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1.
This paper explores the role of synoptic eddy feedback in the air-sea interaction in the North Atlantic region, particularly the interaction between the North Atlantic Oscillation (NAO) and the North Atlantic sea surface temperature anomalies (SSTA) tripole. A linearized five-layer primitive equation atmospheric model with synoptic eddy and low-frequency flow (SELF) interaction is coupled with a linearized oceanic mixed-layer model to investigate this issue. In this model, the “climatological” storm track/activity (or synoptic eddy activity) is characterized in terms of spatial structures, variances, decay time scales and propagation speeds through the complex empirical orthogonal function (CEOF) analysis on the observed data, which provides a unique tool to investigate the role of synoptic eddy feedback in the North Atlantic air–sea coupling. Model experiments show that the NAO-like atmospheric circulation anomalies can produce tripole-like SSTA in the North Atlantic Ocean, and the tripole-like SSTA can excite a NAO-like dipole with an equivalent barotropic structure in the atmospheric circulation, which suggests a positive feedback between the NAO and the SSTA tripole. This positive feedback makes the NAO/SSTA tripole-like mode be the leading mode of the coupled dynamical system. The synoptic eddy feedback plays an essential role in the origin of the NAO/SSTA tripole-like leading mode and the equivalent barotropic structure in the atmosphere. Without synoptic eddy feedback, the atmosphere has a baroclinic structure in the response field to the tripole-like SSTA forcing, and the leading mode of the dynamic system does not resemble NAO/SSTA tripole pattern.  相似文献   

2.
A high-resolution pre-industrial control simulation with the regional climate model REMO is analyzed in detail for different European subregions. To our knowledge, this is the first long pre-industrial control simulation by a regional climate model as well as at comparable resolution. We assess the ability of the climate model to reproduce the observed climate variability in various parts of the continent. In order to investigate the representation of extreme events in the model under pre-industrial greenhouse gas concentrations, selected seasons are examined with regard to the atmospheric circulation and other climatic characteristics that have contributed to the occurrences. A special focus is dedicated to land-atmosphere interactions. Extreme seasons are simulated by the model under various circumstances, some of them strongly resemble observed periods of extraordinary conditions like the summer 2003 or autumn 2006 in parts of Europe. The regional perspective turns out to be of importance when analyzing events that are constituted by meso-scale atmospheric dynamics. Moreover, the predictability of the European climate on seasonal to decadal time scales is examined by relating the statistics of surface variables to large-scale modes of variability impacting the North Atlantic sector like the Meridional Overturning Circulation, the El Niño Southern Oscillation, and the North Atlantic Oscillation. For this purpose, we introduce a measure of tail dependence that quantifies the correlation between extreme values in two variables that describe the state of the climate system. Significant dependence of extreme events can be detected in various situations.  相似文献   

3.
春季北大西洋三极型海温异常变化及其与NAO和ENSO的联系   总被引：1，自引：0，他引：1  
利用1951—2016年HadISST逐月海表温度(Sea Surface Temperature,SST)资料,NCEP/NCAR再分析资料以及1958—2016年美国伍兹霍尔海洋研究所(Woods Hole Oceanographic Institution,WHOI)提供的OAFlux数据集,运用经验正交函数分解(Empirical Orthogonal Function,EOF)和偏相关分析等统计方法,研究了春季北大西洋海温异常的主要特征及其与春季NAO和前期冬季ENSO联系。结果表明:春季北大西洋海温异常EOF的第一模态是自北而南出现的三极结构的海温距平型,其方差贡献率为35.7%。春季北大西洋三极型海温异常的形成主要受到春季NAO主导作用,还受到前期冬季热带中东太平洋海温异常的影响。消除前期冬季Niňo3.4的影响后,春季北大西洋三极型海温异常指数与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数的偏相关系数分别为0.50,通过了99%置信度水平的显著性检验。消除春季NAO的影响后,春季北大西洋三极型海温异常指数与前期冬季Niňo3.4指数的偏相关系数为-0.26,通过了95%信度水平的显著性检验。春季NAO正(负)位相引起的海表风场和海表湍流热通量的异常,进而激发出正(负)位相的北大西洋三极型海温异常。前期冬季ENSO事件可以引起春季大气环流异常和热带外海温异常,进而调制春季NAO对北大西洋三极型海温异常的影响。  相似文献   

4.
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.  相似文献   

5.
Correlation of water temperature and latent heat fluxes in winter in the North Atlantic with the atmospheric circulation in the subsequent months are analyzed based on the NCEP/NCAR reanalysis data. Monthly and daily indices of the North Atlantic Oscillation are used as characteristics of the atmosphere circulation. It is shown that conditions of the heat exchange between ocean and atmosphere in the western North Atlantic in February can influence the atmospheric circulation and air temperature in Europe in March.  相似文献   

6.
基于1970—2016年Hadley中心海温资料、NCEP/NCAR再分析资料和ECHAM4模式,研究了各海盆海表温度异常（SSTA）对1998和2016年这两个超级厄尔尼诺衰减年8月西北太平洋热带气旋（TC）生成及大尺度环流变化的可能影响。结果表明,热带印度洋和大西洋在1998与2016年几乎相反的SSTA型态是导致TC生成频数显著差异的主要原因之一,而热带和北太平洋SSTA在1998与2016年均分别在珠江三角洲和日本以南形成气旋性环流。1998年8月热带印度洋和大西洋SSTA产生的西北太平洋反气旋环流响应强于太平洋SSTA产生的气旋性环流异常,使西北太平洋受异常反气旋控制,减少TC的生成。2016年在三个大洋SSTA共同作用下,西北太平洋受异常气旋控制导致TC生成频数偏多。太平洋经向SSTA模在北半球副热带强迫出东西反向的跷跷板形势,在西北太平洋对流层产生的响应与实际变化相反,因此太平洋经向模对西北太平洋TC生成没有正的贡献。  相似文献   

7.
Extreme precipitation events in the upper Yangtze River Valley (YRV) have recently become an increasingly important focus in China because they often cause droughts and floods. Unfortunately, little is known about the climate processes responsible for these events. This paper investigates factors favorable to frequent extreme precipitation events in the upper YRV. Our results reveal that a weakened South China Sea summer monsoon trough, intensified Eurasian-Pacific blocking highs, an intensified South Asian High, a southward subtropical westerly jet and an intensified Western North Pacific Subtropical High (WNPSH) increase atmospheric instability and enhance the convergence of moisture over the upper YRV, which result in more extreme precipitation events. The snow depth over the eastern Tibetan Plateau (TP) in winter and sea surface temperature anomalies (SSTAs) over three key regions in summer are important external forcing factors in the atmospheric circulation anomalies. Deep snow on the Tibetan Plateau in winter can weaken the subsequent East Asian summer monsoon circulation above by increasing the soil moisture content in summer and weakening the land–sea thermal contrast over East Asia. The positive SSTA in the western North Pacific may affect southwestward extension of the WNPSH and the blocking high over northeastern Asia by arousing the East Asian-Pacific pattern. The positive SSTA in the North Atlantic can affect extreme precipitation event frequency in the upper YRV via a wave train pattern along the westerly jet between the North Atlantic and East Asia. A tripolar pattern from west to east over the Indian Ocean can strengthen moisture transport by enhancing Somali cross-equatorial flow.  相似文献   

8.
Observations show that at middle and high latitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that of the seasonal cycle and will likely exert a significant influence on the ocean circulation. The focus of this work will be the contribution of the North Atlantic Oscillation (NAO) to the stochastic forcing in the North Atlantic and its influence on the large-scale, wind-driven ocean circulation. To this end, a QG model of the North Atlantic Ocean was forced with the stochastic component of wind stress curl associated with the NAO signal. The ocean response is localized primarily in the western boundary region and can be conveniently understood using generalized stability analysis. Much of the variability is associated with the nonnormal influence of the bathymetry and inhomogeneities in the western boundary flow on the large-scale circulation. A more traditional statistical analysis of the circulation, however, reveals that there are very small and insignificant correlations between the NAO forcing and the ocean response within the western boundary region. This suggests that the dynamics of the ocean response to stochastic forcing may obscure any obvious coherence between the forcing and the response which is equally difficult to identify from observations.  相似文献   

9.
Increased evidence has shown the important role of Atlantic sea surface temperature (SST) in modulating the El Niño–Southern Oscillation (ENSO). Persistent anomalies of summer Madden–Julian Oscillation (MJO) act to link the Atlantic SST anomalies (SSTAs) to ENSO. The Atlantic SSTAs are strongly correlated with the persistent anomalies of summer MJO, and possibly affect MJO in two major ways. One is that an anomalous cyclonic (anticyclonic) circulation appears over the tropical Atlantic Ocean associated with positive (negative) SSTA in spring, and it intensifies (weakens) the Walker circulation. Equatorial updraft anomaly then appears over the Indian Ocean and the eastern Pacific Ocean, intensifying MJO activity over these regions. The other involves a high pressure (low pressure) anomaly associated with the North Atlantic SSTA tripole pattern that is transmitted to the mid- and low-latitudes by a circumglobal teleconnection pattern, leading to strong (weak) convective activity of MJO over the Indian Ocean. The above results offer new viewpoints about the process from springtime Atlantic SSTA signals to summertime atmospheric oscillation, and then to the MJO of tropical atmosphere affecting wintertime Pacific ENSO events, which connects different oceans.  相似文献   

10.
Observations show that at middle and high latitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that of the seasonal cycle and will likely exert a significant influence on the ocean circulation. The focus of this work will be the contribution of the North Atlantic Oscillation (NAO) to the stochastic forcing in the North Atlantic and its influence on the large-scale, wind-driven ocean circulation. To this end, a QG model of the North Atlantic Ocean was forced with the stochastic component of wind stress curl associated with the NAO signal. The ocean response is localized primarily in the western boundary region and can be conveniently understood using generalized stability analysis. Much of the variability is associated with the nonnormal influence of the bathymetry and inhomogeneities in the western boundary flow on the large-scale circulation. A more traditional statistical analysis of the circulation, however, reveals that there are very small and insignificant correlations between the NAO forcing and the ocean response within the western boundary region. This suggests that the dynamics of the ocean response to stochastic forcing may obscure any obvious coherence between the forcing and the response which is equally difficult to identify from observations.  相似文献   

11.
In this study, we investigated the features of Arctic Oscillation （AO） and Antarctic Oscillation （AAO）, that is, the annular modes in the extratropics, in the internal atmospheric variability attained through an ensemble of integrations by an atmospheric general circulation model （AGCM） forced with the global observed SSTs. We focused on the interannual variability of AO/AAO, which is dominated by internal atmospheric variability. In comparison with previous observed results, the AO/AAO in internal atmospheric variability bear some similar characteristics, but exhibit a much clearer spatial structure： significant correlation between the North Pacific and North Atlantic centers of action, much stronger and more significant associated precipitation anomalies, and the meridional displacement of upper-tropospheric westerly jet streams in the Northern/Southern Hemisphere. In addition, we examined the relationship between the North Atlantic Oscillation （NAO）/AO and East Asian winter monsoon （EAWM）. It has been shown that in the internal atmospheric variability, the EAWM variation is significantly related to the NAO through upper-tropospheric atmospheric teleconnection patterns.  相似文献   

12.
A survey of the spatial and temporal behavior of the atmospheric general circulation as it relates to both polar regions is presented. The review is based on the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-year reanalysis (ERA-40), updated using ECMWF operational analyses. The analysis spans 1960–2005 in the Northern Hemisphere, but is restricted to 1979–2005 in the Southern Hemisphere because of difficulties experienced by ERA-40 prior to the modern satellite era.The seasonal cycle of atmospheric circulation is illustrated by focusing on winter and summer. The huge circulation contrasts between the land-dominated Northern Hemisphere and the ocean-dominated Southern Hemisphere stand out. The intensification of the North Atlantic Oscillation/Northern Annular Mode and the Southern Annular Mode in DJF is highlighted and likely due to warming of the tropical Indian Ocean. The Arctic frontal zone during northern summer and the semi-annual oscillation throughout the year in the Southern Hemisphere are prominent features of the high latitude circulation in the respective hemispheres.Rotated principal component analysis (RPCA) is used to describe the primary modes of temporal variability affecting both polar regions, especially the links with the tropical forcing. The North Atlantic Oscillation is a key modulator of the atmospheric circulation in the North Atlantic sector, especially in winter, and is the dominant control on the moisture transport into the Arctic Basin. The Pacific-South American teleconnection patterns are primary factors in the high southern latitude circulation variability throughout the year, especially in the Pacific sector of Antarctica where the majority of moisture transport into the continent occurs.  相似文献   

13.
The behaviour of precipitation and maximum temperature extremes in the Mediterranean area under climate change conditions is analysed in the present study. In this context, the ability of synoptic downscaling techniques in combination with extreme value statistics for dealing with extremes is investigated. Analyses are based upon a set of long-term station time series in the whole Mediterranean area. At first, a station-specific ensemble approach for model validation was developed which includes (1) the downscaling of daily precipitation and maximum temperature values from the large-scale atmospheric circulation via analogue method and (2) the fitting of extremes by generalized Pareto distribution (GPD). Model uncertainties are quantified as confidence intervals derived from the ensemble distributions of GPD-related return values and described by a new metric called “ratio of overlapping”. Model performance for extreme precipitation is highest in winter, whereas the best models for maximum temperature extremes are set up in autumn. Valid models are applied to a 30-year period at the end of the twenty-first century (2070–2099) by means of ECHAM5/MPI-OM general circulation model data for IPCC SRES B1 scenario. The most distinctive future changes are observed in autumn in terms of a strong reduction of precipitation extremes in Northwest Iberia and the Northern Central Mediterranean area as well as a simultaneous distinct increase of maximum temperature extremes in Southwestern Iberia and the Central and Southeastern Mediterranean regions. These signals are checked for changes in the underlying dynamical processes using extreme-related circulation classifications. The most important finding connected to future changes of precipitation extremes in the Northwestern Mediterranean area is a reduction of southerly displaced deep North Atlantic cyclones in 2070–2099 as associated with a strengthened North Atlantic Oscillation. Thus, the here estimated future changes of extreme precipitation are in line with the discourse about the influence of North Atlantic circulation variability on the changing climate in Europe.  相似文献   

14.
Extreme midlatitude cyclone characteristics, precipitation, wind speed events, their inter-relationships, and the connection to large-scale atmospheric patterns are investigated in simulations of a prolonged cold period, known as the Maunder Minimum from 1640 to 1715 and compared with today. An ensemble of six simulations for the Maunder Minimum as well as a control simulation for perpetual 1990 conditions are carried out with a coupled atmosphere-ocean general circulation model, i.e., the Climate Community System Model (CCSM). The comparison of the simulations shows that in a climate state colder than today the occurrence of cyclones, the extreme events of precipitation and wind speed shift southward in all seasons in the North Atlantic and the North Pacific. The extremes of cyclone intensity increases significantly in winter in almost all regions, which is related to a stronger meridional temperature gradient and an increase in lower tropospheric baroclinicity. Extremes of cyclone intensity in subregions of the North Atlantic are related to extremes in precipitation and in wind speed during winter. Moreover, extremes of cyclone intensity are also connected to distinct large-scale atmospheric patterns for the different subregions, but these relationships vanish during summer. Analyzing the mean 1,000 hPa geopotential height change of the Maunder Minimum simulations compared with the control simulation, we find a similar pattern as the correlation pattern with the cyclone intensity index of the southern Europe cyclones. This illustrates that changes in the atmospheric high-frequency, i.e., the simulated southward shift of cyclones in the North Atlantic and the related increase of extreme precipitation and wind speed in particular in the Mediterranean in winter, are associated with large-scale atmospheric circulation changes.  相似文献   

15.
The river ice regime is considered a sensitive indicator of climate change and within this study long term changes (in case of River Daugava starting from 1530, but for other studied rivers starting from first half of twentieth century) river ice regimes in the Baltic region have been studied. The ice cover duration on the rivers (17 rivers) in the Baltic countries and Belarus has decreased during the recent decades. In addition to this, long term observational records of ice break on the rivers of the studied region exhibit a pattern of periodic changes in the intensity of ice regime. Both the ice regime and the seasonal river discharge are shown to be strongly influenced by large-scale atmospheric circulation processes over North Atlantic that manifests through close correlation with North Atlantic Oscillation index.  相似文献   

16.
This paper investigates the relationship between mei-yu and North Atlantic sea surface temperature anomalies (SSTA). Results show that they are significantly associated with each other on the decadal timescale. Both mei-yu precipitation and mei-yu duration are characterized by significant decadal variability. Their decadal components are closely correlated with a triple mode of North Atlantic SSTA in the preceding winter. Regression analysis demonstrates that the wintertime North Atlantic SSTA may impose a delayed impact on East Asia Summer Monsoon (EASM) circulation and mei-yu on the decadal timescale. The persistency of SSTA plays an important role during this course. The triple SSTA mode can persist from winter until late spring. It is suggested that the springtime SSTA may excite a stationary wave-train propagating from west Eurasia to East Asia and exert an impact on mei-yu.  相似文献   

17.
回顾了青藏高原雪盖的季节内变化及其影响研究的新进展。高原大部分地区雪盖不稳定且持续时间短,导致高原雪盖具有显著的季节内快速变化特征。局地气温和降水的季节内变化是控制高原雪盖季节内变化的直接原因,这种直接关系是区域大气环流季节内活动的结果。高原雪盖季节内变化还与大尺度大气环流的季节内活动有关,热带季节内振荡、北极涛动和北大西洋涛动引起的大气季节内过程可解释部分高原雪盖季节内变率。高原雪盖季节内变化通过雪-反照率效应迅速对大气施加影响,雪盖造成的冷异常通过大气平流过程影响高原及其下游地区,造成东亚高空急流和东亚大槽增强。由于高原雪盖季节内变化的重要影响,数值预报中高原雪盖的初始场和预报场会影响次季节预报技巧。  相似文献   

18.
热带西太平洋海温异常对东北亚阻塞形势影响的数值模拟   总被引：6，自引：0，他引：6  
本文利用ＩＡＰ－ＡＧＣＭ模拟研究了实测海温异常和热带西太平洋假想海温异常对夏季东亚地区环流异常的影响,并与观测结果进行了比较,籍此间接地研究了海温异常对东北亚阻塞形势的影响。首先,利用１９８０年实测海温异常,分三种试验（全球海温异常试验、热带海温异常试验、热带外海温异常试验）模拟了该年７月和８月的大气环流异常,指出海温异常是致使东北亚阻塞形势得以形成和维持的一个重要原因,同时也表明在对东亚大气环流异常型的影响方面,热带海温异常比热带经温异常更为重要。此外,我们还模拟研究了热带西太平洋的假想海温异常对东亚大气环流异常的影响,表明了热带西太平浃海温负异常对东北亚阻塞形势形成和维持有着重要作用  相似文献   

19.
A complex analysis is made of the variability of climatic, hydrological, and oceanological processes and environmental factors in the Baltic Sea region in the 20th century and at the beginning of the 21st century. The cause-and-effect relations between the climate dynamics over the North Atlantic and hydrometeorological conditions in the Baltic basin are established. Different variants of the North Atlantic Oscillation (NAO) indices were used as an atmospheric circulation intensity measure.  相似文献   

20.
X. Rodó 《Climate Dynamics》2001,18(3-4):203-217
Analysis of upper tropospheric relative humidity (UTH) variations from 1979 through 1992 shows the existence of a non-linear reversal in the tropical atmospheric anomalous circulation patterns characteristic of an El Niño (EN) phase. In addition to the tropics, both the original structures and their reversal affect midlatitudes. These changes begin a few months after the decaying of a strong EN episode and end one to two years after the peak in sea surface temperatures (SST) in the Pacific. This anomalous atmospheric circulation is matched by parallel reversals in cloud cover anomalies and absorbed solar radiation. At present, the limited time span covered by accurate satellite records prevents any conclusion regarding this reversal. An extension of coherent El Niño-Southern Oscillation (ENSO) signals in these correlation fields to higher latitudes is also reported. (By approximately between one to two years after the onset, the anomalous warming in the eastern equatorial Pacific had almost vanished, and the anomalies associated with the local Hadley cells and the Walker circulation revert towards a pre-El Niño stage). The coherence of these bimodal patterns might be responsible for certain climatic phenomena associated with ENSO in midlatitude regions. Though a relation between SST anomalies in the Pacific and the Indian and Atlantic oceans has already been postulated, this work shows how the return to near-normal conditions might yield a reversal in atmospheric patterns even in the absence of La Niña (LN) episodes. This in turn, might also produce climatic responses and feedbacks altering the net heat flux entering these remote regions, leading to droughts and floods. The results obtained strongly reinforce, for the most recent record, the hypothesis of a tropical atmospheric bridge connecting with the extratropics. In addition, it has been possible to isolate the ENSO signal from western Mediterranean (WM) SSTA, by means of advanced statistical techniques. The existence of anomalous atmospheric structures, that might link W Mediterranean SSTA and SSTA in the tropical North Atlantic (TNA), remains an open question, worthy of further investigation. Similarly, the role of ENSO in forcing this TNA region, and the subsequent atmospheric changes, will have to be further analysed with the aid of regional coupled models.  相似文献   

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