Tropical-temperate interactions over southern Africa simulated by a regional climate model

The Weather Research and Forecasting model (WRF) forced by ERA40 re-analyses, is used to examine, at regional scale, the role of key features of the local atmospheric circulation on the origin and development of Tropical Temperate Troughs (TTTs) representing a major contribution to South African rainfall during austral summer. A cluster analysis applied on 1971–2000 ERA40 and WRF simulated daily outgoing longwave radiation reveals for the November–February season three coherent regimes characteristic of TTTs over the region. Analyses of WRF simulated TTTs suggest that their occurrence is primarily linked with mid-latitude westerly waves and their phasing. Ensemble experiments designed for the case of austral summer 1996/1997 allow to examine the reproducibility of TTT events. The results obtained illustrate the importance of westerly waves phasing regarding the persistence of rain-producing continental TTT events. Moreover, oceanic surface conditions prevailing over the Agulhas current regions of the South West Indian Ocean (SWIO) are also found to influence TTT persistence for regional experiments with an oceanic mixed layer, warmer sea surface temperatures being associated with increased moisture advection from the SWIO where latent heat release is enhanced, favoring baroclinic instability and thus sustaining convection activity locally.     

Including tropical croplands in a terrestrial biosphere model: application to West Africa

Studying the large-scale relationships between climate and agriculture raises two different issues: the impact of climate on crops, and the potential feedbacks to climate from croplands. A relevant and consistent framework to address this twofold issue is to extend existing Dynamic Global Vegetation Models, which can be coupled to climate models, in order to explicitly account for croplands. Here we present the first results of such a strategy applied to tropical croplands over West Africa. We introduce into the terrestrial biosphere model ORCHIDEE (IPSL) adequate processes and parameterisations taken from the crop model SARRAH (CIRAD), which is calibrated for millet over this region. The resulting model, ORCH-mil, realistically simulates the growth and yield of millet when tested on an experimental station in Senegal. The model is then applied over West Africa using a 36-year climate reanalysis dataset. First the model is tested in terms of yield simulation, against national millet yields from the FAO database. The ability of the model to reproduce the spatial and temporal variability of millet yields is assessed. Then, the effects on land surface fluxes of explicitly accounting for croplands are examined: significant differences between ORCH-mil and ORCHIDEE appear, through changes in sensible and latent heat fluxes, surface albedo, and water resources. These differences encompass a potential impact on the monsoon system, mainly during the retreat of monsoon rains.     

Analysis of the probability distribution of the late Pleistocene climatic record implications for model validation

Some statistical properties of the late Pleistocene climatic changes, as inferred from the analysis of the Probability Density Function (PDF) of the deep-sea ¹⁸O record, are discussed. Estimates of the probability distribution of the SPECMAP record seem to reveal a bimodal character over the last 600 000 years BP, with a transition to a unimodal distribution for a longer time interval. Based on knowledge of the accuracy of the measurements, we try to remove the arbitrariness of such estimates, and by employing a number of different estimators we show that the major features observed are not affected by the particular method used. Possible dynamic interpretations are discussed. The abruptness of many transitions between the two modes and their symmetric occurrence seem to be significant features of the past climatic changes, which should be reproduced by climate models. The distribution of this indicator is then used to compare the performances of two climate models. Various statistical tests are employed and it is shown that, for a proper determination of the physical significance of such tests, information about the accuracy and reliability of the data must be included. For example, the comparison of two generalized isotopic curves, independently developed, shows a correlation coefficient of the order of 70%; therefore any account of the variance of any particular target curve beyond this level cannot be considered significant. These types of analyses suggest how the standard data evaluation and model validation techniques can be improved when discussing the essential features of observed and simulated dynamical systems.     

A model investigation of recent ENSO impacts over southern Africa

Summary This study investigates the impacts of five recent ENSO events on southern Africa, the associated circulation anomalies and the ability of an atmospheric general circulation model (UKMO HadAM3) to represent these impacts when forced by observed sea-surface temperature (SST). It is found that the model is most successful for the 1997/8 El Niño but does less well for the 1991/2 and 2002/3 El Niños and the 1995/6 and 1999/00 La Niña events. Diagnostics from the model and NCEP re-analyses suggest that modulations to the Angola low, an important centre of tropical convection over southern Africa during austral summer, are often important for influencing the rainfall impacts of ENSO over subtropical southern Africa. Since the model has difficulty in adequately representing this regional circulation feature and its variability, it has problems in capturing ENSO rainfall impacts over southern Africa. During 1997/8, modulations to the Angola low were weak and Indian Ocean SST forcing strong and the model is relatively successful. The implications of these results for dynamical model based seasonal forecasting of the region are discussed.Current affiliation: CSIR Centre for Mathematical Modelling and Computer Simulation, Bangalore, India.     

The transition from the present-day climate to a modern Snowball Earth

We use the coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM to investigate the transition from the present-day climate to a modern Snowball Earth, defined as the Earth in modern geography with complete sea-ice cover. Starting from the present-day climate and applying an abrupt decrease of total solar irradiance (TSI) we find that the critical TSI marking the Snowball Earth bifurcation point is between 91 and 94% of the present-day TSI. The Snowball Earth bifurcation point as well as the transition times are well reproduced by a zero-dimensional energy balance model of the mean ocean potential temperature. During the transition, the asymmetric distribution of continents between the Northern and Southern Hemisphere causes heat transports toward the more water-covered Southern Hemisphere. This is accompanied by an intensification of the southern Hadley cell and the wind-driven subtropical ocean cells by a factor of 4. If we set back TSI to 100% shortly before the transition to a modern Snowball Earth is completed, a narrow band of open equatorial water is sufficient for rapid melting. This implies that for 100% TSI the point of unstoppable glaciation separating partial from complete sea-ice cover is much closer to complete sea-ice cover than in classical energy balance models. Stable states can have no greater than 56.6% sea-ice cover implying that ECHAM5/MPI-OM does not exhibit stable states with near-complete sea-ice cover but open equatorial waters.     

Composite climatic patterns associated with extreme modes of summer rainfall over southern Africa: 1975–1984

Summary Climatic patterns associated with extreme modes of summer rainfall over southern Africa are investigated using composite techniques. Differences between the wet summers of the mid-1970s and the dry summers of the early 1980s are highlighted. In dry summers both the Southern Oscillation Index (SOI) and Quasi-Biennial Oscillation (QBO) are negatively biased. Composite difference fields of outgoing longwave radiation (OLR), sea surface temperature (SST), and upper and lower tropospheric wind are analysed. The OLR difference field indicates the widespread nature of convective variations with a consistent sign in the domain 15–33° S, 0–40° E. An area of opposing sign is conspicuous over the southwest Indian Ocean and represents a dipole, whereby wet summers over southern Africa coincide with dry summers over the adjacent ocean. This dipole behaviour is an expression of the primary mode of interannual climatic variability in the region. SST composite differences are negative over a wide portion of the central equatorial Indian Ocean and SE Atlantic, and positive to the south of Africa where the Agulhas Current flows. Wind composites reveal distinctive circulation differences in the extreme summers considered. In the tropical zone off the east coast of Africa difference vectors indicate upper westerly and lower easterly circulation anomalies, and distinguish a pathway for moist Indian Ocean air. A deep anticyclonic gyre is located over the region of positive SST differences in the sub-tropics to the SE of Africa. The identification of climatic patterns in extreme summers offers some useful guidelines in seasonal forecasts.With 6 Figures     

A description of persistent climatic anomalies in a 1000-year climatic model simulation

 The Mark 2 version of the CSIRO coupled global climatic model has been used to generate a 1000-year simulation of natural (i.e. unforced) climatic variability representative of “present conditions”. The annual mean output from the simulation has been used to investigate the occurrence of decadal and longer trends over the globe for a number of climatic variables. Here trends are defined to be periods of years with a climatic anomaly of a given sign. The analysis reveals substantial differences between the trend characteristics of the various climatic variables. Trends longer than 12 years duration were unusual for rainfall. Such trends were fairly uniformly distributed over the globe and had an asymmetry in the rate of occurrence for wet or dry conditions. On the other hand, trends in surface wind stress, and especially the atmospheric screen temperature, were of longer duration but primarily confined to oceanic regions. The trends in the atmospheric screen temperature could be traced deep into the oceanic mixed layer, implying large changes in oceanic thermal inertia. This thermal inertia then constituted an important component of the `memory' of the climatic system. While the geographic region associated with a given trend could be identified over several adjacent grid boxes of the model, regional plots for individual years of the trend revealed a range of variations, suggesting that a consistent forcing mechanism may not be responsible for a trend at a given location. Typical return periods for 12-year rainfall trends were once in 1000 years, highlighting the rarity of such events. Using a looser definition of a trend revealed that drying trends up to 50 years duration were also possible, attributable solely to natural climatic variability. Significant (∼20% to 40%) rainfall reductions per year can be associated with a long-term drying trend, hence such events are of considerable climatic significance. It can take more than 100 years for the hydrologic losses associated with such a trend to be overcome. Overall, the simulation provides new and useful insights into climatic trends, and quantifies a number of poorly observed characteristics. The results highlight the extensive and pervasive influence of unforced natural climatic variability as an omnipresent generator of climatic trends. Received: 20 January 2000 / Accepted: 21 September 2000     

Effects of the southern oscillation on the probability for climatic categories of monthly rainfall, in a semi-arid region in the southern mid-latitudes

In many regions of the world, planning agricultural and water management activities is usually done based on probabilities for monthly rainfall, taking on values on specified intervals of values. These intervals of monthly rainfall amounts are commonly grouped into three categories: drought, normal rainfall, and abundant rainfall. Changes in the probabilities for occurrence of monthly rainfall amounts within these climatic rainfall categories will influence the decisions farmers and water managers will take (for example, crops to cultivate, flood preparedness, and operations of water reservoirs). This research explores the changes produced by the SO (Southern Oscillation) on the probability that the areal average of monthly rainfall (AAvMR) takes on values belonging to specified climatic rainfall categories. The semi-arid region under study is a major agricultural region in central Argentina; weather effects on agriculture in this region influence the world market of several crops. The evolution of the Southern Oscillation was divided into three phases: LSOI (low Southern Oscillation index phase, that includes ENSO events), NSOI (neutral SOI phase), and HSOI (high SOI phase that includes La Niña–SO events). The following are the criteria defining the three phases of the SO: (1) low SOI (ENSO), where the five-month moving average of the SO index, SOI, is less than −0.5 standard deviation during at least five consecutive months, and is equal to or less than −1 standard deviation during at least one month; (2) high SOI (La Niña–SO), where the SOI is greater than 0.5 standard deviation during at least five consecutive months, and is equal to or greater than 1 standard deviation during at least one month; and (3) neutral SOI (transition between extremes), where the SOI does not correspond to low SOI nor to high SOI. It was found that the Southern Oscillation influences the probability distribution of monthly rainfall only in four months of the year. Findings show that monthly rainfall has a complex response to the evolution of the SO. The response is not restricted to higher probability for occurrence of abundant rainfall or drought categories during low SOI (ENSO) or high SOI (La Niña–SO) episodes, respectively. The LSOI (ENSO) phase influences the AAvMR in several ways: depending on the month, it increases or decreases the probability of the abundant rainfall category. LSOI (ENSO) also increases or decreases, depending on the month, the probability of the normal rainfall category. It also decreases the probability that AAvMR takes on values in the drought category. A similar kind of complex response of monthly rainfall amounts occurs when the active phase is the HSOI (La Niña–SO). The responses are: (1) the probability of the category `drought' increases only in three months of the year, (2) increase or decrease of the probability of the normal rainfall category, depending on the month, and (3) decrease of the probability of the abundant rainfall category. Finally, the effects of NSOI (neutral phase of the SO) are not negligible. Depending on the month, NSOI episodes increase or decrease the probability of drought, or abundant rainfall, or normal rainfall categories.     

桂南农业气候资源及其开发利用

分析了桂南地区光、温及雨水3大气候资源，主要作物气候生产潜力，指出了影响作物气候生产潜力的主要气候因素，并提出了开发对策。     

近35a六盘水倒春寒气候特征分析

该文利用1971-2005年六盘水市3个测站的平均气温资料,采用线性倾向估计、小波分析等统计方法,对六盘水地区倒春寒天气的分布情况、年际变化及倒春寒发生的总次数及总日数的气候变化特征等进行了分析,结果表明:近35a倒春寒天气东部、北部较重,南部较轻;倒春寒天气总日数和总次数近35a来均呈下降趋势;倒春寒天气总日数在年代际尺度上存在显著的16a、准10a周期振荡,在年际尺度上,存在准6a和2~4a的振荡周期;倒春寒天气总次数存在显著的准10a周期振荡和一个较弱的准4a振荡周期;目前倒春寒天气由偏少向偏多的趋势转变。     

Orbital forcing in a stochastic resonance model of the Late-Pleistocene climatic variations

Applying a number of simplifying assumptions, we have used a zero-dimensional energy balance climate model (EBM) of the Budyko-Sellers type to investigate the degree to which the climatic variability of the Late Pleistocene can be explained by the mechanism of stochastic resonance. As a logical extension of an earlier version of this model we have included a more complete representation for the orbital forcings to show that a reasonably good agreement with the paleoclimatic record can be obtained. This zerodimensional EBM is appealing because of its simplicity and its ability to reproduce some of the most striking qualitative features of the observed climatic variability with a very limited number of parameters, several of which can be derived from either the observations or general circulation model (GCM) simulations of ice-age conditions.     

电子表格在短期气候预测评分工作中的应用

介绍了怎样利用电子表格进行短期气候预测评分：首先统计其本参数，建立基本参数表格，然后录入预测结果和实况资料，最后再建一个过渡表格。另外，以水城2003年1月降水趋势预测为例，详细介绍了具体怎样利用气候预测来评分的。     

南印度洋热带气旋生成频数气候变率对海温异常模响应的研究

基于1980—2014年的哈德莱中心海冰及海温的月平均SST资料,美国联合台风警报中心(JTWC)的best-track资料以及NCEP/NCAR再分析月平均资料,利用广义平衡反馈方法(GEFA)研究南印度洋热带气旋(TC)生成频数对海表温度异常的响应特征。研究表明:(1)南印度TC生成频数对北太平洋第一模态(NP1)和热带大西洋第二模态(TA2)有显著响应,分别通过了置信度为99%和96%的Monte-Carlo检验,对应的响应振幅分别为0.67和0.49。(2)局地环境要素对关键SSTA模的GEFA响应结果显示:当NP1出现类似于太平洋年代际振荡(PDO)的正位相时,850 h Pa相对涡度在15°S附近的印度洋海域上都有一个自西向东的显著正响应带,垂直风切变在马达加斯加以东的大部分海域都表现为显著的负响应,600 h Pa相对湿度在马达加斯加以东的部分海域表现为显著的正响应;当TA2对应的时间系数为正异常时,850 h Pa相对涡度和600 h Pa相对湿度在澳大利亚的西北部印度洋海域表现为显著的正响应,垂直风切变在澳大利亚的西北部印度洋海域表现为显著的负响应。     

A sensitivity study on the role of the swamps of southern Sudan in the summer climate of North Africa using a regional climate model

We used the regional climate model RegCM3 to investigate the role of the swamps of southern Sudan in affecting the climate of the surrounding region. Towards this end, we first assessed the performance of a high resolution version of the model over northern Africa. RegCM3 shows a good skill in simulating the climatology of rainfall and temperature patterns as well as the related circulation features during the summer season, outperforming previous coarser resolution applications of the model over this region. Sensitivity experiments reveal that, relative to bare soil conditions, the swamps act to locally modify the surface energy budget primarily through an increase of surface latent heat flux. Existence of the swamps leads to lower ground temperature (up to 2 °C), a larger north–south temperature gradient, and increased local rainfall (up to 40 %). Of particular importance is the impact on rainfall in the surrounding regions. The swamps have almost no impact on the rainfall over the source region of the Nile in Ethiopia or in the Sahel region; however, they favor wetter conditions over central Sudan (up to 15 %) in comparison to the bare desert soil conditions.     

Enhanced climatic warming in the Tibetan Plateau due to doubling CO2: a model study

The National Center for Atmospheric Research (NCAR) regional climate model (RegCM2), together with initial conditions and time-dependent lateral boundary conditions provided by a 130-year transient increasing CO₂ simulation of the NCAR Climate System Model (CSM), has been used to investigate the mechanism of ground warming over the Tibetan Plateau (TP). The model results show that when CO₂ in the atmosphere is doubled, a strong ground warming occurs in the TP. Two regions within it with the largest warming are in the eastern TP (region I) and along the southwestern and western slopes (region II). Moreover, in region I the ground warming in the winter half year is stronger than that in the summer half year, but in region II the warming difference between the seasons becomes opposite to that in region I, i.e., the warming is strong in the summer half year and weak in the winter half year. There are indications that the summer monsoon enhances but the winter monsoon weakens when CO₂ is doubled. A strong elevation dependency of ground warming is found in region I for the winter half year, and in region II for both winter and summer half years at elevations below 5 km. The simulated characteristics of ground warming in the TP are consistent with the observations. In region I, when CO₂ is doubled, the cloud amount increases at lower elevations and decreases at higher elevation for the winter half year. As a consequence, at lower elevations the short wave solar radiation absorbed at the surface declines, and the downward long wave flux reaching the surface enhances; on the other hand, at higher elevations the surface solar radiation flux increases and the surface infrared radiation flux shows a more uniform increase. The net effect of the changes in both radiation fluxes is an enhanced surface warming at higher elevations, which is the primary cause of the elevation dependency in the surface warming. In the summer half year the cloud amount reduces as a result of doubling CO₂ in region I for all elevations, and there is no elevation dependency detected in the ground warming. Furthermore, there is little snow existing in region I for both summer and winter half years, and the impact of snow-albedo feedback is not significant. In region II, although the changes in the cloud amount bear a resemblance to those in region I, the most significant factor affecting the surface energy budget is the depletion of the snow cover at higher elevations, which leads to a reduction of the surface albedo. This reduction in turn leads to an enhancement in the solar radiation absorbed in the surface. The snow-albedo feedback mechanism is the most essential cause of the elevation dependency in the surface warming for region II.