Decadal climate variability in the Mediterranean region: roles of large-scale forcings and regional processes

We analyze decadal climate variability in the Mediterranean region using observational datasets over the period 1850–2009 and a regional climate model simulation for the period 1960–2000, focusing in particular on the winter (DJF) and summer (JJA) seasons. Our results show that decadal variability associated with the winter and summer manifestations of the North Atlantic Oscillation (NAO and SNAO respectively) and the Atlantic Multidecadal Oscillation (AMO) significantly contribute to decadal climate anomalies over the Mediterranean region during these seasons. Over 30% of decadal variance in DJF and JJA precipitation in parts of the Mediterranean region can be explained by NAO and SNAO variability respectively. During JJA, the AMO explains over 30% of regional surface air temperature anomalies and Mediterranean Sea surface temperature anomalies, with significant influence also in the transition seasons. In DJF, only Mediterranean SST still significantly correlates with the AMO while regional surface air temperature does not. Also, there is no significant NAO influence on decadal Mediterranean surface air temperature anomalies during this season. A simulation with the PROTHEUS regional ocean–atmosphere coupled model is utilized to investigate processes determining regional decadal changes during the 1960–2000 period, specifically the wetter and cooler 1971–1985 conditions versus the drier and warmer 1986–2000 conditions. The simulation successfully captures the essence of observed decadal changes. Model set-up suggests that AMO variability is transmitted to the Mediterranean/European region and the Mediterranean Sea via atmospheric processes. Regional feedbacks involving cloud cover and soil moisture changes also appear to contribute to observed changes. If confirmed, the linkage between Mediterranean temperatures and the AMO may imply a certain degree of regional decadal climate predictability. The AMO and other decadal influences outlined here should be considered along with those from long-term increases in greenhouse gas forcings when making regional climate out-looks for the Mediterranean 10–20?years out.     

1953～2017年黄河源区气温变化的多尺度特征

基于黄河源区8个站点的年平均气温序列,利用集合经验模态分解（Ensemble Empirical Mode Decomposition,EEMD）方法,揭示了以玛多站为代表的黄河源区1953~2017年气温演变的多时间尺度特征,探讨不同时间尺度上的周期振荡对气温变化总体特征的影响程度,分析了黄河源区不同时间尺度的气温变化与海温指数,尤其是与北大西洋多年代际振荡（Atlantic Multidecadal Oscillation,AMO）间的关系。结果表明:（1）1953年以来黄河源区玛多站年平均气温以0.31 ℃/10 a的变化率表现为明显的增暖趋势,20世纪80年代后期开始转暖,尤其是进入20世纪90年代后期变暖更加明显。（2）1953~2017年,黄河源区年平均气温呈现3 a、6 a、11 a、25 a、64 a及65 a以上时间尺度的准周期变化,其中以准3 a和65 a以上时间尺度的振荡最显著,准3 a的年际振荡在21世纪以前振幅较大,而进入21世纪后年际振荡振幅减弱,65 a以上时间尺度的年代际振荡振幅明显加大。（3）1998年气候显著变暖以前,以准3 a周期为代表的年际振荡在气温演变过程中占据主导地位,1998年气候显著变暖以后,65 a以上时间尺度周期振荡的贡献率增加近5倍,与准3 a周期振荡的贡献相当。（4）气温与Nino3.4指数和PDO（Pacific Decadal Oscillation）指数的同期相关均不显著,但当气温领先PDO指数22 a时正相关最大且显著,不同于PDO指数,气温原始序列及其3个年代际尺度分量滞后AMO指数3~7 a或二者同期时相关性最高,这就意味着AMO对黄河源区气温具有显著影响。（5）AMO的正暖位相对应着包括中国的整个东亚地区偏暖,黄河源区只是受影响区域的一部分,20世纪60年代至90年代初期AMO的负冷位相期、20世纪90年代中后期至今AMO的正暖位相与黄河源区气温距平序列的负距平、正距平相对应,气温在65 a以上时间尺度的变化与AMO指数相关性更高,可见,AMO是影响黄河源区气温变化的一个重要的气候振荡,这种影响主要表现在年代际时间尺度上。     

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 20th century cooling trend over the southeastern United States

Portions of the southern and southeastern United States, primarily Mississippi, Alabama, and Georgia, have experienced century-long (1895–2007) downward air temperature trends that occur in all seasons. Superimposed on them are shifts in mean temperatures on decadal scales characterized by alternating warm (1930s–1940s, 1990s) and cold (1900s; 1960s–1970s) regimes. Regional atmospheric circulation and SST teleconnection indices, station-based cloud cover and soil moisture (Palmer drought severity index) data are used in stepwise multiple linear regression models. These models identify predictors linked to observed winter, summer, and annual Southeastern air temperature variability, the observed variance (r²) they explain, and the resulting prediction and residual time series. Long-term variations and trends in tropical Pacific sea temperatures, cloud cover, soil moisture and the North Atlantic and Arctic oscillations account for much of the air temperature downtrends. Soil moisture and cloud cover are the primary predictors of 59.6 % of the observed summer temperature variance. While the teleconnections, cloud cover and moisture data account for some of the annual and summer Southeastern cooling trend, large significant downward trending residuals remain in winter and summer. Comparison is made to the northeastern United States where large twentieth century upward air temperature trends are driven by cloud cover increases and Atlantic Multidecadal Oscillation (AMO) variability. Differences between the Northeastern warming and the Southeastern cooling trends in summer are attributable in part to the differing roles of cloud cover, soil moisture, the Arctic Oscillation and the AMO on air temperatures of the 2 regions.     

基于信息流理论的因果分析在辨析大西洋多年代际振荡物理机制中的应用

大西洋多年代际振荡（AMO）是指发生在北大西洋的海表温度（SST）冷暖异常多年代际（50～80年）振荡的现象。通常AMO被认为是受大西洋经向翻转环流（AMOC）及其对应的海洋动力过程（经向热量输运）的影响。近年来有观点认为,AMO是大气随机热力强迫的产物,大气主导了海气间的热量交换进而影响AMO。弄清AMO和北大西洋海表热通量的因果关系是辨析AMO动力和热力驱动机制的关键。本文利用基于信息流理论的因果分析方法,研究了1880年以来观测的AMO与北大西洋海表热通量间的因果关系。结果表明,在多年代际尺度上,从AMO到海表热通量的信息流要远大于二者相反方向的信息流,这说明AMO是北大西洋海表热通量异常的因,海洋主导了海气间的热量交换。大气随机热力强迫机制无法解释AMO与热通量两者因果分析的结果。对泛大西洋地区的陆地气温和AMO指数进行分析,进一步表明由于海洋主导了海气热量交换,AMO的海温异常加热/冷却控制了绝大多数地区气温的多年代际变化。利用海温驱动的大气环流模式的模拟结果验证了AMO的海温异常对周边陆地气温强迫作用。本文的结果为辨析AMO的动力和热力驱动机制提供了新线索,进一步表明AMO并非是大气随机热力强迫的产物,海洋环流可能是AMO的主要驱动因子。     

Abrupt summer warming and changes in temperature extremes over Northeast Asia since the mid-1990s: Drivers and physical processes

This study investigated the drivers and physical processes for the abrupt decadal summer surface warming and increases in hot temperature extremes that occurred over Northeast Asia in the mid-1990 s. Observations indicate an abrupt increase in summer mean surface air temperature(SAT) over Northeast Asia since the mid-1990 s. Accompanying this abrupt surface warming, significant changes in some temperature extremes, characterized by increases in summer mean daily maximum temperature(Tmax), daily minimum temperature(Tmin), annual hottest day temperature(TXx), and annual warmest night temperature(TNx) were observed. There were also increases in the frequency of summer days(SU) and tropical nights(TR).Atmospheric general circulation model experiments forced by changes in sea surface temperature(SST)/ sea ice extent(SIE),anthropogenic greenhouse gas(GHG) concentrations, and anthropogenic aerosol(AA) forcing, relative to the period 1964–93, reproduced the general patterns of observed summer mean SAT changes and associated changes in temperature extremes,although the abrupt decrease in precipitation since the mid-1990 s was not simulated. Additional model experiments with different forcings indicated that changes in SST/SIE explained 76% of the area-averaged summer mean surface warming signal over Northeast Asia, while the direct impact of changes in GHG and AA explained the remaining 24% of the surface warming signal. Analysis of physical processes indicated that the direct impact of the changes in AA(through aerosol–radiation and aerosol–cloud interactions), mainly related to the reduction of AA precursor emissions over Europe, played a dominant role in the increase in TXx and a similarly important role as SST/SIE changes in the increase in the frequency of SU over Northeast Asia via AA-induced coupled atmosphere–land surface and cloud feedbacks, rather than through a direct impact of AA changes on cloud condensation nuclei. The modelling results also imply that the abrupt summer surface warming and increases in hot temperature extremes over Northeast Asia since the mid-1990 s will probably sustain in the next few decades as GHG concentrations continue to increase and AA precursor emissions over both North America and Europe continue to decrease.     

Estimation of contemporary observed variations of air temperature and wind speed in the troposphere of the Northern Hemisphere

Carried out is the statistical analysis of contemporary observed variations of air temperature and wind speed in the troposphere of the Northern Hemisphere based on the data on global surface air temperature for 1850–2013 obtained from the University of East Anglia Climate Research Unit (HadCRUT4) and NCEP/NCAR reanalysis (1948–2013). Revealed are the long-term trends of air temperature and wind speed at different constant-pressure levels. Established is the anticipatory role of the zonal atmospheric circulation in the long-term variability of air temperature in the lower troposphere averaged for the zone of 30°–70° N. According to the results of correlation analysis, in some areas of the Northern Hemisphere the contribution of the wind speed to air temperature variability makes up not less than 60%.     

An Observational Analysis of the Oceanic and Atmospheric Structure of Global-Scale Multi-decadal Variability简

The aim of the present study was to identify multi-decadal variability （MDV） relative to the current centennial global warming trend in available observation data.The centennial global wanning trend was first identified in the global mean surface temperature （STgm） data.The MDV was identified based on three sets of climate variables,including sea surface temperature （SST）,ocean temperature from the surface to 700 m,and the NCEP and ERA40 reanalysis datasets,respectively.All variables were detrended and low-pass filtered.Through three independent EOF analyses of the filtered variables,all results consistently showed two dominant modes,with their respective temporal variability resembling the Pacific Decadal Oscillation/Inter-decadal Pacific Oscillation （PDO/IPO） and the Atlantic Multi-decadal Oscillation （AMO）.The spatial structure of the PDO-like oscillation is characterized by an ENSO-like structure and hemispheric symmetric features.The structure associated with the AMO-like oscillation exhibits hemispheric asymmetric features with anomalous warm air over Eurasia and warm SST in the Atlantic and Pacific basin north of 10°S,and cold SST over the southern oceans.The Pacific and Atlantic MDV in upper-ocean temperature suggest that they are mutually linked.We also found that the PDO-like and AMO-like oscillations are almost equally important in global-scale MDV by EOF analyses.In the period 1975-2005,the evolution of the two oscillations has given rise to strong temperature trends and has contributed almost half of the STgm warming.Hereon,in the next decade,the two oscillations are expected to slow down the global warming trends.     

太平洋年代际振荡、大西洋年代际振荡和全球变暖对北美地区降水的相对贡献

本文研究了1934～2018年期间太平洋年代际振荡（Interdecadal Pacific Oscillation,IPO）、大西洋年代际振荡（Atlantic Multidecadal Oscillation,AMO）以及全球变暖（Global Warming,GW）对北美地区陆地降水年代际变化的相对贡献。首先通过对冬（12至次年2月）、夏季（6～8月）北美地区的陆地降水与中低纬地区的海表面温度进行奇异值分解分析,得到对北美陆地冬季降水相对贡献较大的主要海温模态为IPO（42.33%）和AMO（23.21%）,夏季则为AMO（32.66%）和IPO（21.60%）。其次利用线性回归模型,分析三种信号分别对北美冬、夏季陆地降水的相对贡献及对北美陆地不同区域降水的相对重要性,结果表明AMO对夏季北美陆地降水变化的贡献最大,IPO次之,冬季则相反,GW对冬夏季北美陆地降水都有一定的贡献。夏季期间阿拉斯加地区AMO的贡献最大,约占65.8%,加拿大地区GW的贡献最大,约占44.5%,美国本土及墨西哥地区三者贡献基本一致;冬季期间阿拉斯加和加拿大地区GW的贡献最大,分别为62.3%和44.7%,美国本土和墨西哥地区IPO的贡献最大,分别为47.9%和71.5%。进一步利用信息流方法,验证了IPO、AMO、GW对降水的敏感性区域。最后运用全球大气环流模式ECHAM 4.6进一步确定了太平洋和大西洋海温异常对北美地区陆地降水变化的影响途径,结果表明印度洋海表面温度异常在AMO和IPO对北美陆地降水变化的作用中至关重要。     

The Climate Response to Global Forest Area Changes under Different Warming Scenarios in China

Human activities have notably affected the Earth’s climate through greenhouse gases(GHG), aerosol, and land use/land cover change(LULCC). To investigate the impact of forest changes on regional climate under different shared socioeconomic pathways(SSPs), changes in surface air temperature and precipitation over China under low and medium/high radiative forcing scenarios from 2021 to 2099 are analyzed using multimodel climate simulations from the Coupled Model Intercomparison Project Phase 6(CMIP...     

Quantifying the contributions of anthropogenic and natural forcings to climate changes over arid-semiarid areas during 1946–2005

In this study, the contributions from changes in man-made greenhouse gases (GHG), anthropogenic aerosols (AA), and land use (LU), as well as natural solar and volcanic (NAT) forcing changes, to observed changes in surface air temperature (T) and precipitation (P) over global land, especially over arid-semiarid areas, during 1946–2005 are quantified using observations and climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results show that the anthropogenic (ANT) forcings dominate the ubiquitous surface warming seen in observations and lead to slight increases in precipitation over most land areas, while the NAT forcing leads to small cooling over land. GHG increases are the primary factor responsible for the anthropogenic climate change, while the AA forcing offsets a large part of the GHG-induced warming and P changes. The LU forcing generally contributes little to the T and P changes from 1946 to 2005 over most land areas. Unlike the consistent temperature changes among most model simulations, precipitation changes display a large spread among the models and are incomparable with the observations in spatial distributions and magnitude, mainly due to its large internal variability that varies among individual model runs. Using an optimal fingerprinting method, we find that the observed warming over land during 1946–2005 can be largely attributed to the ANT forcings, and the combination of the ANT and NAT forcings can explain about 85~95% of the observed warming trend over global land as well as over most arid-semiarid regions such as Northern China. However, the anthropogenic influences on precipitation over the past 60 years are generally undetectable over most land areas, including most arid-semiarid regions. This indicates that internal variability is still larger than the forced change for land precipitation.     

全球变暖、AMO、IPO对全球陆地降水变化的相对贡献

本文探究了不同海表温度（SST）模态对6—8月和12月—次年2月全球陆地降水的趋势以及年代际变化的相对贡献。首先对热带地区陆地降水和SST进行SVD分析,得到影响陆地降水的趋势和年代际变化主要的海洋模态为：海洋中的全球变暖（Global Warming,GW）、大西洋多年代际振荡（Atlantic Multidecadal Oscillation,AMO）和太平洋多年代际振荡（Interdecadal Pacific Oscillation,IPO）。其次利用多元线性回归模型进一步定量评估了全球变暖、AMO和IPO对不同地区陆地降水的相对贡献大小。结果表明,全球变暖对陆地降水变化的贡献在冬夏季都是最大的,AMO在6—8月的贡献次之。IPO在12月—次年2月的贡献次之。不同纬度带,三者的贡献不同。GW的贡献在6—8月期间对10°N以北地区较大,南半球受GW的贡献相对较小,GW在12月—次年2月对40°N以北降水贡献异常显著;AMO主要在6—8月对10°～40°S和50°～60°S纬度带上的降水变化的贡献比较大;而IPO主要在12月—次年2月对北半球中纬度降水变化的贡献比较大。GW对许多地区降水变化的方差贡献都是最大的,例如6—8月期间,对北美洲东北部和亚洲降水变化贡献最大,12月—次年2月期间,对欧洲降水变化贡献最大。AMO对6—8月降水变化的方差贡献最大的区域为非洲萨赫勒、西伯利亚和南美洲。12月—次年2月期间,IPO对美国西南部的降水变化贡献最大,此外,北美洲东北部、南美洲西北部、非洲南部、澳大利亚东部、南亚季风区和我国北部的降水在12月—次年2月期间同样受IPO影响显著。进一步利用信息流的方法,探究了GW、AMO和IPO与陆地降水变化之间的因果关系,验证了上述结论。     

中国降水的年代际变化及全球变暖、太平洋年代际振荡、大西洋多年代际振荡对其的相对贡献

虽然中国降水以年际变化为主,但可利用奇异谱分析辨析出10—20 a、20—50 a 年代际变化的显著性区域以及>50 a 的长期趋势的显著性区域。本研究通过奇异值分解、多元线性回归等方法探究了1934—2018年不同海洋模态对6—8月（夏季）和12月—翌年2月（冬季）中国陆地降水趋势以及年代际振荡的相对贡献。通过对中国降水及中低纬度地区海温进行奇异值分解发现,不论冬夏,影响中国降水的主要模态是全球变暖,其次是太平洋年代际振荡。利用多元线性回归模型定量评估全球变暖、太平洋年代际振荡、大西洋多年代际振荡对中国不同区域降水的方差贡献及各因子的相对贡献,结果表明:夏季,三者可以解释西北和华北大约30%的年代际降水,其中全球变暖的相对贡献最大、太平洋年代际振荡次之;冬季,三者可以解释东北42%、西北和华北30%左右的年代际降水,东北和西北以全球变暖的相对贡献为主、大西洋多年代际振荡为辅,华北仍以全球变暖的影响为主、太平洋年代际振荡为辅。      

陆面过程对全球变暖的响应及可能机制—基于CMIP3的多模式集合分析

利用“国际耦合模式比较计划” (Phase 3 of the Coupled Model Intercomparison Project, CMIP3) 12个模式对20世纪 (The Twentieth-Century Climate in Coupled Models, 20C3M) 和21世纪SRES (Special Report on Emissions Scenarios) A1B 情景下的模拟结果, 通过21世纪 (2001～2099年) 与20世纪 (1901～1999年) 陆面能量和水文变量的对比分析, 揭示了陆面过程对全球变暖响应的基本特征, 并探讨了其可能的响应机制。结果表明, 与20世纪相比, 21世纪全球陆面平均的表面温度、 地表净辐射、 潜热通量明显增加; 而感热通量有所减小。降水、 径流、 蒸发等地表水循环分量也表现出不同程度的增加, 而土壤含水量有减小趋势。通过分析近地层主要大气强迫变量与陆面变量之间的联系, 发现陆面能量平衡过程对全球变暖的响应主要受向下长波辐射和气温变化的影响, 而温度的变化对陆面水文过程的影响起决定性的作用。进一步分析表明, 陆面过程对全球变暖的响应存在明显的区域性差异, 陆面温度和感热对全球变暖响应最显著的区域位于北半球中高纬, 而净辐射和潜热对全球变暖的响应在亚洲中部和非洲大陆最显著。相对于20世纪, 21世纪主要是长波辐射和温度对陆面能量平衡过程的贡献重要。对于陆面水文过程, 径流和土壤含水量对全球变暖的响应在亚洲中部以及北美最显著。在全球变暖背景下, 21世纪相对于20世纪, 温度对陆面水循环的影响更加显著, 主要体现在北半球中纬度地区。     

MULTIPLE TIME SCALE ANALYSIS OF CLIMATE VARIATION IN MACAU DURING THE LAST 100 YEARS

The multiple time scale climate changes are studied and calculated with statistical analysis and wavelet transformation on the basis of daily series of observed data over the period 1901-2007 in Macau.The result shows that statistically significant oscillations with 2 to 5 years of period generally exist in the series of climate variables(e.g.annual mean surface air temperature and precipitation as well as evaporation etc.),but with obvious locality in time domain.The variation of annual mean surface air temperature has a quasi 60-year period.The phases of the 60-year variation approximately and consistently match that of Atlantic Multidecadal Oscillation(AMO).The oscillations of seasonal mean surface air temperature in summer and winter have the periods of quasi 30-year and quasi 60-year,respectively.These two periods of oscillations have statistically significant correlation with Pacific decadal oscillation(PDO) and AMO,individually.The multidecadal variations of the precipitation of the annually first flood period and annual evaporation are dominated by periods of quasi 30-year and quasi 50-year,respectively.     

长江流域1961-2000年蒸发量变化趋势研究

利用长江流域115个气象站点1961-2000年的观测数据,计算了各站点的参照蒸发量和实际蒸发量,并进行了20 cm蒸发皿蒸发量、参照蒸发量和实际蒸发量时空变化趋势分析。结果表明,近40 a来,长江流域蒸发皿蒸发量、参照蒸发量和实际蒸发量的年平均变化均呈现显著下降趋势。就季节平均变化而言,春季和秋季,三者的变化趋势都不明显,而夏季三者均具有显著的下降趋势,冬季蒸发皿蒸发量和参照蒸发量均显著下降,实际蒸发量却明显上升。蒸发量的变化趋势具有空间分布差异,长江流域中下游地区蒸发量的变化趋势明显比上游地区显著,尤其表现在夏季。尽管近20余年长江流域气温不断升高,但太阳净辐射和风速的显著下降,可能是导致蒸发量持续降低的主要原因。     

The response of the South Asian Summer Monsoon circulation to intensified irrigation in global climate model simulations

Agricultural intensification in South Asia has resulted in the expansion and intensification of surface irrigation over the twentieth century. The resulting changes to the surface energy balance could affect the temperature contrasts between the South Asian land surface and the equatorial Indian Ocean, potentially altering the South Asian Summer Monsoon (SASM) circulation. Prior studies have noted apparent declines in the monsoon intensity over the twentieth century and have focused on how altered surface energy balances impact the SASM rainfall distribution. Here, we use the coupled Goddard Institute for Space Studies ModelE-R general circulation model to investigate the impact of intensifying irrigation on the large-scale SASM circulation over the twentieth century, including how the effect of irrigation compares to the impact of increasing greenhouse gas (GHG) forcing. We force our simulations with time-varying, historical estimates of irrigation, both alone and with twentieth century GHGs and other forcings. In the irrigation only experiment, irrigation rates correlate strongly with lower and upper level temperature contrasts between the Indian sub-continent and the Indian Ocean (Pearson’s r = ?0.66 and r = ?0.46, respectively), important quantities that control the strength of the SASM circulation. When GHG forcing is included, these correlations strengthen: r = ?0.72 and r = ?0.47 for lower and upper level temperature contrasts, respectively. Under irrigated conditions, the mean SASM intensity in the model decreases only slightly and insignificantly. However, in the simulation with irrigation and GHG forcing, inter-annual variability of the SASM circulation decreases by ~40 %, consistent with trends in the reanalysis products. This suggests that the inclusion of irrigation may be necessary to accurately simulate the historical trends and variability of the SASM system over the last 50 years. These findings suggest that intensifying irrigation, in concert with increased GHG forcing, is capable of reducing the variability of the simulated SASM circulation and altering the regional moisture transport by limiting the surface warming and reducing land–sea temperature gradients.     

A GCM study of future climate response to aerosol pollution reductions

We use the global atmospheric GCM aerosol model ECHAM5-HAM to asses possible impacts of future air pollution mitigation strategies on climate. Air quality control strategies focus on the reduction of aerosol emissions. Here we investigate the extreme case of a maximum feasible end-of-pipe abatement of aerosols in the near term future (2030) in combination with increasing greenhouse gas (GHG) concentrations. The temperature response of increasing GHG concentrations and reduced aerosol emissions leads to a global annual mean equilibrium temperature response of 2.18 K. When aerosols are maximally abated only in the Industry and Powerplant sector, while other sectors stay with currently enforced regulations, the temperature response is 1.89 K. A maximum feasible abatement applied in the Domestic and Transport sector, while other sectors remain with the current legislation, leads to a temperature response of 1.39 K. Increasing GHG concentrations alone lead to a temperature response of 1.20 K. We also simulate 2–5% increases in global mean precipitation among all scenarios considered, and the hydrological sensitivity is found to be significantly higher for aerosols than for GHGs. Our study, thus highlights the huge potential impact of future air pollution mitigation strategies on climate and supports the need for urgent GHG emission reductions. GHG and aerosol forcings are not independent as both affect and are influenced by changes in the hydrological cycle. However, within the given range of changes in aerosol emissions and GHG concentrations considered in this study, the climate response towards increasing GHG concentrations and decreasing aerosols emissions is additive.     

Elevational Dependence of Air Temperature Variability and Trends in British Columbia's Cariboo Mountains, 1950–2010

Pristine mountain environments are more sensitive to climate change than other land surfaces. Climatic variations in mountainous terrain are still poorly understood. Previous studies revealed inconsistent findings on the elevational dependence of warming in the mountains. In this study, the trends and elevational dependence of air temperature in the Cariboo Mountains Region (CMR) of British Columbia are explored using a surface air temperature dataset with a spatial resolution of five arc minutes over the 1950–2010 period. A Mann-Kendall test is performed for evaluation of trends and their significance. In recent decades the CMR has been warming at a faster rate than regional and global warming. The minimum air temperature trend shows significant amplified warming at higher elevations. The snow–albedo feedback and changes in cloud cover over the CMR may possibly be the major physical mechanisms responsible for these trends. The implications of such changes on the endangered mountain caribou and water resources of the area are also discussed.     

An overview of decadal climate predictability in a multi-model ensemble by climate model MIROC

Decadal climate predictability is examined in hindcast experiments by a multi-model ensemble using three versions of the coupled atmosphere-ocean model MIROC. In these hindcast experiments, initial conditions are obtained from an anomaly assimilation procedure using the observed oceanic temperature and salinity with prescribed natural and anthropogenic forcings on the basis of the historical data and future emission scenarios in the Intergovernmental Panel of Climate Change. Results of the multi-model ensemble in our hindcast experiments show that predictability of surface air temperature (SAT) anomalies on decadal timescales mostly originates from externally forced variability. Although the predictable component of internally generated variability has considerably smaller SAT variance than that of externally forced variability, ocean subsurface temperature variability has predictive skills over almost a decade, particularly in the North Pacific and the North Atlantic where dominant signals associated with Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are observed. Initialization enhances the predictive skills of AMO and PDO indices and slightly improves those of global mean temperature anomalies. Improvement of these predictive skills in the multi-model ensemble is higher than that in a single-model ensemble.