Twentieth century north atlantic climate change. Part I: assessing determinism

Boreal winter North Atlantic climate change since 1950 is well described by a trend in the leading spatial structure of variability, known as the North Atlantic Oscillation (NAO). Through diagnoses of ensembles of atmospheric general circulation model (AGCM) experiments, we demonstrate that this climate change is a response to the temporal history of sea surface temperatures (SSTs). Specifically, 58 of 67 multi-model ensemble members (87%), forced with observed global SSTs since 1950, simulate a positive trend in a winter index of the NAO, and the spatial pattern of the multi-model ensemble mean trend agrees with that observed. An ensemble of AGCM simulations with only tropical SST forcing further suggests that variations in these SSTs are of primary importance. The probability distribution function (PDF) of 50-year NAO index trends from the forced simulations are, moreover, appreciably different from the PDF of a control simulation with no interannual SST variability, although chaotic atmospheric variations are shown to yield substantial 50-year trends. Our results thus advance the view that the observed linear trend in the winter NAO index is a combination of a strong tropically forced signal and an appreciable noise component of the same phase. The changes in tropical rainfall of greatest relevance include increased rainfall over the equatorial Indian Ocean, a change that has likely occurred in nature and is physically consistent with the observed, significant warming trend of the underlying sea surface.     

瓦克环流指数与哈得莱环流指数

利用地面风场资料定义了瓦克环流指数与哈得莱环流指数,并研究指数的变化及其与厄尔尼诺的关系。     

Twentieth century North Atlantic climate change. Part II: Understanding the effect of Indian Ocean warming

Ensembles of atmospheric general circulation model (AGCM) experiments are used in an effort to understand the boreal winter Northern Hemisphere (NH) extratropical climate response to the observed warming of tropical sea surface temperatures (SSTs) over the last half of the twentieth Century. Specifically, we inquire about the origins of unusual, if not unprecedented, changes in the wintertime North Atlantic and European climate that are well described by a linear trend in most indices of the North Atlantic Oscillation (NAO). The simulated NH atmospheric response to the linear trend component of tropic-wide SST change since 1950 projects strongly onto the positive polarity of the NAO and is a hemispheric pattern distinguished by decreased (increased) Arctic (middle latitude) sea level pressure. Progressive warming of the Indian Ocean is the principal contributor to this wintertime extratropical response, as shown through additional AGCM ensembles forced with only the SST trend in that sector. The Indian Ocean influence is further established through the reproducibility of results across three different models forced with identical, idealized patterns of the observed warming. Examination of the transient atmospheric adjustment to a sudden “switch-on” of an Indian Ocean SST anomaly reveals that the North Atlantic response is not consistent with linear theory and most likely involves synoptic eddy feedbacks associated with changes in the North Atlantic storm track. The tropical SST control exerted over twentieth century regional climate underlies the importance of determining the future course of tropical SST for regional climate change and its uncertainty. Better understanding of the extratropical responses to different, plausible trajectories of the tropical oceans is key to such efforts.     

Regional climate change experiments over southern South America. II: Climate change scenarios in the late twenty-first century

We present an analysis of climate change over southern South America as simulated by a regional climate model. The regional model MM5 was nested within time-slice global atmospheric model experiments conducted by the HadAM3H model. The simulations cover a 10-year period representing present-day climate (1981–1990) and two future scenarios for the SRESA2 and B2 emission scenarios for the period 2081–2090. There are a few quantitative differences between the two regional scenarios. The simulated changes are larger for the A2 than the B2 scenario, although with few qualitative differences. For the two regional scenarios, the warming in southern Brazil, Paraguay, Bolivia and northeastern Argentina is particularly large in spring. Over the western coast of South America both scenarios project a general decrease in precipitation. Both the A2 and B2 simulations show a general increase in precipitation in northern and central Argentina especially in summer and fall and a general decrease in precipitation in winter and spring. In fall the simulations agree on a general decrease in precipitation in southern Brazil. This reflects changes in the atmospheric circulation during winter and spring. Changes in mean sea level pressure show a cell of increasing pressure centered somewhere in the southern Atlantic Ocean and southern Pacific Ocean, mainly during summer and fall in the Atlantic and in spring in the Pacific. In relation to the pressure distribution in the control run, this indicates a southward extension of the summer mean Atlantic and Pacific subtropical highs.     

Initialization and data assimilation experiments with a primitive equation model

The process of combining models of the ocean circulation with large data sets is known in meteorology as model initialization and data assimilation. This process is new to oceanographers, who only now are on the verge of having available world-wide synoptic maps of dynamic variables. In this paper we carry out a series of idealized initialization/assimilation experiments with a primitive equation (PE) model, which constitute a first step in developing a realistic process model and data assimilation techniques for the Gulf Stream system. The PE model is used in a spin-down mode and initialized with an analytic jet profile with geostrophically balanced fields.Two major questions are addressed in the present study. The first concerns the initialization process of a PE model during which internal/inertial gravity wave noise is produced. We ask: are the initialization shocks equally crucial for ocean models as they have been for their atmospheric counterparts? The results of an extensive series of balanced versus unbalanced initializations indicate that, for a PE model with a rigid lid, a brutally unbalanced initialization is required to produce strong internal gravity wave shocks. A geostrophically balanced initialization is sufficient to ensure smooth jet evolutions, with no apparent gravity waves, over long time durations in the spin-down mode. No sophisticated initialization procedures seem, therefore, to be required.The second question addressed is: which component of the flow is the most important in data assimilation to drive the model response towards a baseline reference ocean? We specifically compare the knowledge of the depth-integrated flow only, corresponding to measurements of the total transport, with the knowledge of the density field only, or equivalently the velocity shear. The knowledge of the interior density field is much more effective in decreasing the root-mean-square (r.m.s.) errors relative to the reference ocean. If the baroclinic structure is known, coarse horizontal resolutions of data insertion can be reached before significantly worsening the model estimates. If only the depth-averaged flow is known, a decrease in the horizontal resolution of data assimilation has an immediate effect: the r.m.s. errors sharply increase and the assimilation run diverges from the reference ocean. In the assimilation of the barotropic flow alone, even with dense resolution, the errors in the deep layers always show an increasing trend. The relative effectiveness of baroclinic versus barotropic data insertion can be rationalized in the context of geostrophic adjustment theory.     

Pacific Walker Circulation variability in coupled and uncoupled climate models

There is still considerable uncertainty concerning twentieth century trends in the Pacific Walker Circulation (PWC). In this paper, observational datasets, coupled (CMIP5) and uncoupled (AGCM) model simulations, and additional numerical sensitivity experiments are analyzed to investigate twentieth century changes in the PWC and their physical mechanisms. The PWC weakens over the century in the CMIP5 simulations, but strengthens in the AGCM simulations and also in the observational twentieth century reanalysis (20CR) dataset. It is argued that the weakening in the CMIP5 simulations is not a consequence of a reduced global convective mass flux expected from simple considerations of the global hydrological response to global warming, but is rather due to a weakening of the zonal equatorial Pacific sea surface temperature (SST) gradient. Further clarification is provided by additional uncoupled atmospheric general circulation model simulations in which the ENSO-unrelated and ENSO-related portions of the observed SST changes are prescribed as lower boundary conditions. Both sets of SST forcing fields have a global warming trend, and both sets of simulations produce a weakening of the global convective mass flux. However, consistent with the strong role of the zonal SST gradient, the PWC strengthens in the simulations with the ENSO-unrelated SST forcing, which has a strengthening zonal SST gradient, despite the weakening of the global convective mass flux. Overall, our results suggest that the PWC strengthened during twentieth century global warming, but also that this strengthening was partly masked by a weakening trend associated with ENSO-related PWC variability.     

春季赤道印度洋纬向-垂直环流的变化特征及其与Walker环流的关系

利用1951~2010年20CR（20th Century Reanalysis Version 2）再分析资料和NOAA海表温度资料研究了赤道印度洋纬向-垂直环流的季节性差异、变化特征及其与Walker环流的关系。本文首先分析了四个季节赤道印度洋上空纬向-垂直环流的结构特征,发现春季和秋季存在严格东西方向上的赤道印度洋纬向-垂直环流。随后,针对春季赤道印度洋纬向-垂直环流变化特征作进一步的分析,研究结果表明,春季纬向-垂直环流的强度及其变率在1951~2010年间持续增强,而该垂直环流的中心位置则表现出明显的年代际变化特征:1981年之前垂直环流的中心位置表现为向西移动,而在1981年后则转为向东移。春季赤道印度洋纬向-垂直环流与Walker环流之间的相关关系同样存在年代际转折,1981年之前两者之间并不存在显著的相关,而在1981年之后,两者之间的关系显著增强。不同年代际时段内赤道印度洋纬向-垂直环流与海温的关系也发生了明显的改变,1981~2010年赤道印度洋纬向-垂直环流主要受到前期和同期太平洋上的ENSO型海温信号的影响,而在此之前该垂直环流主要受到前期和同期赤道东印度洋海温的影响。     

Climate change in Bangladesh: a spatio-temporal analysis and simulation of recent temperature and rainfall data using GIS and time series analysis model

In this paper, temperature and rainfall data series were analysed from 34 meteorological stations distributed throughout Bangladesh over a 40-year period (1971 to 2010) in order to evaluate the magnitude of these changes statistically and spatially. Linear regression, coefficient of variation, inverse distance weighted interpolation techniques and geographical information systems were performed to analyse the trends, variability and spatial patterns of temperature and rainfall. Autoregressive integrated moving average time series model was used to simulate the temperature and rainfall data. The results confirm a particularly strong and recent climate change in Bangladesh with a 0.20 °C per decade upward trend of mean temperature. The highest upward trend in minimum temperature (range of 0.80–2.4 °C) was observed in the northern, northwestern, northeastern, central and central southern parts while greatest warming in the maximum temperature (range of 1.20–2.48 °C) was found in the southern, southeastern and northeastern parts during 1971–2010. An upward trend of annual rainfall (+7.13 mm per year) and downward pre-monsoon (?0.75 mm per year) and post-monsoon rainfall (?0.55 mm per year) trends were observed during this period. Rainfall was erratic in pre-monsoon season and even more so during the post-monsoon season (variability of 44.84 and 85.25 % per year, respectively). The mean forecasted temperature exhibited an increase of 0.018 °C per year in 2011–2020, and if this trend continues, this would lead to approximately 1.0 °C warmer temperatures in Bangladesh by 2020, compared to that of 1971. A greater rise is projected for the mean minimum (0.20 °C) than the mean maximum (0.16 °C) temperature. Annual rainfall is projected to decline 153 mm from 2011 to 2020, and a drying condition will persist in the northwestern, western and southwestern parts of the country during the pre- and post-monsoonal seasons.     

盘县半个多世纪的气温变化特征分析

利用贵州省六盘水市盘县气象观测站1951-01—2007-02连续、可靠的地面月平均气温、月平均最高、最低气温以及日极端最高、最低气温资料,对盘县年及各季平均气温、年平均最高、最低气温的变化进行分析。结果表明:盘县半个多世纪气温总体呈上升趋势,并且具有冬暖夏热或冬冷夏凉的典型气候特征,同时极端最高气温主要发生在20世纪50年代,极端最低气温主要发生在20世纪60年代。近10 a盘县气温升高趋势更加突出,暖冬气候特征更加明显。     

Twentieth century Sahel rainfall variability as simulated by the ARPEGE AGCM, and future changes

The ability of the ARPEGE AGCM in reproducing the twentieth century Sahelian drought when only forced by observed SST time evolution has been characterized. Atmospheric internal variability is shown to have a strong contribution in driving the simulated precipitation variability over the Sahel at decadal to multi-decadal time scales. The simulated drought is associated with a southward shift of the continental rainbelt over central and eastern Sahel, associated with an inter-hemispheric SST mode (the southern hemisphere oceans warming faster than the northern ones after 1970). The analysis of idealized experiments further highlights the importance of the Pacific basin. The related increase of the tropospheric temperature (TT) over the tropics is then suggested to dry the margin of convection zones over Africa, in agreement with the so-called “upped-ante” mechanism. A simple metric is then defined to determine the ability of the CMIP3 coupled models in reproducing both the observed Sahel drying and these mechanisms, in order to determine the reliability of the twenty-first century scenarios. Only one model reproduces both the observed drought over the Sahel and consistent SST/TT relationships over the second half of the twentieth century. This model predicts enhanced dry conditions over the Sahel at the end of the twenty-first century. However, as the mechanisms highlighted here for the recent period are not stationary during the twenty-first century when considering the trends, similarities between observed and simulated features of the West African monsoon for the twentieth century are a necessary but insufficient condition for a trustworthy prediction of the future.     

基于R/S方法的榕江县近半个世纪气候变化分析

通过对榕江县近54a气候指标进行的R/S分析,结果表明:榕江县近54a年平均气温呈变暖的趋势,增温率为0.11℃/10a.计算出的Hurst指数表明未来气温仍然呈整体上升的趋势,但这种升温趋势持续性不强;未来日照时数整体呈减少趋势,且这种减少趋势持续性较显著;降水量和相对湿度上升变化的趋势比较缓慢,H值表明降水量具有不显著的持续性,相对湿度具有显著持续性,未来继续维持这种趋势;本站气压变化趋势不明显,H=0.8248,表明未来维持这种趋势,且持续性较强.     

Mid-twenty-first century warm season climate change in the Central United States. Part I: regional and global model predictions

A regional climate model (RCM) constrained by future anomalies averaged from atmosphere–ocean general circulation model (AOGCM) simulations is used to generate mid-twenty-first century climate change predictions at 30-km resolution over the central U.S. The predictions are compared with those from 15 AOGCM and 7 RCM dynamic downscaling simulations to identify common climate change signals. There is strong agreement among the multi-model ensemble in predicting wetter conditions in April and May over the northern Great Plains and drier conditions over the southern Great Plains in June through August for the mid-twenty-first century. Projected changes in extreme daily precipitation are statistically significant over only a limited portion of the central U.S. in the RCM constrained with future anomalies. Projected changes in monthly mean 2-m air temperature are generally consistent across the AOGCM ensemble average, North American Regional Climate Change Assessment Program RCM ensemble average, and RCM constrained with future anomalies, which produce a maximum increase in August of 2.4–2.9 K over the northern and southern Great Plains and Midwest. Changes in extremes in daily 2-m air temperature from the RCM downscaled with anomalies are statistically significant over nearly the entire Great Plains and Midwest and indicate a positive shift in the warm tail of the daily 2-m temperature distribution that is larger than the positive shift in the cold tail.     

Critical analysis of simulated daily temperature data from the ARPEGE-climate model: application to climate change in the Champagne wine-producing region

As global warming is scientifically and widely accepted, its impacts at regional scales are raising many questions for wine producers. In particular, climate parameters, especially temperature, play a decisive role in vine growth and grape ripening. An overview of expected climate change in terms of bioclimatic indexes (Winkler and Huglin) and thermal extremes in the wine-producing region of Champagne is presented. A variable-grid atmospheric general circulation model, ARPEGE-Climate, with a local zoom at 50 km over the area of interest, is used to investigate potential future changes in thermal extremes and bioclimatic indexes. Changes in daily maximum and minimum temperatures at key stages are discussed for three emission scenarios (B1, A1B, A2) that are currently used in studies of impacts of climate change. Model outputs are analyzed and critically assessed for a control period (1971–2000) and for changes in extreme events in relation to future scenarios, such as a decrease in extreme low temperatures in spring (April) during bud break and an increase in extreme high temperatures in summer, associated with more frequent heat waves during ripening.     

Circulation statistics and climate change in Central Europe: PRUDENCE simulations and observations

PRUDENCE simulations of the climate in Central Europe are analysed with respect to mean temperature, mean precipitation and three monthly mean geostrophic circulation indices. The three global models show important circulation biases in the control climate, in particular in the strength of the west-circulations in winter and summer. The nine regional models inherit much of the circulation biases from their host model, especially in winter. In summer, the regional models show a larger spread in circulation statistics, depending on nesting procedures and other model characteristics. Simulated circulation biases appear to have a significant inluence on simulated temperature and precipitation. The PRUDENCE ensemble appears to be biased towards warmer and wetter than observed circulations in winter, and towards warmer and dryer circulations in summer. A2-scenario simulations show important circulation changes, which have a significant impact on changes in the distributions of monthly mean temperature and precipitation. It is likely that interactions between land–surface processes and atmospheric circulation play an important role in the simulated changes in the summer climate in Central Europe.     

Twentieth century simulation of the southern hemisphere climate in coupled models. Part II: sea ice conditions and variability

We examine the representation of the mean state and interannual variability of Antarctic sea ice in six simulations of the twentieth century from coupled models participating in the Intergovernmental Panel on Climate Change fourth assessment report. The simulations exhibit a largely seasonal southern hemisphere ice cover, as observed. There is a considerable scatter in the monthly simulated climatological ice extent among different models, but no consistent bias when compared to observations. The scatter in maximum winter ice extent among different models is correlated to the strength of the climatological zonal winds suggesting that wind forced ice transport is responsible for much of this scatter. Observations show that the leading mode of southern hemisphere ice variability exhibits a dipole structure with anomalies of one sign in the Atlantic sector associated with anomalies of the opposite sign in the Pacific sector. The observed ice anomalies also exhibit eastward propagation with the Antarctic circumpolar current, as part of the documented Antarctic circumpolar wave phenomenon. Many of the models do simulate dipole-like behavior in sea ice anomalies as the leading mode of ice variability, but there is a large discrepancy in the eastward propagation of these anomalies among the different models. Consistent with observations, the simulated Antarctic dipole-like variations in the ice cover are led by sea-level pressure anomalies in the Amundsen/ Bellingshausen Sea. These are associated, to different degrees in different models, with both the southern annular mode and the El Nino-Southern Oscillation (ENSO). There are indications that the magnitude of the influence of ENSO on the southern hemisphere ice cover is related to the strength of ENSO events simulated by the different models.     

Climate-related uncertainties in projections of the twenty-first century terrestrial carbon budget: off-line model experiments using IPCC greenhouse-gas scenarios and AOGCM climate projections

A terrestrial ecosystem model (Sim-CYCLE) was driven by multiple climate projections to investigate uncertainties in predicting the interactions between global environmental change and the terrestrial carbon cycle. Sim-CYCLE has a spatial resolution of 0.5°, and mechanistically evaluates photosynthetic and respiratory CO₂ exchange. Six scenarios for atmospheric-CO₂ concentrations in the twenty-first century, proposed by the Intergovernmental Panel on Climate Change, were considered. For each scenario, climate projections by a coupled atmosphere–ocean general circulation model (AOGCM) were used to assess the uncertainty due to socio-economic predictions. Under a single CO₂ scenario, climate projections with seven AOGCMs were used to investigate the uncertainty stemming from uncertainty in the climate simulations. Increases in global photosynthesis and carbon storage differed considerably among scenarios, ranging from 23 to 37% and from 24 to 81 Pg C, respectively. Among the AOGCM projections, increases ranged from 26 to 33% and from 48 to 289 Pg C, respectively. There were regional heterogeneities in both climatic change and carbon budget response, and different carbon-cycle components often responded differently to a given environmental change. Photosynthetic CO₂ fixation was more sensitive to atmospheric CO₂, whereas soil carbon storage was more sensitive to temperature. Consequently, uncertainties in the CO₂ scenarios and climatic projections may create additional uncertainties in projecting atmospheric-CO₂ concentrations and climates through the interactive feedbacks between the atmosphere and the terrestrial ecosystem.     

Twentieth century simulation of the southern hemisphere climate in coupled models. Part 1: large scale circulation variability

The ability of five, global coupled climate models to simulate important atmospheric circulation characteristics in the Southern Hemisphere for the period 1960–1999 is assessed. The circulation features examined are the Southern Hemisphere annular mode (SAM), the semi-annual oscillation (SAO) and the quasi-stationary zonal wave 3 (ZW3). The models assessed are the National Center for Atmospheric Research Community Climate System Model Version 3 (CCSM3), the Commonwealth Scientific and Industrial Research Organisation Mark 3, the Geophysical Fluid Dynamics Laboratory Model, the Goddard Institute for Space Studies Model ER (GISS-ER) and the UK Meteorological Office Hadley Center Coupled Model Version 3. The simulations were compared to the NCAR–NCEP reanalyses. The models simulate a SAO which differs spatially from the observed over the Pacific and Indian oceans. The amplitudes are too high over the southern ocean and too low over the midlatitudes. These differences are attributed to a circumpolar trough which is too deep and extends too far north, and to the inability of the models to simulate the middle to high latitude temperature gradient. The SAM is well-represented spatially by most models but there are important differences which may influence the flow over the Pacific and in the region extending from the Ross to Weddell Seas. The observed trend towards positive polarity in the SAM is apparent in the ensemble averages of the GISS-ER and CCSM3 simulations, suggesting that the trend is due to external forcing by changes in the concentration of ozone and greenhouse gases. ZW3 is well-represented by the models but the observed trend towards positive phases of ZW3 is not apparent in the simulations suggesting that the observed trend may be due to natural variability, not external forcing.     

Mid-twenty-first century climate change in the Central United States. Part II: Climate change processes

Ensemble regional model simulations over the central US with 30-km resolution are analyzed to investigate the physical processes of projected precipitation changes in the mid-twenty-first century under greenhouse gas forcing. An atmospheric moisture balance is constructed, and changes in the diurnal cycle are evaluated. Wetter conditions over the central US in April and May occur most strongly in the afternoon and evening, supported primarily by moisture convergence by transient eddy activity, indicating enhanced daytime convection. In June, increased rainfall over the northern Great Plains is strongest from 0000 to 0600 LT. It is supported by positive changes in stationary meridional moisture convergence related to a strengthening of the GPLLJ accompanied by an intensification of the western extension of the North Atlantic subtropical high. In the Midwest, decreased rainfall is strongest at 1500 LT and 0000 LT. Both a suppression of daytime convection as well as changes in the zonal flow in the GPLLJ exit region are important. Future drying over the northern Great Plains in summer is triggered by weakened daytime convection, and persists throughout August and September when a deficit in soil moisture develops and land–atmosphere feedbacks become increasingly important.     

Climate change estimates for the regions of Russia in the 20th century and in the beginning of the 21st century based on the observational data

Compared are the estimates or regional changes in temperature and precipitation on the territory of Russia for two methods of the spatial averaging of meteorological station data, one of which is adapted to the sparse observational network and takes account of the station weights proportional to the area of their influence. Considered are several variants of the zoning with the separation of the different number of regions. Formulated is a criterion of the zoning adequacy to the problem of the revelation and analysis of regional climate changes. Estimated is the representativeness of the network of observations of temperature and precipitation for separate regions. Presented are the estimates of regional trends of air temperature and precipitation for the century interval and for the recent decades obtained on the basis of the full archive of available data for the zoning attached to the administrative division of the Russian Federation into the federal districts.     

Multiple factors causing Holocene lake-level change in monsoonal and arid central Asia as identified by model experiments

Lake-level records provide a rich resource of information about past changes in effective moisture, but water-balance fluctuations can be driven by a number of different climate variables and it is often difficult to pinpoint their exact cause. This understanding is essential, however, for reconciling divergent paleo-records or for making predictions about future lake-level variations. This research uses a series of models, the NCAR CCSM3, a lake energy-balance and a lake water-balance model, to examine the reasons for lake-level changes in monsoonal Asia and arid central Asia between the early (8.5 ka), middle (6.0 ka) and late (ca. 1800 AD) Holocene. Our results indicate that the components of the lake water balance responsible for lake-level changes varied by region and through time. High lake levels at 8.5 and 6.0 ka in the monsoon region were caused by the combined effects of low lake evaporation and high precipitation. The low lake evaporation resulted from low winter solar radiation and high summer cloud cover. Precipitation associated with the mid-latitude westerlies increased from the early to middle Holocene and maintained high lake levels throughout most of arid central Asia ca. 6 ka. The modeled evolution of lake level in arid central Asia from the mid to late Holocene was spatially heterogeneous, due to different sensitivities of the northern and southern parts of the region to seasonally-changing insolation, particularly regarding the duration of lake ice cover. The model results do not suggest that precipitation and lake evaporation changes compete with one another in forcing lake-level change, as has been hypothesized.