南极涛动异常及其对冬春季北半球大气环流影响的数值模拟试验

利用IAP9L-AGCM模式考察了模式中与南极涛动异常相关的海温敏感区,发现南半球高纬海温异常能够强迫出南极涛动异常,而赤道东太平洋海温异常与太平洋南美型密切相关.研究了南极涛动异常对冬春季北半球大气环流及亚洲北部气温的影响,结果表明,南极涛动加强,能够引起北半球高纬环流异常和欧亚西风加强,以及亚洲北部地表气温和850 hPa气温显著增温.数值模拟支持了已有的诊断结果,也证实了冬春季节南极涛动异常下两半球高纬间的经向遥相关存在.     

南极涛动的年际变化及其对东亚冬春季气候的影响

研究了南极涛动的年际变化及其对冬春两季东亚气候的影响, 结果表明, 南极涛动强年不利于东亚冬春两季冷空气的活动. 通过平均经圈环流分析发现, 在南极涛动异常年有从南极到北极分布的经向遥相关, 并且该遥相关具有正压的结构. 此遥相关冬季在欧亚地区显著, 春季在太平洋地区显著. 研究还表明, 南半球高纬的平均纬圈西风, 在冬季与欧亚西风有显著的正相关关系, 因而在一定程度上证实了冬季经向遥相关的存在; 在春季, 则与太平洋北美型遥相关有显著的反相关关系. 因此, 局地经向遥相关是冬、春两季中南北半球中高纬大气环流相互作用的一个可能途径.     

高空大气涛动现象与太阳活动的联系

本文根据全球高空10 hPa位势高度距平场EOF分析得知,存在于地面层大气中的南北向涛动现象~北极高空大气涛动和南极涛动,在高空大气中更为清楚,而且这种高空南北向涛动现象是波及全球的;存在于地面层大气中著名的纬向涛动现象~南方涛动(Southern Oscillation,SO)和北方涛动(North Oscillation,NO),在高空大气中则变得不甚清楚.表征北极高空大气涛动的第一模态与表征南极涛动的第二模态的方差贡献率分别为41.47%和27.04%,二者累积方差贡献率达到68.51%,构成了平流层高空大气年代际振荡的主要形式;另外还存在两半球对称性中高纬度南极涛动模态和两半球不对称性中高纬度南极涛动模态,是高空大气中出现概率比较小的振荡形式.谱分析表明,无论北极高空大气涛动模态、南极涛动模态还是中高纬度纬向涛动模态,都存在与太阳磁场磁性指数相一致的22年准周期变化以及与太阳黑子相对数相一致的11年准周期变化;采用逐次滤波法的滤波分析和对比分析表明,高空大气涛动现象的重要影响因子乃太阳活动,其中太阳磁场的大幅度涨落及其磁性变化是主要因素,太阳黑子相对数的变化为次要因素.     

南半球夏季纬向平均环流的垂直结构异常及其与AAO和ENSO的联系

利用NCEP/NCAR月平均再分析资料,研究了南半球夏季（12~2月）纬向平均环流的垂直结构异常及其与南极涛动（AAO）和ENSO的联系.结果表明,南半球夏季纬向平均［u］的异常分布的主要模态（EOF1）显示出极区、50°S~70°S、以及50°S以北的区间内“三极”型振荡.EOF1 既反映了AAO的特征又与ENSO有着显著的关系.由于AAO指数与Nino3区指数之间存在着统计相关,为进一步弄清AAO和ENSO在南半球纬向平均气流变动的相关分量及其结构,利用Nino3区指数使用一元回归方法滤除ENSO影响,再对剩下的部分作EOF分解,得到了独立于ENSO的纬向平均［u］的第一模态AEOF1.相关分析表明AEOF1为与AAO相对应的纬向平均［u］异常的分布.用南半球纬向平均［u］去掉其与AAO相联系的模态AEOF1,进行EOF分解得到的第一模SEOF1,其与Nino3 区指数的相关高达0.9.由此给出了纬向平均气流的变动与ENSO无关的模态和与ENSO有关的模态.时间变化分析表明,近30年中,除了年代际变化和3~7年的年际变化外,纬向平均的纬向基本气流尚有极地西风减弱、副极地西风加强、副热带西风减弱、热带东风加强的长期趋势.     

南半球环流异常与长江中下游夏季旱涝的关系

本文对长江中下游夏季旱涝年前期（3、4、5月的季节平均）和同期（6、7、8月的季节平均）的南半球环流作对比分析,探讨南半球环流异常与长江中下游夏季旱涝的关系. 结果表明前期和同期南半球环流均有显著差异,春季南极涛动对长江中下游旱涝的影响较夏季显著,南半球副热带高压在春、夏两季中有很好季节持续性. 因此,春季南半球环流异常可以作为长江中下游夏季旱涝主要短期气候预测因子. 南、北半球中高纬环流相互作用是长江中下游夏季降水的一个重要因素,其可能的联系机制是从南半球高纬到北半球东亚沿岸经向分布的正压遥相关. 研究中还发现在长江中下游的涝年,整个对流层中南半球春、夏两季有持续增温,这说明了南北半球的温度梯度减弱也是东亚夏季风减弱的原因之一.     

全球纬向平均大气环流基本模态年代际变化的凝聚小波分析

应用1871-2008年NCEP／NCAR月平均再分析资料,研究了1948-2008年期间全球纬向平均大气环流基本模态的年代际变化.小波凝聚谱的结果表明全球纬向平均大气环流基本模态存在显著的20年左右周期的年代际变化.小波凝聚位相的结果清楚地显示了纬向平均大气环流基本模态的变化顺序.在20年左右的年代际变化时间尺度上,全球纬向平均温度超前纬向平均位势高度2个月,同时超前纬向平均流10个月出现变化;全球纬向平均位势高度又超前纬向平均流8个月出现变化.全球温度上升(下降), 将使高纬度的纬向平均位势高度降低(升高),中低纬度的纬向平均位势高度升高(降低);进而使得中高纬和热带的纬向平均西风加(减)速或东风减(加)速,同时使极地和副热带的西风减(加)速或东风加(减)速.20世纪70年代末期以来全球显著增暖的异常信号最早出现在南半球对流层顶附近,其次出现在南半球对流层低层、北半球对流层顶附近和北半球对流层低层.     

决定北半球冬季哈得莱环流年际变率的三维大气环流图像:观测分析和数值模拟

本文利用NCEP/NCAR再分析资料,分析了1979~2008年北半球冬季哈得莱(Hadley)环流年际变化的特征,在此基础上,讨论了在观测海温驱动下大气环流模式的模拟结果.观测分析表明,近30年北半球冬季哈得莱环流年际变率的主导模态呈现出空间上的非均匀变化,哈得莱环流圈位于热带部分与其位于副热带部分的强度变化符号相反,这在表征其年际变化特征的另一指标——经向风垂直切变中亦有显著体现.大气环流模式AMIP积分试验结果表明,北半球冬季哈得莱环流强度的上述年际变化源于海温强迫.分析发现,热带中东太平洋和南印度洋暖海温距平强迫导致了哈得莱环流强度年际变化的主导模态呈现出空间上的非均匀变化.ElNio的局地作用和大气桥作用激发的太平洋局地哈得莱环流(30°S~30°N,150°E~90°W)和大西洋局地哈得莱环流(30°S~30°N,90°W~10°W)并非呈现出整体一致的变化,尽管二者纬向平均后分别使气候平均的哈得莱环流圈强度加强和减弱.ElNio遥强迫作用激发的西北太平洋反气旋(0°~30°N,100°E~150°E)使北半球Hadley环流圈强度减弱,ElNio和南印度洋暖海温距平共同强迫出的南印度洋反气旋(30°S~0°,60°E~100°E)使南半球Hadley环流圈的强度亦减弱.上述局地哈得莱环流的变化叠加后,因纬向平均的太平洋局地哈得莱环流强度在(副)热带部分的增强大(小)于纬向平均的大西洋局地哈得莱环流和西北太平洋、南印度洋局地哈得莱环流在(副)热带地区的减弱,结果使得哈得莱环流圈的强度在(副)热带部分偏强(弱);较之南半球,北半球强度变化稍强.因此,北半球冬季哈得莱环流年际变率的主导模态在空间上呈现出非均匀变化.     

冬季太阳11年周期活动对大气环流的影响

利用气象场的再分析资料和太阳辐射活动资料,对太阳11年周期活动影响北半球冬季(11月~3月)大气环流的过程进行了统计分析和动力学诊断.根据赤道平流层纬向风准两年振荡(QBO)的东、西风状态对太阳活动效应进行了分类讨论,结果表明：东风态QBO时,太阳活动效应主要集中在赤道平流层中、高层和南半球平流层,强太阳活动时增强的紫外辐射加热了赤道地区的臭氧层,造成平流层低纬明显增温,同时加强了南半球的Brewer-Dobson(B-D)环流,引起南极高纬平流层温度增加;而北半球中高纬的环流主要受行星波的影响,太阳活动影响很小.西风态QBO时,太阳活动效应在北半球更为重要,初冬时强太阳活动除了加热赤道地区臭氧层外,还抑制了北半球的B-D环流,造成赤道平流层温度增加和纬向风梯度在垂直方向的变化,从而改变了对流层两支行星波波导的强度;冬末时在太阳活动调制下,行星波向极波导增强,B-D环流逐渐恢复,造成北半球极地平流层明显增温,同时伴随着赤道区域温度的下降.     

500 hPa高度场上两极和赤道位势高度气候演变规律的研究

本文运用近50 a来500 hPa层次上南极、赤道和北极位势高度以及南北半球西风指数的资料,分析了它们的时间演变规律及其相互间的关系.结果表明,南极位势高度显著下降;赤道位势高度显著上升.南极、赤道和北极位势高度都存在着显著的年际、年代际变化特征.总体上,南极位势高度与赤道位势高度有极其显著的负相关关系,北极与赤道的位势高度之间以及两极位势高度之间相关关系不显著;在共振的特定频率中,北极位势高度振荡落后于南极位势高度,赤道位势高度振荡又落后于两极位势高度振荡,气候变化最先开始的区域为南极地区.进一步分析表明,伴随着以上三个区域的位势高度趋势变化及其周期振荡,必然引起高空西风的增强与周期振荡.研究表明,南北两半球西风指数都存在显著的上升趋势,且存在显著的年代际变化特征.其中,南半球西风指数上升幅度较北半球大,其振荡落后于南极位势高度.     

南半球平流层极涡崩溃早晚年环流异常特征

极涡崩溃是平流层大气环流一个重要的变化过程,本文利用31年的再分析资料研究了南半球平流层极涡崩溃早晚年的异常特征.研究结果表明,南半球平流层极涡崩溃偏早年极涡崩溃前后平流层环流场异常表现为整层一致的变化,即都为正温度异常、正位势高度异常和负纬向风异常;而南半球平流层极涡崩溃偏晚年极涡崩溃前后平流层环流场异常的整层一致性的变化不典型,而在符号上与极涡崩溃偏早年的异常相反.与北半球平流层极涡崩溃前后环流异常相反明显不同,南半球平流层极涡崩溃偏早或偏晚年在极涡崩溃前后的环流异常保持相同的性质.进一步分析表明行星波活动在南极极涡的崩溃过程中起到了重要作用,极涡崩溃早年上传行星波比极涡崩溃晚年强,并且持续时间长.通过波流相互作用,行星波的异常使得极涡崩溃早年和晚年10月的平流层高纬地区分别为位势高度正异常和负异常,环流异常持续保持可能最终影响了南半球平流层极涡的崩溃时间.分析显示南半球极涡崩溃偏晚与La Niña事件之间可能存在一定的联系,但在极涡崩溃偏早年与赤道太平洋海表温度异常(SSTA)并无明显关系.     

北京沙尘频次的年际变化及其全球环流背景分析

本文采用相关和合成的分析方法,研究北京沙尘频次的年际变化及其冬、春季的全球环流背景.结果表明北京沙尘频次有年际变化的特点,并与全球范围内的环流异常相联系,特别是南、北半球的中高纬的环流异常.南半球环流异常与沙尘的联系在冬、春两季有很好的持续性和显著性,北半球中高纬环流异常与沙尘的联系冬季较春季显著.春季对流层高层的东亚西风急流增强能使低空的蒙古气旋加强和锋生,从而引起地面大风,为沙尘天气频次的增加提供动力条件.     

Asymmetry in the interhemispheric planetary wave-tide link between the two hemispheres

This study assesses the relation between the year-to-year variability of the semidiurnal tides (SDT) observed at high latitudes of both hemispheres and the global stratospheric stationary planetary wave (SPW) with zonal wavenumber S=1 (SPW1) derived from the UKMO temperature data. No significant positive correlation can be identified between the interannual variability of the Northern Hemisphere (NH) SDT and the Southern Hemisphere (SH) SPW1 for austral late-winter months. In contrast, a good consistency is evident for the interannual variations between the SDT observed at Rothera (68°S, 68°W) and the Arctic SPW1 for NH mid-winter months. Since it has been observed that during austral summer the non-migrating SDT often plays a significant role at the latitude of Rothera, a physical link between the SH SDT and the NH SPW is suggested. This asymmetry in the interhemispheric link is also noted in a recent study.     

Spectral characteristics of the meridional transport of angular momentum in the mid-troposhere of the Southern Hemisphere

Summary The wavenumber-frequency spectra of the meridional flux of angular momentum at 20°, 30°, 40°, 50°, 60° and 70°S, at 500 mb, show a definite domain of wave interactions between the zonal and meridional components of the velocity at various latitudes. In middle latitudes, the spectral band of the meridional flux of angular momentum is oriented from a region of low wavenumbers and low frequencies to a region of high wavenumbers and negative frequencies assigned for waves moving from west to east. In low latitudes, however, the spectral domain is confined to a narrow band centered near the zero frquency.In contrast to the meridional flux of angular momentum in the Northern Hemisphere in which the intensity in winter is about twice that in Summer, in the Southern Hemisphere the meridional flux shows same intensity for all seasons.In the Southern Hemisphere, most of the meridional flux of angular momentum is directed toward the south pole and is accomplished by the eastward moving waves. In the Northern Hemisphere, however, most of the meridional flux is directed toward the north pole and is contributed by the stationary waves.The National Center for Atmospheric Research, Boulder, Colorado 80302, (USA).     

Meridional eddy flux of enthalpy in the Southern Hemisphere during the IGY

Summary From meteorological IGY data for the calendar year 1958, the mean meridional eddy transport of enthalpy was evaluated for the Southern Hemisphere. Levels chosen for the study were 1000, 850, 700, 500, 400, 300, 200, 150 and 100 mb. Data from 84 Southern Hemisphere and 25 equatorial Northern Hemisphere stations were used. Yearly mean quantities related to meridional eddy enthalpy flux were computed and analyzed.It was found that around 40° S there is a double-maximum zone of poleward, meridional, transient eddy enthalpy flux, the stronger transport occurring at 850 mb, and the weaker near 200 mb. The countergradient transient eddy flux regions in the low latitude mid-troposphere and in the middle and upper latitude lower stratosphere, found in previous Northern Hemisphere investigations, were observed to exist in the Southern Hemisphere also. The standing eddy heat transport, as expected, was very weak except at high latitudes where Antarctic continentality effected a large double-maximum poleward flux centered near the surface and in the lower stratosphere. The total vertically integrated enthalpy transport by the eddies was found to be poleward everywhere, reaching a maximum between 35° and 40° S.     

Wavenumber frequency spectra of the meridional transport of sensible heat in the mid-troposphere of the Southern Hemisphere

Summary The wavenumber-frequency spectra of the meridional transport of sensible heat at 20°, 30°, 40°, 50°, 60°, and 70°S, at 500 mb in the Southern Hemisphere, show a definite spectral domain for the transport at various latitudes, which is dominated by the wave motion of the meridional component of the velocity. In middle latitudes, the spectral band of the meridional flux of sensible heat is oriented from a region of low wavenumbers and low frequencies to a region of high wavenumbers and negative frequencies assigned for waves moving from west to east. In low latitudes, the spectral band is confined to a narrow band centered near the zero frequency. It is found that most of the meridional transport of sensible heat at 500 mb in the Southern Hemisphere is accomplished by waves of medium wavelengths moving from west to east in middle and high latitudes. The meridional flux of sensible heat at 500 mb in the summer of the Southern Hemisphere is about three times that in the summer of the Northern Hemisphere. However, the meridional flux of sensible heat at 500 mb is about the same in the winter of both hemispheres. In the Southern Hemisphere practically all the meridional flux of sensible heat is associated with the moving waves in all seasons, whereas in the Northern Hemisphere the stationary waves contribute about 40% of the transport in winter.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The meridional mode of global tropospheric–stratospheric circulation

The spatio–temporal variations of major meridional modes are studied by using the ECMWF and NCEP/NCAR reanalyzed geopotential data between 70 and 10 hPa during 1979 and 2001. The variance contribution rates from the first and second modes are 56–69% and 14–22%, respectively, for ECMWF, but 76–85% and 9–10% for NCEP/NCAR. The climatic trend coefficients are positive and negative in the troposphere and stratosphere, respectively. The reversal is remarkably correlated with the AO/NAM, AAO/SAM and polar vortex, suggesting their important role in connecting the mid-low and upper circulation and the interaction between SH and NH.     

Interannual variability of Antarctic Oscillation and its influence on East Asian climate during boreal winter and spring

1 Introduction Antarctic Oscillation is a major mode of Southern Hemispheric (SH) extratropical atmospheric circula- tion. The SH Annual Mode represents a zonally sym- metric exchange of mass between polarward of 60°S and 40°S[1], therefore AAO indicates both the intensity of circumpolar low and zonal mean west wind at mid-high latitudes in SH. Positive phase of AAO tends to deepen circumpolar low and enhance west wind at mid-high latitudes in SH. Many studies show that AAO has bar…     

Effects of winter atmospheric circulation on temporal and spatial variability in annual streamflow in the western United States

Abstract

Winter mean 700-hectoPascal (hPa) height anomalies, representing the average atmospheric circulation during the snow season, are compared with annual streamflow measured at 140 streamgauges in the western United States. Correlation and anomaly pattern analyses are used to identify relationships between winter mean atmospheric circulation and temporal and spatial variability in annual streamflow. Results indicate that variability in winter mean 700-Hpa height anomalies accounts for a statistically significant portion of the temporal variability in annual streamflow in the western United States. In general, above-average annual streamflow is associated with negative winter mean 700-Hpa height anomalies over the eastern North Pacific Ocean and/or the western United States. The anomalies produce an anomalous flow of moist air from the eastern North Pacific Ocean into the western United States that increases winter precipitation and snowpack accumulations, and subsequently streamflow. Winter mean 700-hPa height anomalies also account for statistically significant differences in spatial distributions of annual streamflow. As part of this study, winter mean atmospheric circulation patterns for the 40 years analysed were classified into five winter mean 700-hPa height anomaly patterns. These patterns are related to statistically significant and physically meaningful differences in spatial distributions of annual streamflow.     

North Atlantic Oscillation – Concepts And Studies

This paper aims to provide a comprehensive review of previous studies and concepts concerning the North Atlantic Oscillation. The North Atlantic Oscillation (NAO) and its recent homologue, the Arctic Oscillation/Northern Hemisphere annular mode (AO/NAM), are the most prominent modes of variability in the Northern Hemisphere winter climate. The NAO teleconnection is characterised by a meridional displacement of atmospheric mass over the North Atlantic area. Its state is usually expressed by the standardised air pressure difference between the Azores High and the Iceland Low. ThisNAO index is a measure of the strength of the westerly flow (positive with strong westerlies, and vice versa). Together with the El Niño/Southern Oscillation (ENSO) phenomenon, the NAO is a major source of seasonal to interdecadal variability in the global atmosphere. On interannual and shorter time scales, the NAO dynamics can be explained as a purely internal mode of variability of the atmospheric circulation. Interdecadal variability maybe influenced, however, by ocean and sea-ice processes.