一类非线性海气耦合波的研究

建立了一类非线性海气耦合波的模式,并用该模式作了解析研究,讨论了该类海气耦合波的存在条件,发现当海气耦合较强时分别存在以海洋为主导方面及以大气为主导方面的两支海气耦合波,而耦合较弱时则仅有前者存在。还求得了该模式中该类海气耦合波的椭圆余弦波解及孤立波解,并对以海洋为主导的海气耦合波的性质作了讨论,认为海气相互作用耦合也是产生大气季节内振荡的机制之一。     

Coupled ocean-atmosphere models with flux correction

A method is proposed for removing the drift of coupled atmosphere-ocean models, which in the past has often hindered the application of coupled models in climate response and sensitivity experiments. The ocean-atmosphere flux fields exhibit inconsistencies when evaluated separately for the individual sub-systems in independent, uncoupled mode equilibrium climate computations. In order to balance these inconsistencies a constant ocean-atmosphere flux correction field is introduced in the boundary conditions coupling the two sub-systems together. The method ensures that the coupled model operates at the reference climate state for which the individual model subsystems were designed without affecting the dynamical response of the coupled system in climate variability experiments. The method is illustrated for a simple two component box model and an ocean general circulation model coupled to a two layer diagnostic atmospheric model.     

Unstable interannual oscillation modes in a linearized coupled ocean-atmosphere model and their associations with ENSO

Summary By using a coupled ocean-atmosphere model with an oceanic surface boundary layer, including linear atmospheric and oceanic dynamics and linearized SST prognostic equation with respect to spatially varying climatological background states, we have investigated the eigenvalue problem of the linearized coupled system in the tropical Pacific, including the characteristic periods, horizontal structures, temporal-spatial evolution and instability of the unstable interannual oscillation characteristic modes and their associations with ENSO. The main results show that the quasi-biennial (QB) oscillation was found to act as the most unstable mode in the tropical Pacific coupled air-sea system. Only the most unstable QB mode displays the ENSO-like structure and temporalspatial evolution, and its existence seems likely to have no essential dependence on the climatological annual cycle (AC). Unfortunately, from the linearized coupled system we have not derived a most unstable mode relevant to the observed principle mode with the preferred 3–4 year lower-frequency (LF) oscillation period in the real world ENSO variability. Therefore, we infer that the LF mode would likely result from certain nonlinear interaction, in which the QB mode that acts as the shortest ENSO cycle could be fundamentally important. Also, we believe that the results in present work could be helpful to fully understand the multiple time scales and the associated mechanism responsible for the real world ENSO variability.With 7 Figures     

A simple quasi-geostrophic coupled ocean-atmosphere model

The quasi-geostrophic atmospheric and oceanic equations of momentum and thermodynamics with dissipation factors are used to create a simple coupled ocean-atmosphere model describing the large-scale shallow-water mo-tion. We discuss the ocean-atmosphere coupling effect in mid-high and low latitudes separately and analyze charac-teristics of which the oscillatory periods of coupled low-frequency modes (ocean mode) vary with the coupling fre-quency and latitudinal number. This can interpret the correlation between low-frequency oscillation and ocean-at-mosphere interaction. Then from the dispersion curves of atmosphere and ocean, we reveal effect of the coupling strength on the propagation of Rossby waves. The convection mechanism between the two modes is also discussed in view of the slowly varying wave train.The results show that Newtonian cooling and Rayleigh friction play a stable rule in oceanic Rossby waves, the period of coupled low-frequency mode grows with the increment of the coupling frequency. The larger the latitudinal number is, the more rapidly it grows. When the coupling frequency tends to critical value, the oceanic Rossby waves become static. When the ocean-atmosphere coupling strength grows to some degree, the propagation of oceanic Rossby waves will become opposite to its original direction. One part of the oceanic Rossby waves is converted into atmospheric Rossby waves, the energy conversion coefficient is also solved out.     

Overturning and wind-driven circulation in a low-order ocean–atmosphere model

A low-order ocean–atmosphere model is presented which combines coupling through heat exchange at the interface and wind stress forcing. The coupling terms are derived from the boundary conditions and the forcing terms of the constituents. Both the ocean and the atmosphere model are based on Galerkin truncations of the basic fluid dynamical equations. Hence, the coupled model can readily be extended to include more physics and more detail. The model presented here is the simplest of a hierarchy of low-order ocean–atmosphere models. The behaviour of the coupled model is investigated by means of geometric singular perturbation theory and bifurcation analysis. Two ways are found in which the slow time scales can play a role in the coupled dynamics. In the first scenario, a limit cycle on the overturning time scale is created. The associated oscillatory behaviour is governed by internal ocean dynamics. In the second scenario, intermittent behaviour occurs between periodic and chaotic regimes in parameter space.     

一个简单的准地转海气耦合模式

文章采用准地转带耗散因子的大气和海洋的动量、热力学方程,建立了一个简单的描写大尺度运动的准地转海气耦台浅水模式,分别在中高纬和低纬地区讨论了海气的耦合效应,分析了耦合低频模(海洋模)的振荡周期随耦合频率和经向波数的变化转征,并由此而说明低频振荡与海气相互作用有关。然后从海洋和大气的频散曲线中揭示出耦合强度对Rossby波传播的影响,还从缓变波列的观点,讨论了两种模之间的转换机制。 结果表明:Newton冷却Rayleigh摩擦对海洋Rossby波起稳定作用。随着耦合频率的增加,耦合低频模的周期也相应增加;经向波数越大,这种增加就越迅速。当耦合频率趋近于临界值时,海洋Rossby波趋于静止。当海气耦合强度增加到一定程度时,海洋Rossby波的传播方向变成与原来相反。通过海气相互作用,海洋Rossby波的一部分将转换成大气Rossby波,本文求解了其能量转换系数。      

Modeling long-term climate changes with equilibrium asynchronous coupling

 The use of the equilibrium asynchronous coupling (EAC) scheme is proposed as a strategy to better understand long-term climate changes in a fully coupled ocean-atmosphere general circulation model. The EAC scheme requires each component model to be integrated to its equilibrium before being coupled to the other component. Use of this scheme has the distinct advantage of being able to clarify the nature of the coupling between the ocean and atmosphere, because each asynchronous iteration takes the form of a sensitivity experiment. Basic features of the EAC scheme are first studied in an energy balance model. It is found that the convergence rate of the EAC scheme is proportional to the damping rate in the atmosphere or surface ocean, but is inversely proportional to the coupling strength between the ocean and atmosphere. Furthermore, the seasonal cycle response converges much faster than the annual mean response. Using realistic parameters, the seasonal cycle response should converge in a few iterations. The EAC scheme is further applied to a coupled ocean-atmosphere general circulation model to study the tropical monsoon climate of the early Holocene. The convergence behavior of the sea surface temperature is found to agree with the theory derived from the energy balance model study. The EAC scheme is further used to investigate the role of ocean-atmosphere feedback in modifying the response of monsoons to orbital forcings in the early Holocene. It is found that the ocean exerts a positive feedback on the North African monsoon, but a negative feedback on the Indian monsoon. Received: 16 March 1998 / Accepted: 24 December 1998     

热带大气和海洋运动的频率差异在海气系统演变中的作用

对热带太平洋地区850 hPa纬向风场、海表温度场月平均距平进行了谱分析。结果表明:纬向风距平与海表温度距平的振幅在El Ni?o或La Ni?a期间同时增长。而在其他期间,波振幅随时间呈相反变化趋势。在El Ni?o或La Ni?a事件发展期间,纬向风距平的位相角与海表温度距平位相角的差值在90o左右,而在事件的衰减期间,它们的位相角的差值在0o左右。本文还利用长波近似、海洋对大气加热取局地热力平衡近似时的简单热带海气耦合模式,研究了大气变量和海洋变量位相差随时间变化对海气耦合解的影响。在海气耦合模式中,当大气模式取为非定常时,大气和海洋Kelvin波之间以及Rossby波之间存在着能量转换,使大气和海洋波振幅呈相反变化趋势。此时,耦合波振幅随位相角差的变化没有共同的增长或衰减区间。大气Kelvin波与海洋Rossby波或大气Rossby波与海洋Kelvin波相互作用时,波振幅随位相差的变化存在着相同增长和衰减区间,它们的振幅要么同时增长,要么同时减少。当大气模式取定常时,因为相互作用波之间的位相差是一常数,波振幅随时间无限制增长。本文还在大气模式取为非定常和定常两种情况下,对海气耦合模式进行了数值求解。结果表明,当大气模式取为非定常时,数值解随时间的变化趋势跟观测结果有较好一致性。当大气模式取为定常时,数值解随时间的变化趋势跟观测结果差别较大。     

Climatology and interannual variability simulated by the ARPEGE-OPA coupled model

A 10-year simulation with a coupled ocean-atmosphere general circulation model (CGCM) is presented. The model consists of the climate version of the Météo-France global forecasting model, ARPEGE, coupled to the LODYC oceanic model, OPA, by the CERFACS coupling package OASIS. The oceanic component is dynamically active over the tropical Pacific, while climatological time-dependent sea surface temperatures (SSTs) are prescribed outside of the Pacific domain. The coupled model shows little drift and exhibits a very regular seasonal cycle. The climatological mean state and seasonal cycle are well simulated by the coupled model. In particular, the oceanic surface current pattern is accurately depicted and the location and intensity of the Equatorial Undercurrent (EUC) are in good agreement with available data. The seasonal cycle of equatorial SSTs captures quite realistically the annual harmonic. Some deficiencies remain including a weak zonal equatorial SST gradient, underestimated wind stress over the Pacific equatorial band and an additional inter-tropical convergence zone (ITCZ) south of the equator in northern winter and spring. Weak interannual variability is present in the equatorial SST signal with a maximum amplitude of 0.5°C.     

经向风应力在热带不稳定海气相互作用中的角色

利用1951-2000年北太平洋海温和副高特征量资料，探讨夏半年副高与前期海温的关系，发现影响副高活动的海温关键区赤道东太平洋前期信息最明显，从前期秋季持续至春季都保持与夏半年各月副高强度、西伸脊点位置的高相关；ENS0事件的形成超前于副高强度的转折约半年时间；El Nino年副高持续偏强偏西，La Nina年则相反，脊线位置也有所反映，但不如强度与西伸显著。在此基础上进一步分析表明，福建雨季降水、副高强度与Nino 3区海温存在着大约相一致的年代际振荡与突变时间；雨季降水分布的异常直接受到副高活动的影响，而副高活动又明显受ENS0循环的制约，因而关注赤道东太平洋海温异常及ENS0信息对福建雨季降水分布趋势的预测旦有重要意义。     

A GLOBAL COUPLED OCEAN-ATMOSPHERE MODEL OF SHALLOW WATER WAVE AND ITS NUMERICAL EXPERIMENTS

A global coupled air-sea model of shallow water wave is developed based on coupled ocean-atmospheredynamics.The coupling is realized through the air-sea interaction process that the atmosphere acts on theocean by wind stress and the ocean acts on the atmosphere with heating proportional to sea surface temperature(SST)anomaly.The equation is harotropic primitive one.Response experiments of coupling system arealso carried out SSTA in two categories of intensities.Compared with the results of AGCM simulation ex-periment in which only the dynamic change of air system is considered,it demonstrates that the air-seainteraction between the tropical ocean and the global atmosphere plays a very important role in the evolutionof climate system.The results of numerical simulation show that it is encouraging.     

THE ROLE OF MERIDIONAL WIND STRESS IN THE TROPICAL UNSTABLE AIR-SEA INTERACTION

1INTRODUCTIONAsshowninobservationalfacts,theevolutionofElNi駉displaysastheamplitudeexpansionandmigrationofanomalousdisturbancesinair-seacouplingsystems.Theoreticaladdressofthephenomenonhasbeenattemptedfromtheaspectofcouplingdynamics.Theresultsshowthattheair-seainteractionsystemincludesakindofunstablemode,whichispropagatingindirectionsdeterminedbyfactorsgoverningSSTchanges.Withanair-seacouplingmodel,Lau[1]wasamongtheearliestpeoplewhodiscoveredunstablestationarywavemodesintheair-seaintera…     

STUDY OF CHARACTERISTIC MODES IN SIMPLE TROPICAL COUPLED AIR-SEA MODEL

The propagation features, stabilities and dynamical characteristic structures of coupled Kelvin inner modes andsecond order Rossby inner modes are studied using a simple tropical coupled air-sea model in this paper. It is shownthat there is mechanism of selecting scale and frequency in the tropical air-sea system. The effects of air-sea couplingare mainly on the large-scale modes and nonuniform. These effects make the frequency of Kelvin modes decrease andeven excite the eastward propagating Rossby inner modes. These effects make the unstable development of Kelvinmodes and result in the decay of Rossby modes. The effects of atmospheric damp are opposite to those of air-sea coup-ling. The oceanic damp only make the wave amplitudes decay. Simutaneously, this paper shows the dynamical character-istic structures of air-sea coupled system and the phase relations between the atmospheric and the oceanic wave compo-nent.     

On the attractors of an intermediate coupled ocean-atmosphere model

Techniques of numerical bifurcation theory are used to study stationary and periodic solutions of an intermediate coupled model for tropical ocean-atmosphere interaction. The qualitative dynamical behavior is determined for a volume in parameter space spanned by the atmospheric damping length, the coupling parameter, the surface layer feedback strength and the relative adjustment time coefficient. Time integration methods have previously shown much interesting dynamics, including multiple steady states, eastward- or westward-propagating orbits and relaxation oscillations. The present study shows how this dynamics arises in parameter space through the interaction of the different branches of equilibrium solutions and the singularities on these branches. For example, we show that westward-propagating periodic orbits arise through an interaction of two unstable stationary modes and that relaxation oscillations occur through a limit cycle-saddle node interaction. There are several dynamical regimes in the coupled model which are determined by the primary bifurcation structure; this structure depends strongly on the parameters in the model. Although much of the dynamics may be studied in the fast-wave limit, it is shown that ocean wave dynamics introduces additional oscillatory instabilities and how these relate to propagating oscillations.     

A mechanism for formation of the western North Pacific monsoon trough: nonlinear upscale cascade

Linear and nonlinear barotropic vorticity model frameworks are constructed to understand the formation of the monsoon trough in boreal summer over the western North Pacific. The governing equation is written with respect to specified zonal background flows, and a wave perturbation is prescribed in the eastern boundary. Whereas a uniform background mean flow leads no scale contraction, a confluent background zonal flow causes the contraction of zonal wavelength. Under linear dynamics, the wave contraction leads to the development of smaller scale vorticity perturbations. As a result, there is no upscale cascade. Under nonlinear dynamics, cyclonic (anticyclonic) wave disturbances shift northward (southward) away from the central latitude due to the vorticity segregation process. The merging of small-scale cyclonic and anticyclonic perturbations finally leads to the generation of a pair of large-scale cyclonic and anti-cyclonic vorticity gyres, straddling across the central latitude. The large-scale cyclonic circulation due to nonlinear upscale cascade can be further strengthened through a positive convection-circulation feedback.

     

Midlatitude unstable air-sea interaction with atmospheric transient eddy dynamical forcing in an analytical coupled model

While recent observational studies have shown the critical role of atmospheric transient eddy (TE) activities in midlatitude unstable air-sea interaction, there is still a lack of a theoretical framework characterizing such an interaction. In this study, an analytical coupled air-sea model with inclusion of the TE dynamical forcing is developed to investigate the role of such a forcing in midlatitude unstable air-sea interaction. In this model, the atmosphere is governed by a barotropic quasi-geostrophic potential vorticity equation forced by surface diabatic heating and TE vorticity forcing. The ocean is governed by a baroclinic Rossby wave equation driven by wind stress. Sea surface temperature (SST) is determined by mixing layer physics. Based on detailed observational analyses, a parameterized linear relationship between TE vorticity forcing and meridional second-order derivative of SST is proposed to close the equations. Analytical solutions of the coupled model show that the midlatitude air-sea interaction with atmospheric TE dynamical forcing can destabilize the oceanic Rossby wave within a wide range of wavelengths. For the most unstable growing mode, characteristic atmospheric streamfunction anomalies are nearly in phase with their oceanic counterparts and both have a northeastward phase shift relative to SST anomalies, as the observed. Although both surface diabatic heating and TE vorticity forcing can lead to unstable air-sea interaction, the latter has a dominant contribution to the unstable growth. Sensitivity analyses further show that the growth rate of the unstable coupled mode is also influenced by the background zonal wind and the air–sea coupling strength. Such an unstable air-sea interaction provides a key positive feedback mechanism for midlatitude coupled climate variabilities.

     

Teleconnections and predictive characteristics of Australian seasonal rainfall

A new methodology is proposed that allows patterns of interannual covariability, or teleconnections, between the intraseasonal and slow components of seasonal mean Australian rainfall and the corresponding components in the Southern Hemisphere atmospheric circulation to be estimated. In all seasons, the dominant rainfall–circulation teleconnections in the intraseasonal component are shown to have the characteristic features associated with well-known intraseasonal dynamical and statistical atmospheric modes and their relationship with rainfall. Thus, for example, there are patterns of interannual covariability that reflect rainfall relationships with the intraseasonal Southern Annular Mode, the Madden-Julian Oscillation and wavenumber 3 and 4 intraseasonal modes of variability. The predictive characteristics of the atmospheric circulation–rainfall relationship are shown to reside with the slow components. In all seasons, we find rainfall–circulation teleconnections in the slow components related to the El Niño-Southern Oscillation. Each season also has a coupled mode, with a statistically significant trend in the time series of the atmospheric component that appears to be related to recent observed trends in rainfall. The slow Southern Annular Mode also features in association with southern Australian rainfall, especially during austral winter and spring. There is also evidence of an influence of Indian Ocean sea surface temperature variability on rainfall in southeast Australia during austral winter and spring.     

Ensemble data assimilation in a simple coupled climate model: The role of ocean-atmosphere interaction

A conceptual coupled ocean-atmosphere model was used to study coupled ensemble data assimilation schemes with a focus on the role of ocean-atmosphere interaction in the assimilation. The optimal scheme was the fully coupled data assimilation scheme that employs the coupled covariance matrix and assimilates observations in both the atmosphere and ocean. The assimilation of synoptic atmospheric variability that captures the temporal fluctuation of the weather noise was found to be critical for the estimation of not only the atmospheric, but also oceanic states. The synoptic atmosphere observation was especially important in the mid-latitude system, where oceanic variability is driven by weather noise. The assimilation of synoptic atmospheric variability in the coupled model improved the atmospheric variability in the analysis and the subsequent forecasts, reducing error in the surface forcing and, in turn, in the ocean state. Atmospheric observation was able to further improve the oceanic state estimation directly through the coupled covariance between the atmosphere and ocean states. Relative to the mid-latitude system, the tropical system was influenced more by ocean-atmosphere interaction and, thus, the assimilation of oceanic observation becomes more important for the estimation of the ocean and atmosphere.