论东亚夏季风的特征、驱动力与年代际变化

本文是以新的资料和研究结果对东亚夏季风的基本特征、驱动力和年代际变化所作的重新分析与评估。内容包括四个部分:（1）东亚夏季风的基本特征;（2）东亚夏季风的驱动力;（3）东亚夏季风的年代际变率与原因;（4）东亚夏季风与全球季风的关系。结果表明:东亚夏季风是亚洲夏季风的一个重要有机部分,主要由来源于热带的季风气流组成,并随季节由南向北呈阶段性推进,它是形成夏季东亚天气与气候的主要环流和降水系统。驱动夏季风的主要强迫有三部分:外部强迫、耦合强迫与内部变率,其中人类活动引起的外强迫（气候变暖、城市化、气溶胶增加等）是新出现的外强迫,它正不断改变着东亚夏季风的特征与演变趋势。海洋与陆面耦合强迫作为自然因子是引起东亚夏季风年际和年代际变化的主要原因,其中太平洋年代尺度振荡（PDO）与北大西洋多年代尺度振荡（AMO）的协同作用是造成东亚夏季风30~40年周期振荡的主要原因。1960年代以后,东亚夏季风经历了强—弱—强的年代际变化,相应的中国东部夏季降水型出现了“北多南少”向“南涝北旱”以及“北方渐增”的转变。最近的研究表明,上述东亚夏季风年代际变化与整个亚非夏季风系统的变化趋势是一致的。在本世纪主要受气候变暖的影响,夏季风雨带将持续北移,中国北方和西部地区出现持续性多雨的格局。最后本文指出,亚非夏季风系统相比于其他区域季风系统更适合全球季风的概念。     

亚洲夏季风动力学研究综述

亚洲夏季风按照气候带可以分为东亚副热带夏季风和亚洲热带夏季风。就气候平均而言,东亚副热带夏季风于4月初在我国江南(泛称“华南”)地区建立,而亚洲热带夏季风首先于5月初在孟加拉湾东北部建立,之后向东推进,于5月第4候到达南海,然而夏季风无法直接西传至印度地区,因此印度夏季风的爆发表现为热带对流在阿拉伯海上空自赤道向北逐步推进的特征。东亚副热带夏季风与亚洲热带夏季风的爆发机制和时空变率都存在明显差异。亚洲夏季风的建立与青藏高原的动力和热力强迫作用联系紧密,其中东亚副热带夏季风的建立又与东亚大陆-西北太平洋的纬向海陆热力差异的季节转换紧密联系,而亚洲热带夏季风的爆发则与亚洲南部地区对流层中上部经向温度梯度的季节变化有关。同时,亚洲热带夏季风的建立过程还与亚洲南部高、低空环流的垂直耦合密切相关。就季节内变化而言,东亚副热带夏季风在4月份表现出10～20天季节内振荡,这与青藏高原表面感热的季节内变化有关,而盛夏的东亚副热带夏季风则存在准双周和21～30天两种振荡信号。亚洲热带夏季风的季节内振荡包含30～60天的北传信号和10～20天的西传信号,其中北传信号与环境气流的垂直切变、边界层辐合以及暖SST下垫面有关。亚洲夏季风年际变率的主要外强迫是ENSO事件,同时印度洋和大西洋海温异常、南极海冰以及青藏高原的冬、春季积雪和感热异常也影响着亚洲夏季风的年际变率。而亚洲夏季风的年代际变化既与气候系统的自然变率有关,又受热带海温强迫、人为排放气溶胶浓度和青藏高原表面热状况长期变化影响。      

过去1500年典型暖期东亚夏季风年代际变化特征对比及其可能成因

利用通用地球系统模式开展的过去1500年气候模拟全强迫试验和对照试验结果，在验证模式模拟性能的基础上，采用多变量经验正交函数分解等方法，对比分析了典型暖期东亚夏季风年代际变化特征及其成因机制。结果表明，两个典型暖期东亚夏季风变化的主周期均为准10 a和准20 a。中世纪暖期黄河流域至日本南部一带降水偏多，长江流域以南和西北太平洋一带降水偏少；现代暖期东亚夏季风降水表现为“南涝北旱”型分布特征。内部变率是影响典型暖期东亚夏季风变化的主控因子之一，其中太平洋年代际振荡起决定性作用。当太平洋年代际振荡处于正位相时，热带西太平洋（东亚大陆）变暖（变冷），东亚地区海、陆热力差减小，对应弱的东亚夏季风。另外，中世纪暖期海平面气压的动态变化对应850 hPa风场在西北太平洋（日本海）一带均出现了经向排列的异常反气旋（气旋），从而导致中国南部（北部）降水偏多（偏少）。      

东亚夏季风次季节变化研究进展

东亚夏季风次季节（10~90 d）变化是中国夏季持续性强降水、高温热浪等高影响天气事件的重要环流载体,处于天气预报上限和气候季节预测下限之间的预报过渡区。研究表明:东亚夏季风次季节变化是东亚夏季风的固有物理特征,它和季节进程之间的时间锁相关系是东亚夏季风次季节变化潜在可预报性的重要来源。东亚夏季风次季节变化与Madden-Julian振荡（MJO）存在显著差异,试图通过MJO来预测东亚夏季风次季节变化的不确定性较大。东亚夏季风次季节预测的另一重要来源是下垫面外强迫,包括欧亚大陆春季积雪、中国东部春季土壤湿度和厄尔尼诺-南方涛动（ENSO）事件。此外,去趋势偏-交叉相关分析统计方法能够分析东亚夏季风多因子和多时间尺度问题。目前,亟需解决的科学问题包括:东亚夏季风次季节模态的客观定量描述、造成东亚夏季风次季节模态年际变化的关键物理过程、不同外强迫因子对东亚夏季风次季节模态的共同影响。     

FGOALS-g3模拟的南亚夏季风:气候态和年际变率

南亚夏季风的变化决定着印度半岛的旱涝状况,气候系统模式则是研究南亚夏季风变化规律的重要工具。本文基于观测和JRA55再分析资料,系统评估了FGOALS-g3模式模拟的南亚夏季风气候态和年际变率,并重点关注FGOALS-g3与FGOALS-g2以及是否考虑海气相互作用的模拟差异。结果表明,由于局地海温模拟的变化,相比于FGOALS-g2,FGOALS-g3模拟的南亚夏季风在气候态热带印度洋信风和El Ni?o期间沃克环流下沉支上有明显改进。同时,由于对流层系统性冷偏差持续存在并且中心位于副热带300 hPa附近,造成气候态上经向温度梯度减弱,使季风环流减弱,导致FGOALS-g3中陆地季风槽的水汽辐散偏差和降水干偏差仍然存在;在年际变率上,FGOALS-g3模拟的El Ni?o期间赤道西太平洋海温冷异常偏弱,印度洋偶极子偏强,导致印度半岛下沉运动减弱,FGOALS-g3中ENSO—印度降水负相关关系也依然偏弱。研究表明,耦合过程导致的气候态海温偏差通过改变环流和水汽输送,有效补偿了大气模式中印度半岛中部和中南半岛的降水湿偏差;在年际变率上,耦合模式由于考虑了海温—降水—云短波辐射的负反馈过程,能够减小大气模式模拟偏差的强度,但印太暖池区海温模拟偏差导致沃克环流下沉支偏西,使得印度半岛的降水响应出现更大的湿偏差。     

东亚夏季风和中国东部夏季降水年代际变化的模拟

利用中国科学院大气物理研究所发展的第四代大气环流模式模拟了1970年代末东亚夏季风和相关的中国东部夏季降水年代际变化。结果表明,在给定的观测海温强迫下,模式能模拟出东亚夏季风的年代际减弱及 相关的环流场变化,包括东亚沿海的偏北风异常以及西太平洋副高的形态变化,模式还较好再现了中国东部夏季降水的雨型变化,即长江流域降水偏多,而华北和华南偏少,但位置略偏南。基于奇异值分解(SVD)的分析表明,热带海洋变暖是这次东亚夏季风的年代际减弱的主要因素,这与太平洋年代际振荡(PDO)在1970年代末期的位相转变有关。此外,模式还较好模拟了长江流域的变冷趋势,进而减弱了海陆温差,使东亚夏季风减弱。     

东亚季风近几十年来的主要变化特征

本文简要综述了关于东亚夏季风和冬季风近几十年来的主要变化特征的若干研究结果,特别是关于其年代际变化方面.夏季风及夏季气候的主要变化特征有:1970年代末之后东亚夏季风的年代际时间尺度的减弱以及相应的我国夏季降水江淮流域增多而华北减少、1992年之后我国华南夏季降水增多、1999年之后我国长江中下游夏季降水减少而淮河流域夏季降水增多、东亚夏季风和ENSO之间的年际变化相关性存在不稳定性.而关于东亚冬季风与冬季气候的主要变化特征有:1980年代中期之后东亚冬季风及其年际变率减弱、1970年代中期之后冬季风和ENSO的年际变化相关性较弱、近年来的北极秋季海冰减少对北半球冬季积雪增多有显著贡献、东北冬季积雪在1980年代中期以后增多.与上述变化有关的极端气候和物候都发生了多方面的变化.     

亚洲夏季风的年际和年代际变化及其未来预测

本文是对我们近五年在亚洲夏季风年代际与年际变率及其未来预测方面研究的一个综述.主要包括下列三个问题:(1)根据123年中国夏季降水资料和印度学者的分析,检测出亚洲夏季风具有明显的年代际尺度减弱,这种年代际变化使中国东部(包括东亚)和南亚夏季降水的格局在过去60年中发生了明显变化.在东亚,从1970年代后期开始,主要异常雨带有不断南移的趋势,结果造成了南涝北旱的降水分布,这主要受到60～80年年代际振荡的影响.青藏高原前冬和春季积雪的年代际减少与热带中东太平洋海表温度的年代际增加是东亚降水型改变的主要原因,这是通过减弱亚洲地区夏季海陆温差与夏季风强度而实现的.未来亚洲夏季风的预测表明,东亚夏季风和南亚夏季风对气候变暖有十分不同的响应.东亚夏季风在本世纪将增强,雨带北推,尤其在2040年代之后;而南亚夏季风环流将继续减弱.这种不同的变化是由于两者对高低层海陆热力差异的不同响应造成.(2)年际尺度的变率在亚洲夏季风区主要表现为2年与4～7年的振荡.本文着重分析了2年振荡(TBO)形成的过程、机理及其对东亚降水的影响.对TBO-海洋机理进行了具体的改进,说明了东亚夏季风降水深受TBO影响的原因,尤其是阐明了长江型(YRV) TBO和淮河型(HRV) TBO的特征及其形成的循环过程.(3)在总结亚洲夏季风时期遥相关型的基础上,本文提出了季节内和年际尺度的低空遥相关型:即西北太平洋季风的遥相关型与印度“南支”和“北支”遥相关型.它们基本上反映了沿低空夏季风强风速带Rossby波群速度传播的结果.据此可以根据西北太平洋和印度夏季风的变化分别预测中国梅雨和华北雨季来临和降水异常.最后研究还表明,在本世纪亚洲夏季风可能更显著地受到人类活动造成的全球变暖的影响,未来的亚洲夏季风活动是人类排放的CO2引起的全球变暖与自然变化(海洋和陆面过程(积雪))共同作用的结果.     

北极涛动的年代际变化及其气候影响

北极涛动(Arctic Oscillation,AO)是北半球热带外地区大气环流变率的主导模态,对北半球以及区域尺度气温变化具有重要影响。AO可在没有外强迫条件下通过波流相互作用形成,因此它被认为是全球气候系统内部变率的重要组成部分。研究年代际尺度上AO的变化及其气候影响,可加深对当前北半球气候变化规律的物理理解,也可为预估未来年代际尺度上气候变化及其不确定性提供科学依据。本文从AO影响东亚冬季风年代际变化的物理机制、AO对北半球冬季气温长期趋势的贡献、AO年代际影响的不确定性三个方面出发,简要回顾和总结了近年来有关年代际尺度上冬季AO时空变化及其对北半球气候影响的研究成果,并初步展望一些值得继续深入研究的问题。     

东亚夏季风降水年代际变异模态及其与太平洋年代际振荡的关系

利用全球陆地月平均降水资料、英国气象局哈德莱中心的月平均海表温度距平(SSTA)资料及NCEP/NCAR再分析大气环流资料,探讨东亚夏季风降水年代际变率及其与太平洋年代际振荡(PDO)的联系。研究指出:东亚夏季风降水年代际变异模态以及PDO均在1976年前后呈现显著的年代际转折,并且东亚夏季风降水与PDO在年代际尺度上具有较好的相关关系。PDO能够在对流层低层激发出与年代际东亚夏季风环流较为相似的大气环流异常特征,表明东亚夏季风环流的年代际变化可能受大气外强迫因子PDO在对流层低层的外源强迫作用影响,最终导致东亚夏季风降水发生年代际变化。     

Responses of East Asian summer monsoon to historical SST and atmospheric forcing during 1950–2000

The East Asian summer monsoon (EASM) circulation and summer rainfall over East China have experienced large decadal changes during the latter half of the 20th century. To investigate the potential causes behind these changes, a series of simulations using the national center for atmospheric research (NCAR) community atmospheric model version 3 (CAM3) and the geophysical fluid dynamics laboratory (GFDL) atmospheric model version 2.1 (AM2.1) are analyzed. These simulations are forced separately with different historical forcing, namely tropical sea surface temperature (SSTs), global SSTs, greenhouse gases plus aerosols, and a combination of global SSTs and greenhouse gases plus aerosols. This study focuses on the relative roles of these individual forcings in causing the observed monsoon and rainfall changes over East Asia during 1950–2000. The simulations from both models show that the SST forcing, primarily from the Tropics, is able to induce most of the observed weakening of the EASM circulation, while the greenhouse gas plus (direct) aerosol forcing increases the land-sea thermal contrast and thus enhances the EASM circulation. The results suggest that the recent warming in the Tropics, especially the warming associated with the tropical interdecadal variability centered over the central and eastern Pacific, is a primary cause for the weakening of the EASM since the late 1970s. However, a realistic simulation of the relatively small-scale rainfall change pattern over East China remains a challenge for the global models.     

Recent Progress in Studies of the Variabilities and Mechanisms of the East Asian Monsoon in a Changing Climate

Located in a monsoon domain,East Asia suffers devastating natural hazards induced by anomalous monsoon behaviors.East Asian monsoon(EAM)research has traditionally been a high priority for the Chinese climate community and is particularly challenging in a changing climate where the global mean temperature has been rising.Recent advances in studies of the variabilities and mechanisms of the EAM are reviewed in this paper,focusing on the interannual to interdecadal time scales.Some new results have been achieved in understanding the behaviors of the EAM,such as the evolution of the East Asian summer monsoon(EASM),including both its onset and withdrawal over the South China Sea,the changes in the northern boundary activity of the EASM,or the transitional climate zone in East Asia,and the cycle of the EASM and the East Asian winter monsoon and their linkages.In addition,understanding of the mechanism of the EAM variability has improved in several aspects,including the impacts of different types of ENSO on the EAM,the impacts from the Indian Ocean and Atlantic Ocean,and the roles of mid-to high-latitude processes.Finally,some scientific issues regarding our understanding of the EAM are proposed for future investigation.     

Forced response of the East Asian summer rainfall over the past millennium: results from a coupled model simulation

The centennial?Cmillennial variation of the East Asian summer monsoon (EASM) precipitation over the past 1000?years was investigated through the analysis of a millennium simulation of the coupled ECHO-G model. The model results indicate that the centennial?Cmillennial variation of the EASM is essentially a forced response to the external radiative forcing (insolation, volcanic aerosol, and green house gases). The strength of the response depends on latitude; and the spatial structure of the centennial?Cmillennial variation differs from the interannual variability that arises primarily from the internal feedback processes within the climate system. On millennial time scale, the extratropical and subtropical precipitation was generally strong during Medieval Warm Period (MWP) and weak during Little Ice Age (LIA). The tropical rainfall is insensitive to the effective solar radiation forcing (insolation plus radiative effect of volcanic aerosols) but significantly responds to the modern anthropogenic radiative forcing. On centennial time scale, the variation of the extratropical and subtropical rainfall also tends to follow the effective solar radiation forcing closely. The forced response features in-phase rainfall variability between the extratropics and subtropics, which is in contrast to the anti-correlation on the interannual time scale. Further, the behavior of the interannual?Cdecadal variation in the extratropics is effectively modulated by change of the mean states on the millennial time scale, suggesting that the structure of the internal mode may vary with significant changes in the external forcing. These findings imply that on the millennial time scale, (a) the proxy data in the extratropical EA may more sensitively reflect the EASM rainfall variations, and (b) the Meiyu and the northern China rainfall provide a consistent measure for the EASM strength.     

Review of simulations of climate variability and change with the GFDL R30 coupled climate model

A review is presented of the development and simulation characteristics of the most recent version of a global coupled model for climate variability and change studies at the Geophysical Fluid Dynamics Laboratory, as well as a review of the climate change experiments performed with the model. The atmospheric portion of the coupled model uses a spectral technique with rhomboidal 30 truncation, which corresponds to a transform grid with a resolution of approximately 3.75° longitude by 2.25° latitude. The ocean component has a resolution of approximately 1.875° longitude by 2.25° latitude. Relatively simple formulations of river routing, sea ice, and land surface processes are included. Two primary versions of the coupled model are described, differing in their initialization techniques and in the specification of sub-grid scale oceanic mixing of heat and salt. For each model a stable control integration of near millennial scale duration has been conducted, and the characteristics of both the time-mean and variability are described and compared to observations. A review is presented of a suite of climate change experiments conducted with these models using both idealized and realistic estimates of time-varying radiative forcing. Some experiments include estimates of forcing from past changes in volcanic aerosols and solar irradiance. The experiments performed are described, and some of the central findings are highlighted. In particular, the observed increase in global mean surface temperature is largely contained within the spread of simulated global mean temperatures from an ensemble of experiments using observationally-derived estimates of the changes in radiative forcing from increasing greenhouse gases and sulfate aerosols.     

THE ANALYSIS OF MECHANISM OF IMPACT OF AEROSOLS ON EAST ASIAN SUMMER MONSOON INDEX AND ONSET

RegCM4.3, a high-resolution regional climate model, which includes five kinds of aerosols(dust, sea salt,sulfate, black carbon and organic carbon), is employed to simulate the East Asian summer monsoon(EASM) from 1995 to 2010 and the simulation data are used to study the possible impact of natural and anthropogenic aerosols on EASM.The results show that the regional climate model can well simulate the EASM and the spatial and temporal distribution of aerosols. The EASM index is reduced by about 5% by the natural and anthropogenic aerosols and the monsoon onset time is also delayed by about a pentad except for Southeast China. The aerosols heat the middle atmosphere through absorbing solar radiation and the air column expands in Southeast China and its offshore areas. As a result, the geopotential height decreases and a cyclonic circulation anomaly is generated in the lower atmosphere. Northerly wind located in the west of cyclonic circulation weakens the low-level southerly wind in the EASM region. Negative surface radiative forcing due to aerosols causes downward motion and an indirect meridional circulation is formed with the low-level northerly wind and high-level southerly wind anomaly in the north of 25° N in the monsoon area, which weakens the vertical circulation of EASM. The summer precipitation of the monsoon region is significantly reduced,especially in North and Southwest China where the value of moisture flux divergence increases.     

Impacts of Coastal SST Variability on the East Asian Summer Monsoon

The impacts of the seasonal and interannual SST variability in the East Asia coastal regions （EACRSST） on the East Asian summer monsoon （EASM） have been examined using a regional climate model （PδRCM9） in this paper. The simulation results show that the correlation between the EACRSST and the EASM is strengthened after the mid-1970s and also the variability of the EACRSST forcing becomes much more important to the EASM interannual variability after the mid-1970s. The impacts of the EACRSST on the summer precipitation over each sub-region in the EASM region become weak gradually from south to north, and the temporal evolution features of the summer precipitation differences over North and Northeast China agree well with those of the index of EASM （IEASM） differences.
The mechanism analyses show that different EACRSST forcings result in the differences of sensible and latent heat flux exchanges at the air-sea interface, which alter the heating rate of the atmosphere. The heating rate differences induce low level air temperature differences over East Asia, resulting in the differences of the land-sea thermal contrast （LSTC） which lead to 850 hPa geopotential height changes. When the 850 hPa geopotential height increases over the East Asian continent and decreases over the coast of East China and the adjacent oceans during the weakening period of weakens consequently. On the contrary, the EASM enhances during the strengthening period of the LSTC.     

Using climate impacts indicators to evaluate climate model ensembles: temperature suitability of premium winegrape cultivation in the United States

We explore the potential to improve understanding of the climate system by directly targeting climate model analyses at specific indicators of climate change impact. Using the temperature suitability of premium winegrape cultivation as a climate impacts indicator, we quantify the inter- and intra-ensemble spread in three climate model ensembles: a physically uniform multi-member ensemble consisting of the RegCM3 high-resolution climate model nested within the NCAR CCSM3 global climate model; the multi-model NARCCAP ensemble consisting of single realizations of multiple high-resolution climate models nested within multiple global climate models; and the multi-model CMIP3 ensemble consisting of realizations of multiple global climate models. We find that the temperature suitability for premium winegrape cultivation is substantially reduced throughout the high-value growing areas of California and the Columbia Valley region (eastern Oregon and Washington) in all three ensembles in response to changes in temperature projected for the mid-twenty first century period. The reductions in temperature suitability are driven primarily by projected increases in mean growing season temperature and occurrence of growing season severe hot days. The intra-ensemble spread in the simulated climate change impact is smaller in the single-model ensemble than in the multi-model ensembles, suggesting that the uncertainty arising from internal climate system variability is smaller than the uncertainty arising from climate model formulation. In addition, the intra-ensemble spread is similar in the NARCCAP nested climate model ensemble and the CMIP3 global climate model ensemble, suggesting that the uncertainty arising from the model formulation of fine-scale climate processes is not smaller than the uncertainty arising from the formulation of large-scale climate processes. Correction of climate model biases substantially reduces both the inter- and intra-ensemble spread in projected climate change impact, particularly for the multi-model ensembles, suggesting that—at least for some systems—the projected impacts of climate change could be more robust than the projected climate change. Extension of this impacts-based analysis to a larger suite of impacts indicators will deepen our understanding of future climate change uncertainty by focusing on the climate phenomena that most directly influence natural and human systems.     

A method to study the impact of climate change on variability of river flow: an example from the Guadalupe River in Texas

This work introduced a method to study river flow variability in response to climate change by using remote sensing precipitation data, downscaled climate model outputs with bias corrections, and a land surface model. A meteorological forcing dataset representing future climate was constructed via the delta change method in which the modeled change was added to the present-day conditions. The delta change was conducted at a fine spatial and temporal scale to contain the signals of weather events, which exhibit substantial responses to climate change. An empirical transformation technique was further applied to the constructed forcing to ensure a realistic range. The meteorological forcing was then used to drive the land surface model to simulate the future river flow. The results show that preserving fine-scale processes in response to climate change is a necessity to assess climatic impacts on the variability of river flow events.     

Multi-fingerprint detection and attribution analysis of greenhouse gas, greenhouse gas-plus-aerosol and solar forced climate change

 A multi-fingerprint analysis is applied to the detection and attribution of anthropogenic climate change. While a single fingerprint is optimal for the detection of climate change, further tests of the statistical consistency of the detected climate change signal with model predictions for different candidate forcing mechanisms require the simultaneous application of several fingerprints. Model-predicted climate change signals are derived from three anthropogenic global warming simulations for the period 1880 to 2049 and two simulations forced by estimated changes in solar radiation from 1700 to 1992. In the first global warming simulation, the forcing is by greenhouse gas only, while in the remaining two simulations the direct influence of sulfate aerosols is also included. From the climate change signals of the greenhouse gas only and the average of the two greenhouse gas-plus-aerosol simulations, two optimized fingerprint patterns are derived by weighting the model-predicted climate change patterns towards low-noise directions. The optimized fingerprint patterns are then applied as a filter to the observed near-surface temperature trend patterns, yielding several detection variables. The space-time structure of natural climate variability needed to determine the optimal fingerprint pattern and the resultant signal-to-noise ratio of the detection variable is estimated from several multi-century control simulations with different CGCMs and from instrumental data over the last 136 y. Applying the combined greenhouse gas-plus-aerosol fingerprint in the same way as the greenhouse gas only fingerprint in a previous work, the recent 30-y trends (1966–1995) of annual mean near surface temperature are again found to represent a significant climate change at the 97.5% confidence level. However, using both the greenhouse gas and the combined forcing fingerprints in a two-pattern analysis, a substantially better agreement between observations and the climate model prediction is found for the combined forcing simulation. Anticipating that the influence of the aerosol forcing is strongest for longer term temperature trends in summer, application of the detection and attribution test to the latest observed 50-y trend pattern of summer temperature yielded statistical consistency with the greenhouse gas-plus-aerosol simulation with respect to both the pattern and amplitude of the signal. In contrast, the observations are inconsistent with the greenhouse-gas only climate change signal at a 95% confidence level for all estimates of climate variability. The observed trend 1943–1992 is furthermore inconsistent with a hypothesized solar radiation change alone at an estimated 90% confidence level. Thus, in contrast to the single pattern analysis, the two pattern analysis is able to discriminate between different forcing hypotheses in the observed climate change signal. The results are subject to uncertainties associated with the forcing history, which is poorly known for the solar and aerosol forcing, the possible omission of other important forcings, and inevitable model errors in the computation of the response to the forcing. Further uncertainties in the estimated significance levels arise from the use of model internal variability simulations and relatively short instrumental observations (after subtraction of an estimated greenhouse gas signal) to estimate the natural climate variability. The resulting confidence limits accordingly vary for different estimates using different variability data. Despite these uncertainties, however, we consider our results sufficiently robust to have some confidence in our finding that the observed climate change is consistent with a combined greenhouse gas and aerosol forcing, but inconsistent with greenhouse gas or solar forcing alone. Received: 28 April 1996 / Accepted: 27 January 1997     

The expectation of future precipitation change over the Mediterranean region is different from what we observe

In this study we assess the role of anthropogenic forcing (greenhouse gases and sulphate aerosols, GS) in recently observed precipitation trends over the Mediterranean region. We investigate whether the observed precipitation trends (1966–2005 and 1979–2008) are consistent with what 22 models project as response of precipitation to GS forcing. Significance is estimated using 9,000-year control runs derived from the CMIP3 archive. The results indicate that externally forced changes are detectable in observed precipitation trends in winter, late summer and in autumn. Natural internal climate variability cannot explain these changes. However, the observed trends (derived from 3 sources) are markedly inconsistent with expected changes due to GS forcing. While the influence of GS signal is detectable in winter and early spring, observed changes are several times larger than the projected response to GS forcing. The most striking inconsistency, however, is the contradiction between projected drying and the observed increase in precipitation in late summer and autumn, irrespective of the data set used. Natural (internal) variability as estimated from the models cannot account for these inconsistencies, which are already present in the large scale circulation patterns (Geopotential height at 500 hPa). The obtained results are robust to the removal of the fingerprint of the North Atlantic Oscillation. The detection of an outright sign mismatch of observed and projected trends in autumn and late summer, leads us to conclude that the recently observed trends can not be used as an illustration of plausible future expected change in the Mediterranean region. These significant shortcomings in our understanding of recent observed changes complicate communication of future expected changes in Mediterranean precipitation.