The nonlinear association between ENSO and the Euro-Atlantic winter sea level pressure

A nonlinear projection of the tropical Pacific sea surface temperature anomalies (SSTA) onto the Northern Hemisphere winter sea level pressure (SLP) anomalies by neural networks (NN) was performed to investigate the nonlinear association between El Niño-Southern Oscillation (ENSO) and the Euro-Atlantic winter climate. While the linear impact of ENSO on the Euro-Atlantic winter SLP is weak, the NN projection reveals statistically significant SLP anomalies over the Euro-Atlantic sector during both extreme cold and warm ENSO episodes, suggesting that the Euro-Atlantic climate mainly responds to ENSO nonlinearly. The nonlinear response, mainly a quadratic response to the SSTA, reveals that regardless of the sign of the SSTA, positive SLP anomalies are found over the North Atlantic, stretching from eastern Canada to Europe (with anomaly center located just northwestward of Portugal), and negative anomalies centered over Scandinavia and Norwegian Sea, consistent with the excitation of the positive North Atlantic Oscillation pattern.     

The influence of the Arctic Oscillation on winter temperatures in Iran

Summary The impact of the Arctic Oscillation (AO) on the winter surface air temperature (SAT) over Iran is demonstrated. Winter SAT data for 50 years (1951–2000) are analyzed for the negative and the positive AO phases. Using the Median Sequential Correlation Analysis (MSCA) technique it is shown that the winter SAT is negatively correlated to the winter AO index for most parts of Iran. The winter AO index accounts for about 14% to 46% of the winter SAT variance. The positive (negative) SAT anomaly is found to be associated with the onset of the negative (positive) phase. The overall probability of below long-term mean temperature during the positive and the negative phases are estimated to be around 70% and 25%, respectively. For the negative phase, westerly winds that originate from the warm Atlantic regions increase over Iran and consequently positive temperature anomalies are found across the country. The positive AO phase is accompanied by northerly winds that allow continental polar and arctic air masses to move into Iran, producing below normal temperatures. The summer AO is found to explain about 25–32% of the winter SAT variance in Iran. The reason for this is explained by the significant correlation (+0.38) between the summer and the following winter AO indices. These results indicate that the summer climate is linked to changes in atmospheric circulation which persist through to the following autumn and winter.     

A possible cause of the AO polarity reversal from winter to summer in 2010 and its relation to hemispheric extreme summer weather

In 2010, the Northern Hemisphere, in particular Russia and Japan, experienced an abnormally hot summer characterized by record-breaking warm temperatures and associated with a strongly positive Arctic Oscillation (AO), that is, low pressure in the Arctic and high pressure in the midlatitudes. In contrast, the AO index the previous winter and spring (2009/2010) was record-breaking negative. The AO polarity reversal that began in summer 2010 can explain the abnormally hot summer. The winter sea surface temperatures (SST) in the North Atlantic Ocean showed a tripolar anomaly pattern—warm SST anomalies over the tropics and high latitudes and cold SST anomalies over the midlatitudes—under the influence of the negative AO. The warm SST anomalies continued into summer 2010 because of the large oceanic heat capacity. A model simulation strongly suggested that the AO-related summertime North Atlantic oceanic warm temperature anomalies remotely caused blocking highs to form over Europe, which amplified the positive summertime AO. Thus, a possible cause of the AO polarity reversal might be the “memory” of the negative winter AO in the North Atlantic Ocean, suggesting an interseasonal linkage of the AO in which the oceanic memory of a wintertime negative AO induces a positive AO in the following summer. Understanding of this interseasonal linkage may aid in the long-term prediction of such abnormal summer events.     

Interannual Variations of the Blocking High over the Ural Mountains and Its Association with the AO/NAO in Boreal Winter

This paper analyzes interannual variations of the blocking high over the Ural Mountains in the boreal winter and their association with the Arctic Oscillation/North Atlantic Oscillation (AO/NAO).In Jan...     

中国东北暖季气温变化特征及其与海温和大尺度环流的关系

利用1980~2014年CRU TS3.24月平均气温数据和NCEP/NCAR再分析资料,分析了中国东北暖季（5~9月）气温的时空变化特征及其相应的大气环流状况。结果表明:中国东北暖季气温主要表现为全区一致型和南北反位相型两个模态,二者总解释方差高达86%。全区一致型具有明显的年代际变化特征,并在1990年代中期发生了显著的年代际突变,而南北反位相型具有明显的年际和年代际变化特征。全区一致型增暖对应着中国东北地区上空500 hPa位势高度的正异常和850 hPa的反气旋环流异常。当500 hPa位势高度南北反相时,对应于中国东北暖季气温的南北反位相型。进一步分析表明中国东北暖季气温的全区一致型及其1990年代中期的年代际突变与日本海及黑潮延伸区的海温异常及太平洋年代际振荡和大西洋多年代际振荡指数紧密相关。菲律宾以东的西太平洋、北太平洋中部、我国东南沿海、靠近北美东北部的北大西洋等海域的海温异常对中国东北暖季气温全区一致型的出现具有一定的预测作用。而南北反位相变化型与黑潮延伸区的海温异常关系显著,与大尺度指数的相关普遍不明显。在1990年代中期突变前,南北反位相型受到ENSO事件的影响,之后影响不显著。     

An Asymmetric Spatiotemporal Connection between the Euro-Atlantic Blocking within the NAO Life Cycle and European Climates

This paper examines an asymmetric spatiotemporal connection and climatic impact between the winter atmospheric blocking activity in the Euro-Atlantic sector and the life cycle of the North Atlantic Oscillation(NAO) during the period 1950–2012. Results show that, for positive NAO(NAO+) events, the instantaneous blocking(IB) frequency exhibits an enhancement along the southwest–northeast(SW–NE) direction from the eastern Atlantic to northeastern Europe(SW–NE pattern, hereafter), which is particularly evident during the NAO+decaying stage. By contrast, for negative NAO(NAO-)events, the IB frequency exhibits a spatially asymmetric southeast–northwest(SE–NW) distribution from central Europe to the North Atlantic and Greenland(SE–NW pattern, hereafter). Moreover, for NAO-(NAO+) events, the most marked decrease(increase) in the surface air temperature(SAT) in winter over northern Europe is in the decaying stage. For NAO+events, the dominant positive temperature and precipitation anomalies exhibit the SW–NE-oriented distribution from western to northeastern Europe, which is parallel to the NAO+-related blocking frequency distribution. For NAO-events, the dominant negative temperature anomaly is in northern and central Europe, whereas the dominant positive precipitation anomaly is distributed over southern Europe along the SW–NE direction. In addition, the downward infrared radiation controlled by the NAO's circulation plays a crucial role in the SAT anomaly distribution. It is further shown that the NAO's phase can act as an asymmetric impact on the European climate through producing this asymmetric spatiotemporal connection with the Euro-Atlantic IB frequency.     

An Observed Connection Between Wintertime Temperature Anomalies over Northwest China and Weather Regime Transitions in North Atlantic

In this study, the association between wintertime temperature anomalies over Northwest China and the weather regime transitions in North Atlantic on synoptic scale is analyzed by using observational surface air temperature(SAT) data and atmospheric reanalysis data. Daily SAT anomaly and duration time are used in order to define SAT anomaly cases. Differences with regard to the circulation anomalies over the Ural Mountains and the upstream North Atlantic area are evident. It is found that the colder than normal SAT is caused by the enhanced Ural high and associated southward flow over Northwest China. Time-lagged composites reveal possible connections between the SAT anomalies and the different development phases of the North Atlantic Oscillation(NAO). The Ural highs tend to be strengthened during the negative phase of NAO(NAO–) to Atlantic ridge transition, which are closely related to the downstream-propagating Rossby wave activity. The opposite circulation patterns are observed in the warm SAT cases. A cyclonic circulation anomaly is distinctly enhanced over the Urals during the positive phase of NAO(NAO+) to Scandinavian blocking transition, which would cause warmer SAT over Northwest China. Further analyses suggest that the intensified zonal wind over North Atlantic would favor the NAO– to Atlantic ridge transition, while the weakened zonal wind may be responsible for the transition between NAO+ and Scandinavian blocking.     

2011～2012年冬季欧亚大陆低温严寒事件与平流层北极涛动异常下传的影响

利用NCEP-NCAR 再分析资料分析了2011～2012 年冬季发生在欧亚大陆的一次异常低温严寒事件的大气环流演变过程以及可能的成因。这次低温事件,主要出现在2012 年1 月下旬至2 月上旬,持续大约3 周左右,非常强的低温异常覆盖了几乎整个欧洲以及东亚的西伯利亚、蒙古国和我国东北、华北等地。这次低温事件的演变与对流层北极涛动(AO)由正位相转变为负位相的时间相匹配,意味着AO 可能发挥重要作用。进一步分析表明,前期行星波的异常上传导致平流层发生爆发性增温现象,极夜急流减弱,AO 位相首先在平流层由正变负;在2～3 周左右的时间内,平流层AO 异常信号逐渐下传,使得对流层AO 也转为负位相;随后,乌拉尔山阻塞高压异常发展,极区的冷空气不断向南爆发,先后在东亚和欧洲造成剧烈的降温,导致低温严寒事件。因此,考虑平流层环流的异常可能有助于提高欧亚大陆冬季低温严寒事件的预测能力。     

Impact of the Winter North Pacific Oscillation on the Surface Air Temperature over Eurasia and North America: Sensitivity to the Index Definition

This study analyzes the impact of the winter North Pacific Oscillation(NPO) on the surface air temperature(SAT)variations over Eurasia and North America based on six different NPO indices. Results show that the influences of the winter NPO on the SAT over Eurasia and North America are sensitive to the definition of the NPO index. The impact of the winter NPO on the SAT variations over Eurasia(North America) is significant(insignificant) when the anticyclonic anomaly associated with the NPO index over the North Pacific midlatitudes shifts westward and pronounced northerly wind anomalies appear around Lake Baikal. By contrast, the impact of the winter NPO on the SAT variations over Eurasia(North America)is insignificant(significant) when the anticyclonic anomaly over the North Pacific related to the NPO index shifts eastward and the associated northerly wind anomalies to its eastern flank extend to North America. The present study suggests that the NPO definition should be taken into account when analyzing the impact of the winter NPO on Eurasian and North American SAT variations.     

北方涛动同北半球温带大气环流的遥相关(一)——基本结构

本文利用1951—1980年逐季的平均值资料(共120个季)讨论了北方涛动和与其相联系的北太平洋海温与北半球海平面气压场、500hPa位势高度场遥相关的基本结构,并与南方涛动和赤道东太平洋海温的结果进行了对比分析.发现北太平洋Namias海区和加利福尼亚海流区海温的变化与北方涛动具有很密切的联系;北方涛动和这两个海区的海温同北半球中高纬度大气环流特别是PNA型和NAO型环流异常存在明显的遥相关关系;南方涛动和赤道太平洋海温同WP型或NPO型环流异常关系比较密切,而与PNA型和NAO型的关系不如北方涛动和Namias海区及加利福尼亚海流区海温的显著.     

两类ENSO对中国北方冬季平均气温和极端低温的不同影响

利用1961-2012年观测、再分析资料以及全球大气环流模式数值试验,探讨了中国北方冬季平均气温对于不同类型（即东部型和中部型）ENSO事件的气候响应,并分析了不同类型ENSO对极端低温事件的可能影响,重点关注了北大西洋涛动（NAO）在其中的桥梁作用。结果表明,ENSO信号能通过调制北大西洋地区的大气环流改变欧亚中高纬度地区的纬向温度平流输送和西伯利亚高压的强度,进而影响中国北方冬季气温,由于不同类型ENSO事件海温分布的差异,这种影响具有明显的非线性特征。在两类厄尔尼诺和东部型拉尼娜事件冬季,北大西洋涛动均呈现负位相,不利于北大西洋的暖湿空气向欧亚大陆输送,西伯利亚高压偏强,因而中国北方地区较气候态偏冷。中部型厄尔尼诺和东部型拉尼娜事件冬季气温负异常的显著区域分别位于东北大范围地区、内蒙古河套附近;东部型厄尔尼诺事件冬季显著的冷异常信号仅局限于黑龙江北部与大兴安岭地区;而中部型拉尼娜事件冬季虽伴随北大西洋涛动正位相,但其空间结构向西偏移,对下游中国北方地区气温的直接影响并不显著,可能受局地信号干扰较大。数值试验再现了北大西洋涛动以及中国北方冬季气温对不同类型ENSO的响应,进一步佐证了上述结论。此外,两类厄尔尼诺事件冬季中国东北地区日平均气温容易偏低,极端低温事件的发生频次增多;而两类拉尼娜事件对极端低温的影响较弱。      

The effect of the wintertime Arctic Oscillation on springtime vegetation over the northern high latitude region

The winter Arctic Oscillation (AO), a major source of climate variability in the Northern Hemisphere, affects winter and the subsequent spring climate over northern high latitude. Such effects are evident even in the 1st eigenmode of the normalized difference vegetation index (NDVI). The impacts of the winter AO is a dipole pattern between Eurasia and North America; positive (negative) values of the winter AO induce warmer (cooler) and high (low) vegetation activity in the following spring over Eurasia (North America). Regarding the time-lagged response of vegetation, the sea surface temperature (SST) and snow cover contribute to maintaining the large-scale circulation anomaly associated with the AO.     

冬季平流层北极涛动对江南气温的影响

利用ERA—interim及NCEP—DOE两种再分析资料,分析了冬季北极涛动（Arctic Oscillation,AO）与我国江南地区地表气温相关的时空结构。结果表明：1）2月30hPa的Ao指数与江南地区地表气温的同期相关系数最高,这与AO指数和江南地区地表气温的标准差均在2月极大有关。2）30hPa的正AO事件加强时,贝加尔湖地区对流层易出现显著的正位势高度异常,有利于东亚地区出现向南、向下的异常风场,与之对应,西伯利亚高压向南扩展,有利于北方冷空气南侵至我国江南地区,造成局地气温负距平;反之亦然。     

南极海冰涛动对北半球夏季大气环流的影响

南极海冰首要模态呈现偶极子型异常,正负异常中心分别位于别林斯高晋海/阿蒙森海和威德尔海。过去研究表明冬春季节南极海冰涛动异常对后期南极涛动（Antarctic Oscillation,AAO）型大气环流有显著影响,而AAO可以通过经向遥相关等机制影响北半球大气环流和东亚气候。本文中我们利用观测分析发现南极海冰涛动从5～7月（May–July,MJJ）到8～10月（August–October, ASO）有很好的持续性,并进一步分析其对北半球夏季大气环流的可能影响及其物理过程。结果表明,MJJ南极海冰涛动首先通过冰气相互作用在南半球激发持续性的AAO型大气环流异常,使得南半球中纬度和极地及热带之间的气压梯度加大,在MJJ至JAS,纬向平均纬向风呈现显著的正负相间的从南极到北极的经向遥相关型分布。对流层中层位势高度场上,在澳大利亚北部到海洋性大陆区域,出现显著的负异常,在东亚沿岸从低纬到高纬呈现南北走向的“? + ?”太平洋—日本（Pacific–Japan,PJ）遥相关波列,其对应赤道中部太平洋及赤道印度洋存在显著的降水和海温负异常,西北太平洋至我国东部沿海地区存在显著降水正异常和温度负异常;低纬度北美洲到大西洋一带存在的负位势高度异常和北大西洋附近存在的正位势高度异常中心,构成一个类似于西大西洋型遥相关（Western Atlantic,WA）的结构,对应赤道南大西洋降水增加和南撒哈拉地区降水减少。从物理过程来看,南极海冰涛动首先通过局地效应影响Ferrel环流,进而通过经圈环流调整使得海洋性大陆区域和热带大西洋上方的Hadley环流上升支得到增强,海洋性大陆区域特别是菲律宾附近的热带对流活动偏强,激发类似于负位相的PJ波列,影响东亚北太平洋地区的大气环流,而热带大西洋对流增强和北传特征,则通过激发WA遥相关影响大西洋和欧洲地区的大气环流。以上两种通道将持续性MJJ至ASO南极海冰涛动强迫的大气环流信号从南半球中高纬度经热带地区传递到北半球中高纬地区,从而对热带和北半球夏季大气环流产生显著影响。     

The influence of tropical Pacific forcing on the Arctic Oscillation

The potential role of tropical Pacific forcing in driving the seasonal variability of the Arctic Oscillation (AO) is explored using both observational data and a simple general circulation model (SGCM). A lead–lag regression technique is first applied to the monthly averaged sea surface temperature (SST) and the AO index. The AO maximum is found to be related to a negative SST anomaly over the tropical Pacific three months earlier. A singular value decomposition (SVD) analysis is then performed on the tropical Pacific SST and the sea level pressure (SLP) over the Northern Hemisphere. An AO-like atmospheric pattern and its associated SST appear as the second pair of SVD modes. Ensemble integrations are carried out with the SGCM to test the atmospheric response to different tropical Pacific forcings. The atmospheric response to the linear fit of the model’s empirical forcing associated with the SST variability in the second SVD modes strongly projects onto the AO. Idealized thermal forcings are then designed based on the regression of the seasonally averaged tropical Pacific precipitation against the AO index. Results indicate that forcing anomalies over the western tropical Pacific are more effective in generating an AO-like response while those over the eastern tropical Pacific tend to produce a Pacific-North American (PNA)-like response. The physical mechanisms responsible for the energy transport from the tropical Pacific to the extratropical North Atlantic are investigated using wave activity flux and vorticity forcing formalisms. The energy from the western tropical Pacific forcing tends to propagate zonally to the North Atlantic because of the jet stream waveguide effect while the transport of the energy from the eastern tropical Pacific forcing mostly concentrates over the PNA area. The linearized SGCM results show that nonlinear processes are involved in the generation of the forced AO-like pattern.     

Opposing Trends of Winter Cold Extremes over Eastern Eurasia and North America under Recent Arctic Warming

Under recent Arctic warming, boreal winters have witnessed severe cold surges over both Eurasia and North America,bringing about serious social and economic impacts. Here, we investigated the changes in daily surface air temperature(SAT) variability during the rapid Arctic warming period of 1988/89–2015/16, and found the daily SAT variance, mainly contributed by the sub-seasonal component, shows an increasing and decreasing trend over eastern Eurasia and North America, respectively. Increasing c...     

A synoptic and dynamical characterization of wave-train and blocking cold surge over East Asia

Through an agglomerative hierarchical clustering method, cold surges over East Asia are classified into two distinct types based on the spatial pattern of the geopotential height anomalies at 300 hPa. One is the wave-train type that is associated with developing large-scale waves across the Eurasian continent. The other is the blocking type whose occurrence accompanies subarctic blocking. During the wave-train cold surge, growing baroclinic waves induce a southeastward expansion of the Siberian High and strong northerly winds over East Asia. Blocking cold surge, on the other hand, is associated with a southward expansion of the Siberian High and northeasterly winds inherent to a height dipole consisting of the subarctic blocking and the East Asian coastal trough. The blocking cold surge tends to be more intense and last longer compared to the wave-train type. The wave-train cold surge is associated with the formation of a negative upper tropospheric height anomaly southeast of Greenland approximately 12 days before the surge occurrence. Further analysis of isentropic potential vorticity reveals that this height anomaly could originate from the lower stratosphere over the North Atlantic. Cold surge of the blocking type occurs with an amplifying positive geopotential and a negative potential vorticity anomaly over the Arctic and the northern Eurasia in stratosphere. These anomalies resemble the stratospheric signature of a negative phase of the Arctic Oscillation. This stratospheric feature is further demonstrated by the observation that the blocking type cold surge occurs more often when the Arctic Oscillation is in its negative phase.     

Spring Arctic Oscillation-East Asian summer monsoon connection through circulation changes over the western North Pacific

In the present study the links between spring Arctic Oscillation (AO) and East Asian summer monsoon (EASM) was investigated with focus on the importance of the North Pacific atmospheric circulation and sea surface temperature (SST). To reduce the statistical uncertainty, we analyzed high-pass filtered data with the inter-annual time scales, and excluded the El Ni?o/Southern Oscillation signals in the climate fields using a linear fitting method. The significant relationship between spring AO and EASM are supported by the changes of multi-monsoon components, including monsoon indices, precipitation, and three-dimensional atmospheric circulations. Following a stronger positive spring AO, an anomalous cyclonic circulation at 850?hPa appears in southeastern Asia and the western North Pacific in summer, with the easterly anomalies spanning from the Pacific to Asian continent along 25°N?C30°N and the westerly anomalies south of 15°N. At the same time, the summer western North Pacific subtropical high becomes weaker. Consistently, the positive precipitation anomalies are developed over a broad region south of 30°N stretching from southern China to the western Pacific and the negative precipitation anomalies appear in the lower valley of the Yangtze River and southern Japan. The anomalous cyclone in the western North Pacific persisting from spring to summer plays a key role in modulating EASM and monsoon precipitation by a positive air-sea feedback mechanism. During spring the AO-associated atmospheric circulation change produces warmer SSTs between 150°E?C180° near the equator. The anomalous sensible and latent heating, in turn, intensifies the cyclone through a Gill-type response of the atmosphere. Through this positive feedback, the tropical atmosphere and SST patterns sustain their strength from spring to summer, that consequently modifies the monsoon trough and the western North Pacific subtropical high and eventually the EASM precipitation. Moreover, the SST response to AO-circulation is supported by the numerical simulations of an ocean model, and the anomalous atmospheric circulation over the western North Pacific is also reproduced by the dedicated numerical simulations using the coupled atmosphere?Cocean model. The observation evidence and numerical simulations suggest the spring AO can impact the EASM via triggering tropical air-sea feedback over the western North Pacific.     

Fast responses of Northern Hemisphere winter daily circulation to anthropogenic aerosols

Anthropogenic aerosols (AA) have significantly caused anomalous winter mean atmospheric circulation over the Northern Hemisphere, but the main daily patterns of winter large-scale circulation change are not well understood. Here a self-organizing map analysis is applied to identify the leading patterns in AA-induced winter daily geopotential height (Z) anomaly fields simulated by three atmospheric general circulation models, with a focus on fast adjustments. Two winter daily circulation response patterns with a synoptic time scale are found: one pattern shows concurring Z anomalies over North America and North Asia with the same sign and the Bering Sea seeing the opposite, resembling the Asia–Bering–North American teleconnection; while the other is the Arctic Oscillation-like pattern with similar Z anomalies over North Pacific and North Atlantic and the opposite over the Arctic region. The AA-induced anomalous precipitation over the tropics and anomalous synoptic eddy activities over the extratropical oceans concur to support and maintain these circulation anomaly patterns. The winter-mean climate responses to AA can be understood as a result of these daily anomaly patterns, especially over the higher latitudes. Specifically, the associated changes in surface air temperature (SAT) over the mid-high latitudes are caused by the AA-driven meridional movements of polar (cold and dry) airmass and midlatitude (warm and moist) airmass in the regions, mainly through the relevant surface downward longwave radiation. This study highlights the role of AA in altering daily weather patterns, which is not sufficiently captured by seasonal mean responses.     

Modulation of the Arctic Oscillation and the East Asian winter climate relationships by the 11-year solar cycle

The modulation of the relationship between the Arctic Oscillation (AO) and the East Asian winter climate by the 11-year solar cycle was investigated.During winters with high solar activity (HS),robust ...