北太平洋副热带逆流区长Rossby波动力特性

利用2.5层海洋模式分析了北太平洋副热带逆流(STCC)区波长在450-840 km之间的长Rossby波的动力特性.不考虑耗散时,STCC区域的长Rossby波是中性波,其快模态的周期是88-110 d,相速度为0.06-0.09 m/s,群速度几乎与相速度相同.弱耗散作用使长Rossby波为弱衰减波,但几乎不影响其相速度与频率.观测的STCC区海平面高度(SSH)的准90d振荡对应长Rossby波的第一斜压模,"快"模态的波动振幅是"慢"模态的约5倍.准90d振荡存在的3个必要条件是:(1)第2层向西平均流取值为0.02-0.04m/s;(2)2个活动层的厚度超过480 m;(3)第2层的密度介于24.8-26.8之间.正因为北太平洋STCC区满足这3个条件,才在该海域出现SSH的准90d振荡.海面热通量的变化对快模态长Rossby波的传播特性影响不大,但对慢模态的传播特性有一定的影响.     

北太平洋海表面高度的年际变化及其机制

利用15年（1993～2007年）月平均的海表面高度（SSH）异常资料,分析了北太平洋海表面高度的年际变化的时空结构,并研究了热通量和风应力两个因子对其的强迫作用.结果表明,北太平洋年际时间尺度SSH变化的大值区在黑潮延伸区和西太平洋暖池区.EOF分解第一模态的空间结构沿纬向呈带状分布,第二模态为沿经向呈带状分布.热通量强迫作用在中纬度的东北太平洋可以解释SSH年际变化40%以上.风应力对SSH的作用包括正压和斜压两个方面.正压Sverdrup平衡模型模拟的SSH年际变化较弱,仅能解释高纬度副极地环流西部的20%~40%.由大尺度风应力强迫的第一阶斜压Rossby波模型可以解释热带地区的20%~60%,中纬度中部的20%~40%,以及阿拉斯加环流东部和副极地环流西部的20%~60%.风应力强迫的一阶斜压Rossby波模型对SSH的强迫机理又可分为局地风应力强迫和西传Rossby波作用.其中,风应力的局地强迫作用（Ekman抽吸）在东北太平洋、白令海以及热带中部有显著的预报技巧,可以解释SSH年际变异的40%以上.Rossby波的传播作用在中纬度海域的副热带环流中西部和夏威夷岛以东起着重要作用,可解释20%~60%.     

北太平洋副热带高压年际变异与ENSO 循环之间的选择性相互作用

利用NCEP/NCAR大气再分析资料以及Hadley中心海表温度资料,针对北太平洋副热带高压(简称副高)的完整系统,通过分析超前于ENSO事件的海平面副高年际异常特征及其对ENSO事件的触发作用以及ENSO事件对500 hPa副高和海平面副高的滞后影响,结果表明了北太平洋副热带高压年际变异和ENSO循环之间存在选择性相互作用.即在大多数情况下,一方面,前期海平面副高减弱会导致热带西太平洋表面西风异常,通过海洋平流过程触发El Nino事件在夏季发生发展,在秋冬季成熟; 而另一方面,El Nino事件在秋冬季发展成熟后,增强了赤道中太平洋的对流性热源,通过对异常热源的动力响应,同期和次年夏季500 hPa副高增强,又通过增强的Hadley环流作用,副热带地区下沉运动增强,从而使得次年夏季海平面副高增强,增强的海平面副高又有利于触发下一个La Nina事件.副高年际变异和ENSO循环之间相互作用的选择性主要取决于副高异常是否接近于赤道以及ENSO事件本身的持续性.这种相互作用有利于在热带太平洋海气系统产生准两年振荡.     

FGOALS－g快速耦合模式模拟的北太平洋年代际变率

本文分析了由中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室（LASG/IAP）最新发展的FGOALS_－g快速耦合模式300 a积分模拟结果,通过与多种观测资料的对比分析,讨论了北太平洋年代际变率的时空结构、主要年代际模态的演变特征以及与ENSO的联系等研究内容. 结果表明：该模式能成功模拟出北太平洋年代际变率的主要空间分布特征;模拟的年代际模态具有多时间尺度性,其中最显著的是周期约为10～20 a左右的准20年振荡模态,该模态上层海洋热容量异常的演变过程主要表现为大致沿副热带海洋涡旋做海盆尺度顺时针旋转的特征,相应的大气异常不仅与阿留申低压的变异有关,而且与太平洋-北美PNA）遥相关型以及上游的欧亚大气环流异常有密切关系;模拟的北太平洋年代际变率对年际ENSO循环的发生频率和强度有明显的调制作用. 但模拟的KOE区和阿拉斯加湾SST异常振幅比观测偏强,这与模式海冰偏多、高纬度SST偏冷的误差有关.     

FGOALS－g快速耦合模式模拟的北太平洋年代际变率

本文分析了由中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室（LASG/IAP）最新发展的FGOALS_－g快速耦合模式300 a积分模拟结果,通过与多种观测资料的对比分析,讨论了北太平洋年代际变率的时空结构、主要年代际模态的演变特征以及与ENSO的联系等研究内容. 结果表明：该模式能成功模拟出北太平洋年代际变率的主要空间分布特征;模拟的年代际模态具有多时间尺度性,其中最显著的是周期约为10～20 a左右的准20年振荡模态,该模态上层海洋热容量异常的演变过程主要表现为大致沿副热带海洋涡旋做海盆尺度顺时针旋转的特征,相应的大气异常不仅与阿留申低压的变异有关,而且与太平洋-北美PNA）遥相关型以及上游的欧亚大气环流异常有密切关系;模拟的北太平洋年代际变率对年际ENSO循环的发生频率和强度有明显的调制作用. 但模拟的KOE区和阿拉斯加湾SST异常振幅比观测偏强,这与模式海冰偏多、高纬度SST偏冷的误差有关.     

风生边界急流稳定性的渐近理论

观测表明，当冬季盛吹北风时，在南中国海西边界附近将形成一支向南的急流，在一定条件下这支急流可弯曲成波动甚至形成涡旋.本文应用等值浅水模式，采用截断模方法，分析了急流的稳定性，并给出急流上不稳定波出现的条件.分析表明只有当向南的风生急流很强很窄时，由变性的Kelvin波和风应力强迫出的地形Rossby波在长波波段耦合而出现不稳定，不稳定波在波长约200 km时向北传播的相速度约为02 m·s-1，波振幅增长到e倍所需的时间约15天.分析进一步表明，夏季向北的风生流在海洋的西边界附近是稳定的.这些结果在一定程度上解释了观测结果.     

切变基本纬向流中非线性赤道Rossby长波

为了解决观测和理论研究中的一些问题以及更好地了解热带大气动力学 ,有必要进一步研究基本气流的变化对大气中赤道Rossby波动的影响 .本文研究分析基本气流对赤道Rossby长波的影响 ,利用一个简单赤道 β平面浅水模式和摄动法 ,研究纬向基本气流切变中非线性赤道Rossby波 ,推导出在切变基本纬向流中赤道Rossby长波振幅演变所满足的非线性KdV方程并得到其孤立波解 .分析表明 ,孤立波存在的必要条件是基本气流有切变 ,而且基流切变不能太强 ,否则将产生正压不稳定 .     

厄尔尼诺/南方涛动与赤道远西太平洋准两年周期振荡之间相互作用的概念模式

建立了一个反映厄尔尼诺/南方涛动(ENSO)与热带远西太平洋准两年振荡(QBOWP)相互作用最基本物理过程的新概念模式. 在此概念模式中, QBOWP对ENSO的影响通过两种途径: (1) 沿赤道太平洋海洋Kelvin波和 (2) 大气的Walker环流; 而ENSO对QBOWP的影响则可通过大气的Walker环流异常来实现. 对该模式结果的分析诊断表明: 在ENSO与QBOWP相互作用过程中, 大气桥(Walker环流)的作用比海洋桥(沿赤道太平洋的Kelvin波)更重要; 通过QBOWP与ENSO的相互作用, 一个3~5年周期的ENSO振荡可以变为准两年振荡, 而赤道远西太平洋年际变化的主要周期也会变长; 热带太平洋大气-海洋耦合系统的多时间尺度的年际变化可以通过ENSO与QBOWP的相互作用来实现.     

海洋表面盐度与海洋环流和气候变化的关系

依据Argo的海洋盐度观测及降水、蒸发、海面高度等数据分析了全球海洋表层盐度的平均态和低频变化特征及其与海洋环流和气候变化的关系,并结合前人成果对海洋盐度的研究进行了回顾和展望.蒸发减降水在最低阶上决定了海洋表层盐度的分布,海洋环流动力过程在大部分海域调整了海洋表层盐度的空间结构,并影响了海洋表层盐度的低频变率.全球变暖的情况下,海洋表层盐度表现为盐度高的地方会变得更高、盐度低的地方会变得更低,表征为整个水循环系统的加强.在最近20年全球变暖停滞期,海表盐度变化与全球变暖的趋势不同,更多地体现了对气候系统的年代际及多年代际振荡的响应,此阶段对应着Walker环流加强;西太平洋和东南印度洋海表盐度的反向变化揭示了跨海盆尺度海洋动力过程的重要性,海洋Rossby波和印尼贯穿流的共同作用使得东南印度洋海表盐度显著降低;在东南太平洋和北大西洋更多地体现了全球变暖下的海表盐度单调升高或降低的响应.期待未来的盐度卫星能够提供更高时空分辨率的盐度数据,这将和Argo观测一起,拓展我们在中尺度动力学、区域海洋学以及气候变化等方面的认知.     

中国近海海平面季节尺度变化的时频分析

用连续小波变换方法，对中国近海验潮站资料(20～30a)和Topex/Poseidon卫星测高海平面变化资料(1992年10月～1998年9月)进行分析，给出了南海、黄海和东海海平面变化在季节尺度上的时频特征. 结果表明，黄海和东海的海平面变化较相似，存在显著的周年和准双月的振荡信号；南海具有较明显的周年信号和较弱的半年周期信号；浅海区和深海区的海平面季节尺度变化在周期性与强度上存在明显差异.     

Seasonal variations of the Hawaiian Lee Countercurrent induced by the meridional migration of the trade winds

Seasonal variations of the Hawaiian Lee Countercurrent (HLCC) are investigated using satellite observations of sea surface height and wind stress as well as eddy-resolving ocean model simulations. The HLCC is strong from summer to winter and weak in spring between the dateline and the Hawaiian Islands. In response to the seasonal migration of the northeast trade winds in the meridional direction, the wind curl dipole lee of Hawaii varies in strength, exciting westward-propagating Rossby waves. The analyses of both observations and simulations show that the propagation of Rossby waves south of the HLCC, driven by the southern pole of the wind curl dipole in the lee of the islands, contributes the most to the seasonal variations of the HLCC. Unlike the wind-driven seasonal variations, our analysis suggests that other mechanisms such as mode water intrusion or air–sea interaction may cause the interannual variations of the HLCC.     

Effect of atmospheric action center in low-frequency teleconnection mechanism: an explicit analytic study

In consideration of the characteristics of spectral average of the Rossby wave trains and the adoption of a climatic mean stream field as the basic stream field, an approximate analytical formula for the period of atmospheric low-frequency oscillation (LFO) and the group velocity is deduced from a barotropic and non-divergent linearized vorticity equation. All the action centers of atmosphere are found to be the oscillators of low frequency. The LFO propagates southward across the streamlines in the wind field with a southward component or propagates northward across the streamlines in the wind field with northward component instead of along a great circle. The switch of the propagation direction takes place near the top of ridge or the bottom of trough. The angle between the wave rays and the zonal direction can be determined. Project supported by the National Natural Science Foundation of China (Grant No. 49175241).     

The eastward subtropical countercurrent on isopycnal surface in the western North Pacific

The classic Sverdrup theory suggests that the water movement in the central subtropical gyre of North Pa-cific be slowly westward or southwestward.In the late sixties of the20th century,the existence of a peculiar eastward narrow flow between20°N and25°N in spring was theoretically predicted.It was named the Subtropical Countercurrent(STCC),although direct observational evidences were not yet sufficient to con-firm whether or not such eastward flow between20°N and25°N was a persistent…     

Excitation of Rossby waves by HF electromagnetic seismic origin emissions in the earth's mesosphere

Interaction of high-frequency seismo-electromagnetic emissions with the weakly ionized gas of the ionospheric D-layer is considered. It is shown that through the earth's ionosphere weakly damped high-frequency electron cyclotron electromagnetic waves can propagate. These new type of waves easily reach the ionospheric D-layer where they interact with the existing electrons and ions. Acting on electrons ponderomotive force is taken into account and corresponding modified Charney equation is obtained. It is shown that only nonlinear vortical structures with negative vorticity (anticyclone) can be excited. The amplitude modulation of electromagnetic waves can lead to the excitation of Rossby waves in the weakly ionized gas. The corresponding growth rate is defined. Depending on the intensity of the pumping waves generated by seismic activity different stable and unstable branches of oscillations are found. Detection of the new oscillation branches and energetically reinforcing Rossby solitary vortical anticyclone structures may be serve as precursors to earthquake.     

Targeted observation analysis of a Northwestern Tropical Pacific Ocean mooring array using an ensemble-based method

An important supplement for ocean observing systems, the Northwestern Tropical Pacific Ocean (NWTPO) mooring array including 15 moorings equipped with Acoustic Doppler Current Profilers (ADCP) devices was developed by the Chinese Academy of Sciences and deployed in 2013. This study assessed the performance of this mooring array in monitoring the intra-seasonal and low-frequency (above 91 days) variability of oceanic currents by conducting targeted observation analyses using an ensemble-based method. Key regions for monitoring intra-seasonal variability of the NWTPO circulation are the equator, Indonesian throughflow (ITF), headstream of the North Equatorial Countercurrent (NECC), and Subtropical Countercurrent (STCC). For monitoring intra-seasonal variability, the range of each mooring is confined to a local scale. Therefore, NWTPO moorings cannot adequately resolve intra-seasonal variability in areas of the ITF, the headstream of the NECC, and STCC due to location constraints of the moorings. For monitoring low-frequency variability of NWTPO circulation, the key regions are the Western Boundary Current (WBC), NECC, and the Equatorial Undercurrent (EUC). NWTPO moorings performed relatively well in monitoring the low-frequency variability, as indicated by the strong background correlations between each of the currents. The NWTPO mooring array plays an important role in monitoring the location and intensity of background currents. Because moorings are costly and require a high-density distribution for optimal performance, understanding the multi-timescale dynamical nature of the NWTPO current system is critical for the deploying future moorings in this region.     

Analysis of STCC eddies using the Okubo–Weiss parameter on model and satellite data

The North Pacific Subtropical Counter Current (STCC) is a weak zonal current comprising of a weak eastward flow near the surface (with speeds of less than 0.1 m/s and a thickness of approximately 50–100 m) and westward flow (the North Equatorial Current) beneath. Previous studies (e.g., Qiu J Phys Oceanogr 29: 2471–2486, 1999) have shown that the STCC is baroclinically unstable. Therefore, despite its weak mean speeds, nonlinear STCC eddies with diameters ~300 km or larger and rotational speeds exceeding the eddy propagation speeds develop (Samelson J Phys Oceanogr 27: 2645–2662, 1997; Chelton et al. Prog Oceanogr 91: 167–216, 2011). In this study, the authors present numerical experiments to describe and explain the instability and eddy-generation processes of the STCC and the seasonal variation. Emphasis is on finite-amplitude eddies which are analyzed based on the parameter of Okubo (Deep-Sea Res 17: 445–454, 1970) and Weiss (Physica D 48: 273–294, 1991). The temperature and salinity distribution in March and April offer the favorable condition for eddies to grow, while September and October are unfavorable seasons for the generation of eddies. STCC is maintained not only by subsurface front but also by the sea surface temperature (SST) front. The seasonal variation of the vertical shear is dominated by the seasonal surface STCC velocity. The SST front enhances the instability and lead to the faster growth of STCC eddies in winter and spring. The near-surface processes are therefore crucial for the STCC system.     

On the generation of baroclinic rossby waves in the presence of a semi-circular boundary

Abstract

An analytical model is constructed for the generation of baroclinic Rossby waves by a vorticity source in the presence of a semi-circular boundary. The vorticity source is used to represent the effect of the Agulhas retroflection to the south of Southern Africa. The displacement of the interface between the two layers of the model ocean consists of quantized waves near the coast and a train of Rossby waves drifting westward further offshore.     

Forcing of the ionosphere from above and below during the Arctic winter of 2005/2006

The present paper focuses on planetary wave type responses of the thermosphere/ionosphere system to forcing from above and below during the Arctic winter of 2005/2006. The forcing from above is described by the sunspot numbers, the solar wind speed, the B_z-component of the IMF and the geomagnetic Kp-index, while the forcing from below, i.e. by upward propagating atmospheric waves, is represented by the SABER/TIMED temperatures. The observed global ionospheric zonally symmetric oscillations with periods of ～9, ～14 and ～24–27 days were approved to be of solar origin. The most persistent ～9-day oscillation is linked to a triad of solar coronal holes distributed roughly 120° apart in solar longitude. The ～18-day westward propagating wave with zonal wavenumber 1, observed in the ionospheric currents (detected by magnetometer data), and in the F-region plasma (foF2 and TEC) could be allocated to a simultaneous 18-day westward propagating planetary wave observed in the stratosphere/mesosphere/lower thermosphere region with large (～70 km) vertical wavelength.