非绝热加热对大气局地扰动位能的影响和机理

本文运用统计相关和奇异值分解方法,系统地考察了非绝热加热对大气局地扰动位能的影响特征和机理问题.分析结果表明,热带地区海表温度异常和扰动位能的耦合相关特征与厄尔尼诺和南方涛动变率的关系密切,扰动位能在热带外地区的耦合模态空间型呈现出与北太平洋-北美大气遥相关型极为相似的分布特征.通过进一步考察大气中视热源与扰动位能的耦合特征,发现在低纬地区仍主要反映了厄尔尼诺和南方涛动的影响机制,在热带外地区,视热源异常的耦合模态则表现出与北半球环状模类似的特点,这表明热带外地区大气的局地能量有效性与热带非绝热加热的遥强迫以及局地性热源加热强迫有着极为密切的联系.     

分层扰动位能理论及其应用——以南海夏季风的年际变化为例

在扰动位能(PPE)理论的基础上,针对不同高度上局地环流能量转换问题,本文提出了分层扰动位能(LPPE)的概念.研究表明850hPa的分层扰动位能一阶矩(LPPE1)在热带地区为正,高纬度地区为负,200 hPa高度LPPE1在北美高纬度地区出现正值分布,100 hPa及以上LPPE1热带地区为负,高纬度为正.LPPE1冬季半球的分布与年平均相似,北半球夏季大陆上出现正的极大值.在局地,LPPE1在数值上远远大于分层扰动位能二阶矩(LPPE2)及更高阶矩,因此,LPPE的分布与LPPE1的分布相似.南海季风区低层动能的季节变化与LPPE呈现反向变化关系.相关分析表明,南海夏季风(SCSSM)与春季的LPPE1偶极型分布之间存在着显著的年际(正)相关关系,可以作为SCSSM强度的一个预报因子.春季赤道印度洋、西太平洋海表温度(SST)的负(正)异常对应春季、夏季LPPE1的南负北正(南正北负)偶极型分布,夏季(JJAS) LPPE1的偶极型分布与南海季风区动能的一致增大(减小)是两者耦合模态的主导模态,夏季南海季风区的西风增强(减弱), SCSSM增强(减弱),这是能量异常影响SCSSM的一个可能的机制.     

北半球大气遥相关型与区域尺度大气扰动

北极涛动(AO)、北大西洋涛动(NAO)和太平洋-北美型(PNA)等北半球大气遥相关型,可以用大气位势高度的物理分解扰动分量解释.结果发现,AO反映的是北极地区行星尺度纬圈平均扰动分量的变化,PNA与持续性天气尺度扰动分量相联系,NAO是行星尺度纬圈平均扰动与天气尺度扰动共同作用的结果.对行星尺度纬圈平均扰动分量和天气尺度扰动分量用旋转经验正交函数(REOF)展开,不但可以证实人们已经命名的区域性大气涛动,还新发现了北极地区的两对偶极涛动、欧亚涛动(EAO)和"大西洋-欧亚型"(AEA)波列.这些涛动连接了相邻地区的异常天气和异常气候.     

全球大气环流的三型分解理论

文章回顾了近年来发展的全球大气环流的三型分解理论,主要包括全球大气环流三型分解模型及全球大尺度水平型环流、经圈型环流和纬圈型环流的动力学方程组理论.与传统二维环流分解方法的对比表明:全球大气环流三型分解将垂直涡度中由水平涡旋运动与辐合辐散运动引起的垂直涡度分量有效地分解开来,也将垂直速度中的经向垂直环流与纬向垂直环流分量分解开来,为研究辐合辐散过程对垂直涡度场的演变作用及局地垂直环流的准确描述问题提供了新的方法.全球大气环流的三型分解是一种基于实际大气运动特征的三维环流分解方法,其分解后的水平型、经圈型以及纬圈型环流可分别看作是中高纬度Rossby波及低纬度Hadley和Walker环流在全球的推广.因此,新的环流分解模型及其动力学方程组为中高纬度大气环流与低纬度大气环流之间的相互作用问题研究以及全球变暖背景下大尺度环流异常演变的物理机制问题研究提供了新的理论与方法.     

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.     

耦合模式长期积分中东亚夏季风与ENSO联系的不稳定性

通过对挪威卑尔根全球大气-海洋-海冰耦合模式300a控制积分结果进行交叉子波分析,揭示了东亚夏季风（EASM）与同期Nio3区（90°W～150°W,5°S～5°N）海洋表面温度异常的相关关系在长期变化中是不稳定的,呈现出明显的阶段性特征.气候要素场在二者联系的紧密（HCP）和微弱（LCP）时期差别显著,在HCP时期,西北太平洋对流层低层出现一对耦合的异常气旋和反气旋性环流系统;东南亚地区对流层低层表现为强东风异常,风速的年际变率加大;热带西太平洋对流层温度和位势高度场的年际变率普遍加强.此外,中国夏季降水与同期Nio3区海洋表面温度异常的相关关系在上述两种时期也存在较大差别.     

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.     

春季Hadley环流异常对夏季西北太平洋热带气旋频数影响的数值模拟试验

观测事实揭示出春季Hadley环流与夏季西北太平洋热带气旋频数之间存在显著的负相关关系.由春季Hadley环流异常引起的西北太平洋地区夏季纬向风垂直切变、大气辐合辐散等的异常变化是这一关系存在的内在原因.本文通过数值试验对这一关系的真实性进行了验证,即利用中国科学院大气物理研究所发展的9层大气环流模式（IAP9L-AGCM）模拟了春季Hadley环流异常偏强情景,并分析了该情景下影响西北太平洋热带气旋生成的环境场的响应.结果表明,在春季Hadley环流偏强情景下,夏季西北太平洋地区纬向风垂直切变幅度加大,低空大气异常辐散,高空大气异常辐合,东亚夏季风减弱,这种环流背景不利于热带气旋生成和发展,因此,西北太平洋热带气旋频数异常偏少.数值模拟结果与已有的诊断结果相吻合,进而证实了春季Hadley环流与夏季西北太平洋热带气旋频数负相关关系的存在.因此,春季Hadley环流信号可以用于西北太平洋热带气旋活动的气候预测.     

全球纬向平均大气环流基本模态年代际变化的凝聚小波分析

应用1871-2008年NCEP／NCAR月平均再分析资料,研究了1948-2008年期间全球纬向平均大气环流基本模态的年代际变化.小波凝聚谱的结果表明全球纬向平均大气环流基本模态存在显著的20年左右周期的年代际变化.小波凝聚位相的结果清楚地显示了纬向平均大气环流基本模态的变化顺序.在20年左右的年代际变化时间尺度上,全球纬向平均温度超前纬向平均位势高度2个月,同时超前纬向平均流10个月出现变化;全球纬向平均位势高度又超前纬向平均流8个月出现变化.全球温度上升(下降), 将使高纬度的纬向平均位势高度降低(升高),中低纬度的纬向平均位势高度升高(降低);进而使得中高纬和热带的纬向平均西风加(减)速或东风减(加)速,同时使极地和副热带的西风减(加)速或东风加(减)速.20世纪70年代末期以来全球显著增暖的异常信号最早出现在南半球对流层顶附近,其次出现在南半球对流层低层、北半球对流层顶附近和北半球对流层低层.     

印度洋经圈环流的结构及其与风应力的耦合关系

利用惯性理论计算了印度洋经圈环流的结构,并分析风应力与经圈环流的耦合关系．结果表明,惯性理论能够较好地表征经圈环流的特点——副热带环流圈与跨赤道环流圈,北印度洋经圈环流会随着季风的季节性反向而翻转．风应力异常和经向流函函数异常可以分解为夏季风模态、冬季风模态及异常模态．当异常呈现冬夏季风期间环流的结构特征时,经圈环流与风应力的同时相关最大;在异常模态时,二者的关系较为复杂．通过20°S和赤道断面的输送均表现为多时间尺度变化的特征,主要的周期集中在年际和年代际尺度上．     

Structure of the Indian Ocean Meridional Overturning Circulation and its relationship with the zonal wind stress

We studied the structure of the Indian Ocean(IO)Meridional Overturning Circulation(MOC)by applying a nonlinear inertia theory and analyzed the coupled relationship between zonal wind stress and MOC anomalies.Our results show that the inertia theory can represent the main characteristics of the IO MOC:the subtropical cell(STC)and cross-equator cell(CEC).The stream function in equatorial and northern IO changes a sign from winter to summer.The anomalies of the zonal wind stress and stream function can be decomposed into summer monsoon mode,winter monsoon mode,and abnormal mode by using the singular vector decomposition(SVD)analysis.The first two modes correlate with the transport through 20°S and equator simultaneously whereas the relationship obscures between the third mode and transports across 20°S and equator,showing the complex air-sea interaction process.The transport experiences multi-time scale variability according to the continuous power spectrum analysis,with major periods in inter-annual and decadal scale.     

A theoretical investigation of the tropical Indo-Pacific tripole mode

The El Ni o-Southern Oscillation(ENSO)phenomenon in the tropical Pacific has been a focus of ocean and climate studies in the last few decades.Recently,the short-term climate variability in the tropical Indian Ocean has attracted increasingly more attention,especially with the proposition of the Indian Ocean Dipole(IOD)mode.However,these phenomena are often studied separately without much consideration of their interaction.Observations reveal a striking out-of-phase relationship between zonal gradients of sea surface height anomaly(SSHA)and sea surface temperature anomaly(SSTA)in the tropical Indian and Pacific Oceans.Since the two oceans share the ascending branch of the Walker cells over the warm pool,the variation within one of them will affect the other.The accompanied zonal surface wind anomalies are always opposite over the two basins,thus producing a tripole structure with opposite zonal gradients of SSHA/SSTA in the two oceans.This mode of variability has been referred to as Indo-Pacific Tripole(IPT).Based on observational data analyses and a simple ocean-atmosphere coupled model,this study tries to identify the characteristics and physical mechanism of IPT with a particular emphasis on the relationships among ENSO,IOD,and IPT.The model includes the basic oceanic and atmospheric variables and the feedbacks between them,and takes into account the inter-basin connection through an atmospheric bridge,thus providing a valuable framework for further research on the short-term tropical climate variability.     

南印度洋副热带偶极模在ENSO事件中的作用

南印度洋副热带偶极模（Subtropical Dipole Pattern,SDP）是印度洋存在的另一种很明显的偶极型海温差异现象,在年际和年代际尺度上均有十分明显的表现.而目前有关印度洋海气相互作用的研究主要集中在赤道印度洋地区,针对南印度洋地区的工作还比较少,特别是有关南印度洋海温与ENSO（El NiDo-Southern Oscillation）事件关系的研究.本文初步探讨了年际尺度上南印度洋副热带偶极型海温变化差异与ENSO事件的关系,发现SDP与ENSO事件有密切的联系,SDP事件就像连接正负ENSO位相转换的一个中间环节,SDP事件前后期ENSO的位相刚好完全相反.进一步,本文通过分析SDP事件前后期海温、高低层风、低层辐合辐散、高空云量和辐射等的变化特征研究了南印度洋偶极型海温异常在ENSO事件中的作用,结果表明：SDP在ENSO事件中的作用不仅涉及海气相互作用的正负反馈过程,还与热带和副热带大气环流之间的相互作用有关,特别是与东南印度洋海温变化所引起的异常纬向风由赤道印度洋向赤道太平洋传播的过程等有十分直接的关系;同时,SDP对ENSO事件的影响在很大程度上还依赖于大尺度平均气流随季节的变换.     

Modes of low-frequency climate variability and their relationships with land precipitation and surface temperature: application to the Northern Hemisphere winter climate

Variations in the Earth's climate have had considerable impact on society sectors such as energy, agriculture, fisheries, water resources, and environmental quality. This natural climate variability must be documented and understood in order to assess its potential impacts, its predictability and relationships with human-induced changes. Understanding the mechanisms responsible for natural variability proceeds through a strategy based on the use of a hierarchy of climate models and careful data analysis. In this paper, we examine primarily climate fluctuations on interannual-to-decadal time scales and their climate signature in terms of precipitation and temperature. First, space and time characteristics of two of the major variability modes, the Southern Oscillation (and its associated teleconnection patterns) and the North Atlantic Oscillation, are documented with a focus onto the midlatitudes of the Northern Hemisphere. Then, the current hypothesis regarding the nature of these modes (forced, coupled or internal) are reviewed based on both simulation results and statistical data analyses. Next, we address the potential predictability of seasonal surface temperature and land precipitation using an ensemble of atmospheric model simulations forced by observed sea surface temperatures. Finally, we review the relationships between the atmospheric variability modes and the recent low-frequency trends and suggest a possible influence of anthropogenic effects on these low-frequency variations.     

The storm tracks and the energy cycle of the Southern Hemisphere: sensitivity to sea-ice boundary conditions

The effect of sea-ice on various aspects of the Southern Hemisphere (SH) extratropical climate is examined. Two simulations using the LMD GCM are performed: a control run with the observed sea-ice distribution and an anomaly run in which all SH sea-ice is replaced by open ocean. When sea-ice is removed, the mean sea level pressure displays anomalies predominantly negatives near the Antarctic coast. In general, the meridional temperature gradient is reduced over most of the Southern Ocean, the polar jet is weaker and the sea level pressure rises equatorward of the control ice edge. The high frequency filtered standard deviation of both the sea level pressure and the 300-hPa geopotential height decreases over the southern Pacific and southwestern Atlantic oceans, especially to the north of the ice edge (as prescribed in the control). In contrast, over the Indian Ocean the perturbed simulation exhibits less variability equatorward of about 50°S and increased variability to the south. The zonal averages of the zonal and eddy potential and kinetic energies were evaluated. The effect of removing sea-ice is to diminish the available potential energy of the mean zonal flow, the available potential energy of the perturbations, the kinetic energy of the growing disturbances and the kinetic energy of the mean zonal flow over most of the Southern Ocean. The zonally averaged intensity of the subpolar trough and the rate of the baroclinic energy conversions are also weaker.     

Instability of planetary waves in a high vertical resolution model

Abstract

A high vertical resolution model is used to examine the instability of a baroclinic zonal flow and a finite amplitude topographically forced wave. Two families of unstable modes are found, consisting of zonally propagating most unstable modes, and stationary unstable modes. The former have time scale and spatial structure similar to baroclinic synoptic disturbances, but are localized in space due to interaction with the zonally asymmetric forcing. These modes transport heat efficiently in both the zonal and meridional directions. The second family of stationary unstable modes has characteristics of modes of low frequency variability of the atmosphere. They have time scales of 10 days and longer, and are of planetary scale with an equivalent barotropic vertical structure. The horizontal structure resembles blocking flows. They are maintained by available potential energy of the basic wave, and have large zonal heat fluxes. The results for both families of modes are interpreted in terms of an interaction between forcing and baroclinic instability to create favoured regions for eddy development. Applications to baroclinic planetary waves are also considered.     

Large scale climate and rainfall seasonality in a Mediterranean Area: Insights from a non‐homogeneous Markov model applied to the Agro‐Pontino plain

In the context of climate change and variability, there is considerable interest in how large scale climate indicators influence regional precipitation occurrence and its seasonality. Seasonal and longer climate projections from coupled ocean–atmosphere models need to be downscaled to regional levels for hydrologic applications, and the identification of appropriate state variables from such models that can best inform this process is also of direct interest. Here, a Non‐Homogeneous Hidden Markov Model (NHMM) for downscaling daily rainfall is developed for the Agro‐Pontino Plain, a coastal reclamation region very vulnerable to changes of hydrological cycle. The NHMM, through a set of atmospheric predictors, provides the link between large scale meteorological features and local rainfall patterns. Atmospheric data from the NCEP/NCAR archive and 56‐years record (1951–2004) of daily rainfall measurements from 7 stations in Agro‐Pontino Plain are analyzed. A number of validation tests are carried out, in order to: 1) identify the best set of atmospheric predictors to model local rainfall; 2) evaluate the model performance to capture realistically relevant rainfall attributes as the inter‐annual and seasonal variability, as well as average and extreme rainfall patterns. Validation tests show that the best set of atmospheric predictors are the following: mean sea level pressure, temperature at 1000 hPa, meridional and zonal wind at 850 hPa and precipitable water, from 20°N to 80°N of latitude and from 80°W to 60°E of longitude. Furthermore, the validation tests show that the rainfall attributes are simulated realistically and accurately. The capability of the NHMM to be used as a forecasting tool to quantify changes of rainfall patterns forced by alteration of atmospheric circulation under climate change and variability scenarios is discussed.