正压大气中基态定常波对强迫响应的影响

本文利用正压涡度方程模式研究了热带不同经度位置异常强迫激发的中纬响应类型与北半球冬季基本气流定常波结构的关系。发现对热带印度洋和太平洋的大部分地区,中纬的响应类型主要由基态定常波的结构决定,并且基态定常波通过向扰动转换动能维持了中纬的响应。通过这些结果解释了GCM研究中发现的中综异常响应类型对El Nino成熟期海表面温度异常位置变化的不敏感性。此外还讨论了观测和GCM中出现的热带印度洋——太平洋地区低频尺度上的纬向偶极强迫源对GCM响应结果的作用。     

大气对外强迫低频遥响应的数值模拟 II:对欧亚中高纬“寒潮”异常的响应

本文通过用IAP GCM所作的数值模拟研究了欧亚大陆中高纬度地区的外强迫在全球大气中激发的响应.结果清楚表明,同赤道地区的热源强迫一样,中高纬度地区的外强迫也可以在全球大气中产生低频遥响应;通过低频波列EAP,欧亚大陆中高纬度地区的寒潮异常可以对赤道中西太平洋地区的大气运动及全球大气环流有重要影响.对30—60天振荡的强迫激发来讲,地球大气的气候基本态是极为重要的,热带大气对于全球大气的低频振荡活动具有尤为突出的作用.     

亚洲夏季风环流结构与热带印度洋偶极型海温异常

使用T42L28大气环流模式就夏季风时期大气对印度洋海温偶极子型异常的响应进行了数值试验研究,结果表明,印度洋偶极子型海温异常可以引起感热和潜热加热异常并进而形成异常辐合辐散,导致热带印度洋及其邻近地区夏季降水异常。同时此热带扰动可激发或造成中纬度异常波列。通过改变季风区温度场分布,偶极子型海温强迫可以影响大气的正／斜压环流结构和斜压性强弱。强的纬向风垂直切变趋向于靠近海洋异常偏暖的地区。不论是正偶极子型强迫或负偶极子型强迫,西太平洋暖池和东亚地区的大气环流均出现异常并激发出中纬度的异常波列,但异常类型并未显著反相。     

大气对热带热源低频响应的数值研究

本文设计—关于基本气流线性化的二层球面模式研究大气对不同类型赤道热源的低频响应,分析了全球大气40—50天低频振荡的结构和传播特点。结果表明:全球大气对热带热源响应特征是在热带地区表现为东西向非对称的热源东侧的Kelvin响应和西侧的Rossby响应,高低层反位相;在中高纬响应是具有低频振荡的二维Rossby波列,且具有相当正压结构。低频振荡的传播特征表明热带大气对移动性热源的响应更有利于振荡的向东传播,且该振荡可以沿波列路径向中高纬度传播,然后再折向低纬。二维Rossby波列是联系着热带和中高纬度地区低频振荡的基本动力学途径。该波列的形成取决于基本气流的分布,冬半球波列振幅较大。因此本文认为40—50天低频振荡是全球尺度的现象,它是一涉及到热带对流加热、赤道Kelvin波、亦道Rossby波、二维Rossby波列以及基本气流等动力学相互作用的过程。     

澳洲大陆热力强迫对南北半球环流异常的影响效应

本文采用OSU-AGCM动力框架加入牛顿加热项构成的简化大气环流模式,研究澳洲大陆热力强迫与南北半球环流异常的相关关系。本文对不同平衡温度模拟结果的差异(偏差场)进行了分析,探讨某局地热力强迫对全球其它区域环流异常的影响效应.数值试验结果表明,南北半球海陆热力结构有利于两半球行星尺度经向环流的加强及其低纬跨赤道气流的形成:南半球澳洲大陆热力强迫可以通过东、西风带侧向藕合效应,显著地影响北半球中纬西风带急流状况;二维Rossby波能量频散径向传播可能足澳洲热力强迫与北半球常定环流系统的异常变化相关现象的重要成因,且澳洲大陆强迫产生的径向波列路经与PNA、EU型相似.     

地形对大气低频振荡影响的数值研究

使用五层全球谱模式研究地形对月、季尺度低频振荡的影响。该模式为三角形截断，截断波数为１０，并含有较为完善的物理过程参数化，对分别用有地形和无地形模式来模拟的低通滤波资料进行了分析和比较。结果表明，地形强迫产生的月、季尺度低频波列有明显的季节特征。北半球夏季由地形影响产生的热带热源是南北半球低频波列的源。模拟结果进一步指出北半球地形强迫产生的波列扰动可以引起赤道纬向流的异常，这种纬向流的瞬变强迫可以     

平流层大气对对流层大尺度热力强迫的线性、定常响应

本文采用一个关于大气定常波的16层线性化原始方程三维谱模式,研究了冬季平流层大气对于对流层各种定常大尺度热力强迫的响应。研究结果表明,对流层中的热力强迫作用对于冬季平流层中垂直向上传播的行星定常波的维持有显著的贡献,平流层中强迫扰动的水平结构及其与对流层中扰动的差异与加热场的垂直结构有密切的关系。相对于强度和空间结构相同的强迫源而言,中纬强迫对于平流层的作用比低纬强迫更为显著。     

大气对南极冰异常的遥响应及其季节内振荡

运用ＩＡＰＡＧＣＭ模式证实了大气对南极冰异常的强迫遥响应是激发产生全球大气季节内振荡的重要机制,进而着重考察了候平均偏差结果的时间序列,并且通过带通滤波处理,特别分析了响应场中３０～６０ｄ低频振荡的特征及其活动。通过分析发现：大气对南极冰减退的响应是一种具有３０～６０ｄ周期的低频遥响应,并呈现出清楚的二维Ｒｏｓｓｂｙ波列特征;强迫场中的３０～６０ｄ季节内振荡具有着同实际大气中的低频振荡相类似的垂直结构和传播特征。大气响应场中３０～６０ｄ振荡能量在垂直方向上随高度的增加而增加,在纬向上表现出明显的区域性特征,即季节内振荡的最大动能区（由于ＣＩＳＫ机制）分布在大洋内;ＥＵＰ,ＰＮＡ,ＡＳＡ和ＲＳＡ波列可能是全球大气低频扰动传播的主要路径,３０～６０ｄ低频扰动在波列路径上的传播具有很大的一致性和系统性,从而使中高纬和热带地区、以及南北半球的３０～６０ｄ大气振荡相互联系起来,而且可以认为,赤道中太平洋和赤道中大西洋地区是南北半球３０～６０ｄ低频振荡间相互作用和相互联系的重要通道。     

七月大气环流对南极海冰异常的响应

本文用一个全球大气九层谱模式，模拟了七月份南极两个不同海区海冰区异常对大气环流的影响。主要讨论了大气环流对南极海冰异常存在的局地性的及全球性的响应。细致分析了二个区域极冰异常导致的南北半球低频波列分布的差异，以及它们对热带区域及亚洲季风区降水、越赤道气流的不同影响。最后则依据我们的模拟结果，讨论了南极海冰异常影响全球大气环流的动力学机制。     

强迫二维Rossby波传播特征的数值试验

Bjerknes提出了大气对外界强迫响应的概念。大量研究已证明这一遥响应机制的存在,且大气对强迫源的遥响应及其低频活动的共同特征都表现为类似大圆路径的波列,Hoskins的理论研究揭示了这现象的实质,他把叶笃正所提出的Rossby波的频散理论推广到球面上。黄荣辉曾利用波折射指数平方与EP通量系统地研究了北半球冬夏准定常行星波的传播规律。邹晓蕾、叶笃正、吴国雄提出了中高纬的相关链、定常地形波链,并指出北美与东亚相关型的差异与两区域地形准定常波在不同纬向流中传播的差异有关。观测分析亦表明,青藏高原是大气低频振荡的强迫源,且在此区域可产生向北、向南或向东、向西传播的波列。     

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.

     

Interactions between the tropical ISO and midlatitude low-frequency flow

In this study, we investigate the interaction between the tropical Intraseasonal Oscillation (ISO) and midlatitude atmospheric low-frequency variability, using observational data and numerical models, with a special emphasis on the role of the synoptic eddy feedback. A statistical closure for the synoptic eddy-to-low frequency flow feedback is constructed, based on a singular value decomposition (SVD) method with observational data. Applying this statistical closure to a barotropic model and a baroclinic 2½-layer model, we study the role of the synoptic eddy feedback in the midlatitude response to the tropical ISO forcing. Both observational and modeling studies show that the strongest synoptic eddy forcing appears at the Pacific and Atlantic storm-track regions, and the synoptic eddy exerts a positive feedback to the midlatitude low-frequency flow induced by tropical ISO forcing. Our numerical experiments demonstrate the possible role of midlatitude disturbance forcing in the ISO initiation at the equator. The signal of the midlatitude perturbations propagates southeastward in the form of a Rossby wave package. It may reach the equator within several days under either easterly or westerly basic flow regimes. The response at the equator has observed ISO-like structure and eastward propagation characteristics.     

Tropical/extratropical forcing on wintertime variability of the extratropical temperature and circulation

The secular trends and interannual variability of wintertime temperatures over northern extratropical lands and circulations over the northern hemisphere are examined using the NCEP/NCAR reanalysis from 1961 to 2010. A primitive equation dry atmospheric model, driven by time-averaged forcing in each winter diagnosed from the NCEP reanalysis, is then employed to investigate the influences of tropical and extratropical forcing on the temperature and circulation variability. The model has no topography and the forcing is thus model specific. The dynamic and thermodynamic maintenances of the circulation and temperature anomalies are also diagnosed. Distinct surface temperature trends over 1961–1990 and 1991–2010 are found over most of the extratropical lands. The trend is stronger in the last two decades than that before 1990, particularly over eastern Canadian Arctic, Greenland, and Asia. The exchange of midlatitude and polar air supports the temperature trends. Both the diagnosed extratropical and tropical forcings contribute to the temperature and circulation trends over 1961–1990, while the extratropical forcing dominates tropical forcing for the trends over 1991–2010. The contribution of the tropical forcing to the trends is sensitive to the period considered. The temperature and circulation responses to the diagnosed tropical and extratropical forcings are approximately additive and partially offsetting. Covariances between the interannual surface temperature and 500-hPa geopotential anomalies for the NCEP reanalysis from 1961 to 2010 are dominated by two leading modes associated with the North Atlantic Oscillation (NAO) and Pacific-North American (PNA) teleconnection patterns. The diagnosed extratropical forcing accounts for a significant part of the NAO and PNA associated variability, including the interannual variability of stationary wave anomalies, as well as dynamically and thermodynamically synoptic eddy feedbacks over the North Atlantic and North Pacific. The tropical forcing contributes to the PNA related temperature and circulation variability, but has a small contribution to the NAO associated variability. Additionally, relative contributions of tropical Indian and Pacific forcings are also assessed.     

THE FORCING OF ANOMALOUS DIVERGENT WIND AND TRANSIENT VORTICITY FLUXES TO THE ANOMALY OF STATIONARY WAVES

The forcing of anomalous divergent wind and transient vorticity fluxes to the anomaly ofstationary waves of DJF (December—January—February) 1982/1983 is studied.The results showthat the anomaly of the transient vorticity flux convergence tends to cancel or dissipate the anomalyof the vorticity sources generated by divergent wind.The stationary wave anomalies are maintainedby both of these forcings.It is also found that,for the DJF 1982/1983 case,both of the vorticitysource anomaly over the tropical and equatorial eastern Pacific and that over the midlatitude NorthPacific are important to maintain the atmospheric circulation anomaly over the Pacific/NorthAmerica region.     

The Dynamics of ENSO Anomaly as Revealed in Ensemble Climate Simulations-Impact of Mean Stationary Wave

A series of climate ensemble experiments using the climate model from National Centers for Environmental Prediction (NCEP) were performed to exam impact of sea surface temperature (SST) on dynamics of El-Nino/South-crn Oscillation (ENSO).A specific question addressed in this paper is how important the mean stationary wave influences anomalous Rossby wave trains or teleconnection patterns as often observed during ENSO events.Evidences from those ensemble simulations argue that ENSO anomalies,especially over Pacific-North America (PNA) region,appear to be a result of modification for climatological mean stationary wave forced by persistent tropical SST anomalies Therefore,the role of SST forcing in maintaining climate basic state is emphasized.In this argument,the interaction between atmospheric internal dynamics and external forcing,such as SST is a key element to understand and ultimately predict ENSO.     

Contrasting the tropical responses to zonally asymmetric extratropical and tropical thermal forcing

The mechanism is investigated by which extratropical thermal forcing with a finite zonal extent produces global impact. The goal is to understand the near-global response to a weakened Atlantic meridional overturning circulation suggested by paleoclimate data and modeling studies. An atmospheric model coupled to an aquaplanet slab mixed layer ocean, in which the unperturbed climate is zonally symmetric, is perturbed by prescribing cooling of the mixed layer in the Northern Hemisphere and heating of equal magnitude in the Southern Hemisphere, over some finite range of longitudes. In the case of heating/cooling confined to the extratropics, the zonally asymmetric forcing is homogenized by midlatitude westerlies and extratropical eddies before passing on to the tropics, inducing a zonally symmetric tropical response. In addition, the zonal mean responses vary little as the zonal extent of the forced region is changed, holding the zonal mean heating fixed, implying little impact of stationary eddies on the zonal mean. In contrast, when the heating/cooling is confined to the tropics, the zonally asymmetric forcing produces a highly localized response with slight westward extension, due to advection by mean easterly trade winds. Regardless of the forcing location, neither the spatial structure nor the zonal mean responses are strongly affected by wind–evaporation–sea surface temperature feedback.     

The influences of interannual stratification variability and wind stress forcing on ENSO before and after the 1976 climate shift

In order to understand the change in oceanic variability associated with the climate shift of the mid-1970s, we analyze the contribution of momentum forcing to the leading baroclinic modes over the tropical Pacific using Simple Ocean Data Assimilation (SODA, version 2.0.2) for the period of 1958–1997. Specifically, we look at the statistical relationship between the wind projection coefficients and climate indices and attempt to provide a physical explanation for the observed changes. It is found that the wind stress projection coefficients according to the oceanic baroclinic modes are different in terms of their magnitude and phase in the tropical Pacific, reflecting a specific forcing associated with each mode before and after the 1976 climate shift. Compared to that before the 1970s, the first baroclinic mode is had a greater effect on the interannual sea surface temperature due to equatorial wave dynamics, and there was an increased delayed response of the second baroclinic mode variability to the interannual atmospheric forcing after the late 1970s. This reflects changes in ENSO feedback processes associated with the climate shift. Our analysis further indicates that, after the late 1970s, there was a decrease in the wind stress forcing projecting onto the Ekman layer, which is associated with increased mixed-layer depth. This result suggests that the changes in the ENSO properties before and after the late 1970s are largely associated with the changes in the way in which the wind stress forcing is dynamically projected onto the surface layer of the tropical Pacific Ocean over interannual timescales.     

The influence of tropical Pacific forcing on the Arctic Oscillation

The potential role of tropical Pacific forcing in driving the seasonal variability of the Arctic Oscillation (AO) is explored using both observational data and a simple general circulation model (SGCM). A lead–lag regression technique is first applied to the monthly averaged sea surface temperature (SST) and the AO index. The AO maximum is found to be related to a negative SST anomaly over the tropical Pacific three months earlier. A singular value decomposition (SVD) analysis is then performed on the tropical Pacific SST and the sea level pressure (SLP) over the Northern Hemisphere. An AO-like atmospheric pattern and its associated SST appear as the second pair of SVD modes. Ensemble integrations are carried out with the SGCM to test the atmospheric response to different tropical Pacific forcings. The atmospheric response to the linear fit of the model’s empirical forcing associated with the SST variability in the second SVD modes strongly projects onto the AO. Idealized thermal forcings are then designed based on the regression of the seasonally averaged tropical Pacific precipitation against the AO index. Results indicate that forcing anomalies over the western tropical Pacific are more effective in generating an AO-like response while those over the eastern tropical Pacific tend to produce a Pacific-North American (PNA)-like response. The physical mechanisms responsible for the energy transport from the tropical Pacific to the extratropical North Atlantic are investigated using wave activity flux and vorticity forcing formalisms. The energy from the western tropical Pacific forcing tends to propagate zonally to the North Atlantic because of the jet stream waveguide effect while the transport of the energy from the eastern tropical Pacific forcing mostly concentrates over the PNA area. The linearized SGCM results show that nonlinear processes are involved in the generation of the forced AO-like pattern.     

The influence of the Madden-Julian oscillation on high-latitude surface air temperature during boreal winter

By analyzing NCEP-NCAR reanalysis daily data for 1979–2016, the modulation by Madden-Julian Oscillation (MJO) of the wintertime surface air temperature (SAT) over high latitude is examined. The real-time multivariate MJO (RMM) index, which divides the MJO into eight phases, is used. It is found that a significantly negative SAT anomaly over the northern high latitude region of (180°–60 °W, 60°–90 °N) lags the MJO convection for 1∼2 weeks in phase 3, in which the enhanced convective activity exists over the Indian Ocean. While a significantly positive SAT anomaly appears over the same region following the MJO phase 7, as the tropical heating shows an opposite sign. Analysis of the anomalous circulation indicates that the observed SAT signal is probably a result of the northeastward propagating Rossby wave train triggered by MJO-related tropical forcing through Rossby wave energy dispersion. By using an anomalous atmospheric general circulation model (AGCM), the significant effect of tropical forcing on organizing the extratropical circulation anomaly is confirmed. Analysis of a temperature tendency equation further reveals that the intraseasonal SAT anomaly is primarily attributed to the advection of the mean temperature by the wind anomaly associated with the anomalous circulation of the MJO-related variability.     

SEASONAL PREDICTIONS FOR SPRING AND AUTUMN SURFACE AIR TEMPERATURES OVER SOUTHERN CHINA BY THE NCEP CFSV2

In this study, we investigate the variations of spring and autumn air temperatures in southern China (SC) and associated atmospheric circulation patterns. During the boreal spring, the SC air temperature is mainly influenced by tropical sea surface temperature anomalies (SSTAs). On the one hand, the El Ni?o SSTA pattern may induce a stronger-than-normal western Pacific subtropical high, which leads to warming in SC. On the other hand, the warm SSTAs in the tropical Indian Ocean may trigger anomalous Rossby wave trains, which propagate northeastward and result in anomalously high temperature in SC. During the boreal autumn, however, the SC temperature is more likely affected by mid-latitude atmospheric circulation, such as the wave trains forced by the North Atlantic SSTAs. The NCEP Climate Forecast System version 2 (CFSv2) is able to capture the climatology of SC air temperatures during both spring and autumn. For interannual variation, the CFSv2 shows a good skill for predicting the SC temperature in spring, due to the model’s good performance in capturing the associated atmospheric circulation anomalies as responses to tropical SSTAs, in spite of the overestimated relationship with the El Ni?o–Southern Oscillation (ENSO). However, the model has a poor skill for predicting the SC temperature in autumn, primarily due to the unrealistic prediction of its relationship with the ENSO.