共查询到20条相似文献,搜索用时 15 毫秒
1.
The impact of new land surface physics on the GCM simulation of climate and climate sensitivity 总被引:16,自引:4,他引:12
P. M. Cox R. A. Betts C. B. Bunton R. L. H. Essery P. R. Rowntree J. Smith 《Climate Dynamics》1999,15(3):183-203
Recent improvements to the Hadley Centre climate model include the introduction of a new land surface scheme called “MOSES”
(Met Office Surface Exchange Scheme). MOSES is built on the previous scheme, but incorporates in addition an interactive plant
photosynthesis and conductance module, and a new soil thermodynamics scheme which simulates the freezing and melting of soil
water, and takes account of the dependence of soil thermal characteristics on the frozen and unfrozen components. The impact
of these new features is demonstrated by comparing 1×CO2 and 2×CO2 climate simulations carried out using the old (UKMO) and new (MOSES) land surface schemes. MOSES is found to improve the
simulation of current climate. Soil water freezing tends to warm the high-latitude land in the northern Hemisphere during
autumn and winter, whilst the increased soil water availability in MOSES alleviates a spurious summer drying in the mid-latitudes.
The interactive canopy conductance responds directly to CO2, supressing transpiration as the concentration increases and producing a significant enhancement of the warming due to the
radiative effects of CO2 alone.
Received: 16 March 1998 / Accepted: 4 August 1998 相似文献
2.
Agricultural drought in a future climate: results from 15 global climate models participating in the IPCC 4th assessment 总被引:8,自引:0,他引:8
Guiling Wang 《Climate Dynamics》2005,25(7-8):739-753
This study examines the impact of greenhouse gas warming on soil moisture based on predictions of 15 global climate models
by comparing the after-stabilization climate in the SRESA1b experiment with the pre-industrial control climate. The models
are consistent in predicting summer dryness and winter wetness in only part of the northern middle and high latitudes. Slightly
over half of the models predict year-round wetness in central Eurasia and/or year-round dryness in Siberia and mid-latitude
Northeast Asia. One explanation is offered that relates such lack of seasonality to the carryover effect of soil moisture
storage from season to season. In the tropics and subtropics, a decrease of soil moisture is the dominant response. The models
are especially consistent in predicting drier soil over the southwest North America, Central America, the Mediterranean, Australia,
and the South Africa in all seasons, and over much of the Amazon and West Africa in the June–July–August (JJA) season and
the Asian monsoon region in the December–January–February (DJF) season. Since the only major areas of future wetness predicted
with a high level of model consistency are part of the northern middle and high latitudes during the non-growing season, it
is suggested that greenhouse gas warming will cause a worldwide agricultural drought. Over regions where there is considerable
consistency among the analyzed models in predicting the sign of soil moisture changes, there is a wide range of magnitudes
of the soil moisture response, indicating a high degree of model dependency in terrestrial hydrological sensitivity. A major
part of the inter-model differences in the sensitivity of soil moisture response are attributable to differences in land surface
parameterization. 相似文献
3.
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. 相似文献
4.
A version of the National Centre for Atmospheric Research (NCAR) coupled climate model is integrated under current climate
conditions and in a series of experiments with climate forcings ranging from modest to very strong. The purpose of the experiments
is to investigate the nature and behaviour of the climate feedback/sensitivity of the model, its evolution with time and climate
state, the robustness of model parameterizations as forcing levels increase, and the possibility of a “runaway” warming under
strong forcing. The model is integrated for 50 years, or to failure, after increasing the solar constant by 2.5, 10, 15, 25,
35, and 45% of its control value. The model successfully completes 50 years of integration for the 2.5, 10, 15, and 25% solar
constant increases but fails for increases of 35% and 45%. The effective global climate sensitivity evolves with time and
analysis indicates that a new equilibrium will be obtained for the 2.5, 10, and 15% cases but that runaway warming is underway
for the 25% increase in solar constant. Feedback processes are analysed both locally and globally in terms of longwave and
shortwave, clear-sky/surface, and cloud forcing components. Feedbacks in the system must be negative overall and of sufficient
strength to balance the positive forcing if the system is to attain a new equilibrium. Longwave negative feedback processes
strengthen in a reasonably linear fashion as temperature increases but shortwave feedback processes do not. In particular,
solar cloud feedback becomes less negative and, for the 25% forcing case, eventually becomes positive, resulting in temperatures
that “run away”. The conditions under which a runaway climate warming might occur have previously been investigated using
simpler models. For sufficiently strong forcing, the greenhouse effect of increasing water vapour in a warmer atmosphere is
expected to overwhelm the negative feedback of the longwave cooling to space as temperature increases. This is not, however,
the reason for the climate instability experienced in the GCM. Instead, the model experiences a “cloud feedback” warming whereby
the decrease in cloudiness that occurs when temperature increases beyond a critical value results in an increased absorption
of solar radiation by the system, leading to the runaway warming. 相似文献
5.
Incorporating organic soil into a global climate model 总被引:3,自引:1,他引:2
Organic matter significantly alters a soil’s thermal and hydraulic properties but is not typically included in land-surface
schemes used in global climate models. This omission has consequences for ground thermal and moisture regimes, particularly
in the high-latitudes where soil carbon content is generally high. Global soil carbon data is used to build a geographically
distributed, profiled soil carbon density dataset for the Community Land Model (CLM). CLM parameterizations for soil thermal
and hydraulic properties are modified to accommodate both mineral and organic soil matter. Offline simulations including organic
soil are characterized by cooler annual mean soil temperatures (up to ∼2.5°C cooler for regions of high soil carbon content).
Cooling is strong in summer due to modulation of early and mid-summer soil heat flux. Winter temperatures are slightly warmer
as organic soils do not cool as efficiently during fall and winter. High porosity and hydraulic conductivity of organic soil
leads to a wetter soil column but with comparatively low surface layer saturation levels and correspondingly low soil evaporation.
When CLM is coupled to the Community Atmosphere Model, the reduced latent heat flux drives deeper boundary layers, associated
reductions in low cloud fraction, and warmer summer air temperatures in the Arctic. Lastly, the insulative properties of organic
soil reduce interannual soil temperature variability, but only marginally. This result suggests that, although the mean soil
temperature cooling will delay the simulated date at which frozen soil begins to thaw, organic matter may provide only limited
insulation from surface warming. 相似文献
6.
Timothy Andrews Marie Doutriaux-Boucher Olivier Boucher Piers M. Forster 《Climate Dynamics》2011,36(3-4):783-792
An increase in atmospheric carbon dioxide concentration has both a radiative (greenhouse) effect and a physiological effect on climate. The physiological effect forces climate as plant stomata do not open as wide under enhanced CO2 levels and this alters the surface energy balance by reducing the evapotranspiration flux to the atmosphere, a process referred to as ‘carbon dioxide physiological forcing’. Here the climate impact of the carbon dioxide physiological forcing is isolated using an ensemble of twelve 5-year experiments with the Met Office Hadley Centre HadCM3LC fully coupled atmosphere–ocean model where atmospheric carbon dioxide levels are instantaneously quadrupled and thereafter held constant. Fast responses (within a few months) to carbon dioxide physiological forcing are analyzed at a global and regional scale. Results show a strong influence of the physiological forcing on the land surface energy budget, hydrological cycle and near surface climate. For example, global precipitation rate reduces by ~3% with significant decreases over most land-regions, mainly from reductions to convective rainfall. This fast hydrological response is still evident after 5 years of model integration. Decreased evapotranspiration over land also leads to land surface warming and a drying of near surface air, both of which lead to significant reductions in near surface relative humidity (~6%) and cloud fraction (~3%). Patterns of fast responses consistently show that results are largest in the Amazon and central African forest, and to a lesser extent in the boreal and temperate forest. Carbon dioxide physiological forcing could be a source of uncertainty in many model predicted quantities, such as climate sensitivity, transient climate response and the hydrological sensitivity. These results highlight the importance of including biological components of the Earth system in climate change studies. 相似文献
7.
A regional climate model, the Weather Research and Forecasting (WRF) Model, is forced with increased atmospheric CO2 and anomalous SSTs and lateral boundary conditions derived from nine coupled atmosphere–ocean general circulation models to produce an ensemble set of nine future climate simulations for northern Africa at the end of the twenty-first century. A well validated control simulation, agreement among ensemble members, and a physical understanding of the future climate change enhance confidence in the predictions. The regional model ensembles produce consistent precipitation projections over much of northern tropical Africa. A moisture budget analysis is used to identify the circulation changes that support future precipitation anomalies. The projected midsummer drought over the Guinean Coast region is related partly to weakened monsoon flow. Since the rainfall maximum demonstrates a southward bias in the control simulation in July–August, this may be indicative of future summer drying over the Sahel. Wetter conditions in late summer over the Sahel are associated with enhanced moisture transport by the West African westerly jet, a strengthening of the jet itself, and moisture transport from the Mediterranean. Severe drought in East Africa during August and September is accompanied by a weakened Indian monsoon and Somali jet. Simulations with projected and idealized SST forcing suggest that overall SST warming in part supports this regional model ensemble agreement, although changes in SST gradients are important over West Africa in spring and fall. Simulations which isolate the role of individual climate forcings suggest that the spatial distribution of the rainfall predictions is controlled by the anomalous SST and lateral boundary conditions, while CO2 forcing within the regional model domain plays an important secondary role and generally produces wetter conditions. 相似文献
8.
There are many indicators that human activity may change climate conditions all around the globe through emissions of greenhouse
gases. In addition, aerosol particles are emitted from various natural and anthropogenic sources. One important source of
aerosols arises from biomass burning, particularly in low latitudes where shifting cultivation and land degradation lead to
enhanced aerosol burden. In this study the counteracting effects of greenhouse gases and aerosols on African climate are compared
using climate model experiments with fully interactive aerosols from different sources. The consideration of aerosol emissions
induces a remarkable decrease in short-wave solar irradiation near the surface, especially in winter and autumn in tropical
West Africa and the Congo Basin where biomass burning is mainly prevailing. This directly leads to a modification of the surface
energy budget with reduced sensible heat fluxes. As a consequence, temperature decreases, compensating the strong warming
signal due to enhanced trace gas concentrations. While precipitation in tropical Africa is less sensitive to the greenhouse
warming, it tends to decrease, if the effect of aerosols from biomass burning is taken into account. This is partly due to
the local impact of enhanced aerosol burden and partly to modifications of the large-scale monsoon circulation in the lower
troposphere, usually lagging behind the season with maximum aerosol emissions. In the model equilibrium experiments, the greenhouse
gas impact on temperature stands out from internal variability at various time scales from daily to decadaland the same holds
for precipitation under the additional aerosol forcing. Greenhouse gases and aerosols exhibit an opposite effect on daily
temperature extremes, resulting in an compensation of the individual responses under the combined forcing. In terms of precipitation,
daily extreme events tend to be reduced under aerosol forcing, particularly over the tropical Atlantic and the Congo basin.
These results suggest that the simulation of the multiple aerosol effects from anthropogenic sources represents an important
factor in tropical climate change, hence, requiring more attention in climate modelling attempts. 相似文献
9.
The participation of different vegetation types within the physical climate system is investigated using a coupled atmosphere-biosphere model, CCM3-IBIS. We analyze the effects that six different vegetation biomes (tropical, boreal, and temperate forests, savanna, grassland and steppe, and shrubland/tundra) have on the climate through their role in modulating the biophysical exchanges of energy, water, and momentum between the land-surface and the atmosphere. Using CCM3-IBIS we completely remove the vegetation cover of a particular biome and compare it to a control simulation where the biome is present, thereby isolating the climatic effects of each biome. Results from the tropical and boreal forest removal simulations are in agreement with previous studies while the other simulations provide new evidence as to their contribution in forcing the climate. Removal of the temperate forest vegetation exhibits behavior characteristic of both the tropical and boreal simulations with cooling during winter and spring due to an increase in the surface albedo and warming during the summer caused by a reduction in latent cooling. Removal of the savanna vegetation exhibits behavior much like the tropical forest simulation while removal of the grassland and steppe vegetation has the largest effect over the central United States with warming and drying of the atmosphere in summer. The largest climatic effect of shrubland and tundra vegetation removal occurs in DJF in Australia and central Siberia and is due to reduced latent cooling and enhanced cold air advection, respectively. Our results show that removal of the boreal forest yields the largest temperature signal globally when either including or excluding the areas of forest removal. Globally, precipitation is most affected by removal of the savanna vegetation when including the areas of vegetation removal, while removal of the tropical forest most influences the global precipitation excluding the areas of vegetation removal. 相似文献
10.
Vegetation is a major component of the climate system because of its controls on the energy and water balance over land. This functioning changes because of the physiological response of leaves to increased CO2. A climate model is used to compare these changes with the climate changes from radiative forcing by greenhouse gases. For this purpose, we use the Community Earth System Model coupled to a slab ocean. Ensemble integrations are done for current and doubled CO2. The consequent reduction of transpiration and net increase of surface radiative heating from reduction in cloudiness increases the temperature over land by a significant fraction of that directly from the radiative warming by CO2. Large-scale atmospheric circulation adjustments result. In particular, over the tropics, a low-level westerly wind anomaly develops associated with reduced geopotential height over land, enhancing moisture transport and convergence, and precipitation increases over the western Amazon, the Congo basin, South Africa, and Indonesia, while over mid-latitudes, land precipitation decreases from reduced evapotranspiration. On average, land precipitation is enhanced by 0.03 mm day?1 (about 19 % of the CO2 radiative forcing induced increase). This increase of land precipitation with decreased ET is an apparent negative feedback, i.e., less ET makes more precipitation. Global precipitation is slightly reduced. Runoff increases associated with both the increased land precipitation and reduced evapotranspiration. Examining the consistency of the variations among ensemble members shows that vegetation feedbacks on precipitation are more robust over the tropics and in mid to high latitudes than over the subtropics where vegetation is sparse and the internal climate variability has a larger influence. 相似文献
11.
In the framework of the Global Soil Wetness Project (GSWP), the ISBA land-surface scheme of the ARPEGE atmospheric general circulation model has been forced with meteorological observations and analyses in order to produce a two-year (1987–1988) soil moisture climatology at a 1°×1° horizontal resolution. This climatology is model dependent, but it is the climatology that the ARPEGE model would produce if its precipitation and radiative fluxes were perfectly simulated. In the present study, ensembles of seasonal simulations (March to September) have been performed for 1987 and 1988, in which the total soil water content simulated by ARPEGE is relaxed towards the GSWP climatology. The results indicate that the relaxation has a positive impact on both the model's climatology and the simulated interannual variability, thereby confirming the utility of the GSWP soil moisture data for prescribing initial or boundary conditions in comprehensive climate and numerical weather prediction models. They also demonstrate the relevance of soil moisture for achieving realistic simulations of the Northern Hemisphere summer climate. In order to get closer to the framework of seasonal predictions, additional experiments have been performed in which GSWP is only used for initialising soil moisture at the beginning of the summer season (the relaxation towards GSWP is removed on 1st June). The results show a limited improvement of the interannual variability, compared to the simulations initialised from the ARPEGE climatology. However, some regional patterns of the precipitation differences between 1987 and 1988 are better captured, suggesting that seasonal predictions can benefit from a better initialisation of soil moisture. 相似文献
12.
G. C. Hegerl P. A. Stott M. R. Allen J. F. B. Mitchell S. F. B. Tett U. Cubasch 《Climate Dynamics》2000,16(10-11):737-754
Fingerprint techniques for the detection of anthropogenic climate change aim to distinguish the climate response to anthropogenic forcing from responses to other external influences and from internal climate variability. All these responses and the characteristics of internal variability are typically estimated from climate model data. We evaluate the sensitivity of detection and attribution results to the use of response and variability estimates from two different coupled ocean atmosphere general circulation models (HadCM2, developed at the Hadley Centre, and ECHAM3/LSG from the MPI für Meteorologie and Deutsches Klimarechenzentrum). The models differ in their response to greenhouse gas and direct sulfate aerosol forcing and also in the structure of their internal variability. This leads to differences in the estimated amplitude and the significance level of anthropogenic signals in observed 50-year summer (June, July, August) surface temperature trends. While the detection of anthropogenic influence on climate is robust to intermodel differences, our ability to discriminate between the greenhouse gas and the sulfate aerosol signals is not. An analysis of the recent warming, and the warming that occurred in the first half of the twentieth century, suggests that simulations forced with combined changes in natural (solar and volcanic) and anthropogenic (greenhouse gas and sulfate aerosol) forcings agree best with the observations. 相似文献
13.
14.
论东亚夏季风的特征、驱动力与年代际变化 总被引:9,自引:0,他引:9
本文是以新的资料和研究结果对东亚夏季风的基本特征、驱动力和年代际变化所作的重新分析与评估。内容包括四个部分:(1)东亚夏季风的基本特征;(2)东亚夏季风的驱动力;(3)东亚夏季风的年代际变率与原因;(4)东亚夏季风与全球季风的关系。结果表明:东亚夏季风是亚洲夏季风的一个重要有机部分,主要由来源于热带的季风气流组成,并随季节由南向北呈阶段性推进,它是形成夏季东亚天气与气候的主要环流和降水系统。驱动夏季风的主要强迫有三部分:外部强迫、耦合强迫与内部变率,其中人类活动引起的外强迫(气候变暖、城市化、气溶胶增加等)是新出现的外强迫,它正不断改变着东亚夏季风的特征与演变趋势。海洋与陆面耦合强迫作为自然因子是引起东亚夏季风年际和年代际变化的主要原因,其中太平洋年代尺度振荡(PDO)与北大西洋多年代尺度振荡(AMO)的协同作用是造成东亚夏季风30~40年周期振荡的主要原因。1960年代以后,东亚夏季风经历了强—弱—强的年代际变化,相应的中国东部夏季降水型出现了“北多南少”向“南涝北旱”以及“北方渐增”的转变。最近的研究表明,上述东亚夏季风年代际变化与整个亚非夏季风系统的变化趋势是一致的。在本世纪主要受气候变暖的影响,夏季风雨带将持续北移,中国北方和西部地区出现持续性多雨的格局。最后本文指出,亚非夏季风系统相比于其他区域季风系统更适合全球季风的概念。 相似文献
15.
Observed and projected climate change in Taiwan 总被引:1,自引:0,他引:1
Summary
This study examined the secular climate change characteristics in Taiwan over the past 100 years and the relationship with
the global climate change. Estimates for the likelihood of future climate changes in Taiwan were made based on the projection
from the IPCC climate models.
In the past 100 years, Taiwan experienced an island-wide warming trend (1.0–1.4 °C/100 years). Both the annual and daily temperature
ranges have also increased. The warming in Taiwan is closely connected to a large-scale circulation and SAT fluctuations,
such as the “cool ocean warm land” phenomenon. The water vapor pressure has increased significantly and could have resulted
in a larger temperature increase in summer. The probability for the occurrence of high temperatures has increased and the
result suggests that both the mean and variance in the SAT in Taiwan have changed significantly since the beginning of the
20th century. Although, as a whole, the precipitation in Taiwan has shown a tendency to increase in northern Taiwan and to
decrease in southern Taiwan in the past 100 years, it exhibits a more complicated spatial pattern. The changes occur mainly
in either the dry or rainy season and result in an enhanced seasonal cycle. The changes in temperature and precipitation are
consistent with the weakening of the East Asian monsoon.
Under consideration of both the warming effect from greenhouse gases and the cooling effect from aerosols, all projections
from climate models indicated a warmer climate near Taiwan in the future. The projected increase in the area-mean temperature
near Taiwan ranged from 0.9–2.7 °C relative to the 1961–1990 averaged temperature, when the CO2 concentration increased to 1.9 times the 1961–1990 level. These simulated temperature increases were statistically significant
and can be attributed to the radiative forcing associated with the increased concentration of greenhouse gases and aerosols.
The projected changes in precipitation were within the range of natural variability for all five models. There is no evidence
supporting the possibility of precipitation changes near Taiwan based on the simulations from five IPCC climate models.
Received February 5, 2001 Revised July 30, 2001 相似文献
16.
G. C. Hegerl K. Hasselmann U. Cubasch J. F. B. Mitchell E. Roeckner R. Voss J. Waszkewitz 《Climate Dynamics》1997,13(9):613-634
A multi-fingerprint analysis is applied to the detection and attribution of anthropogenic climate change. While a single
fingerprint is optimal for the detection of climate change, further tests of the statistical consistency of the detected climate
change signal with model predictions for different candidate forcing mechanisms require the simultaneous application of several
fingerprints. Model-predicted climate change signals are derived from three anthropogenic global warming simulations for the
period 1880 to 2049 and two simulations forced by estimated changes in solar radiation from 1700 to 1992. In the first global
warming simulation, the forcing is by greenhouse gas only, while in the remaining two simulations the direct influence of
sulfate aerosols is also included. From the climate change signals of the greenhouse gas only and the average of the two greenhouse
gas-plus-aerosol simulations, two optimized fingerprint patterns are derived by weighting the model-predicted climate change
patterns towards low-noise directions. The optimized fingerprint patterns are then applied as a filter to the observed near-surface
temperature trend patterns, yielding several detection variables. The space-time structure of natural climate variability
needed to determine the optimal fingerprint pattern and the resultant signal-to-noise ratio of the detection variable is estimated
from several multi-century control simulations with different CGCMs and from instrumental data over the last 136 y. Applying
the combined greenhouse gas-plus-aerosol fingerprint in the same way as the greenhouse gas only fingerprint in a previous
work, the recent 30-y trends (1966–1995) of annual mean near surface temperature are again found to represent a significant
climate change at the 97.5% confidence level. However, using both the greenhouse gas and the combined forcing fingerprints
in a two-pattern analysis, a substantially better agreement between observations and the climate model prediction is found
for the combined forcing simulation. Anticipating that the influence of the aerosol forcing is strongest for longer term temperature
trends in summer, application of the detection and attribution test to the latest observed 50-y trend pattern of summer temperature
yielded statistical consistency with the greenhouse gas-plus-aerosol simulation with respect to both the pattern and amplitude
of the signal. In contrast, the observations are inconsistent with the greenhouse-gas only climate change signal at a 95%
confidence level for all estimates of climate variability. The observed trend 1943–1992 is furthermore inconsistent with a
hypothesized solar radiation change alone at an estimated 90% confidence level. Thus, in contrast to the single pattern analysis,
the two pattern analysis is able to discriminate between different forcing hypotheses in the observed climate change signal.
The results are subject to uncertainties associated with the forcing history, which is poorly known for the solar and aerosol
forcing, the possible omission of other important forcings, and inevitable model errors in the computation of the response
to the forcing. Further uncertainties in the estimated significance levels arise from the use of model internal variability
simulations and relatively short instrumental observations (after subtraction of an estimated greenhouse gas signal) to estimate
the natural climate variability. The resulting confidence limits accordingly vary for different estimates using different
variability data. Despite these uncertainties, however, we consider our results sufficiently robust to have some confidence
in our finding that the observed climate change is consistent with a combined greenhouse gas and aerosol forcing, but inconsistent
with greenhouse gas or solar forcing alone.
Received: 28 April 1996 / Accepted: 27 January 1997 相似文献
17.
Previous studies have highlighted the crucial role of land degradation in tropical African climate. This effect urgently has
to be taken into account when predicting future African climate under enhanced greenhouse conditions. Here, we present time
slice experiments of African climate until 2025, using a high-resolution regional climate model. A supposable scenario of
future land use changes, involving vegetation loss and soil degradation, is prescribed simultaneously with increasing greenhouse-gas
concentrations in order to detect, where the different forcings counterbalance or reinforce each other. This proceeding allows
us to define the regions of highest vulnerability with respect to future freshwater availability and food security in tropical
and subtropical Africa and may provide a decision basis for political measures. The model simulates a considerable reduction
in precipitation amount until 2025 over most of tropical Africa, amounting to partly more than 500 mm (20–40% of the annual
sum), particularly in the Congo Basin and the Sahel Zone. The change is strongest in boreal summer and basically reflects
the pattern of maximum vegetation cover during the seasonal cycle. The related change in the surface energy fluxes induces
a substantial near-surface warming by up to 7°C. According to the modified temperature gradients over tropical Africa, the
summer monsoon circulation intensifies and transports more humid air masses into the southern part of West Africa. This humidifying
effect is overcompensated by a remarkable decrease in surface evaporation, leading to the overall drying tendency over most
of Africa. Extreme daily rainfall events become stronger in autumn but less intense in spring. Summer and autumn appear to
be characterized by more severe heat waves over Subsaharan West Africa. In addition, the Tropical Easterly Jet is weakening,
leading to enhanced drought conditions in the Sahel Zone. All these results suggest that the local impact of land degradation
and reduction of vegetation cover may be more important in tropical Africa than the global radiative heating, at least until
2025. This implies that vegetation protection measures at a national scale may directly lead to a mitigation of the expected
negative implications of future climate change in tropical Africa. 相似文献
18.
Impact of ocean model resolution on CCSM climate simulations 总被引:1,自引:1,他引:0
Ben P. Kirtman Cecilia Bitz Frank Bryan William Collins John Dennis Nathan Hearn James L. Kinter III Richard Loft Clement Rousset Leo Siqueira Cristiana Stan Robert Tomas Mariana Vertenstein 《Climate Dynamics》2012,39(6):1303-1328
The current literature provides compelling evidence suggesting that an eddy-resolving (as opposed to eddy-permitting or eddy-parameterized) ocean component model will significantly impact the simulation of the large-scale climate, although this has not been fully tested to date in multi-decadal global coupled climate simulations. The purpose of this paper is to examine how resolved ocean fronts and eddies impact the simulation of large-scale climate. The model used for this study is the NCAR Community Climate System Model version 3.5 (CCSM3.5)—the forerunner to CCSM4. Two experiments are reported here. The control experiment is a 155-year present-day climate simulation using a 0.5° atmosphere component (zonal resolution 0.625 meridional resolution 0.5°; land surface component at the same resolution) coupled to ocean and sea-ice components with zonal resolution of 1.2° and meridional resolution varying from 0.27° at the equator to 0.54° in the mid-latitudes. The second simulation uses the same atmospheric and land-surface models coupled to eddy-resolving 0.1° ocean and sea-ice component models. The simulations are compared in terms of how the representation of smaller scale features in the time mean ocean circulation and ocean eddies impact the mean and variable climate. In terms of the global mean surface temperature, the enhanced ocean resolution leads to a ubiquitous surface warming with a global mean surface temperature increase of about 0.2?°C relative to the control. The warming is largest in the Arctic and regions of strong ocean fronts and ocean eddy activity (i.e., Southern Ocean, western boundary currents). The Arctic warming is associated with significant losses of sea-ice in the high-resolution simulation. The sea surface temperature gradients in the North Atlantic, in particular, are better resolved in the high-resolution model leading to significantly sharper temperature gradients and associated large-scale shifts in the rainfall. In the extra-tropics, the interannual temperature variability is increased with the resolved eddies, and a notable increases in the amplitude of the El Ni?o and the Southern Oscillation is also detected. Changes in global temperature anomaly teleconnections and local air-sea feedbacks are also documented and show large changes in ocean–atmosphere coupling. In particular, local air-sea feedbacks are significantly modified by the increased ocean resolution. In the high-resolution simulation in the extra-tropics there is compelling evidence of stronger forcing of the atmosphere by SST variability arising from ocean dynamics. This coupling is very weak or absent in the low-resolution model. 相似文献
19.
Northern African climate at the end of the twenty-first century: an integrated application of regional and global climate models 总被引:1,自引:1,他引:0
A method for simulating future climate on regional space scales is developed and applied to northern Africa. Simulation with
a regional model allows for the horizontal resolution needed to resolve the region’s strong meridional gradients and the optimization
of parameterizations and land-surface model. The control simulation is constrained by reanalysis data, and realistically represents
the present day climate. Atmosphere–ocean general circulation model (AOGCM) output provides SST and lateral boundary condition
anomalies for 2081–2100 under a business-as-usual emissions scenario, and the atmospheric CO2 concentration is increased to 757 ppmv. A nine-member ensemble of future climate projections is generated by using output
from nine AOGCMs. The consistency of precipitation projections for the end of the twenty-first century is much greater for
the regional model ensemble than among the AOGCMs. More than 77% of ensemble members produce the same sign rainfall anomaly
over much of northern Africa. For West Africa, the regional model projects wetter conditions in spring, but a mid-summer drought
develops during June and July, and the heat stoke risk increases across the Sahel. Wetter conditions resume in late summer,
and the likelihood of flooding increases. The regional model generally projects wetter conditions over eastern Central Africa
in June and drying during August through September. Severe drought impacts parts of East Africa in late summer. Conditions
become wetter in October, but the enhanced rainfall does not compensate for the summertime deficit. The risk of heat stroke
increases over this region, although the threat is not projected to be as great as in the Sahel. 相似文献
20.
Detectability of Summer Dryness Caused by Greenhouse Warming 总被引:14,自引:0,他引:14
This study investigates the temporal and spatial variation of soil moisture associated with global warming as simulated by long-term integrations of a coupled ocean-atmosphere model conducted earlier. Starting from year 1765, integrations of the coupled model for 300 years were performed for three scenarios: increasing greenhouse gases only, increasing sulfate-aerosol loading only and the combination of both radiative forcings. The integration with the combined radiative forcings reproduces approximately the observed increases of global mean surface air temperature during the 20th century. Analysis of this integration indicates that both summer dryness and winter wetness occur in middle-to-high latitudes of North America and southern Europe. These features were identified in earlier studies. However, in the southern part of North America where the percentage reduction of soil moisture during summer is quite large, soil moisture is decreased for nearly the entire annual cycle in response to greenhouse warming. A similar observation applies to other semi-arid regions in subtropical to middle latitudes such as central Asia and the area surrounding the Mediterranean Sea. On the other hand, annual mean runoff is greatly increased in high latitudes because of increased poleward transport of moisture in the warmer model atmosphere. An analysis of the central North American and southern European regions indicates that the time when the change of soil moisture exceeds one standard deviation about the control integration occurs considerably later than that of surface air temperature for a given experiment because the ratio of forced change to natural variability is much smaller for soil moisture compared with temperature. The corresponding lag time for runoff change is even greater than that of either precipitation or soil moisture for the same reason. Also according to the above criterion, the inclusion of the effect of sulfate aerosols in the greenhouse warming experiment delays the noticeable change of soil moisture by several decades. It appears that observed surface air temperature is a better indicator of greenhouse warming than hydrologic quantities such as precipitation, runoff and soil moisture. Therefore, we are unlikely to notice definitive CO2-induced continental summer dryness until several decades into the 21st century. 相似文献