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1.
The intraseasonal oscillation in ECHAM4 Part I: coupled to a comprehensive ocean model 总被引：1，自引：0，他引：1
This study discusses the representation of the intraseasonal oscillation (ISO) in three simulations with the ECHAM4 atmosphere general circulation model (GCM). First, the model is forced by AMIP sea surface temperatures (SST), then coupled to the OPYC3 global ocean GCM and third forced by OPYC3 SSTs to clarify possible air-sea interactions and connections of the ISO and the ENSO cycle. The simulations are compared to ECMWF reanalysis data and NOAA outgoing longwave radiation (OLR) observations. Although previous studies have shown that the ECHAM4 GCM simulates an ISO-like oscillation, the main deficits are an overly fast eastward propagation and an eastward displacement of the main ISO activity, which is shown with a composite analysis of daily data between 1984 to 1988 for the reanalysis and the AMIP simulation, 25 years of the coupled integration, and a five year subset of the coupled SST output used for the OPYC3 forced atmosphere GCM experiment. These deficits are common to many atmospheric GCMs. The composites are obtained by principal oscillation pattern (POP). The POPs are also used to investigate the propagation speed and the interannual variability of the main ISO activity. The present coupled model version reveals no clear improvements in the ISO simulation compared to the uncoupled version forced with OPYC3 SSTs, although it is shown that the modeled ISO influences the simulated high-frequency SST variability in the coupled GCM. Within the current analysis, ECHAM4 forced by AMIP SSTs provides the most reasonable ISO simulation. However, it is shown that the maximum amplitudes of the annual cycle of the ISO variability in all analyzed model versions are reached too late in the year (spring and summer) compared to the observations (winter and spring). Additionally, the ENSO cycle influences the interannual variability of the ISO, which is revealed by 20 years of daily reanalysis data and 100 years of the coupled integration. The ENSO cycle is simulated by the coupled model, although there is a roughly 1 K cold bias in the East Pacific in the coupled model. This leads to a diminished influence of the ENSO cycle on the spatial variability of the modeled ISO activity compared to observations. This points out the strong sensitivity of the SST on the ISO activity. Small biases in the SST appear to cause large deterioration in the modeled ISO. 相似文献
2.
利用1979—2019年ECMWF提供的ERA-Interim逐日再分析资料,采用Morlet小波分析、滤波及合成分析等方法,探究了青藏高原夏季对流层高层纬向风季节内振荡（IntraSeasonal Oscillation,ISO）的主要周期及其传播特征。结果表明：青藏高原夏季高层纬向风季节内振荡的主要周期为10～30 d,其强度存在明显的年际差异。在纬向风ISO强年,振荡过程持续时间长、振幅强,ISO方差中心从对流层高层向下影响到对流层中层,表现为相当正压结构。其传播在纬向上主要表现为ISO中心从高原东部3次向东传,可达西太平洋地区;经向上分别有4次自中高纬向南传播的10～30 d ISO中心与来自低纬地区的ISO中心在高原南侧汇合,其强度在高原南侧有所加强,强振荡中心可向南传播到达低纬地区。ISO的位相演变主要表现为低频反气旋和低频气旋中心在高原东部交替出现,引起东部地区上空低频东风和低频西风的强度变化。在ISO极端活跃位相,高原东部低频西风达最强。 相似文献
3.
外强迫对热带季节内振荡影响的模拟研究 总被引：3，自引：2，他引：3
应用经过修改的NCAR CCM3模式和CAM2模式进行的数值实验结果以及NCEP的GFS模式的输出结果讨论了海温等外强迫作用对热带季节内振荡的影响.结果表明,热带季节内振荡是热带大气固有的内部变率.它是由大气内部过程的相互作用决定的.但外强迫对热带季节内振荡的强度、传播方向等有明显的影响.当外强迫没有变化时,模式可以模拟出与观测近似的低频振荡.当作为外强迫的海温和太阳辐射有年内季节变化时,模式模拟的季节内振荡则明显减弱.当海温与辐射不仅有季节变化而且有年际变化时,模式模拟的季节内振荡会进一步减弱.具有长周期的外强迫还会削弱季节内振荡中东移波动的能量而增加静止波的强度.在与海洋模式耦合的状态下,模式不受来自海洋的外强迫影响,而是与海洋构成一个耦合系统,可以产生最强的季节内振荡. 相似文献
4.
The performance of ECHAM5 atmospheric general circulation model (AGCM) is evaluated to simulate the seasonal mean and intraseasonal variability of Indian summer monsoon (ISM). The model is simulated at two different vertical resolutions, with 19 and 31 levels (L19 and L31, respectively), using observed monthly mean sea surface temperature and compared with the observation. The analyses examine the biases present in the internal dynamics of the model in simulating the mean monsoon and the evolution of the boreal summer intraseasonal oscillation (BSISO) and attempts to unveil the reason behind them. The model reasonably simulates the seasonal mean-state of the atmosphere during ISM. However, some notable discrepancies are found in the simulated summer mean moisture and rainfall distribution. Both the vertical resolutions, overestimate the seasonal mean precipitation over the oceanic regions, but underestimate the precipitation over the Indian landmass. The performance of the model improves with the increment of the vertical resolution. The AGCM reasonably simulates some salient features of BSISO, but fails to show the eastward propagation of the convection across the Maritime Continent in L19 simulation. The propagation across the Maritime Continent and tilted rainband structure improve as one moves from L19 to L31. The model unlikely shows prominent westward propagation that originates over the tropical western Pacific region. L31 also produces some of the observed characteristics of the northward propagating BSISOs. However, the northward propagating convection becomes stationary in phase 5–7. The simulation of shallow diabatic heating structure and the heavy rainfall activity over the Bay of Bengal indicate the abundance of the premature convection-generated precipitation events in the model. It is found that the moist physics is responsible for the poor simulation of the northward propagating convection anomalies. 相似文献
5.
Aspects of the intraseasonal oscillation simulated by the Hadley Centre Atmosphere Model 总被引：2，自引：0，他引：2
The period and amplitude of the tropical intraseasonal oscillation in the Hadley Centre Global Climate Model have been examined
by applying time filtering and spectral analysis techniques to time series of daily velocity potential. Results from an AMIP
integration of the model, forced with observed sea surface temperatures for the decade 1979-88 show that the model has a reasonable
degree of skill in simulating the main features of the oscillation, although the signal in the model is rather noisy compared
to NWP analyses. The model also shows interannual variability of the oscillation similar in magnitude to that in the analyses.
The impact on the oscillation of including a parametrization of momentum transport by cumulus convection is assessed. This
parametrization significantly improves the mean circulation of the model, reducing systematic errors particularly in the tropics.
However, the strength and coherence of the tropical intraseasonal oscillation are significantly reduced by this parametrization.
It is speculated that this may be related to a reduction in westerly flow in the tropical central and east Pacific and a reduction
in the strength of the SPCZ in DJF.
Received: 17 July 1996/Accepted: 30 January 1997 相似文献
6.
Ravi P. Shukla 《大气与海洋》2014,52(4):321-330
In this study, we examine the characteristics of the boreal summer monsoon intraseasonal oscillation (BSISO) using the second version of the Climate Forecast System (CFSv2) and revisit the role of air–sea coupling in BSISO simulations. In particular, simulations of the BSISO in two carefully designed model experiments are compared: a fully coupled run and an uncoupled atmospheric general circulation model (AGCM) run with prescribed sea surface temperatures (SSTs). In these experiments an identical AGCM is used, and the daily mean SSTs from the coupled run are prescribed as a boundary condition in the AGCM run. Comparisons indicate that air–sea coupling plays an important role in realistically simulating the BSISO in CFSv2. Compared with the AGCM run, the coupled run not only simulates the spatial distributions of intraseasonal rainfall variations better but also shows more realistic spectral peaks and northward and eastward propagation features of the BSISO over India and the western Pacific. This study indicates that including an air–sea feedback mechanism may have the potential to improve the realism of the mean flow and intraseasonal variability in the Indian and western Pacific monsoon region. 相似文献
7.
大气季节内振荡最显著的特征之一就是其复杂多变的传播特征。为了进一步分析大气季节内振荡的传播特征及其不同传播方向分量的不同作用,基于波的传播理论,对时-空谱分析进行了发展,提出了时-空二维波传播分解方法,并用理想函数验证了其正确性和可行性。结果表明:对任一时-空二维序列,采用此方法均可正确地分解得到空间上具有不同传播方向的3部分分量:前进波、后退波和驻波分量。之后,对向外长波辐射(OLR)、200和850hPa纬向风的东传、西传分量以及驻波分量分别进行联合经验正交函数(CEOF)分析。结果表明,在东传分量上,热带大气季节内振荡主要表现为纬向1波的向东传播,与对流层高、低层纬向风呈现反位相的斜压结构,和未分解的情况相比,前两个主要模态重要性的排序出现了颠倒,说明通过时-空二维波传播分解,避开了各分量之间的相互影响,进一步揭示了前两个模态的物理意义及其在季节内振荡中的重要性;在西传分量上,热带大气季节内振荡主要表现为纬向2—3波的向西传播;驻波分量主要表现出印度洋和太平洋的反位相结构。对热带地区大气季节内振荡的东、西分量的分析,深化了对热带大气季节内振荡东传、西传特征的了解,这对于热带大气季节内振荡更加准确预报及其... 相似文献
8.
In this study, satellite-derived outgoing longwave radiation (OLR) and the reanalysis from the National Centers for Environmental
Prediction/National Center for Atmospheric Research are used as verification data in a study of intraseasonal variability
in the Goddard Laboratory for Atmospheres (GLA) and the United Kingdom Meteorological Office (UKMO) atmospheric general circulation
models. These models simulated the most realistic intraseasonal oscillations (IO) of the 15 Atmospheric Model Intercomparison
Project models previously analyzed. During the active phase of the intraseasonal oscillation, convection is observed to migrate
from the Indian Ocean to the western/central Pacific Ocean, and into the South Pacific Convergence Zone (SPCZ). The simulated
convection, particularly in the GLA model, is most realistic over the western/central Pacific Ocean and the SPCZ. In the reanalysis,
the baroclinic structure of the IO is evident in the eddy-stream function, and eastward migration of the anticyclone/cyclone
pairs occurs in conjunction with the eastward development of convection. Both the GLA and UKMO models exhibit a baroclinic
structure on intraseasonal time scales. The GLA model is more realistic than the UKMO model at simulating the eastward migration
of the anticyclone/cyclone pairs when the convection is active over the western/central Pacific. In the UKMO model, the main
heating is located off the equator, which contributes to the irregular structures seen in this model on intraseasonal time
scales. The maintenance and initiation of the intraseasonal oscillation has also been investigated. Analysis of the latent
heat flux indicates that evaporative wind feedback is not the dominant mechanism for promoting the eastward propagation of
the intraseasonal oscillation since evaporation to the west of the convection dominants. The data suggest a wave-CISK (conditional
instability of the secondkind) type mechanism, although the contribution by frictional convergence is not apparent. In the
GLA model, enhanced evaporation tends to develop in-place over the west Pacific warm pool, while in the UKMO simulation westward
propagation of enhanced evaporation is evident. It is suggested that lack of an interactive ocean may be associated with the
models systematic failure to simulate the eastward transition of convection from the Indian Ocean into the western Pacific
Ocean. This hypothesis is based upon the examination of observed sea surface temperature (SST) and its relationship to the
active phase of the intraseasonal oscillation, which indicates that the IO may evolve as a coupled ocean-atmosphere mode.
The eastward propagation of convection appears to be related to the gradient of SST, with above normal SST to the east of
the convection maintaining the eastward evolution, and decreasing SST near the western portion of the convective envelope
being associated with the cessation of convection.
Received: 13 September 1996/Accepted: 14 April 1997 相似文献
9.
IAP AGCM4.0模式对热带大气季节内振荡的模拟评估 总被引：1，自引：1，他引：0
基于中国科学院大气物理所大气环流模式IAP AGCM4.0总共30年(1979~2008年)的模拟结果,评估了IAP AGCM4.0模式对热带大气季节内振荡的模拟能力。分析结果表明IAP AGCM4.0模式可以在一定程度上模拟出热带大气季节内振荡的主要时空谱结构特征,在周期30~80天处存在明显的谱能量中心;模式模拟的季节内振荡东传的主要特征与观测基本一致,东移波的能量远大于西移波。基于RMM指数(All-season Real-time Multivariate MJO Index)的分析表明,模式模拟的850 h Pa和200 h Pa季节内尺度风场和对流活动在赤道地区的空间分布与观测基本一致。但与观测相比,模式模拟的热带大气季节内振荡的周期较短,东传速度快于观测,虚假的西传特征过强,对流活跃区域范围较小、强度较弱。就非绝热加热而言,模式模拟结果与再分析资料比较接近,但最大加热在印度洋和西太平洋地区出现的位相较晚。进一步分析表明,模式中影响对流触发的相对湿度阈值(RHc)的不同取值(RHc分别取为85%、90%、95%和100%),可以显著影响热带大气非绝热加热垂直廓线,从而影响模式对热带大气季节内振荡的模拟;当对流触发相对湿度阈值取为90%时,IAP AGCM4.0模式对热带大气季节内振荡模拟的能力相对最好,非绝热加热垂直廓线在不同位相的分布特征也与再分析资料最为接近。这说明模式对流参数化方案中不同参数的合适选取,可以改进模式对热带大气季节内振荡的模拟能力。 相似文献
10.
The focus is on sensitivity studies to identify factors that increase the skill of the ECHAM4 atmosphere general circulation model (GCM) in representing the intraseasonal oscillation (ISO). The ISO mode is dominated by an eastward propagating oscillation during boreal winter, also known as the Madden-Julian oscillation. A previous study compared observations and reanalysis data to uncoupled and coupled versions of the ECHAM4 GCM. The sensitivity experiments in the present study use those results to assess the importance of the following mechanisms for the simulation of the ISO. First, the vertical resolution is increased to indicate the effect of improved representation of the tropospheric stratification. Second, the horizontal resolution is increased to investigate the importance of the ratio of vertical and horizontal resolution. Third, the effects of the land-sea distribution are studied in an experiment with land points associated with the maritime continent replaced by sea points. Fourth, the ECHAM4 GCM is forced by a T42 version of the observed optimum interpolated SST (OISST) dataset as used in the ECMWF reanalysis to study the influence of the SST as a boundary condition. In addition to integrations with monthly mean SSTs, also an experiment with weekly mean SSTs is examined. The increased vertical resolution slightly slows down the propagation speed of the simulated ISO. Increasing the horizontal resolution from T42 to T106 results in a very noisy and therefore poorer simulation of the ISO. It is suggested that this is due to an inappropriate ratio of vertical and horizontal resolution. Replacing the land points associated with the maritime continent with sea points leads to a more realistic representation of convection over the maritime continent than in the standard model. A consequence is a reduction of the erroneous eastward shift of the simulated ISO activity, although the phase speed of the ISO is simultaneously increased. ECHAM4 forced by an OISST dataset interpolated to the models T42 grid leads to the strongest improvements, since the annual mean AMIP SST averaged over the maritime continent region is 0.14 K warmer than the OISST, and individual grid points can be more than 0.5 K warmer. The reason is that the AMIP SST uses only measurements over water for grid points that are partly covered by land, whereas the OISST also blends these measurements with measurements over the land portions. ECHAM4 forced by the interpolated OISST, which is in the mean only 0.04 K colder over the maritime continent than the OISST, produces a reasonable ISO with a mean period of 40 days for a forcing with monthly mean SSTs and 48 days for a forcing with weekly mean SSTs. These results illustrate the strong influence of the details of the prescribed SST on the simulation of the ISO. 相似文献