skill of the extratropical Northern Hemisphere troposphere during boreal winter. Generation of forecast errors during the course of the integration is artificially reduced by relaxing the ECMWF model toward the 40-yr ECMWF Re-Analysis (ERA-40) in certain regions. Prescribing rather than persisting sea surface temperature and sea ice fields leads to a modest forecast error reduction in the extended range, especially over the North Pacific and North America; no beneficial influence is found in the medium range. Relaxation of the tropical troposphere leads to reduced extended-range forecast errors especially over the North Pacific, North America, and the North Atlantic. It is shown that a better representation of the Madden-Julian oscillation is of secondary importance for explaining the results of the tropical relaxation experiments. The influence from the tropical stratosphere is negligible. Relaxation of the Northern Hemisphere stratosphere leads to forecast error reduction primarily in high latitudes and over Europe. However, given the strong influence from the troposphere onto the Northern Hemisphere stratosphere it is argued that stratospherically forced experiments are very difficult to interpret in terms of their implications for extended-range predictability of the tropospheric flow. The results are discussed in the context of future forecasting system development.
    Jung T., T. Palmer, M. Rodwell, and S. Serrar, 2010b: Understanding the Anomalously Cold European Winter of 2005/06 Using Relaxation Experiments. Mon. Wea. Rev,, 138( 8), 3157- 3174.10.1175/ Experiments with the atmospheric component of the ECMWF Integrated Forecasting System (IFS) have been carried out to study the origin of the atmospheric circulation anomalies that led to the unusually cold European winter of 2005/06. Experiments with prescribed sea surface temperature (SST) and sea ice fields fail to reproduce the observed atmospheric circulation anomalies suggesting that the role of SST and sea ice was either not very important or the atmospheric response to SST and sea ice was not very well captured by the ECMWF model. Additional experiments are carried out in which certain regions of the atmosphere are relaxed toward analysis data thereby artificially suppressing the development of forecast error. The relaxation experiments suggest that both tropospheric circulation anomalies in the Euro閳ユ弬tlantic region and the anomalously weak stratospheric polar vortex can be explained by tropical circulation anomalies. Separate relaxation experiments for the tropical stratosphere and tropic...
    Jung T., F. Vitart, L. Ferranti, and J.-J. Morcrette, 2011: Origin and predictability of the extreme negative NAO winter of 2009/10,Geophys. Res. Lett.,38(7),L07701, winter of 2009/2010 was one of the most negative winters of the North Atlantic Oscillation (NAO) during the last 150 years. While most operational extended-range forecasting systems had difficulties in predicting the onset of the negative NAO phase, once established, extended-range forecasts were relatively skilful in predicting its persistence. Here, the origin and predictability of the unusual winter of 2009/10 are explored through numerical experimentation with the ECMWF Monthly forecasting system. More specifically, the role of anomalies in sea surface temperature (SST) and sea ice, the tropical atmospheric circulation, the stratospheric polar vortex, solar insolation and near surface temperature (proxy for snow cover) are examined. None of these anomalies is capable of producing the observed NAO anomaly, especially in terms of its magnitude. The results of this study support the hypothesis that internal atmospheric dynamical processes were responsible for the onset and persistence of the negative NAO phase during the 2009/10 winter.
    Overland, J. E. and M. Wang, 2016: Recent extreme Arctic temperatures are due to a split polar vortex. J, Climate, 29( 15), 5609- 5616.10.1175/ 488 1] Because animals and humans respond to seasonally and regionally varying climates, it is instructive to assess how much confidence we can have in regional projections of sea ice from the 20 models provided through the International Panel on Climate Change Fourth Assessment Report (AR4) process (IPCC 2007). Based on the selection of a subset models that closely simulate observed regional ice... /react-text react-text: 489 /react-text [Show full abstract]
    Parkinson, C. L. and J. C. Comiso, 2013: On the 2012 record low Arctic sea ice cover: Combined impact of preconditioning and an August storm. Geophys. Res. Lett,, 40( 7), 1356- 1361.10.1002/ new record low Arctic sea ice extent for the satellite era, 3.4 10km, was reached on 13 September 2012; and a new record low sea ice area, 3.0 10km, was reached on the same date. Preconditioning through decades of overall ice reductions made the ice pack more vulnerable to a strong storm that entered the central Arctic in early August 2012. The storm caused the separation of an expanse of 0.4 10kmof ice that melted in total, while its removal left the main pack more exposed to wind and waves, facilitating the main pack's further decay. Future summer storms could lead to a further acceleration of the decline in the Arctic sea ice cover and should be carefully monitored.
    Semmler T., M. A. Kasper, T. Jung, and S. Serrar, 2016: Remote impact of the Antarctic atmosphere on the Southern mid-latitudes. Meteorologische Zeitschrift, 25, 71- 77.10.1127/metz/2015/0685
    Tang Q., X. Zhang, X. Yang, and J. A. Francis, 2013: Cold winter extremes in northern continents linked to Arctic sea ice loss. Environmental Research Letters, 8( 1), 014036.10.1088/1748-9326/8/1/
    Vihma T., 2014: Effects of Arctic Sea Ice Decline on Weather and Climate: A Review. Surveys in Geophysics, 1- 40.10.1007/ areal extent, concentration and thickness of sea ice in the Arctic Ocean and adjacent seas have strongly decreased during the recent decades, but cold, snow-rich winters have been common over mid-latitude land areas since 2005. A review is presented on studies addressing the local and remote effects of the sea ice decline on weather and climate. It is evident that the reduction in sea ice cover has increased the heat flux from the ocean to atmosphere in autumn and early winter. This has locally increased air temperature, moisture, and cloud cover and reduced the static stability in the lower troposphere. Several studies based on observations, atmospheric reanalyses, and model experiments suggest that the sea ice decline, together with increased snow cover in Eurasia, favours circulation patterns resembling the negative phase of the North Atlantic Oscillation and Arctic Oscillation. The suggested large-scale pressure patterns include a high over Eurasia, which favours cold winters in Europe and northeastern Eurasia. A high over the western and a low over the eastern North America have also been suggested, favouring advection of Arctic air masses to North America. Mid-latitude winter weather is, however, affected by several other factors, which generate a large inter-annual variability and often mask the effects of sea ice decline. In addition, the small sample of years with a large sea ice loss makes it difficult to distinguish the effects directly attributable to sea ice conditions. Several studies suggest that, with advancing global warming, cold winters in mid-latitude continents will no longer be common during the second half of the twenty-first century. Recent studies have also suggested causal links between the sea ice decline and summer precipitation in Europe, the Mediterranean, and East Asia.
    Wu B., J. Su, and R. DArrigo, 2015: Patterns of Asian winter climate variability and links to Arctic sea ice. J, Climate, 28( 17), 6841- 6858.10.1175/ paper describes two dominant patterns of Asian winter climate variability: the Siberian high(SH) pattern and the Asia-Arctic(AA) pattern. The former depicts atmospheric variability closely associated with the intensity of the Siberian high, and the latter characterizes the teleconnection pattern of atmospheric variability between Asia and the Arctic, which is distinct from the Arctic Oscillation(AO). The AA pattern plays more important roles in regulating winter precipitation and the 850 h Pa meridional wind component over East Asia than the SH pattern, which controls surface air temperature variability over East Asia. In the Arctic Ocean and its marginal seas, sea ice loss in both autumn and winter could bring the positive phase of the SH pattern, or cause the negative phase of the AA pattern. The latter corresponds to a weakened East Asian winter monsoon(EAWM) and enhanced winter precipitation in the mid-latitudes of the Asian continent and East Asia. For the SH pattern, sea ice loss in the prior autumn emerges in the Siberian marginal seas, and winter loss mainly occurs in the Barents Sea,Labrador Sea, and Davis Strait. For the AA pattern, sea ice loss in the prior autumn is observed in the Barents-Kara Seas, the western Laptev Sea, and the Beaufort Sea, and winter loss only occurs in some areas of the Barents Sea, the Labrador Sea, and Davis Strait. Simulation experiments with observed sea ice forcing also support that Arctic sea ice loss may favor frequent occurrence of the negative phase of the AA pattern. The results also imply that the relationship between Arctic sea ice loss and winter atmospheric variability over East Asia is unstable, which is a challenge for predicting the EAWM based on Arctic sea ice loss.
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Manuscript History

Manuscript received: 29 November 2016
Manuscript revised: 16 March 2017
Manuscript accepted: 20 April 2017
通讯作者: 陈斌,
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Using NWP to Assess the Influence of the Arctic Atmosphere on Midlatitude Weather and Climate

  • 1. Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven 27570, Germany

Abstract: The influence of the Arctic atmosphere on Northern Hemisphere midlatitude tropospheric weather and climate is explored by comparing the skill of two sets of 14-day weather forecast experiments using the ECMWF model with and without relaxation of the Arctic atmosphere towards ERA-Interim reanalysis data during the integration. Two pathways are identified along which the Arctic influences midlatitude weather: a pronounced one over Asia and Eastern Europe, and a secondary one over North America. In general, linkages are found to be strongest (weakest) during boreal winter (summer) when the amplitude of stationary planetary waves over the Northern Hemisphere is strongest (weakest). No discernible Arctic impact is found over the North Atlantic and North Pacific region, which is consistent with predominantly southwesterly flow. An analysis of the flow-dependence of the linkages shows that anomalous northerly flow conditions increase the Arctic influence on midlatitude weather over the continents. Specifically, an anomalous northerly flow from the Kara Sea towards West Asia leads to cold surface temperature anomalies not only over West Asia but also over Eastern and Central Europe. Finally, the results of this study are discussed in the light of potential midlatitude benefits of improved Arctic prediction capabilities.

摘要: 本文采用ECMWF模式进行了两组14天天气预报试验, 两组试验的区别是积分过程中是否采用趋向ERA再分析数据的北极大气松弛系数, 通过比较来分析北极大气对北半球中纬度对流层天气和气候的影响, 并确定了两条北极影响中纬度天气的路径: 主要路径在亚洲和东欧, 次要路径在北美. 总的来说, 北半球冬季(夏季)静止行星波幅度最大(最弱)的情况下, 关联最强(最弱);北大西洋和北太平洋地区的北极影响很弱, 这与西南气流是一致的. 对气流依赖性的分析表明, 北极异常气流加强了北极对中纬度大陆地区的影响, 具体来说, 喀拉海向西亚的偏北异常气流导致了西亚及东欧和中欧地区的异常低温. 最后, 从改进北极预测能力以利于中纬度的角度讨论了本文的研究结果. (翻译: 宋米荣)

1. Introduction
  • Due to the rapid loss of Arctic sea ice and associated Arctic surface warming, the Arctic and its linkages to the midlatitudes has received increased interest in the climate research community in recent years, the progress of which is summarized in several review papers (e.g. Budikova, 2009; Vihma, 2014; Gao et al., 2015; Overland and Wang, 2016). Most previous studies have been based on either observational data, climate model sensitivity experiments with idealized sea-ice conditions, or the analysis of data from phase 5 of the Coupled Model Intercomparison Project (CMIP5). While it is difficult to disentangle cause and effect from observations and CMIP5 data, the use of idealized sea-ice conditions in models may result in changes of variability and/or inconsistencies along the sea-ice edge.

    Recently, higher-lower latitude linkages have been investigated from a different perspective: by employing a relaxation method (Jung et al., 2014; Semmler et al., 2016). This approach was originally introduced to diagnose the origin of forecast errors (Jung et al., 2010a) and to investigate the causes of the anomalously cold European winters in 2005/06 and 2009/10 (Jung et al., 2010b, 2011). The idea is to run two experiments using a Numerical Weather Prediction (NWP) model: a control forecast experiment using a standard set-up for weather prediction, and another experiment in which the NWP model is relaxed towards reanalysis data in the Arctic. Thus, in the relaxation experiment, the observed state is prescribed in the relaxation area. Comparing the relaxation experiment to the standard simulation in which the atmosphere can freely develop everywhere, given a lower boundary forcing, one can diagnose the influence that the atmosphere in the relaxation area has on remote regions. To reduce sampling uncertainty, this must be done several times in an ensemble approach with different start dates taken from the reanalysis data as initial conditions.

    Here, we use the relaxation approach of (Jung et al., 2010a) to identify the main atmospheric pathways along which the Arctic atmosphere influences midlatitude weather and climate. By employing an NWP approach this study will also provide some insight into the potential improvement of medium-range weather forecasting in the midlatitudes that could be obtained by enhancing prediction capabilities in the Arctic (e.g. through an enhanced Arctic observing system). This study is an extension of the work by (Jung et al., 2014), which focused on the winter season and used ERA-40 rather than ERA-Interim data (this study) for relaxation; the latter is much enhanced in terms of the data quality and covers more recent years. Compared to the previous relaxation experiments in which primarily the mid-troposphere large-scale circulation was investigated, in this study we also consider the impact of tropospheric relaxation on surface parameters, which are more socioeconomically relevant. Furthermore, we do not restrict our investigation to the winter season. Rather, we consider the seasonal cycle of Arctic-midlatitude linkages and explore the possible reasons. Another important difference is the use of a clearly smaller relaxation area, restricted to the central Arctic.

    The outline of this paper is as follows: Details of the experimental setup are given in section 2, followed by a description of the results in section 3. Finally, the outcomes of the study are discussed and conclusions drawn in section 4.

2. Methods
  • Numerical experiments were carried out with model cycle 38r1 of the Integrated Forecast System, which has been run operationally at the European Centre for Medium-Range Weather Forecasting (ECMWF) from 19 June 2012 to 18 November 2013. A spatial resolution of T L255 was employed, which corresponds to about 0.7° in the horizontal direction. In the vertical direction, 60 levels were used. Two 14-day forecasts with a time step of 45 min were computed for each month between January 1979 and December 2012, the first (second) forecast being initialized on the 1st (15th) day of the month. SST and sea-ice fields from ERA-Interim were used as lower boundary conditions. ERA-Interim data were also used for initialization of the forecast and as a reference when computing forecast errors. Model results were archived every 6 h and remapped onto a 2.5° grid.

  • To investigate the remote impacts of the Arctic, the development of error during the forecast was artificially reduced by relaxing the model towards reanalysis data in the polar regions north of 75°N (also south of 75°S). This was realized by adding an extra term of the following form to the prognostic equations: \begin{equation} -\lambda(x-x_{\rm ref}), \ \ (1)\end{equation} where x is the prognostic variable, x ref is the reanalysis value towards which the model state is drawn, and Λ is the relaxation strength parameter. In our study, Λ assumes a maximum value of 0.1 per time step. This means that, every time step, the model's tendency is moved towards the reanalysis data by taking 10% of the difference between the model result and reanalysis data. To smooth the border of the relaxation area, a hyperbolic tangent over a 20°-wide zonal belt was applied. In this region, Λ increases smoothly from zero to its maximum value, with the nominal border of the relaxation area in the middle of the 20° belt [for more details see (Jung et al., 2010a)]. The relaxation was applied in the troposphere, up to 300 hPa, to the zonal and meridional wind components, temperature, and the logarithm of surface pressure.

    In this study, two sets of forecasts were produced: one control integration (CTL) without relaxation, and one in which the troposphere was relaxed towards ERA-Interim data north of 75°N and south of 75°S (R75). Note that the relaxation was only applied to the tropospheric prognostic variables described above and not to surface parameters such as sea ice and SST, which were prescribed in the same way in CTL and R75, or snow cover, which freely developed from the initialization state in both CTL and R75. The difference between CTL and R75 was evaluated in terms of forecast skill in the Arctic and in the northern midlatitudes; the influence of the relaxation over Antarctica is described in a companion paper (Semmler et al., 2016). For the time scales considered here, it can be assumed that the relaxation over the Southern Hemisphere has no influence on the Northern Hemisphere, and vice versa. This is a reasonable assumption given that a forecast length of 14 days is hardly long enough for possible signals to cross hemispheres.

  • To study the seasonality of the Arctic influence on midlatitude weather, the year was divided into four seasons: winter (December, January, February); spring (March, April, May); summer (June, July, August); and autumn (September, October, November). In total, 204 forecast members were produced for each season. To reduce the noise level, the data were averaged over a time window of 24 h.

    To quantify the Arctic impact, several midlatitude (40°-60°N) regions have been defined: Europe (EURO; 20°W-40°E); northern Asia (NAS; 60°-120°E); and northern North America (NNAM; 130°-70°W). These regions were selected because they are highly populated areas that show relatively strong reductions in forecast error due to Arctic relaxation.

  • To understand whether the Arctic influence is linked to specific atmospheric situations (i.e. flow-dependence), we performed composite analyses for each region, in which we considered 500-hPa geopotential height (z500) and mean sea level pressure. For each pair of simulations, we considered the difference in the root-mean-square error (RMSE) between R75 and CTL. We calculated the RMSE using ERA-Interim data. We selected forecasts that were improved due to relaxation, considering each time window of 24 h separately. A forecast was considered to be improved for a particular time window if the error reduction was higher than the limit defined as the mean error reduction of the ensemble plus one standard deviation. For the composite of improved forecast members, we extracted corresponding reanalysis fields and averaged them. We did the same for the remaining forecast members to form a composite of neutral forecasts. To examine anomalous flow conditions for improved forecasts, we calculated differences between the two composites.

3. Results
  • The RMSE growth of daily averaged z500 with and without Arctic relaxation, averaged over the entire northern midlatitudes, is shown in Fig. 1a. For both integrations (CTL and R75), the error increases strongly during the first 10 days, after which error growth starts to saturate. The same holds for sub-regions of the northern midlatitudes (Figs. 1b-d), although there are differences in the magnitude of these values, with the largest values found for Europe (around 180 m in winter) and the smallest ones over northern Asia (around 120 m in winter). Over northern North America the values are similar to the average over the entire northern midlatitudes. A feature prevailing over the entire northern midlatitudes is that summer RMSE values are clearly smaller than winter RMSE values, reflecting the fact that day-to-day variability is much larger for the latter. Spring and autumn RMSE values are only slightly lower than those for winter. Over Europe (Asia) seasonal differences are largest (smallest).

    Figure 1.  RMSE of z500 (units: m) as a function of forecast lead time (in days) for different seasons and forecast experiments (solid line: CTL; dashed line: R75): (a) averaged over the whole northern midlatitudes between 40°N and 60°N (MLAT); (b) averaged over Europe (40°-60°N, 20°W-40°E; EURO); (c) averaged over northern Asia (40°-60°N, 60°-120°E; NAS); (d) averaged over northern North America (40°-60°N, 130°-70°W; NNAM).

    Error reductions depicted in Fig. 2 are generally small and amount to around 5% when averaged over the entire northern midlatitudes. However, over northern Asia values are much higher, amounting to about 15% in autumn. In the other seasons, error reductions around 10% are found.

    An important question, arising from these results, is why there are such pronounced seasonal and regional differences. To shed light on this issue, it is worth considering the climatological mean flow and its variability. Figures 3a, c, e and g show the z500 climatologies from the ERA-Interim data used for the relaxation experiments for different seasons. The meridional gradient of z500 is reduced by about a third in summer compared to winter, while spring and autumn take somewhat intermediate values. Furthermore, when taking the standard deviation over all six-hourly ERA-Interim output intervals per season for each gridpoint, it turns out that there is less variability in summer than in winter (not shown). In addition, the deviation from the zonal mean——that is, the strength of the climatological, stationary planetary waves——is weaker in summer than in winter, while spring and autumn are in between (Figs. 3b, d, f and h).

    Figure 2.  RMSE reduction (units: %) of z500 forecasts due to Arctic relaxation as a function of forecast lead time (in days) for different seasons and regions: (a) averaged over the whole northern midlatitudes between 40°N and 60°N (MLAT); (b) averaged over Europe (40°-60°N, 20°W-40°E; EURO); (c) averaged over northern Asia (40°-60°N, 60°-120°E; NAS); (d) averaged over northern North America (40°-60°N, 130°-70°W; NNAM).

    Also, the regional differences in forecast error and its reduction in Figs. 1 and 2 can be explained by the atmospheric circulation (mean and variability). The large RMSE over Europe compared to the other regions can be explained by the large standard deviation of z500 over this region. When considering the deviation from the zonal mean of z500 (Figs. 3b, d, f and h), it becomes obvious that northern Asia and northern North America are the areas with northerly components in the mean westerly flow conducive for a large Arctic influence on the midlatitude weather and climate. For northern Asia, this materialises in the largest RMSE reduction from the relaxation. Interestingly, the same is not true for northern North America. One possible explanation would be the Pacific influence, given the prevailing westerly flow, strong upstream impact from a region known for the importance of midlatitude dynamics (North Pacific), and the southerly component over the Pacific Ocean (Figs. 3a, c, e and g). This may especially influence the western part of the northern North America region, reaching out to 130°W according to our definition.

    Figures 4 and 5 provide a more comprehensive picture of the geographical distribution of the error reduction for the different seasons, both in the mid-troposphere (z500) and close to the surface (2-m temperature: t2m). We consider two forecast ranges: averaging over forecast lead times of 4-7 days, when there is still an influence from the initial conditions and error growth has not yet saturated; and averaging from 8-14 days, when the initial conditions play a smaller role and error saturation is much more pronounced.

    Figures 4 and 5 confirm that the RMSE reduction due to Arctic relaxation shows some strong regional dependency. Perhaps the most striking feature is the relatively strong Arctic influence over the continents, especially over Asia, compared to the oceans. As mentioned above, this can be explained by the climatological troughs over the east coasts of northern Asia and northern North America, leading to transport of Arctic air into northern Asia and Canada (Fig. 3). As argued by (Jung et al., 2014), a possible explanation for a smaller impact over the oceans lies in the fact that the North Atlantic and North Pacific regions are primarily determined by midlatitude dynamics, due to the relatively low-latitude location of the main storm formation regions over the Gulf Stream and Kuroshio regions. Furthermore, from Figs. 3b, d, f and h, it becomes obvious that, over the oceans, there is a southerly component in the mean westerly flow, leading to a stronger influence from lower latitudes over the oceans.

    Figure 3.  z500 (units: m) from the ERA-Interim data used for the relaxation: (a) winter mean; (b) mean stationary wave field (deviation from zonal averages) for winter; (c, d) as in (a, b) but for spring; (e, f) for summer; (g, h) for autumn.

    Figure 4.  RMSE reduction (units: %) of the z500 forecasts for the Northern Hemisphere north of 20°N due to Arctic relaxation and for different seasons: (a) winter averages over forecast lead times of 4 to 7 days; (b) winter averages over forecast lead times of 8 to 14 days; (c, d) as in (a, b) but for spring; (e, f) for summer; (g, h) for autumn. The dashed lines indicate the northern midlatitude region from 40°N to 60°N.

    The Arctic signal propagates southwards relatively quickly over Asia. During the second week, for example, RMSE reduction is evident as far south as 20°-40°N, although the picture becomes somewhat noisy as we go towards longer forecast lead time due to increased sampling variability. Over Europe and North America, and only in winter and spring, consistent improvements of between 5% and 10% are evident for days 4 to 7 and days 8 to 14. During the other seasons, the Arctic impact appears to be smaller and the results are less conclusive in terms of error reduction. The west coasts of North America and Europe, which are marked by maritime climate, show a rather small influence from the Arctic, consistent with the lesser influence over the oceans.

  • Having established the existence of preferred pathways along which the Arctic influences midlatitude weather, it is worth asking whether the strength of this linkage is flow-dependent. Figure 6 shows the z500 anomalies over the Northern Hemisphere that go along with anomalously large improvements in forecast skill over Asia with Arctic relaxation. It turns out that the link is strongest when anomalous northerly flow from the Kara Sea brings air of Arctic origin towards the midlatitudes, as can be deduced from the positive z500 anomalies over northeastern Europe and negative z500 anomalies over parts of Asia——a feature that is especially true during boreal winter. The link is clearly reflected by a substantial cold anomaly close to the surface in winter (Fig. 7). The cold surface anomaly amounts to about 3 K and extends into the areas of Eastern and Central Europe, because the z500 anomalies lead to an anomalous easterly flow to the south of the positive z500 anomalies over northeastern Europe, and is accompanied by warm anomalies over the Barents Sea, Greenland, and northeastern North America. The colder European temperatures are consistent with a weaker zonality of the flow, which weakens the upstream influence from the North Atlantic. The circulation anomalies are similar to the positive phase of the Eurasia-1 pattern (Barnston and Livezey, 1987). In winter, the northerly flow anomaly from the Kara Sea into West Asia is accompanied by a southerly flow anomaly over East Asia, as can be deduced from the z500 anomalies in Fig. 6, indicating a weakening of the East Asian winter monsoon.

    Figure 5.  As in Fig. 4, but for 2-m temperature forecasts (units: %).

    Figure 6.  z500 difference (units: m) between mean composites for improved and neutral forecasts with Arctic relaxation for northern Asia (green box) and considering forecast lead times of 1 to 7 days. Stippled areas indicate statistically significant areas according to the Wilcoxon test.

    The character of the flow-dependence for Europe and North America——that is, anomalous northerly flow associated with cold-air outbreaks into the considered region increases the linkage——is comparable to that over Asia, at least during winter and spring (not shown). In winter and to some extent in spring, unusually skilful forecasts for Europe seem to be produced, especially in situations involving the negative phase of the East Atlantic pattern, as defined by (Barnston and Livezey, 1987). Similarly, like for northern Asia, the anomaly pattern reduces the zonality of the flow and weakens the North Atlantic influence. For northern North America, the anomalous flow pattern does not resemble any well-established teleconnection pattern. However, like in the other regions, it is associated with a change in the meridionality of the flow.

    Figure 7.  The 2m temperature difference (units: K) between mean composites for improved and neutral forecasts (with respect to z500) with Arctic relaxation for northern Asia (green box) in winter and considering forecast lead times of 1 to 7 days. Stippled areas indicate statistical significance according to the Wilcoxon test.

4. Discussion and conclusions
  • While many previous studies have investigated the influence of Arctic surface conditions such as sea ice or snow on the large-scale circulation with climate model experiments or observational data, here we identify links between the Arctic and the northern midlatitude atmosphere by carrying out NWP experiments with and without relaxation towards reanalysis data in the Arctic atmosphere north of 75°N.

    Our Arctic relaxation experiments bring an improvement to forecasts in the northern midlatitudes, which is largest over continental areas——especially during winter and in Asia. It is reassuring that results are consistent with (Jung et al., 2014), despite the clearly smaller relaxation area (north of 75°N instead of north of 70°N). Compared to (Jung et al., 2014), it is a new and important result that the Arctic influence is strongest in winter and weakest in summer. Over Asia, reductions in forecast error of up to 15%, both in z500 and in t2m, could be achieved if one has perfect knowledge of the Arctic atmosphere. Thus, our results suggest that improved weather predictions in the Arctic (e.g. through an improved observing system) have the potential to improve prediction skill in the continental midlatitudes——especially during periods with anomalously northerly flow. In summer, the impact of the Arctic over continental areas is generally weaker due to reduced amplitudes of stationary planetary waves associated with more zonally oriented flow.

    Even though our relaxation approach is different from the methods used in most previous studies on the influence of the Arctic on the midlatitudes, and even if we are investigating the influence of the Arctic troposphere as opposed to Arctic surface conditions such as sea ice or snow cover, it is noteworthy that the main pathways identified along which the Arctic can influence the midlatitudes are consistent. Previous studies suggest that Siberia tends to be most strongly influenced in winter by changes in Arctic surface conditions, such as the sea-ice concentration and snow——especially over the Barents Sea/Kara Sea area and Eurasia, but also over the entire Arctic in the preceding summer/autumn (e.g. Honda et al., 2009; Francis et al., 2009; Cohen et al., 2012). In turn, Siberia has been identified to be a key region that influences the weather of Northern Europe and to some extent the whole northern midlatitudes (Cohen et al., 2001, 2012). Indeed, in cases of a strong pathway from the Kara Sea to West Asia due to northerly flow anomalies from the Kara Sea to West Asia, cold anomalies over West Asia extending into Eastern and Central Europe, and southerly flow anomalies over East Asia occur. The latter indicates a weakening of the East Asian winter monsoon. This has been associated with sea-ice loss in the Barents Sea/Kara Sea in the preceding autumn (Wu et al., 2015). However, in the present study, it is not sea-ice loss driving the stronger pathway from the Kara Sea to West Asia, as the following consideration indicates.

    Given the pronounced loss of Arctic sea ice during recent decades (e.g. Parkinson and Comiso, 2013), it is worth asking the question as to whether associated large-scale circulation changes might alter the teleconnectivity and hence the impact that Arctic prediction has on lower latitudes. In this context, a trend towards enhanced meridionality, especially over the continents, could lead to an intensification of the influence of the Arctic atmosphere on the northern midlatitudes. Therefore, it could be expected that most of the strongest improved forecasts over West and Central Asia would occur towards the end of the considered time period from 1979 to 2012. However, such trend could not be identified in any of the seasons over the past 30 years. Therefore, it can be argued that the recent Arctic sea-ice loss has not prompted any change in the strength of the influence of the Arctic atmosphere on northern midlatitude weather and climate. This also means that we cannot confirm previous findings, such as those of (Francis and Vavrus, 2012) and (Tang et al., 2013), linking stronger meridionality in the flow and more extreme cold and hot events with shrinking Arctic sea ice in winter and summer, respectively. It remains to be seen if possible future circulation changes will be large enough to change the strength of the influence that the Arctic atmosphere exerts on the northern midlatitudes.

    Oceanic areas such as the North Atlantic and North Pacific, as well as the west of North America and Western Europe, are less affected by the Arctic, at least on the time scales considered here. It might be argued that this is a result of the relatively southerly location of the jet stream along with a predominantly southwesterly flow, suggesting that midlatitude (and probably also tropical and subtropical) dynamics play a more important role instead.

    Our experiments show that there is scope for improved weather forecasts, especially in northern Asia, but to some extent in northeastern Europe and northern North America too, if forecasts can be improved in the Arctic off the Siberian coast, and to some extent off the Canadian Arctic coast. In contrast, an improvement in Arctic weather forecasting capabilities does not seem likely to help with improving weather forecasts for the western coasts of Europe and North America.




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