Dominant patterns of winter Arctic surface wind variability     

WU Bingyi  John Walsh  LIU Jiping  ZHANG Xiangdong 《极地研究(英文版)》2014,(4)

Dominant statistical patterns of winter Arctic surface wind(WASW) variability and their impacts on Arctic sea ice motion are investigated using the complex vector empirical orthogonal function(CVEOF) method. The results indicate that the leading CVEOF of Arctic surface wind variability, which accounts for 33% of the covariance, is characterized by two different and alternating spatial patterns(WASWP1 and WASWP2). Both WASWP1 and WASWP2 show strong interannual and decadal variations, superposed on their declining trends over past decades. Atmospheric circulation anomalies associated with WASWP1 and WASWP2 exhibit, respectively, equivalent barotropic and some baroclinic characteristics, differing from the Arctic dipole anomaly and the seesaw structure anomaly between the Barents Sea and the Beaufort Sea. On decadal time scales, the decline trend of WASWP2 can be attributed to persistent warming of sea surface temperature in the Greenland—Barents—Kara seas from autumn to winter, reflecting the effect of the Arctic warming. The second CVEOF, which accounts for 18% of the covariance, also contains two different spatial patterns(WASWP3 and WASWP4). Their time evolutions are significantly correlated with the North Atlantic Oscillation(NAO) index and the central Arctic Pattern, respectively, measured by the leading EOF of winter sea level pressure(SLP) north of 70°N. Thus, winter anomalous surface wind pattern associated with the NAO is not the most important surface wind pattern. WASWP3 and WASWP4 primarily reflect natural variability of winter surface wind and neither exhibits an apparent trend that differs from WASWP1 or WASWP2. These dominant surface wind patterns strongly influence Arctic sea ice motion and sea ice exchange between the western and eastern Arctic. Furthermore, the Fram Strait sea ice volume flux is only significantly correlated with WASWP3. The results demonstrate that surface and geostrophic winds are not interchangeable in terms of describing wind field variability over the Arctic Ocean. The results have important implications for understanding and investigating Arctic sea ice variations: Dominant patterns of Arctic surface wind variability, rather than simply whether there are the Arctic dipole anomaly and the Arctic Oscillation(or NAO), effectively affect the spatial distribution of Arctic sea ice anomalies.  相似文献   

How does the Indian Ocean subtropical dipole trigger the tropical Indian Ocean dipole via the Mascarene high？   总被引：1，自引：0，他引：1  

FENG Junqiao  HU Dunxin  YU Lejiang 《海洋学报(英文版)》2014,33(1):64-76

The variation in the Indian Ocean is investigated using Hadley center sea surface temperature(SST)data during the period 1958–2010.All the first empirical orthogonal function(EOF)modes of the SST anomalies(SSTA)in different domains represent the basin-wide warming and are closely related to the Pacific El Ni o–Southern Oscillation(ENSO)phenomenon.Further examination suggests that the impact of ENSO on the tropical Indian Ocean is stronger than that on the southern Indian Ocean.The second EOF modes in different domains show different features.It shows a clear east-west SSTA dipole pattern in the tropical Indian Ocean(Indian Ocean dipole,IOD),and a southwest-northeast SSTA dipole in the southern Indian Ocean(Indian Ocean subtropical dipole,IOSD).It is further revealed that the IOSD is also the main structure of the second EOF mode on the whole basin-scale,in which the IOD pattern does not appear.A correlation analysis indicates that an IOSD event observed during the austral summer is highly correlated to the IOD event peaking about 9 months later.One of the possible physical mechanisms underlying this highly significant statistical relationship is proposed.The IOSD and the IOD can occur in sequence with the help of the Mascarene high.The SSTA in the southwestern Indian Ocean persists for several seasons after the mature phase of the IOSD event,likely due to the positive wind–evaporation–SST feedback mechanism.The Mascarene high will be weakened or intensified by this SSTA,which can affect the atmosphere in the tropical region by teleconnection.The pressure gradient between the Mascarene high and the monsoon trough in the tropical Indian Ocean increases(decreases).Hence,an anticyclone(cyclone)circulation appears over the Arabian Sea-India continent.The easterly or westerly anomalies appear in the equatorial Indian Ocean,inducing the onset stage of the IOD.This study shows that the SSTA associated with the IOSD can lead to the onset of IOD with the aid of atmosphere circulation and also explains why some IOD events in the tropical tend to be followed by IOSD in the southern Indian Ocean.  相似文献   

Effective Sensitivity of A Non-uniform Phased Array of Short Dipoles     

W. A. Van Cappellen  J. D. Bregman  M. J. Arts 《Experimental Astronomy》2004,17(1-3):101-109

Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band.  相似文献   

Photon escape probabilities in expanding atmospheres     

A. Peraiah  K. E. Rangarajan  D. Mohan Rao 《Journal of Astrophysics and Astronomy》1981,2(1):81-86

A comparison of mean number of scatterings and escape probabilities has been made in isotropic scattering and dipole scattering by using the angle-averaged partial frequency redistribution functionR _I. We have solved the equations of radiative transfer and statistical equilibrium simultaneously in a spherically symmetric expanding atmosphere. Two cases of atmospheric extension (i.e.)B/A=3 and 10 (whereB andA are the outer and inner radii of the atmosphere) have been treated. We find that the partial frequency redistribution gives a larger mean number of scatterings compared to that given by complete redistribution. Velocities tend to reduce the mean number of scatterings and in crease the mean escape probabilities.  相似文献   

春季热带南大西洋海表温度异常与夏季亚澳季风区降水异常的联系          下载免费PDF全文

陈晨  李忠贤  陈兵  于怡秋 《大气科学学报》2021,44(6):918-926

利用1979—2019年Hadley中心的海表温度资料、GPCP的降水资料以及NCEP-DOE的再分析资料等,分析了北半球春季热带南大西洋海表温度异常与北半球夏季亚澳季风区降水异常的联系。研究表明,北半球春季热带南大西洋海表温度异常与随后夏季热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区的降水异常为显著负相关（正相关）关系。北半球春季热带南大西洋的海表温度正异常可以引起热带大西洋和热带太平洋间的异常垂直环流,其中异常上升支（下沉支）位于热带大西洋（热带中太平洋）。热带中太平洋的异常下沉气流和低层辐散气流引起热带中西太平洋低层的异常东风,后者有利于热带中东太平洋海表温度出现负异常。通过Bjerknes正反馈机制,热带中东太平洋海表温度异常从北半球春季到夏季得到发展。热带中东太平洋海表温度负异常激发的Rossby波使得北半球夏季热带西太平洋低层出现一对异常反气旋。此时,850 hPa上热带西太平洋到海洋性大陆地区为显著的异常东风,有利于热带西太平洋到南海（澳大利亚东侧海域到热带东印度洋）地区出现异常的水汽辐散（辐合）,导致该地区降水减少（增加）。  相似文献   

有限导电内核的驱动方式对发电机模型的影响     

丁嘉威  王正涛  詹文臻  刘美琴 《地球物理学报》2021,64(4):1320-1327

本文以两种绝缘内核的发电机为基准,设置内核电导率与外核相同,选择了以固定速度超速旋转和在外核驱动下发生旋转的两种内核旋转模式,比较分析不同模型间的能量差异、磁场强度、磁雷诺数、磁极翻转频率和四个类地发电机参数.结果表明:对于弱偶极子发电机模型,有限导电内核的引入会对其偶极子强度的相对变化量造成较大影响,最高达103.00%;由外核驱动旋转的有限导电内核模型对于本文其他目标研究量所带来的影响比较小,均小于5%;而固定旋转速度的有限导电内核的模型对磁极翻转频率、赤道对称性和纬向性均存在较明显影响,最大变化量达124.62%.综合本文所选用的发电机模型的特征和数值分析结果,可以发现虽然由外核驱动旋转的有限导电内核模型其转速不可控且存在较大波动,但各物理量变化量与实际内核与外核的能量比更为接近,因此可以推断其驱动机制较自驱动模式更为合理可靠.  相似文献   

Towards the interpretation of Uranus's eccentric magnetic field     

R. Hide 《地球物理与天体物理流体动力学》2013,107(1-4):207-209

Abstract

Coriolis forces stimulate dynamo action in a rapidly-rotating fluid by promoting complexities in the pattern of fluid motions, notably departures from symmetry about the axis of rotation. This pattern and its time variations determine the instantaneous form and temporal behaviour of the magnetic field so produced. Instantaneous magnetic fields will usually exhibit in their broad-scale features approximate alignment with the rotation axis. This is borne out by observations of the magnetic fields of the Earth, Jupiter and Saturn, and it is likely on general grounds that Neptune will be found to have an aligned magnetic field. But, as is shown by laboratory and theoretical studies of thermal convection in rapidly-rotating fluids, for some ranges of rotation speed, rate of heating, etc. certain patterns can occur which in electrically-conducting fluids would produce magnetic fields exhibiting departures from alignment with the rotation axis, which instantaneously could be quite pronounced but would average out to very small values over sufficiently long periods of time. These findings indicate obvious strategies for theoretical studies towards the interpretation of Uranus's eccentric magnetic field (which need not invoke departures from axial symmetry in the thermal, mechanical or electrical boundary conditions of the dynamo region within the planet) and for further observational studies.  相似文献   

双频激电偶极在甘肃李子地区快速找矿中的应用     

杨立功 《地质与勘探》2013,49(2):330-336

论述了在植被发育和地形恶劣的李子地区利用双频激电轴向偶极装置快速寻找金矿的应用成果,总结了岩矿石电性参数特征和工作方案及工作装置。认为双频激电法仪及其工作时使用的轴向偶极装置在北秦岭金银多金属成矿带天水段高山林区寻找金属硫化物金矿床具有轻便、快速、成本低、效率高、效果好的优点。结合扫面异常规模,多极距剖面测量数据的有限元最优化法幅频率数字反演,可有效的获得激电异常的半定量解释。异常查证结果表明,激电异常体与金矿体之间有很好的对应关系,文中轻便激电的工作方案在地面工作条件困难的地区值得推广。  相似文献   

磁偶极子模型等值线图剖面截取的Matlab仿真     

李玉祥  刘卿  赵文辉  孙伟民 《物探化探计算技术》2013,35(5)

在航空磁测目标识别过程中,磁性物体可以用磁偶板子模型来表示,应用Matlab软件中二维双线性插值的方法对磁性目标磁偶极子模型空间磁场的剖面截取进行了仿真,得到了磁测剖面曲线和平面等值线图两种仿真结果.这里提出的平面等值线图剖面截取方法,能够提取出任意方向的磁测曲线,此方法计算简单、操作性好,可应用在各种测量数据等值线图的剖面提取中.  相似文献   

Electromagnetic fields from a horizontal electrical dipole buried in ocean     

JIA Dingyu  WENG Aihua  LIU Yunhe  YIN Changchun 《世界地质(英文版)》2012,15(2):176-181

Marine controlled source electromagnetic signal could be used in mineral resource exploration,reservoir appraisal and communicative technique in ocean. It’s necessary to study the electromagnetic generated by MCSEM. The propagation of the electromagnetic fields from a controlled source in the marine environment was studied with virtual interface method combined with discrete complex image method. Transmitter of finite length current source is approximated by dipole (HED) . A three-layered model is accepted,with sea water as intermediate conductive layer under air and a relatively high resistive seabed as basement,possibly containing a hydrogen layer of higher resistivity. The electromagnetic fields in whole space thus computed show that: (1) the spatial distribution of field component depends on its type; (2) inline Ex component is more sensitive to reservoir layer than that in broadside; (3) The airwave affects marine electromagnetic (MEM) exploration when sea water is relatively shallow; in the case of deep water MEM exploration,the airwave influence could be neglected; and (4) an appropriate frequency should be selected in order to balance the signal strength and electromagnetic induction effect.  相似文献