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1.
《Dynamics of Atmospheres and Oceans》2008,44(3-4):123-150
This study examines mid-latitude climate variability in a model that couples turbulent oceanic and atmospheric flows through an active oceanic mixed layer. Intrinsic ocean dynamics of the inertial recirculation regions combines with nonlinear atmospheric sensitivity to sea-surface temperature (SST) anomalies to play a dominant role in the variability of the coupled system.Intrinsic low-frequency variability arises in the model atmosphere; when run in a stand-alone mode, it is characterized by irregular transitions between preferred high-latitude and less frequent low-latitude zonal-flow states. When the atmosphere is coupled to the ocean, the low-latitude state occurrences exhibit a statistically significant signal in a broad 5–15-year band. A similar signal is found in the time series of the model ocean's energy in this coupled simulation. Accompanying uncoupled ocean-only and atmosphere-only integrations are characterized by a decrease in the decadal-band variability, relative to the coupled integration; their spectra are indistinguishable from a red spectrum.The time scale of the coupled interdecadal oscillation is set by the nonlinear adjustment of the ocean's inertial recirculations to the high-latitude and low-latitude atmospheric forcing regimes. This adjustment involves, in turn, SST changes resulting in long-term ocean–atmosphere heat-flux anomalies that induce the atmospheric regime transitions. 相似文献
2.
A simple idealized atmosphere–ocean climate model and an ensemble Kalman filter are used to explore different coupled ensemble data assimilation strategies. The model is a low-dimensional analogue of the North Atlantic climate system, involving interactions between large-scale atmospheric circulation and ocean states driven by the variability of the Atlantic meridional overturning circulation (MOC). Initialization of the MOC is assessed in a range of experiments, from the simplest configuration consisting of forcing the ocean with a known atmosphere to performing fully coupled ensemble data assimilation. “Daily” assimilation (that is, at the temporal frequency of the atmospheric observations) is contrasted with less frequent assimilation of time-averaged observations. Performance is also evaluated under scenarios in which ocean observations are limited to the upper ocean or are non-existent. Results show that forcing the idealized ocean model with atmospheric analyses is inefficient at recovering the slowly evolving MOC. On the other hand, daily assimilation rapidly leads to accurate MOC analyses, provided a comprehensive set of oceanic observations is available for assimilation. In the absence of sufficient observations in the ocean, the assimilation of time-averaged atmospheric observations proves to be more effective for MOC initialization, including the case where only atmospheric observations are available. 相似文献
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C. Frankignoul E. Kestenare N. Sennéchael G. de Coëtlogon F. D'Andrea 《Climate Dynamics》2000,16(5):333-354
The last 810 years of a control integration with the ECHAM1/LSG coupled model are used to clarify the nature of the ocean-atmosphere
interactions at low frequencies in the North Atlantic and the North Pacific. To a first approximation, the atmosphere acts
as a white noise forcing and the ocean responds as a passive integrator. The sea surface temperature (SST) variability primarily
results from short time scale fluctuations in surface heat exchanges and Ekman currents, and the former also damp the SST
anomalies after they are generated. The thermocline variability is primarily driven by Ekman pumping. Because the heat, momentum,
and vorticity fluxes at the sea surface are correlated in space and time, the SST variability is directly linked to that in
the ocean interior. The SST is also modulated by the wind-driven geostrophic fluctuations, resulting in persistent correlation
with the thermocline changes and a slight low-frequency redness of the SST spectra. The main dynamics are similar in the two
oceans, although in the North Pacific the SST variability is more strongly influenced by advection changes and the oceanic
time scales are larger. A maximum covariance analysis based on singular value decomposition in lead and lag conditions indicates
that some of the main modes of atmospheric variability in the two oceans are sustained by a very weak positive feedback between
the atmosphere, SST, and the strength of the subtropical and subpolar gyres. In addition, in the North Atlantic the main surface
pressure mode has a small quasi-oscillatory component at 6-year period, and advective resonance occurs for SST around 10-year
period, both periods being also singled out by multichannel singular spectrum analysis. The ocean-atmosphere coupling is however
much too weak to redden the tropospheric spectra or create anything more than tiny spectral peaks, so that the atmospheric
and oceanic variability is dominated in both ocean sectors by the one-way interactions.
Received: 2 April 1999 / Accepted: 14 October 1999 相似文献
5.
At least two main oceanic fronts (the subarctic and subtropical fronts) exist in the North Pacific. Especially in the subtropical frontal zone (STFZ), the sea subsurface temperature gradient is significantly larger than that of the surface layer in winter. Subseasonal interaction between the subsurface subtropical front and overlaying atmosphere is revealed by using empirical orthogonal function (EOF) analysis of oceanic temperature gradient. The first EOF mode mainly corresponds to the atmosphere-to-ocean influences. With the enhanced westerly wind, a cold sea surface temperature anomaly (SSTA) appears and then passes down to affect the subsurface ocean. However, the second EOF mode indicates the ocean-to-atmosphere forcing. For the second mode, cold oceanic temperature anomaly generates in the subsurface layer and passes up, which makes the SST gradient increasing. Due to the increasing atmospheric baroclinicity, the enhanced westerly wind leads to more heat fluxes from the ocean to the atmosphere, which results in a colder SSTA and a larger SST gradient in the STFZ. Therefore, a positive ocean-atmosphere feedback begins to maintain in the mid-latitude in winter. 相似文献
6.
Spatial patterns of mid-latitude large-scale ocean-atmosphere interaction on monthly to seasonal time scales have been observed to exhibit a similar structure in both the North Pacific and North Atlantic basins. These patterns have been interpreted as a generic oceanic response to surface wind anomalies, whereby the anomalous winds give rise to corresponding anomalous regions of surface heat flux and consequent oceanic cooling. This mechanistic concept is investigated in this study using numerical models of a global atmosphere and a mid-latitude ocean basin (nominally the Atlantic). The models were run in both coupled and uncoupled mode. Model output was used to generate multi-year time series of monthly mean fields. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses were then used to obtain the principal patterns of variability in heat flux, air temperature, wind speed, and sea surface temperature (SST), and to determine the relationships among these variables. SVD analysis indicates that the turbulent heat flux from the ocean to the atmosphere is primarily controlled by the surface scalar wind speed, and to a lesser extent by air temperature and SST. The principal patterns of air-sea interaction are closely analogous to those found in observational data. In the atmosphere, the pattern consists of a simultaneous strengthening (or weakening) of the mid-latitude westerlies and the easterly trades. In the ocean there is cooling (warming) under the anomalously strong (weak) westerlies and trade winds, with a weaker warming (cooling) in the region separating the westerly and easterly wind regimes. These patterns occur in both coupled and uncoupled models and the primary influence of the coupling is in localizing the interaction patterns. The oceanic patterns can be explained by the principal patterns of surface heat flux and the attendant warming or cooling of the ocean mixed layer. 相似文献
7.
国际Argo(Array for Real-time Geostrophic Oceanography)计划的实施,提供了前所未有的全球深海大洋0～2000 m水深范围内的海水温度和盐度观测资料,在大气和海洋科研业务中应用这一全新的资料,是深入认识大气和海洋变异、提高我国气候预测、海洋监测分析和预报能力的一个关键所在.通过开发非线性温—盐协调同化方案和利用同化高度计资料来调整模式的温度和盐度场,建立了可同化包括Argo等多种海洋观测资料的全球海洋资料变分同化系统,提高了对全球海洋的监测分析能力.实现了海洋资料同化系统与全球海气耦合模式的耦合,显著提高了短期气候预测水平.利用Argo资料改进了海洋动力模式中的物理过程参数化方案,有效提高了海洋模式对真实大洋的模拟能力和对厄尔尼诺/拉尼娜的预测能力.开发了利用Argo浮标漂流轨迹推算全球海洋表层和中层流的方法,提高了推算的全球表层流、中层流资料质量,有效弥补了洋流观测的匮乏. 相似文献
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A new hybrid coupled model(HCM) is presented in this study, which consists of an intermediate tropical Pacific Ocean model and a global atmospheric general circulation model. The ocean component is the intermediate ocean model(IOM)of the intermediate coupled model(ICM) used at the Institute of Oceanology, Chinese Academy of Sciences(IOCAS). The atmospheric component is ECHAM5, the fifth version of the Max Planck Institute for Meteorology atmospheric general circulation model. The HCM integrates its atmospheric and oceanic components by using an anomaly coupling strategy. A100-year simulation has been made with the HCM and its simulation skills are evaluated, including the interannual variability of SST over the tropical Pacific and the ENSO-related responses of the global atmosphere. The model shows irregular occurrence of ENSO events with a spectral range between two and five years. The amplitude and lifetime of ENSO events and the annual phase-locking of SST anomalies are also reproduced realistically. Despite the slightly stronger variance of SST anomalies over the central Pacific than observed in the HCM, the patterns of atmospheric anomalies related to ENSO,such as sea level pressure, temperature and precipitation, are in broad agreement with observations. Therefore, this model can not only simulate the ENSO variability, but also reproduce the global atmospheric variability associated with ENSO, thereby providing a useful modeling tool for ENSO studies. Further model applications of ENSO modulations by ocean–atmosphere processes, and of ENSO-related climate prediction, are also discussed. 相似文献
10.
D. W. Pierce T. P. Barnett N. Schneider R. Saravanan D. Dommenget M. Latif 《Climate Dynamics》2001,18(1-2):51-70
Decadal time scale climate variability in the North Pacific has implications for climate both locally and over North America.
A crucial question is the degree to which this variability arises from coupled ocean/atmosphere interactions over the North
Pacific that involve ocean dynamics, as opposed to either purely thermodynamic effects of the oceanic mixed layer integrating
in situ the stochastic atmospheric forcing, or the teleconnected response to tropical variability. The part of the variability
that is coming from local coupled ocean/atmosphere interactions involving ocean dynamics is potentially predictable by an
ocean/atmosphere general circulation model (O/A GCM), and such predictions could (depending on the achievable lead time) have
distinct societal benefits. This question is examined using the results of fully coupled O/A GCMs, as well as targeted numerical
experiments with stand-alone ocean and atmosphere models individually. It is found that coupled ocean/atmosphere interactions
that involve ocean dynamics are important to determining the strength and frequency of a decadal-time scale peak in the spectra
of several oceanic variables in the Kuroshio extension region off Japan. Local stochastic atmospheric heat flux forcing, integrated
by the oceanic mixed layer into a red spectrum, provides a noise background from which the signal must be extracted. Although
teleconnected ENSO responses influence the North Pacific in the 2–7 years/cycle frequency band, it is shown that some decadal-time
scale processes in the North Pacific proceed without ENSO. Likewise, although the effects of stochastic atmospheric forcing
on ocean dynamics are discernible, a feedback path from the ocean to the atmosphere is suggested by the results.
Received: 23 January 2000 / Accepted: 10 January 2001 相似文献
11.
A simple quasi-geostrophic coupled ocean-atmosphere model 总被引：1，自引：0，他引：1
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. 相似文献
12.
The interplay between the North Atlantic Oscillation (NAO) and the large scale ocean circulation is inspected in a twentieth century simulation conducted with a state-of-the-art coupled general circulation model. Significant lead–lag covariance between oceanic and tropospheric variables suggests that the system supports a damped oscillatory mode involving an active ocean–atmosphere coupling, with a typical NAO-like space structure and a 5 years timescale, qualitatively consistent with a mid-latitude delayed oscillator paradigm. The two essential processes governing the oscillation are (1) a negative feedback between ocean gyre circulation and the high latitude SST meridional gradient and (2) a positive feedback between SST and the NAO. The atmospheric NAO pattern appears to have a weaker projection on the ocean meridional overturning, compared to the gyre circulation, which leads to a secondary role for the thermohaline circulation in driving the meridional heat transport, and thus the oscillatory mode. 相似文献
13.
针对未来1~10 a气候状态的近期气候预测(年代际预测)是当前国际气候领域的研究热点。本文综述了中国科学院大气物理研究所发展的基于耦合气候系统模式的年代际气候预测系统IAP-DecPreS相关的研究进展。IAP-DecPreS系统的核心部分是耦合模式海洋分量初始化方案,“集合最优插值-分析增量更新”(EnOI-IAU)方案,该方案将集合最优插值(EnOI)和增量分析更新(IAU)结合起来,能够同化原始的海洋次表层温度廓线观测资料,对耦合模式进行初始化。系统的年代际回报试验表明,IAP-DecPreS对太平洋年代际振荡和大西洋多年代际变率的预测技巧与耦合模式比较计划第五阶段(CMIP5)技巧较高的模式相当。IAP-DecPreS系统被广泛应用于气候预测相关研究,包括火山气溶胶对年代际预测技巧的影响,全场同化和异常场同化两种不同的初始化方法对ENSO、印度洋偶极子模态和印度洋洋盆模态等的预测技巧的影响。最后,结合国际发展态势,对未来IAP-DecPreS的发展进行了讨论。 相似文献
14.
That a model has sensitivity responses to parameter uncertainties is a key concept in implementing model parameter estimation using filtering theory and methodology.Depending on the nature of associated physics and characteristic variability of the fluid in a coupled system,the response time scales of a model to parameters can be different,from hourly to decadal.Unlike state estimation,where the update frequency is usually linked with observational frequency,the update frequency for parameter estimation must be associated with the time scale of the model sensitivity response to the parameter being estimated.Here,with a simple coupled model,the impact of model sensitivity response time scales on coupled model parameter estimation is studied.The model includes characteristic synoptic to decadal scales by coupling a long-term varying deep ocean with a slow-varying upper ocean forced by a chaotic atmosphere.Results show that,using the update frequency determined by the model sensitivity response time scale,both the reliability and quality of parameter estimation can be improved significantly,and thus the estimated parameters make the model more consistent with the observation.These simple model results provide a guideline for when real observations are used to optimize the parameters in a coupled general circulation model for improving climate analysis and prediction initialization. 相似文献
15.
Vikram M Mehta 《Climate Dynamics》1991,6(1):49-65
Meridional, linear, and free modes of global, primitive-equation, ocean-atmosphere models were analyzed to see if they contain multi-year, especially decadal ( 10–30 years), oscillation time scale modes. A two-layer model of the global ocean and a two-level model of the global atmosphere were formulated. Both models were linearized around axially-symmetric basic states containing mean meridional circulations. The linearized perturbation system was solved as an eigenvalue problem. The operator matrix was discretized in the north-south direction with centered finite differences. Uncoupled, meridional modes of oscillation of the ocean and the atmosphere models were calculated. Calculations were performed at three grid spacings (5°, 2.5° and 1.25°) and for two types of basic states (symmetric and asymmetric). Uncoupled, free oceanic modes in the presence of mean meridional circulations have oscillation time scales ranging from two years to several centuries. Such low frequency meridional modes do not exist in the ocean model if there are no mean meridional circulations. A large number of oceanic modes are grouped around decadal oscillation time scales. All the oceanic modes have neutral growth rates. The spatial structures of some of the oceanic modes are comparable to observed spatial structures of sea surface temperature variations in the Pacific Ocean. Most years to decades variability of meridional modes of the ocean model is contained in tropical and midlatitude modes. Some oceanic modes with years to decades periods have standing oscillations in the tropics and poleward propagation of zonal velocity and layer thickness outside the tropics. Uncoupled, free atmospheric modes in the presence of mean meridional circulations have oscillation time scales ranging from a week to several decades. Such low-frequency meridional modes do not exist in the atmospere model if there are no mean meridional circulations. A large number of modes are grouped around intraseasonal time scales. Unlike the oceanic modes, the atmospheric modes are weakly unstable. Most of the intraseasonal variability of atmospheric modes is contained in tropical, midlatitude, and polar modes. Atmospheric modes with oscillation periods longer than about one year have global extent. Meridional ocean-atmospheric modes exist in the models wherever there are mean meridional circulations, i.e., tropical, midlatitude, polar, and global. Oceanic and atmospheric eigenvectors have symmetric (assymetric) latitudinal structures if their basic states are symmetric (asymmetric) around the equator. For both models, models calculated at coarser than 2.5° grid spacing do not accurately represent low-frequency variability. Scale analysis shows taht advection by tge basic state meridional velocities is the primary cause of the meridional oscillations on time scales longer than two years in the ocean model and longer than a few weeks in the atmosphere model. Meridional modes of the coupled ocean-atmosphere models are the subject of a subsequent paper.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil 相似文献
16.
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。 相似文献
17.
A noise reduction technique, namely the interactive ensemble (IE) approach is adopted to reduce noise at the air–sea interface due to internal atmospheric dynamics in a state-of-the-art coupled general circulation model (CGCM). The IE technique uses multiple realization of atmospheric general circulation models coupled to a single ocean general circulation model. The ensembles mean fluxes from the atmospheric simulations are communicated to the ocean component. Each atmospheric simulation receives the same SST coming from the ocean component. The only difference among the atmospheric simulations comes from perturbed initial conditions, thus the atmospheric states are, in principle synoptically independent. The IE technique can be used to better understand the importance of weather noise forcing of natural variability such as El Niño Southern Oscillation (ENSO). To study the impact of weather noise and resolution in the context of a CGCM, two IE experiments are performed at different resolutions. Atmospheric resolution is an important issue since the noise statistics will depend on the spatial scales resolved. A simple formulation to extract atmospheric internal variability is presented. The results are compared to their respective control cases where internal atmospheric variability is left unchanged. The noise reduction has a major impact on the coupled simulation and the magnitude of this effect strongly depends on the horizontal resolution of the atmospheric component model. Specifically, applying the noise reduction technique reduces the overall climate variability more effectively at higher resolution. This suggests that “weather noise” is more important in sustaining climate variability as resolution increases. ENSO statistics, dynamics, and phase asymmetry are all modified by the noise reduction, in particular ENSO becomes more regular with less phase asymmetry when noise is reduced. All these effects are more marked for the higher resolution case. In contrast, ENSO frequency is unchanged by the reduction in the weather noise, but its phase-locking to the annual cycle is strongly dependent on noise and resolution. At low resolution the noise structure is similar to the signal, whereas the spatial structure of the noise deviates from the spatial structure of the signal as resolution increases. It is also suggested that event-to-event differences are largely driven by atmospheric noise as opposed to chaotic dynamics within the context of the large-scale coupled system, suggesting that there is a well-defined “canonical” event. 相似文献
18.
Interannual-to-interdecadal ocean-atmosphere interaction in midlatitudes is studied using an idealized coupled model consisting of eddy resolving two-layer quasi-geostrophic oceanic and atmospheric components with a simple diagnostic oceanic mixed layer. The model solutions exhibit structure and variability that resemble qualitatively some aspects of the observed climate variability over the North Atlantic. The atmospheric climatology is characterized by a zonally modulated jet. The single-basin ocean climatology consists of a midlatitude double jet that represents the Gulf Stream and Labrador currents, which are parts of the subtropical and subpolar gyres, respectively. The leading mode of the atmospheric low-frequency variability consists predominantly of meridional displacements of the zonal jet, with a local maximum over the ocean. The first basin-scale mode of sea-surface temperature has a red power spectrum, is largely of one polarity and bears qualitative similarities with the observed interdecadal mode identified by Kushnir. A warm sea-surface temperature anomaly is accompanied by anomalously low atmospheric pressure, an intensified model Gulf Stream and a weakened Labrador current. This mode is found not to be affected significantly by oceanic coupling. In the western part of the basin, this sea-surface temperature pattern is shown to be forced by the slowest components of the surface-wind anomaly through a delayed modulation of the baroclinic time-dependent boundary currents which advect mean SST, with synchronous variations in the two oceanic jets. The response in the east is found to be dominated by local atmospheric forcing. Basin-scale intrinsic oceanic variability consists of a damped oceanic oscillatory mode in the baroclinic flow field that is excited by the atmospheric noise. Its period is around 5.5 years, but it has a negligible influence on the evolution of sea-surface temperature. Important for this mode's excitation is the meridional position of the atmospheric center of action relative to the ocean gyres. 相似文献
19.
To further explore enthalpy-based sea-ice assimilation, a one-dimensional(1D) enthalpy sea-ice model is implemented into a simple pycnocline prediction model. The 1D enthalpy sea-ice model includes the physical processes such as brine expulsion, flushing, and salt diffusion. After being coupled with the atmosphere and ocean components, the enthalpy sea-ice model can be integrated stably and serves as an important modulator of model variability. Results from a twin experiment show that the sea-ice data assimilation in the enthalpy space can produce smaller root-mean-square errors of model variables than the traditional scheme that assimilates the observations of ice concentration, especially for slow-varying states. This study provides some insights into the improvement of sea-ice data assimilation in a coupled general circulation model. 相似文献
20.
A hybrid coupled model (HCM) for the tropical Pacific ocean-atmosphere system is employed for ENSO prediction. The HCM consists
of the Geophysical Fluid Dynamics Laboratory ocean general circulation model and an empirical atmospheric model. In hindcast
experiments, a correlation skill competitive to other prediction models is obtained, so we use this system to examine the
effects of several initialization schemes on ENSO prediction. Initialization with wind stress data and initialization with
wind stress reconstructed from SST using the atmospheric model give comparable skill levels. In re-estimating the atmospheric
model in order to prevent hindcast-period wind information from entering through empirical atmospheric model, we note some
sensitivity to the estimation data set, but this is considered to have limited impact for ENSO prediction purposes. Examination
of subsurface heat content anomalies in these cases and a case forced only by the difference between observed and reconstructed
winds suggests that at the current level of prediction skill, the crucial wind components for initialization are those associated
with the slow ENSO mode, rather than with atmospheric internal variability. A “piggyback” suboptimal data assimilation is
tested in which the Climate Prediction Center data assimilation product from a related ocean model is used to correct the
ocean initial thermal field. This yields improved skill, suggesting that not all ENSO prediction systems need to invest in
costly data assimilation efforts, provided the prediction and assimilation models are sufficiently close.
Received: 17 April 1998 / Accepted: 22 July 1999 相似文献