Modeling the Influence of Topographic Barriers on Treeline Advance at the Forest-Tundra Ecotone in Northwestern Alaska

The response of terrestrial ecosystems to climate warming has important implications to potential feedbacks to climate. The interactions between topography, climate, and disturbance could alter recruitment patterns to reduce or offset current predicted positive feedbacks to warming at high latitudes. In northern Alaska the Brooks Range poses a complex environmental and ecological barrier to species migration. We use a spatially explicit model (ALFRESCO) to simulate the transient response of subarctic vegetation to climatic warming in the Kobuk/Noatak River Valley (200 × 400 km) in northwest Alaska. The model simulations showed that a significantly warmer (+6 °C) summer climate would cause expansion of forest through the Brooks Range onto the currently treeless North Slope only after a period of 3000–4000 yr. Substantial forest establishment on the North Slope didnot occur until temperatures warmed 9 °C, and only following a 2000 yr time lag. The long time lags between change in climate and change in vegetation indicate current global change predictions greatly over-estimate the response of vegetation to a warming climate in Alaska. In all the simulations warming caused a steady increase in the proportion of early successional deciduous forest. This would reduce the magnitude of the predicted decrease in regional albedo and the positive feedback to climate warming. Simulation of spruce forest refugia on the North Slope showed forest could survive with only a 4 °C warming and would greatly reduce the time lag of forest expansion under warmer climates. Planting of spruce on the North Slope by humans could increase the likelihood of large-scale colonization of currently treeless tundra. Together, the long time lag and deciduous forest dominance would delay the predicted positive regional feedback of vegetation change to climatic warming. These simulated changes indicate the Brooks Range would significantly constrain regional forest expansion under a warming climate, with similar implications for other regions possessing major east-west oriented mountain ranges.     

Simulation of the evolutionary response of global summer monsoons to orbital forcing over the past 280,000 years

We describe the evolutionary response of northern and southern hemisphere summer monsoons to orbital forcing over the past 280,000 years using a fully coupled general circulation ocean-atmosphere model in which the orbital forcing is accelerated by a factor of 100. We find a strong and positive response of northern (southern) summer monsoon precipitation to northern (southern) summer insolation forcing. On average, July (January) precipitation maxima and JJA (DJF) precipitation maxima have high coherence and are approximately in phase with June (December) insolation maxima, implying an average lag between forcing and response of about 30° of phase at the precession period. The average lag increases to over 40° for 4-month precipitation averages, JJAS (DJFM). The phase varies from region to region. The average JJA (DJF) land temperature maxima also lag the June orbital forcing maxima by about 30° of phase, whereas ocean temperature maxima exhibit a lag of about 60° of phase at the precession period. Using generalized measures of the thermal and hydrologic processes that produce monsoons, we find that the summer monsoon precipitation indices for the six regions all fall within the phase limits of the process indices for the respective hemispheres. Selected observational studies from four of the six monsoon regions report approximate in-phase relations of summer monsoon proxies to summer insolation. However other observational studies report substantial phase lags of monsoon proxies and a strong component of forcing associated with glacial-age boundary conditions or other factors. An important next step will be to include glacial-age boundary condition forcing in long, transient paleoclimate simulations, along with orbital forcing.     

Synergistic feedbacks between ocean and vegetation on mid- and high-latitude climates during the mid-Holocene

Simulations with the IPSL atmosphere–ocean model asynchronously coupled with the BIOME1 vegetation model show the impact of ocean and vegetation feedbacks, and their synergy, on mid- and high-latitude (>40°N) climate in response to orbitally-induced changes in mid-Holocene insolation. The atmospheric response to orbital forcing produces a +1.2 °C warming over the continents in summer and a cooling during the rest of the year. Ocean feedback reinforces the cooling in spring but counteracts the autumn and winter cooling. Vegetation feedback produces warming in all seasons, with largest changes (+1 °C) in spring. Synergy between ocean and vegetation feedbacks leads to further warming, which can be as large as the independent impact of these feedbacks. The combination of these effects causes the high northern latitudes to be warmer throughout the year in the ocean–atmosphere-vegetation simulation. Simulated vegetation changes resulting from this year-round warming are consistent with observed mid-Holocene vegetation patterns. Feedbacks also impact on precipitation. The atmospheric response to orbital-forcing reduces precipitation throughout the year; the most marked changes occur in the mid-latitudes in summer. Ocean feedback reduces aridity during autumn, winter and spring, but does not affect summer precipitation. Vegetation feedback increases spring precipitation but amplifies summer drying. Synergy between the feedbacks increases precipitation in autumn, winter and spring, and reduces precipitation in summer. The combined changes amplify the seasonal contrast in precipitation in the ocean–atmosphere-vegetation simulation. Enhanced summer drought produces an unrealistically large expansion of temperate grasslands, particularly in mid-latitude Eurasia.     

Simulated impact of vegetation on climate across the North American monsoon region in CCSM3.5

The influence of prescribed changes in vegetation on the climate of the North American monsoon region is examined using the National Center for Atmospheric Research Community Climate System Model Version 3.5 (NCAR CCSM3.5). Initial value ensemble experiments are performed in which the vegetation cover fraction over the North American monsoon region is reduced by 0.2 and the intra-annual climatic response is assessed probabilistically in each one-year ensemble experiment. Changes in the surface radiation budget include decreases in sensible and latent heat fluxes and increases in upward longwave and downward shortwave radiation fluxes, with small net changes in surface albedo. The climatic responses to reduced vegetation cover fraction include year-round increases in ground and surface air temperature, a dampened hydrologic cycle with decreased springtime evaporation, springtime and autumnal precipitation, and autumnal cloud cover, and enhanced atmospheric subsidence in late autumn. Decreased vegetation shifts the monsoon season over the Southwest United States earlier in the year. Within the North American monsoon region, the most robust vegetation feedbacks to climate are found over woody landscapes.     

Fluctuation of farming-pastoral ecotone in association with changing East Asia monsoon climate

As a monsoon climate dominated region, East Asia has a high rate of climate variation. Previous studies demonstrated that the East Asian monsoon had weakened since the end of 1970’s; however, contrary to the climatic trend, a common scenario of advancing farming-pastoral ecotone (FPE) has been proposed. The objective of this study is to analyze land surface changes in association with monsoon climate variability over past 25 years in East Asia. A combination of intensive ground survey of vegetation and land use, meteorological data, and remote sensing are used to quantify the relationship between vegetation and climate and to analyze the FPE fluctuations associated with changing climate. Field precipitation data from 1981 to 2005, are used to represent climate variations and to delineate the FPE boundary. NDVI data are used to evaluate greenness-precipitation linkages by vegetation type and to create land cover maps depicting spatial pattern fluctuations of the FPE. This study demonstrates that: (1) There was no persistent northwest shifting trend of either the FPE boundary or vegetation cover during last 25 years. (2) Time integrated NDVI (TI-NDVI) varies with precipitation, and the maximum or minimum NDVI may be only sensitive to precipitation for areas with mean annual precipitation lower than approximately 200 mm. (3) A significant relationship exists between NDVI and precipitation variations for areas with mean annual precipitation greater than approximately 300 mm, especially the ecotone with a ΔNDVI of 0.122?±?0.032. (4) The “advances” of FPE closely mimic fluctuations of precipitation in East Asia.     

Fully coupled climate/dynamical vegetation model simulations over Northern Africa during the mid-Holocene

 The climate and vegetation patterns of the middle Holocene (6000 years ago; 6 ka) over Northern Africa are simulated using a fully-synchronous climate and dynamical vegetation model. The coupled model predicts a northward shift in tropical rainforest and tropical deciduous forest vegetation by about 5 degrees of latitude, and an increase in grassland at the present-day simulated Saharan boundaries. The northward expansion of vegetation over North Africa at 6 ka is initiated by an orbitally-induced amplification of the summer monsoon, and enhanced by feedback effects induced by the vegetation. These combined processes lead to a major reduction in Saharan desert area at 6 ka relative to present-day of about 50%. However, as shown in previous asynchronous modelling studies, the coupled climate/vegetation model does not fully reproduce the vegetation patterns inferred from palaeoenvironmental records, which suggest that steppe vegetation may have existed across most of Northern Africa. Orbital changes produce an intensification of monsoonal precipitation during the peak rainy season (July to September), whilst vegetation feedbacks, in addition to producing further increases in the peak intensity, play an important role in extending the rainy season from May/June through to November. The orbitally induced increases in precipitation are relatively uniform from west to east, in contrast to vegetation feedback-induced increases in precipitation which are concentrated in western North Africa. Annual-average precipitation increases caused by vegetation feedbacks are simulated to be of similar importance to orbital effects in the west, whilst they are relatively unimportant farther to the east. The orbital, vegetation and combined orbital and vegetation-induced changes in climate, from the simulations presented in this study, have been compared with results from previous modelling studies over the appropriate North African domain. Consequently, the important role of vegetation parametrizations in determining the magnitude of vegetation feedbacks has been illustrated. Further modelling studies which include the effects of changes in ocean temperature and changes in soil properties may be needed, along with additional observations, to resolve the discrepancy between model predictions of vegetation and palaeorecords for North Africa. Received: 15 June 1999 / Accepted: 14 December 1999     

The global response to Younger Dryas boundary conditions in an AGCM simulation

 Geological evidence points to a global Younger Dryas (YD) climatic oscillation during the last glacial/ present interglacial transition phase. A convincing mechanism to explain this global YD climatic oscillation is not yet available. Nevertheless, a profound understanding of the mechanism behind the YD climate would lead to a better understanding of climate variability. Therefore, the Hamburg atmospheric circulation model was used to perform four numerical experiments on the YD climate. The objective of this study is to improve the understanding of different forcings influencing climate during the last glacial/interglacial transition and to investigate to what extent the model response agrees with global geological evidence of YD climate change. The following boundary conditions were altered: sea surface conditions, ice sheets, insolation and atmospheric CO₂ concentration. Sea surface temperatures based on foraminiferal assemblages proved to produce insufficient winter cooling in the N Atlantic Ocean in two experiments. It is proposed that this discrepancy is caused by uncertainties in the reconstruction method of sea surface temperatures. Therefore, a model-derived set of Atlantic surface ocean conditions was prescribed in a subsequent simulation. However, the latter set represented an Atlantic Ocean without a thermohaline circulation, which is not in agreement with evidence from ocean cores. The global response to the boundary conditions was analysed using three variables, namely surface temperature, zonal wind speed and precipitation. The statistical significance of the changes was tested with a two-tailed t-test. Moreover, the significant responses to cooled oceans were compared with geological evidence of a YD oscillation. This comparison revealed a good match in Europe, Greenland, Atlantic Canada and the N Pacific region, explaining the YD oscillation in these regions as a response to cooled N Atlantic and N Pacific Oceans. However, the results leave the YD climate in other regions completely unexplained. This reflects either an insufficient set of boundary conditions or the important role played by feedbacks within the coupled atmosphere-ocean-ice system. These feedbacks are poorly represented in the used atmospheric model, since ice sheets and the ocean surface conditions have to be prescribed. Received: 30 July 1996 / Accepted: 12 February 1997     

Statistical and dynamical assessment of vegetation feedbacks on climate over the boreal forest

Vegetation feedbacks over Asiatic Russia are assessed through a combined statistical and dynamical approach in a fully coupled atmosphere–ocean–land model, FOAM-LPJ. The dynamical assessment is comprised of initial value ensemble experiments in which the forest cover fraction is initially reduced over Asiatic Russia, replaced by grass cover, and then the climatic response is determined. The statistical feedback approach, adopted from previous studies of ocean–atmosphere interactions, is applied to compute the feedback of forest cover on subsequent temperature and precipitation in the control simulation. Both methodologies indicate a year-round positive feedback on temperature and precipitation, strongest in spring and moderately substantial in summer. Reduced boreal forest cover enhances the surface albedo, leading to an extended snow season, lower air temperatures, increased atmospheric stability, and enhanced low cloud cover. Changes in the hydrological cycle include diminished transpiration and moisture recycling, supporting a reduction in precipitation. The close agreement in sign and magnitude between the statistical and dynamical feedback assessments testifies to the reliability of the statistical approach. An additional statistical analysis of monthly vegetation feedbacks over Asiatic Russia reveals a robust positive feedback on air temperature of similar quantitative strength in two coupled models, FOAM-LPJ and CAM3–CLM3, and the observational record. CCR Contribution # 931.     

1982-2012年北半球荒漠草原过渡带植被物候特征及其与气候因子的关系

基于GIMMS（global inventory modeling and mapping studies）NDVI 3g数据,在提取北半球荒漠草原过渡带每年植被物候期的基础上,研究了1982-2012年物候期的时间演化趋势及空间分异特征,并结合全球气候再分析资料,探讨了物候变化的气候驱动因素。结果表明：在1998年之前,荒漠草原过渡带植被物候期变化地区间差异较大,而在1998年之后,北半球荒漠草原过渡带生长季结束期整体提前,平均提前0.41 d/a;同时,除萨赫勒以外的各地区植被生长季长度普遍缩短,平均缩短0.88 d/a。植被物候期与气候因子的相关分析发现,荒漠草原过渡带植被物候变化受气候变化影响显著,且空间差异明显。在中高纬度地区,气温是限制植被活动的关键因子,温度升高可以促进生长季开始期的提前,而降水增加则会妨碍植被生长;在较低纬度地区,水分是影响植被活动的关键因素,高温造成的水分亏缺会导致植被生长季缩短。从植被物候期对各气候因子响应的时滞性来看,荒漠草原过渡带植被的物候期对气温变化的响应最迅速,对蒸散的响应存在一定的滞后性,而对降水的响应不存在时滞差异。     

气候变化背景下陆地季风与非季风区极端降水特征对比

利用月平均地表气候要素数据集(CRUTS 4.02)的逐月降水和逐月日平均气温资料,采用线性趋势分析、滑动平均和相关分析等方法,研究了 1901-2017年气候变化背景下全球陆地季风区与非季风区的极端降水特征.结果表明:我国所处的亚洲季风区的极端降水频率分布较为稳定,仅在变暖减缓时段出现大范围小值区;非季风区在急剧加速...     

Interannual Climate Variability of the Past Millennium from Simulations

The interannual variability of global temperature and precipitation during the last millennium is analyzed using the results of ten coupled climate models participating in the Paleoclimate Modelling Intercomparison Project Phase 3. It is found that large temperature(precipitation) variability is most dominant at high latitudes(tropical monsoon regions), and the seasonal magnitudes are greater than the annual mean. Significant multi-decadal-scale changes exist throughout the whole period for the zonal mean of both temperature and precipitation variability, while their long-term trends are indistinctive. The volcanic forcings correlate well with the temperature variability at midlatitudes, indicating possible leading drivers for the interannual time scale climate change.     

Assessment of vegetation dynamics and their response to variations in precipitation and temperature in the Tibetan Plateau

The Tibetan Plateau is a region sensitive to climate change, due to its high altitude and large terrain. This sensitivity can be measured through the response of vegetation patterns to climate variability in this region. Time series analysis of Normalized Difference Vegetation Index (NDVI) imagery and correlation analyses are effective tools to study land cover changes and their response to climatic variations. This is especially important for regions like the Tibetan Plateau, which has a complex ecosystem but lacks a lot of detailed in-situ observation data due to its remoteness, vastness and the severity of its climatic conditions. In this research a time series of 315 SPOT VEGETATION scenes, covering the period between 1998 and 2006, has been processed with the Harmonic ANalysis of Time Series (HANTS) algorithm in order to reveal the governing spatiotemporal pattern of variability. Results show that the spatial distribution of NDVI values is in agreement with the general climate pattern in the Tibetan Plateau. The seasonal variation is greatly influenced by the Asian monsoon. Interannual analysis shows that vegetation density (recorded here by the NDVI values) in the entire Tibetan Plateau has generally increased. Using a 1 km resolution land cover map from GLC2000, seven meteorological stations, presenting monthly data on near surface air temperature and precipitation, were selected for correlation analysis between NDVI and climate conditions in this research. A time lag response has also been found between NDVI and climate variables. Except in desert grassland (Shiquanhe station), the NDVI of all selected sites showed strong correlation with air temperature and precipitation, with variations in correlation according to the different land cover types at different locations. The strongest relationship was found in alpine and subalpine plain grass, the weakest in desert grassland.     

Mid-Holocene monsoons: a multi-model analysis of the inter-hemispheric differences in the responses to orbital forcing and ocean feedbacks

The response of monsoon circulation in the northern and southern hemisphere to 6?ka orbital forcing has been examined in 17 atmospheric general circulation models and 11 coupled ocean–atmosphere general circulation models. The atmospheric response to increased summer insolation at 6?ka in the northern subtropics strengthens the northern-hemisphere summer monsoons and leads to increased monsoonal precipitation in western North America, northern Africa and China; ocean feedbacks amplify this response and lead to further increase in monsoon precipitation in these three regions. The atmospheric response to reduced summer insolation at 6?ka in the southern subtropics weakens the southern-hemisphere summer monsoons and leads to decreased monsoonal precipitation in northern South America, southern Africa and northern Australia; ocean feedbacks weaken this response so that the decrease in rainfall is smaller than might otherwise be expected. The role of the ocean in monsoonal circulation in other regions is more complex. There is no discernable impact of orbital forcing in the monsoon region of North America in the atmosphere-only simulations but a strong increase in precipitation in the ocean–atmosphere simulations. In contrast, there is a strong atmospheric response to orbital forcing over northern India but ocean feedback reduces the strength of the change in the monsoon although it still remains stronger than today. Although there are differences in magnitude and exact location of regional precipitation changes from model to model, the same basic mechanisms are involved in the oceanic modulation of the response to orbital forcing and this gives rise to a robust ensemble response for each of the monsoon systems. Comparison of simulated and reconstructed changes in regional climate suggest that the coupled ocean–atmosphere simulations produce more realistic changes in the northern-hemisphere monsoons than atmosphere-only simulations, though they underestimate the observed changes in precipitation in all regions. Evaluation of the southern-hemisphere monsoons is limited by lack of quantitative reconstructions, but suggest that model skill in simulating these monsoons is limited.     

高纬度淡水强迫增强背景下大西洋经向翻转环流的响应及其机制

利用卑尔根海洋－大气－海冰耦合气候模式(Bergen Climate Model, 简称BCM), 研究在北冰洋及北欧海淡水强迫增强的背景下, 大西洋经向翻转环流(Atlantic Meridional Overturning Circulation, 简称AMOC)的响应及其机制, 着重讨论了海表热力性质、北大西洋深层水 (North Atlantic Deep Water, 简称NADW) 的生成率、 海洋内部等密度层间的垂直混合 (Diapycnal Mixing, 简称DM) 以及大气风场等物理过程随AMOC的响应所发生的时间演变特征。结果显示, 在持续150年增强 (强度为0.4 Sv) 的淡水强迫下 (淡水试验, FW1), AMOC的强度表现为前50年的快速减弱和在接下来100年中的逐渐恢复。同时, 在淡水试验的前50年北大西洋高纬度海表盐度 (Sea Surface Salinity, 简称SSS) 减小, 海水密度降低, 冬季对流混合减弱, 导致NADW生成率快速减弱; 在接下来的100年中, 尽管增强的淡水强迫依然维持, 由于海洋内部自身的调节和海气相互作用, 导致了AMOC的逐渐恢复。恢复机制可以概括为: (1) 随着向南的NADW的减少, 大西洋中低纬度海水垂直层结逐渐减弱, DM随之逐渐增强, 有利于中低纬度海盆内深层水的上升; (2) 南半球西风应力增强与东风应力的减弱及北半球东风的增强使得大西洋向北的埃克曼体积通量净传输恢复; (3) 大西洋向北的盐度传输逐渐恢复及次极地回旋区降水的减弱, 导致SSS和NADW生成率的恢复, 与之对应, AMOC逐渐恢复。研究还发现, 淡水试验中, NADW的恢复主要以厄尔明格海 (Irminger Sea) 为主, 冬季北大西洋海平面气压场 (SLP) 呈现类似正北大西洋涛动 (NAO+) 的模态, 热带降水中心移到赤道以南, 大西洋热带SSS增强。     

Predictability of Mediterranean climate variables from oceanic variability. Part II: Statistical models for monthly precipitation and temperature in the Mediterranean area

The objective of this study is to investigate the predictability of monthly climate variables in the Mediterranean area by using statistical models. It is a well-known fact that the future state of the atmosphere is sensitive to preceding conditions of the slowly varying ocean component with lead times being sufficiently long for predictive assessments. Sea surface temperatures (SSTs) are therefore regarded as one of the best variables to be used in seasonal climate predictions. In the present study, SST-regimes which have been derived and discussed in detail in Part I of this paper, are used with regard to monthly climate predictions for the Mediterranean area. Thus, cross-correlations with time lags from 0 up to 12?months and ensuing multiple regression analyses between the large-scale SST-regimes and monthly precipitation and temperature for Mediterranean sub-regions have been performed for the period 1950?C2003. Statistical hindcast ensembles of Mediterranean precipitation including categorical forecast skill can be identified only for some months in different seasons and for some individual regions of the Mediterranean area. Major predictors are the tropical Atlantic Ocean and the North Atlantic Ocean SST-regimes, but significant relationships can also be found with tropical Pacific and North Pacific SST-regimes. Statistical hindcast ensembles of Mediterranean temperature with some categorical forecast skill can be determined primarily for the Western Mediterranean and the North African regions throughout the year. As for precipitation the major predictors for temperature are located in the tropical Atlantic Ocean and the North Atlantic Ocean, but some connections also exist with the Pacific SST variations.     

Impact of vegetation changes on the dynamics of the atmosphere at the Last Glacial Maximum

Much work is under way to identify and quantify the feedbacks between vegetation and climate. Palaeoclimate modelling may provide a mean to address this problem by comparing simulations with proxy data. We have performed a series of four simulations of the Last Glacial Maximum (LGM, 21,000 years ago) using the climate model HadSM3, to test the sensitivity of climate to various changes in vegetation: a global change (according to a previously discussed simulation of the LGM with HadSM3 coupled to the dynamical vegetation model TRIFFID); a change only north of 35°N; a change only south of 35°N; and a variation in stomatal opening induced by the reduction in atmospheric CO₂ concentration. We focus mainly on the response of temperature, precipitation, and atmosphere dynamics. The response of continental temperature and precipitation mainly results from regional interactions with vegetation. In Eurasia, particularly Siberia and Tibet, the response of the biosphere substantially enhances the glacial cooling through a positive feedback loop between vegetation, temperature, and snow-cover. In central Africa, the decrease in tree fraction reduces the amount of precipitation. Stomatal opening is not seen to play a quantifiable role. The atmosphere dynamics, and more specifically the Asian summer monsoon system, are significantly altered by remote changes in vegetation: the cooling in Siberia and Tibet act in concert to shift the summer subtropical front southwards, weaken the easterly tropical jet and the momentum transport associated with it. By virtue of momentum conservation, these changes in the mid-troposphere circulation are associated with a slowing of the Asian summer monsoon surface flow. The pattern of moisture convergence is slightly altered, with moist convection weakening in the western tropical Pacific and strengthening north of Australia.     

Regional hydrological cycle changes in response to an ambitious mitigation scenario

Climate change impacts on the regional hydrological cycle are compared for model projections following an ambitious emissions-reduction scenario (E1) and a medium-high emissions scenario with no mitigation policy (A1B). The E1 scenario is designed to limit global annual mean warming to 2 °C or less above pre-industrial levels. A multi-model ensemble consisting of ten coupled atmosphere–ocean general circulation models is analyzed, which includes five Earth System Models containing interactive carbon cycles. The aim of the study is to assess the changes that could be mitigated under the E1 scenario and to identify regions where even small climate change may lead to strong changes in precipitation, cloud cover and evapotranspiration. In these regions the hydrological cycle is considered particularly vulnerable to climate change, highlighting the need for adaptation measures even if strong mitigation of climate change would be achieved. In the A1B projections, there are significant drying trends in sub-tropical regions, precipitation increases in high latitudes and some monsoon regions, as well as changes in cloudiness and evapotranspiration. These signals are reduced in E1 scenario projections. However, even under the E1 scenario, significant precipitation decrease in the subtropics and increase in high latitudes are projected. Particularly the Amazon region shows strong drying tendencies in some models, most probably related to vegetation interaction. Where climate change is relatively small, the E1 scenario tends to keep the average magnitude of potential changes at a level comparable to current intra-seasonal to inter-annual variability at that location. Such regions are mainly located in the mid-latitudes.     

Dynamical greenhouse-plus feedback and polar warming amplification. Part II: meridional and vertical asymmetries of the global warming

This paper examines several prominent thermodynamic and dynamic factors responsible for the meridional and vertical warming asymmetries using a moist coupled atmosphere–surface radiative transportive four-box climate model. A coupled atmosphere–surface feedback analysis is formulated to isolate the direct response to an anthropogenic greenhouse gas forcing from individual local feedbacks (water vapor, evaporation, surface sensible heat flux, and ice-albedo), and from the non-local dynamical feedback. Both the direct response and response to water vapor feedback are stronger in low latitudes. The joint effect of the ice-albedo and dynamical greenhouse-plus feedbacks acts to amplify the high latitude surface warming whereas both the evaporation and dynamical greenhouse-minus feedbacks cause a reduction of the surface warming in low latitudes. The enhancement (reduction) of local feedbacks in high (low) latitudes in response to the non-local dynamic feedback further strengthens the polar amplification of the surface warming. Both the direct response and response to water vapor feedback lead to an increase of lapse rate in both low and high latitudes. The stronger total dynamic heating in the mean state in high latitudes is responsible for a larger increase of lapse rate in high latitudes in the direct response and response to water vapor feedback. The local evaporation and surface sensible heat flux feedbacks reduce the lapse rate both in low and high latitudes through cooling the surface and warming the atmosphere. The much stronger evaporation feedback leads to a final warming in low latitudes that is stronger in the atmosphere than the surface.     

气候变化对苏尼特草原NDVI的影响

利用苏尼特草原地区1982—2006年NOAA AVHRR的NDVI数字遥感影像,以及1998-2007年逐旬的SPOT VAG-NDVI数据集,结合研究区域内苏尼特左旗、苏尼特右旗、朱日和、二连浩特4个气象站点的同期降水、气温数据,对植被盖度与不同组合方式的降水及气温数据进行了相关分析,探讨了植被盖度与气象因子的关系。结果表明：苏尼特草原生长季平均盖度、逐月盖度与降水呈正相关关系,与气温呈负相关关系,其中降水对盖度的影响存在着时滞及累积效应。