Some Effects of Rotation Rate on Planetary-Scale Wave Flows

A series of experiments were performed in a rotating annulus of fluid to study effects of rotation rate on planeta-ry-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth’s atmosphere, although magnitude of variation in earth’s rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number deter-mining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with the variation in earth’s rotation rale. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.     

Some effects of rotation rate on planetary — scale wave flows

Summary A series of experiments was performed in a rotating annulus of fluid to study effects of rotation rate on planetary-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary Waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth's atmosphere, although magnitude of variation in earth's rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number determining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with variation in earth's rotation rate. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.With 12 Figures     

夏季东亚高空副热带西风急流季节内异常的环流特征及前兆信号

位于东亚中纬度上空的东亚高空副热带西风急流是东亚季风环流系统中的重要成员,我国夏季降水雨带的季节内变化受东亚高空副热带西风急流位置季节内异常变化影响。根据1979~2008年中国降水资料、NCEP/NCAR再分析资料以及NOAA ERSST V3月平均海表温度资料,利用统计分析和物理量诊断方法对夏季东亚高空副热带西风急流位置季节内异常的东亚大气环流特征及外强迫信号的物理过程进行了探讨。研究指出:6月东亚高空副热带西风急流位置异常主要受欧亚大陆中高纬东传的Rossby波列位相变化影响,春季北大西洋海温异常是欧亚大陆中高纬度Rossby波列位相变化的最显著的外强迫信号;7月东亚高空副热带西风急流位置异常主要受西太平洋热带向副热带传播的Rossby波列位相变化影响,春季西太平洋热带海温异常是西太平洋热带向副热带传播的Rossby波列位相变化的最显著的外强迫信号;8月东亚高空副热带西风急流位置异常主要受南亚大陆向东亚大陆热带、副热带传播的Rossby波列位相变化影响,春季印度洋海温异常是南亚大陆向东亚大陆热带、副热带传播的Rossby波列位相变化的最显著的外强迫信号。     

理想海域中台风引起的潮、流及波的分析 Ⅱ.海岸及陆架的影响

采用理想的大陆架地形和台风模型计算了不同方向登陆的台风所激发的海洋响应。结果表明,岸边的潮位变化主要是由于台风引起的强迫振动造成的。而对于登陆型台风来说,在远离台风路径的地方,潮位的变化则是由于边缘波效应。对地平直海岸和二维大陆架,自由边缘波的振幅远小于强迫波的振幅。平行海岸移行台风在岸边产生随台风一起移动的强制波,其中当台风沿着与Kelvin波相同的方向移行时,岸边有陆架波产生,反之则没有陆架波。此外,还讨论了与风暴潮相关的近岸环流。     

地形强迫对大气长波调整的可能影响

地形作为大气的外部强迫,其动力和热力作用对波动结构演变及极端天气出现都有不能忽视的作用。本文通过数值求解考虑地形强迫的β平面正压准地转位势涡度方程,探讨了地形强迫作用对大气长波调整的可能影响,结果表明:同非线性作用和纬向非均匀基流作用一样,无基流情形下具有纬向差异的地形分布影响了大气长波结构的演变,也能强迫出大气长波调整现象。大气长波调整依赖于地形的高度和地形分布,地形越高,长波越容易出现波数的调整;地形波数越大,即地形结构复杂,越不易出现波数变化。大气长波调整还与纬度有关,纬度越高,β越小,地形强迫作用越突出,长波调整容易出现;反之,低纬度以β效应为主的线性波动不易出现波数调整。大气长波调整对波动初始波动的振幅不敏感,但依赖于波动的初始结构。此外,有基流作用时,地形强迫还是诱发定常波的重要因素,且定常波流场结构依赖于地形高度分布,与波动初始结构无关。     

Rossby wave propagation in a barotropic atmosphere

The ideas of ray tracing from geometrical optics and wave propagation in a slowly varying medium are applied to Rossby waves propagating in a barotropic atmosphere.The propagation of low-frequency Rossby waves in a zonally symmetric basic state is compared with that for stationary waves presented by Hoskins and Karoly (1981). These ideas are then used to study the propagation of Rossby waves in a basic state with zonally varying middle latitude or low latitude jets. Conditions which allow cross-equatorial wave propagation are presented. For a zonally varying middle latitude jet, there is weak wave convergence in regions of decreasing jet speed, However, this is not sufficient to explain the enhanced wave amplitude found in numerical-model experiments using a zonally varying basic state.     

Resonance of baroclinic waves in the tropical oceans: The Indian Ocean and the far western Pacific

The Indian Ocean has a particularity, its width is close to half the wavelength of a Rossby wave of biannual frequency, this coincidence having been capitalized on by several authors to give the observations a physical basis. The purpose of this article is to show that this is not the case since the resonance of tropical baroclinic waves occurs in all three oceans. This is because the westward-propagating Rossby wave is retroflexed at the western boundary to form off-equatorial Rossby waves dragged by countercurrents before receding and turning back as a Kelvin wave. Thus a quasi-stationary baroclinic wave is formed, whose mean period is tuned to the forcing period. Two independent basin modes resonantly forced are highlighted – 1) a nearly symmetric zonal 1/2-yr period Quasi-Stationary Wave (QSW) that is resonantly forced by the biannual monsoon. It is formed from first baroclinic mode equatorial-trapped Rossby and Kelvin waves and off-equatorial Rossby waves at the western antinode. This QSW controls the Equatorial Counter Current at the node. The Indian Ocean Dipole (IOD) results from a subharmonic mode locking resulting from the coupling of this QSW and the 2nd, 3rd and 4th baroclinic modes - 2) a 1-yr period QSW formed from an off-equatorial baroclinic Rossby wave, which is induced from the southernmost current of the Indonesian Throughflow through the Timor passage, propagating in the southern and northern hemispheres: the drivers are south-easterlies in the southern hemisphere and monsoon wind in the northern hemisphere.     

Eastward propagating MJO during boreal summer and Indian monsoon droughts

Improved understanding of underlying mechanism responsible for Indian summer monsoon (ISM) droughts is important due to their profound socio-economic impact over the region. While some droughts are associated with ‘external forcing’ such as the El-Niño and Southern Oscillation (ENSO), many ISM droughts are not related to any known ‘external forcing’. Here, we unravel a fundamental dynamic process responsible for droughts arising not only from external forcing but also those associated with internal dynamics. We show that most ISM droughts are associated with at least one very long break (VLB; breaks with duration of more than 10 days) and that the processes responsible for VLBs may also be the mechanism responsible for ISM droughts. Our analysis also reveals that all extended monsoon breaks (whether co-occurred with El-Niño or not) are associated with an eastward propagating Madden–Julian Oscillation (MJO) in the equatorial Indian Ocean and western Pacific extending to the dateline and westward propagating Rossby waves between 10° and 25°N. The divergent Rossby wave associated with the dry phase of equatorial convection propagates westward towards Indian land, couple with the northward propagating dry phase and leads to the sustenance of breaks. Thus, the propensity of eastward propagating MJO during boreal summer is largely the cause of monsoon droughts. While short breaks are not accompanied by westerly wind events (WWE) over equatorial western Pacific favorable for initiating air–sea interaction, all VLBs are accompanied by sustained WWE. The WWEs associated with all VLB during 1975–2005 initiate air–sea interaction on intraseasonal time scale, extend the warm pool eastward allowing the convectively coupled MJO to propagate further eastward and thereby sustaining the divergent circulation over India and the monsoon break. The ocean–atmosphere coupling on interannual time scale (such as El-Niño) can also produce VLB, but not necessary.     

Nonlinear waves in barotropic model

In this paper, from the system of equation describing a barotropic atmosphere using the method of Taylor expansion for the nonlinear terms, the periodic solutions of the nonlinear inertio-surface gravity waves and Rossby waves have been obtained.The finite-amplitude nonlinear inertio-surface gravity waves and Rossby waves with horizontal divergence satisfy all the KdV equation. The solutions are all the cnoidal function, i, e, the cnoidal waves which in-clude the linear waves and form the solitary waves under certain conditions. For the finite-amplitude Rossby waves with horizontal divergence, we find the new dispersive relation including both the wave number and the amplitude parameter. In case of small amplitude it is reduced to the Yeh formula. It is shown that the larger the amplitude and width, the faster the finite-amplitude inertio-surface gravity waves and the slower the finite-amplitude Rossby waves with horizontal divergence propagate. The blocking or cut-off system in which the amplitude and width are large may be considered as Rossby solitary waves.     

El Nio事件发生和消亡中热带太平洋纬向风应力的动力作用 Ⅱ.模式结果分析

为了分析 ＥＩ Ｎｉｏ事件发生和消亡中热带太平洋纬向风应力的动力作用，建立一个类似于Ｚｅｂｉａｋ的简单热带海洋数值模式，在观测到的风应力异常的强迫下，模拟赤道太平洋地区 １９７１年１月至 １９９８年８月海表温度异常的变化。结果表明，模式对观测的Ｎｉｏ３区海表温度异常（ＳＳＴＡ）有很好的模拟能力。模拟和观测Ｎｉ区ＳＳＴＡ之间的相关系数可达 ０．９０。模式对 Ｅｌ Ｎｉｏ事件期间赤道太平洋海表温度异常随时间变化也有较好的模拟能力。为了分析Ｅｌ Ｎｉｏ期间ＳＳＴＡ的空间分布及其随时间变化的动力学机制，还对１９８６～１９８９年 ＥＮＳＯ循环期间赤道太平洋地区观测的 ＳＳＴＡ的传播特征及其形成机制进行了分析。模式较好地模拟出了观测到的赤道太平洋地区ＳＳＴＡ的传播特征，即从１９８６年底至１９８７年 ４月， ＳＳＴＡ具有向东传播的特征，从 １９８７年 ６月至 １９８８年 ２月具有向西传播的特征。动力学分析的结果表明，赤道中西太平洋地区的缔向风应力异常对 Ｅｌ Ｎｉｏ事件的发生和消亡具有重要作用。赤道中西太平洋地区的西风异常可强迫出东传的Ｋｅｌｖｉｎ波，这个东传的 Ｋｅｌｖｉｎ波对正 ＳＳＴＡ的东传起主要作用，当这个东传的 Ｋｅｌｖ     

On the influences of large-scale inhomogeneity of sea temperature upon the oceanic waves in the tropical regions—part II: Linear numerical experiments

By using a linear oceanic mixed layer model, the long period waves in the tropical ocean are investi-gated numerically. Due to the inhomogeneity of the large-scale average sea temperature field of the ocean in tropical regions, besides the westward propagating equatorial Rossby wave to be modified, there will be a kind of long period thermal wave which propagates eastward under certain oceanic background conditions. Under the influences of these two kinds of waves, the propagating and evolving processes of the sea surface temperature anomalies (SSTA) are dearly shown by numerical experiments. The results of numerical ex-periments are consistent with the ones obtained by the theoretical analysis in Part I. The possible relation-ship between these two kinds of waves and El Nino events is also discussed indirectly.     

El Nino事件发生和消亡中热带太平洋纬向风应力的动力作用 II.模式结果分析

为了分析ElNio事件发生和消亡中热带太平洋纬向风应力的动力作用，建立一个类似于Zebiak的简单热带海洋数值模式，在观测到的风应力异常的强迫下，模拟赤道太平洋地区1971年1月至1998年8月海表温度异常的变化。结果表明，模式对观测的Nio3区海表温度异常(SSTA)有很好的模拟能力。模拟和观测Nio3区SSTA之间的相关系数可达0.90。模式对ElNio事件期间赤道太平洋海表温度异常随时间变化也有较好的模拟能力。为了分析ElNio期间SSTA的空间分布及其随时间变化的动力学机制，还对19861989年ENSO循环期间赤道太平洋地区观测的SSTA的传播特征及其形成机制进行了分析。模式较好地模拟出了观测到的赤道太平洋地区SSTA的传播特征，即从1986年底至1987年4月，SSTA具有向东传播的特征，从1987年6月至1988年2月具有向西传播的特征。动力学分析的结果表明，赤道中西太平洋地区的纬向风应力异常对ElNio事件的发生和消亡具有重要作用。赤道中西太平洋地区的西风异常可强迫出东传的Kelvin波，这个东传的Kelvin波对正SSTA的东传起主要作用，当这个东传的Kelvin波到达东边界，由于东边界的反射作用，在东边界产生西传的Rossby波，这个西传的Rossby波对赤道中东太平洋地区正SSTA的西传起主要作用。东传Kelvin波和反射的Rossby波对ElNio期间赤道东太平洋正SSTA二次峰值的形成具有重要作用。     

Low‐frequency motions on the Scotian shelf and slope

An examination of current‐meter data gathered in 1967/68 on the continental shelf and slope off Nova Scotia has shown that meteorological forcing is an important source of energy. The response of currents to wind forcing is concentrated in a frequency band of 2.5 to 7 days. Daily mean currents of up to 25 cm/s appear to be associated with wind‐stress events. The highest correlations between wind and current are for the alongshore components of these variables. Wind‐induced currents may have been responsible for an intrusion of slope water onto the shelf which was observed in hydrographic sections from October and December 1968. Long data series (up to 167 days) formed by patching together shorter records demonstrate the existence of distinct low‐frequency variability at periods greater than 10 days. Some aspects of these motions suggest the presence of topographic Rossby waves on the shelf and slope. However, spatial and temporal coverage of data are not sufficient to define the sources of this variability.     

青藏高原热力作用下的非绝热Rossby波

从含非绝热项的准地转运动方程组出发，分析了青藏高原大尺度热力作用下非绝热Rossby波的一些性质，从理论上证明当背景西风气流为正压时，冬季高原冷却作用有利于Rossby波的经向传播，夏季高原大尺度热力作用不利于波动的经向传播。非绝热Rossby波的频率方程说明冬季高原的热力作用是中纬季节内振荡的重要激发机制。同时，在背景西风气流为纯斜压条件下，求解了高原热力作用下非绝热Rossby波的频率，并由频率方程说明冬季高原热力作用有利于波动向不稳定方向发展，而夏季高原的大尺度热力作用对波动稳定性的影响存在临界值。     

Variations in Wave Energy and Amplitudes along the Energy Dispersion Paths of Nonstationary Barotropic Rossby Waves

The variations in the wave energy and the amplitude along the energy dispersion paths of the barotropic Rossby waves in zonally symmetric basic flow are studied by solving the wave energy equation,which expresses that the wave energy variability is determined by the divergence of the group velocity and the energy budget from the basic flow.The results suggest that both the wave energy and the amplitude of a leading wave increase significantly in the propagating region that is located south of the jet axis and enclosed by a southern critical line and a northern turning latitude.The leading wave gains the barotropic energy from the basic flow by eddy activities.The amplitude continuously climbs up a peak at the turning latitude due to increasing wave energy and enlarging horizontal scale(shrinking total wavenumber).Both the wave energy and the amplitude eventually decrease when the trailing wave continuously approaches southward to the critical line.The trailing wave decays and its energy is continuously absorbed by the basic flow.Furthermore,both the wave energy and the amplitude oscillate with a limited range in the propagating region that is located near the jet axis and enclosed by two turning latitudes.Both the leading and trailing waves neither develop nor decay significantly.The jet works as a waveguide to allow the waves to propagate a long distance.     

QBO-like Oscillations Induced by Local Thermal Forcing

A zonal-vertical two-dimensional equatorial model is used to study the possibility that the long period oscillation of the zonal mean flow occurring in the lower equatorial stratosphere (QBO) is caused by local thermal ac-tivities at the tropical tropopause. The model successfully reproduces QBO-like oscillations of the zonal mean flow, suggesting that the local heating or cooling at the tropical tropopause is probably the main reason of QBO’s genera-tion. The analysis of the dependence of the oscillation on the wave fencing indicates that the oscillation is not sensible to the forcing scale. The model can reproduce QBO-like oscillations with any forcing scale if the fencing period and amplitude take appropriate values, proving that the internal gravity waves generated by local thermal source take much important roles in QBO.     

Unstable tropical air-sea interaction waves and their physical mechanisms

In this paper, the tropical air-sea interaction is discussed by using a simple air-sea coupled model, in which the inertia-gravity waves are filtered off and only the equatorial Rossby waves are reserved in both the atmosphere and the ocean. There exist two kinds of air-sea interaction waves in the coupled model, that is, the high-frequency fast waves and the low-frequency slow waves. The phase speed of the fast waves is westward and the frequencies are close to those of the equatorial Rossby waves in the atmosphere. The slow waves propagate westward in the part of short wavelengths and eastward in that of long wavelengths. There exist instabilities for both the westward and eastward propagating slow waves. If the fast waves are filtered off, there is little effect on the slow waves which have great in-fluence on the long range process in the tropical air-sea coupled system. According to the tropical air-sea interaction waves we obtain here, a possible explanation to the propagating process of ENSO events is given.     

INTERACTION OF TOPOGRAPHY WITH SOLITARY ROSSBY WAVES IN A NEAR-RESONANT FLOW

An inhomogeneous KdV equation including topographic forcing is derived by usingperturbation expansions and stretching transforms of time and space.The generation of forcedsolitary Rossby waves by topography in a near-resonant flow and their interactions with freesolitary waves are discussed,and some interesting results are obtained.The numerical resultsshow that the topography has obvious effect on enhancing the amplitude of disturbances,and itmay explain to some degree the formation of blocking by localized topography.     

The response of currents in Trinity Bay,Newfoundland, to local wind forcing

Abstract

Analysis of current measurements taken between June and October 1984 at four moorings in Trinity Bay, Newfoundland, is discussed. The alongshore component of current exhibits baroclinic fluctuations coherent with the along‐bay component of wind stress at periods between 3 and 7 days. A two‐layer model of internal Kelvin waves propagating around the perimeter of an elongated bay and forced by a spatially uniform wind stress is presented. The observations support several features of the model response to wind forcing. Along the side of the bay on which Kelvin waves are incoming, the amplitude of the response increases into the bay and decreases with increasing frequency. Along the outgoing side of the bay the amplitude of the response generally shows a maximum at a frequency between 0.2 and 0.3 cpd. The phase lag between current and wind is consistent with a forced response. An example is given of upwelling and downwelling on opposite sides of the bay in agreement with the model behaviour.     

线性准地转模型中副热带环流对潜热加热的定常响应I.基本性质及特征分析

利用线性理论模型进行解析和模拟是大气科学的重要基础研究方法之一, 其简单明了, 解释了如Rossby波形成等重要的环流现象。近年, 有研究 (Chen et al., 2001) 运用定常线性准地转模型研究副热带高压的形成, 在相似加热强迫下, 其结果与理论研究和GCM的研究不同。本文运用该模型系统研究了由季风降水产生的潜热加热所激发的副热带定常波的结构特征, 以理解其结果与GCM不一致的原因。研究表明基本流对热强迫的定常波的结构有重要的影响: (1) 当基本流为东风或为零时, 定常波在垂直方向上表现为第一斜压结构, 波动振幅随东风的增大而减小; (2) 当基本流为西风时, 呈现向上的传播特征, 振幅随高度的升高而增大。而且存在一个临界风速Uc(约3 m/s), 此时, 波动振幅最强; 基本流的平流作用具有非对称性, 西风时平流作用远远强于东风。由此揭示了该线性模型解的本质: 加热在热源区所激发的经向地转涡度输送必须能够平衡加热的位涡制造和纬向位涡平流; 基本流的经向和垂直方向的变化使得气旋和反气旋中心偏向热源中心北侧, 并进一步发现当基本流为夏季亚洲季风区纬向平均的纬向风场时, 线性模型的解中位于中低层的反气旋中心显著抬升, 而不出现在地面上, 与完整的大气环流模式的结果接近。说明即使在线性模式中, 洋面副高的形成也不能完全用季风潜热加热来解释; 另外, 静力稳定度对热强迫的副热带环流的影响也很重要, 使热源的强迫作用放大。