北极涛动与北大西洋涛动的低频变化特征

本文主要利用小波分析方法研究北极涛动(AO)和北大西洋涛动(NAO)的低频变化和周期特征。结果表明:AO在1965年以前有准两年周期振荡,1975年后转为准8 a周期和准16 a周期;NAO也存在相似的周期转变,但其准8 a周期出现在60年代末到1976年前后,并且没有明显的准16 a周期;AO和NAO准8 a周期的交叉小波能谱在1975年前后达到极大值,这与北半球气候突变时间基本吻合。另外,AO和NAO模态的北大西洋中心在1975年后表现出不同程度的东移,AO中心移到地中海地区,且太平洋中心明显增强,使其纬向对称的环形模态较突变前更显著。     

On NAO’s predictability through the DFA method

Summary In this paper, the authors address a predictability study of the NAO index by using the time series analysis method known as “detrended fluctuation analysis” (DFA). This technique provides a quantitative measure of predictability by computing several piecewise fits (either linear or higher degree polynomial ones) to the cumulative series of fluctuations associated to the original series. The measurements, made on several examples of data available in the Internet, agree with other previous analyses (Fernández et al, 2003) assessing that the NAO signal is a slightly red one, whose prediction needs a deeper understanding of the underlying physics.     

Targeted Observations for Improving Prediction of the NAO Onset

Based on the viewpoint that the North Atlantic Oscillation(NAO) has an intrinsic timescale of approximate two weeks and can be treated as an initial value problem, targeted observations for improving the prediction of the onset of NAO events are investigated by using the conditional nonlinear optimal perturbation(CNOP) method with a quasigeostrophic model. The results show that flow-dependent sensitive areas for the prediction of NAO onset are mainly located over North Atlantic and its upstream regions. Targeted observations over the main sensitive areas could improve NAO onset prediction in most cases(approximately 75%) due to reduced errors in anomalous eddy vorticity forcing(EVF) projection in the typical NAO mode. Moreover, a flow-independent sensitive area is determined based on the winter climatological flow, which is located over North America and its adjacent ocean. The NAO onset prediction can also be improved by targeted observations over the flow-independent sensitive area, but the skill improvement is somewhat lower than that derived from observations over the flow-dependent sensitive area. The above results indicate that targeted observations over sensitive areas identified by the CNOP method can help to improve the onset prediction of NAO events.     

Atmospheric blocking: space-time links to the NAO and PNA

In the Northern hemisphere, regions characterized by an enhanced frequency of atmospheric blocking overlap significantly with those associated with the major extra-tropical patterns of large-scale climate variability—namely the North Atlantic Oscillation (NAO) and the Pacific North American (PNA) pattern. There is likewise an overlap in the temporal band-width of blocks and these climate patterns. Here the nature of the linkage between blocks and the climate patterns is explored by using the ERA-40 re-analysis data set to examine (1) their temporal and spatial correlation and (2) the interrelationship between blocks and the NAO/PNA. It is shown that a strong anti-correlation exists between blocking occurrence and the phase of the NAO (PNA) in the North Atlantic (western North Pacific), and that there are distinctive inter-basin differences with a clear geographical (over North Atlantic) and quantitative (over North Pacific) separation of typical blocking genesis/lysis regions during the opposing phases of the climate patterns. An Empirical Orthogonal Function (EOF) analysis points to a significant influence of blocking upon the NAO pattern (identifiable as the leading EOF in the Euro-Atlantic), and a temporal analysis indicates that long-lasting blocks are associated with the development of negative NAO/PNA index values throughout their life-time. In addition an indication of a cause-and effect relationship is set-out for the North Atlantic linkage.     

南方涛动(SO)与北大西洋涛动(NAO)百年变化特征的正交小波分析

利用“南海季风科学试验”（SCSMEX）所获得的可靠资料，对1998年区域海陆热力状况进行了分析与对比，并着重讨论了西太平洋和青藏高原地区热力作用对1998年季风爆发及发展的可能影响。结果发现：大气热源分布与海陆分布有密切联系，由于地形的阻挡使得在包含有南北海陆分布差异的地区，热源主要大值带较全海洋区域明显偏南。表面加热存在明显的季节变化和海陆差异。西太平洋地区与夏季风爆发之间的联系主要表现在海温和潜热加热的变化上。高原在亚洲夏季风爆发过程中的作用机制不同：在南海季风爆发期间以感热加热为主，印度季风爆发期间以水汽凝结释放潜热为主。     

Global SST influence on twentieth century NAO variability

Recent studies have suggested that sea surface temperature (SST) is an important source of variability of the North Atlantic Oscillation (NAO). Here, we deal with four basic aspects contributing to this issue: (1) we investigate the characteristic time scales of this oceanic influence; (2) quantify the scale-dependent hindcast potential of the NAO during the twentieth century as derived from SST-driven atmospheric general circulation model (AGCM) ensembles; (3) the relevant oceanic regions are identified, corresponding SST indices are defined and their relationship to the NAO are evaluated by means of cross spectral analysis and (4) our results are compared with long-term coupled control experiments with different ocean models in order to ensure whether the spectral relationship between the SST regions and the NAO is an intrinsic mode of the coupled climate system, involving the deep ocean circulation, rather than an artefact of the unilateral SST forcing. The observed year-to-year NAO fluctuations are barely influenced by the SST. On the decadal time scales the major swings of the observed NAO are well reproduced by various ensembles from the middle of the twentieth century onward, including the negative state in the 1960s and part of the positive trend afterwards. A six-member ECHAM4-T42 ensemble reveals that the SST boundary condition affects 25% of total decadal-mean and interdecadal-trend NAO variability throughout the twentieth century. The most coherent NAO-related SST feature is the well-known North Atlantic tripole. Additional contributions may arise from the southern Pacific and the low-latitude Indian Ocean. The coupled climate model control runs suggest only the North Atlantic SST-NAO relationship as being a true characteristic of the coupled climate system. The coherence and phase spectra of observations and coupled simulations are in excellent agreement, confirming the robustness of this decadal-scale North Atlantic air–sea coupled mode.     

Dependence of NAO variability on coupling with sea ice

The variance of the North Atlantic Oscillation index (denoted N) is shown to depend on its coupling with area-averaged sea ice concentration anomalies in and around the Barents Sea (index denoted B). The observed form of this coupling is a negative feedback whereby positive N tends to produce negative B, which in turn forces negative N. The effects of this feedback in the system are examined by modifying the feedback in two modeling frameworks: a statistical vector autoregressive model (F _VAR) and an atmospheric global climate model (F _CAM) customized so that sea ice anomalies on the lower boundary are stochastic with adjustable sensitivity to the model??s evolving N. Experiments show that the variance of N decreases nearly linearly with the sensitivity of B to N, where the sensitivity is a measure of the negative feedback strength. Given that the sea ice concentration field has anomalies, the variance of N goes down as these anomalies become more sensitive to N. If the sea ice concentration anomalies are entirely absent, the variance of N is even smaller than the experiment with the most sensitive anomalies. Quantifying how the variance of N depends on the presence and sensitivity of sea ice anomalies to N has implications for the simulation of N in global climate models. In the physical system, projected changes in sea ice thickness or extent could alter the sensitivity of B to N, impacting the within-season variability and hence predictability of N.     

冬季北大西洋涛动与中国西南地区降水的不对称关系

利用1951-2010年NCEP/NCAR再分析资料和中国160个站逐月降水资料,探讨了冬季北大西洋涛动(NAO)与同期中国西南地区降水的关系.结果表明,冬季北大西洋涛动与西南地区降水存在显著的正相关关系,并且,正相关具有不对称性,即当冬季北大西洋涛动处于负位相时,东亚地区环流形势不利于中国西南地区降水的形成,对应着中国西南地区冬季降水的显著减少.而当冬季北大西洋涛动处于正位相时,北大西洋涛动与中国西南地区降水的正相关关系并不显著.进一步的分析表明,与中国西南冬季降水变化密切相关的主要环流结构是里海和中东—阿拉伯海—青藏高原及其下游的遥相关型(CAT遥相关型).北大西洋涛动与里海和中东阿拉伯海—青藏高原及其下游的遥相关结构存在不对称关系,两者的关系仅在北大西洋涛动负位相时显著.冬季北大西洋涛动高、低指数年分别合成的波射线和波作用通量的结果表明,当冬季北大西洋涛动为负位相时,冬季地中海地区的扰动源会形成与里海和中东—阿拉伯海—青藏高原及其下游的遥相关路径一致的波射线,同时波作用通量的结果表明,定常波由里海和中东、阿拉伯海一直传播到青藏高原及下游地区,而冬季北大西洋涛动高指数年,地中海地区的扰动源所形成的波射线偏北,波动传播到达印度半岛地区之后不再向下游传播.冬季北大西洋涛动对里海和中东—阿拉伯海—青藏高原及其下游的遥相关波列的不对称影响决定了北大西洋涛动与西南冬季降水的不对称关系.     

The NPO/ NAO and interdecadal climate variation in China

This article discusses the interannual variation of the North Atlantic Oscillation (NAO) and North Pacific Oscillation (NPO), its relationship with the interdecadal climate variation in China which is associated with the climate jump in the Northern Hemisphere in the 1960’s, using the data analyses. It is clearly shown that both the amplitudes of the NAO and NPO increase obviously in the 1960’s and the main period of the oscillations changes from 3-4 years before the 1960’s to 8-15 years after the 1960’s. Therefore, interdecadal climate variation in China or the climate jump in the 1960’s is closely related to the anomalies of the NAO and NPO.     

Signatures of the NAO in the atmospheric circulation during wet winter months over the Mediterranean region

Summary Positive trend of the North Atlantic Oscillation (NAO) during last several decades was also accompanied by a positive trend of the East Atlantic Western Russia (EAWR) pattern. Decline of the Mediterranean precipitation during the period has also been noted. The precipitation decline over the western part of the region has been linked to the positive trend of the NAO. Explanation for the precipitation decline over the eastern Mediterranean by the role of the EAWR trend has also been suggested. An evaluation of the hypothesis is performed in the current study. A methodology for the determination of the characterizing typical low troposphere circulation during wet-months large-scale correlation-circulation patterns is suggested. The large-scale circulation patterns for three target areas over the northwestern, north-eastern, and southeastern Mediterranean regions are constructed separately for the low and high phase periods of the teleconnection regimes. According to the results, the precipitation decline over the Mediterranean region during the last several decades of the past century is explained by the positive trend of the EAWR, which in its turn was induced by that of the NAO. The trends have lead to the changes in the typical for the wet periods of the year low-troposphere circulation regimes associated with a decline in the water vapor transport from Atlantic.     

On the relationship of regional meteorological drought with SOI and NAO over southwest Iran

A fuzzy hierarchical clustering technique using the pairwise similarity matrix is employed to find the homogenous climate subregions over southwest Iran, based on the similarity of meteorological drought characteristics (i.e., duration, intensity, onset, and ending dates). The representative subregions are recognized for different rainy seasons; for each, the regional rainfall anomalies are computed. To find appropriate drought predictors, the lag relationships of regional rainfall with seasonal Southern Oscillation Index (SOI) and North Atlantic Oscillation (NAO) are examined using a conditional probability approach. The results suggest a significant negative correlation between autumn rainfall and June–August SOI. The NAO is also negatively correlated with autumn rainfall such that it is least likely for an extreme autumn drought to occur when June–August NAO is negative. A spring drought is preceded by an October–December NAO greater than 0.5. However, winter droughts do not appear to be lag-correlated with either SOI or NAO. In addition to the findings for droughts, these indices also emerged having considerable influence on wet seasons. A wet autumn tends to occur when either May–July SOI is less than ?0.5 or June–August NAO is less than about ?0.3. It is also apparent that the extreme wet springs are absent when October–December NAO is positive. This season is influenced most by NAO in both dry and wet spells. However, similar to droughts, the wet winter seasons are not found to be associated with either SOI or NAO.     

Physical mechanisms of European winter snow cover variability and its relationship to the NAO

Annual snow cover in the Northern Hemisphere has decreased in the past two decades, an effect associated with global warming. The regional scale changes of snow cover during winter, however, vary significantly from one region to another. In the present study, snow cover variability over Europe and its connection to other atmospheric variables was investigated using Cyclostationary Empirical Orthogonal Function (CSEOF) analysis. The evolution of atmospheric variables related to each CSEOF mode of snow cover variability was derived via regression analysis in CSEOF space. CSEOF analysis clearly shows that the North Atlantic Oscillation (NAO) is related to European snow cover, particularly in January and February. A negative NAO phase tends to result in a snow cover increases, whereas a positive NAO phase results in snow cover decreases. The temporal changes in the connection between the NAO and European snow cover are explained by time-dependent NAO-related temperature anomalies. If the NAO phase is negative, the temperature is lower in Europe and snow cover increases; by contrast, when the NAO phase is positive, the temperature is higher and snow cover decreases. Temperature and snow cover variations in Europe are associated with the thermal advection by anomalous wind by NAO. CSEOF analysis also shows an abrupt increase of snow cover in December and January and a decrease in February and March since the year 2000, approximately. This abrupt change is associated with sub-seasonal variations of atmospheric circulation in the study region.