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1.
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
2.
Earlier GCM studies have expressed the concern that an enhancement of greenhouse warming might increase the occurrence of summer droughts in mid-latitudes, especially in southern Europe and central North America. This could represent a severe threat for agriculture in the regions concerned, where summer is the main growing season. These predictions must however be considered as uncertain, since most studies featuring enhanced summer dryness in mid-latitudes use very simple representations of the land-surface processes ("bucket" models), despite their key importance for the issue considered. The current study uses a regional climate model including a land-surface scheme of intermediate complexity to investigate the sensitivity of the summer climate to enhanced greenhouse warming over the American Midwest. A surrogate climate change scenario is used for the simulation of a warmer climate. The control runs are driven at the lateral boundaries and the sea surface by reanalysis data and observations, respectively. The warmer climate experiments are forced by a modified set of initial and lateral boundary conditions. The modifications consist of a uniform 3 K temperature increase and an attendant increase of specific humidity (unchanged relative humidity). This strategy maintains a similar dynamical forcing in the warmer climate experiments, thus allowing to investigate thermodynamical impacts of climate change in comparative isolation. The atmospheric CO 2 concentration of the sensitivity experiments is set to four times its pre-industrial value. The simulations are conducted from March 15 to October 1st, for 4 years corresponding to drought (1988), normal (1986, 1990) and flood (1993) conditions. The numerical experiments do not present any great enhancement of summer drying under warmer climatic conditions. First, the overall changes in the hydrological cycle (especially evapotranspiration) are of small magnitude despite the strong forcing applied. Second, precipitation increases in spring lead to higher soil water recharge during this season, compensating for the enhanced soil moisture depletion occurring later in the year. Additional simulations replacing the plant control on transpiration with a bucket-type formulation presented increased soil drying in 1988, the drought year. This suggests that vegetation control on transpiration might play an important part in counteracting an enhancement of summer drying when soil water gets limited. Though further aspects of this issue would need investigating, our results underline the importance of land-surface processes in climate integrations and suggest that the risk of enhanced summer dryness in the region studied might be less acute than previously assumed, provided the North American general circulation does not change markedly with global warming. 相似文献
3.
Climate change in the 21st century simulated by HadGEM2-AO under representative concentration pathways 总被引：2，自引：0，他引：2
Hee-Jeong Baek Johan Lee Hyo-Shin Lee Yu-Kyung Hyun ChunHo Cho Won-Tae Kwon Charline Marzin Sun-Yeong Gan Min-Ji Kim Da-Hee Choi Jonghwa Lee Jaeho Lee Kyung-On Boo Hyun-Suk Kang Young-Hwa Byun 《Asia-Pacific Journal of Atmospheric Sciences》2013,49(5):603-618
We present climate responses of Representative Concentration Pathways (RCPs) using the coupled climate model HadGEM2-AO for the Coupled Model Intercomparison Project phase 5 (CMIP5). The RCPs are selected as standard scenarios for the IPCC Fifth Assessment Report and these scenarios include time paths for emissions and concentrations of greenhouse gas and aerosols and land-use/land cover. The global average warming and precipitation increases for the last 20 years of the 21st century relative to the period 1986-2005 are +1.1°C/+2.1% for RCP2.6, +2.4°C/+4.0% for RCP4.5, +2.5°C/+3.3% for RCP6.0 and +4.1°C/+4.6% for RCP8.5, respectively. The climate response on RCP 2.6 scenario meets the UN Copenhagen Accord to limit global warming within two degrees at the end of 21st century, the mitigation effect is about 3°C between RCP2.6 and RCP8.5. The projected precipitation changes over the 21st century are expected to increase in tropical regions and at high latitudes, and decrease in subtropical regions associated with projected poleward expansions of the Hadley cell. Total soil moisture change is projected to decrease in northern hemisphere high latitudes and increase in central Africa and Asia whereas near-surface soil moisture tends to decrease in most areas according to the warming and evaporation increase. The trend and magnitude of future climate extremes are also projected to increase in proportion to radiative forcing of RCPs. For RCP 8.5, at the end of the summer season the Arctic is projected to be free of sea ice. 相似文献
4.
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
5.
E. Roeckner P. Stier J. Feichter S. Kloster M. Esch I. Fischer-Bruns 《Climate Dynamics》2006,27(6):553-571
The past and future evolution of atmospheric composition and climate has been simulated with a version of the Max Planck Institute Earth System Model (MPI-ESM). The system consists of the atmosphere, including a detailed representation of tropospheric aerosols, the land surface, and the ocean, including a model of the marine biogeochemistry which interacts with the atmosphere via the dust and sulfur cycles. In addition to the prescribed concentrations of carbon dioxide, ozone and other greenhouse gases, the model is driven by natural forcings (solar irradiance and volcanic aerosol), and by emissions of mineral dust, sea salt, sulfur, black carbon (BC) and particulate organic matter (POM). Transient climate simulations were performed for the twentieth century and extended into the twenty-first century, according to SRES scenario A1B, with two different assumptions on future emissions of carbonaceous aerosols (BC, POM). In the first experiment, BC and POM emissions decrease over Europe and China but increase at lower latitudes (central and South America, Africa, Middle East, India, Southeast Asia). In the second experiment, the BC and POM emissions are frozen at their levels of year 2000. According to these experiments the impact of projected changes in carbonaceaous aerosols on the global mean temperature is negligible, but significant changes are found at low latitudes. This includes a cooling of the surface, enhanced precipitation and runoff, and a wetter surface. These regional changes in surface climate are caused primarily by the atmospheric absorption of sunlight by increasing BC levels and, subsequently, by thermally driven circulations which favour the transport of moisture from the adjacent oceans. The vertical redistribution of solar energy is particularly large during the dry season in central Africa when the anomalous atmospheric heating of up to 60 W m−2 and a corresponding decrease in surface solar radiation leads to a marked surface cooling, reduced evaporation and a higher level of soil moisture, which persists throughout the year and contributes to the enhancement of precipitation during the wet season. 相似文献
6.
N. Elguindi A. Grundstein S. Bernardes U. Turuncoglu J. Feddema 《Climatic change》2014,122(4):523-538
A modified Thornthwaite Climate Classification is applied to a 32-member ensemble of CMIP5 GCMs in order to 1) evaluate model performance in the historical climate and 2) assess projected climate change at the end of the 21 s t century following two greenhouse gas representative concentration pathways (RCP4.5 and RCP8.5). This classification scheme differs from the well-known Köppen approach as it uses potential evapotranspiration for thermal conditions, a moisture index for moisture conditions, and has even intervals between climate classes. The multi-model ensemble (MME) reproduces the main spatial features of the global climate reasonably well, however, in many regions the climate types are too moist. Extreme climate types, such as those found in polar and desert regions, as well as the cool- and cold-wet types of eastern North America and the warm and cool-moist types found in the southern U.S., eastern South America, central Africa and Europe are reproduced best by the MME. In contrast, the cold-dry and cold-semiarid climate types characterizing much of the high northern latitudes and the warm-wet type found in parts of Indonesia and southeast Asia are poorly represented by the MME. Regionally, most models exhibit the same sign in moisture and thermal biases, varying only in magnitude. Substantial changes in climate types are projected in both the RCP4.5 and RCP8.5 scenarios. Area coverage of torrid climate types expands by 11 % and 19 % in the RCP4.5 and RCP8.5 projections, respectively. Furthermore, a large portion of these areas in the tropics will experience thermal conditions which exceed the range of historical values and fall into a novel super torrid climate class. The greatest growth in moisture types in climate zones is among those with dry climates (moisture index values < 0) with increased areas of more than 8 % projected by the RCP8.5 MME. 相似文献
7.
Future change of climate in South America in the late twenty-first century: intercomparison of scenarios from three regional climate models 总被引：2，自引：1，他引：1
Jose A. Marengo Tercio Ambrizzi Rosmeri P. da Rocha Lincoln M. Alves Santiago V. Cuadra Maria C. Valverde Roger R. Torres Daniel C. Santos Simone E. T. Ferraz 《Climate Dynamics》2010,35(6):1073-1097
Regional climate change projections for the last half of the twenty-first century have been produced for South America, as
part of the CREAS (Cenarios REgionalizados de Clima Futuro da America do Sul) regional project. Three regional climate models
RCMs (Eta CCS, RegCM3 and HadRM3P) were nested within the HadAM3P global model. The simulations cover a 30-year period representing
present climate (1961–1990) and projections for the IPCC A2 high emission scenario for 2071–2100. The focus was on the changes
in the mean circulation and surface variables, in particular, surface air temperature and precipitation. There is a consistent
pattern of changes in circulation, rainfall and temperatures as depicted by the three models. The HadRM3P shows intensification
and a more southward position of the subtropical Pacific high, while a pattern of intensification/weakening during summer/winter
is projected by the Eta CCS/RegCM3. There is a tendency for a weakening of the subtropical westerly jet from the Eta CCS and
HadRM3P, consistent with other studies. There are indications that regions such of Northeast Brazil and central-eastern and
southern Amazonia may experience rainfall deficiency in the future, while the Northwest coast of Peru-Ecuador and northern
Argentina may experience rainfall excesses in a warmer future, and these changes may vary with the seasons. The three models
show warming in the A2 scenario stronger in the tropical region, especially in the 5°N–15°S band, both in summer and especially
in winter, reaching up to 6–8°C warmer than in the present. In southern South America, the warming in summer varies between
2 and 4°C and in winter between 3 and 5°C in the same region from the 3 models. These changes are consistent with changes
in low level circulation from the models, and they are comparable with changes in rainfall and temperature extremes reported
elsewhere. In summary, some aspects of projected future climate change are quite robust across this set of model runs for
some regions, as the Northwest coast of Peru-Ecuador, northern Argentina, Eastern Amazonia and Northeast Brazil, whereas for
other regions they are less robust as in Pantanal region of West Central and southeastern Brazil. 相似文献
8.
To investigate the hydrologic changes of climate in response to an increase of CO2-concentration in the atmosphere, the results from numerical experiments with three climate models are analyzed and compared
with each other. All three models consist of an atmospheric general circulation model and a simple mixed layer ocean with
a horizontally uniform heat capacity. The first model has a limited computational domain and simple geography with a flat
land surface. The second model has a global computational domain with realistic geography. The third model is identical to
the second model except that it has a higher computational resolution. In each numerical experiment, the CO2-induced change of climate is evaluated based upon a comparison between the two climates of a model with normal and four times
the normal concentration of carbon dioxide in air.
It is noted that the zonal mean value of soil moisture in summer reduces significantly in two separate zones of middle and
high latitudes in response to the increase of the CO2-concentration in air. This CO2-induced summer dryness results not only from the earlier ending of the snowmelt season, but also from the earlier occurrence
of the spring to summer reduction in rainfall rate. The former effect is particularly important in high latitudes, whereas
the latter effect becomes important in middle latitudes. Other statistically significant changes include large increases in
both soil moisture and runoff rate in high latitudes of a model during most of the annual cycle with the exception of the
summer season. The penetration of moisture-rich, warm air into high latitudes is responsible for these increases. 相似文献
9.
气候干湿状况是表征区域气候特征的重要指标,是在全球气候变暖背景下,水循环与陆面蒸散发作用的综合结果。本文从湿润度指数入手,结合降水与潜在蒸散的时空变化,分析了我国干旱半干旱区气候特点与干湿变化特征及对土壤湿度的影响。分析发现：近50年来,我国干旱与半干旱区均呈变湿趋势。干旱区与半干旱区潜在蒸散与降水月差值在年内出现时间上存在不一致,且干旱区明显大于半干旱区;3～9月为干旱气候区潜在蒸散与降水差值大值期,3～6月半干旱区潜在蒸散明显大于降水,7月起差值明显减小。作用分析表明,在干旱区,降水对湿润度指数的影响更大,而对于半干旱区,降水与潜在蒸散作用相当。长期以来,我国整个干旱与半干旱区大部分土壤湿度在逐渐变干,尤其是农业耕作层的浅层土壤,几乎全区域一致呈现变干趋势,说明我国干旱半干旱区农牧业生产存在较大的潜在干旱风险。 相似文献
10.
We report the analysis of two 20-year simulations performed with the low resolution version of the IPSL coupled ocean-atmosphere
model, with no flux correction at the air-sea interface. The simulated climate is characterized by a global sea surface temperature
warming of about 4 °C in 20 years, driven by a net heat gain at the top of the atmosphere. Despite this drift, the circulation
is quite realistic both in the ocean and the atmosphere. Several distinct periods are analyzed. The first corresponds to an
adjustment during which the heat gain weakens both at the top of the atmosphere and at the ocean surface, and the tropical
circulation is slightly modified. Then, the surface warming is enhanced by an increase of the greenhouse feedback. We show
that the mechanisms involved in the model share common features with sensitivity experiments to greenhouse gases or to SST
warming. At the top of the atmosphere, most of the longwave trapping in the atmosphere is driven by the tropical circulation.
At the surface, the reduction of longwave cooling is a direct response to increased temperature and moisture content at low
levels in the atmospheric model. During the last part of the simulation, a regulation occurs from evaporation at the surface
and longwave cooling at TOA. Most of the model drift is attributed to a too large heating by solar radiation in middle and
high latitudes. The reduction of the north–south temperature gradient, and the related changes in the meridional equator-to-pole
ocean heat transport lead to a warming of equatorial and subtropical regions. This is also well demonstrated by the difference
between the two simulations which differ only in the parametrization of sea-ice. When the sea-ice cover is not restored to
climatology the model does not maintain sea-ice at high latitudes. The climate warms more rapidly and the water vapor and
clouds feedback occurs earlier.
Received: 24 May 1996 / Accepted: 29 November 1996 相似文献
11.
In recent years, a substantial reduction of the sea ice in the Arctic has been observed. At the same time, the near-surface
air in this region is warming at a rate almost twice as large as the global average—this phenomenon is known as the Arctic
amplification. The role of the ice-albedo feedback for the Arctic amplification is still a matter of debate. Here the effect
of the surface-albedo feedback (SAF) was studied using a coupled climate model CCSM3 from the National Center for Atmospheric
Research. Experiments, where the SAF was suppressed by locking the surface albedo in the entire coupled model system, were
conducted. The results reveal polar temperature amplification when this model, with suppressed albedo, is forced by a doubling
of the atmospheric CO2 content. Comparisons with variable albedo experiments show that SAF amplifies the surface-temperature response in the Arctic
area by about 33%, whereas the corresponding value for the global-mean surface temperature is about 15%. Even though SAF is
an important process underlying excessive warming at high latitudes, the Arctic amplification is only 15% larger in the variable
than in the locked-albedo experiments. It is found that an increase of water vapour and total cloud cover lead to a greenhouse
effect, which is larger in the Arctic than at lower latitudes. This is expected to explain a part of the Arctic surface–air-temperature
amplification. 相似文献
12.
An analysis of simulated future surface climate change over the southern half of Korean Peninsula using a RegCM3-based high-resolution
one-way double-nested system is presented. Changes in mean climate as well as the frequency and intensity of extreme climate
events are discussed for the 30-year-period of 2021–2050 with respect to the reference period of 1971–2000 based on the IPCC
SRES B2 emission scenario. Warming in the range of 1–4°C is found throughout the analysis region and in all seasons. The warming
is maximum in the higher latitudes of the South Korean Peninsula and in the cold season. A large reduction in snow depth is
projected in response to the increase of winter minimum temperature induced by the greenhouse warming. The change in precipitation
shows a distinct seasonal variation and a substantial regional variability. In particular, we find a large increase of wintertime
precipitation over Korea, especially in the upslope side of major mountain systems. Summer precipitation increases over the
northern part of South Korea and decreases over the southern regions, indicating regional diversity. The precipitation change
also shows marked intraseasonal variations throughout the monsoon season. The temperature change shows a positive trend throughout
2021–2050 while the precipitation change is characterized by pronounced interdecadal variations. The PDF of the daily temperature
is shifted towards higher values and is somewhat narrower in the scenario run than the reference one. The number of frost
days decreases markedly and the number of hot days increases. The regional distribution of heavy precipitation (over 80 mm/day)
changes considerably, indicating changes in flood vulnerable regions. The climate change signal shows pronounced fine scale
signal over Korea, indicating the need of high-resolution climate simulations 相似文献
13.
Relative contribution of soil moisture and snow mass to seasonal climate predictability: a pilot study 总被引：1，自引：1，他引：0
Hervé Douville 《Climate Dynamics》2010,34(6):797-818
Land surface hydrology (LSH) is a potential source of long-range atmospheric predictability that has received less attention
than sea surface temperature (SST). In this study, we carry out ensemble atmospheric simulations driven by observed or climatological
SST in which the LSH is either interactive or nudged towards a global monthly re-analysis. The main objective is to evaluate
the impact of soil moisture or snow mass anomalies on seasonal climate variability and predictability over the 1986–1995 period.
We first analyse the annual cycle of zonal mean potential (perfect model approach) and effective (simulated vs. observed climate)
predictability in order to identify the seasons and latitudes where land surface initialization is potentially relevant. Results
highlight the influence of soil moisture boundary conditions in the summer mid-latitudes and the role of snow boundary conditions
in the northern high latitudes. Then, we focus on the Eurasian continent and we contrast seasons with opposite land surface
anomalies. In addition to the nudged experiments, we conduct ensembles of seasonal hindcasts in which the relaxation is switched
off at the end of spring or winter in order to evaluate the impact of soil moisture or snow mass initialization. LSH appears
as an effective source of surface air temperature and precipitation predictability over Eurasia (as well as North America),
at least as important as SST in spring and summer. Cloud feedbacks and large-scale dynamics contribute to amplify the regional
temperature response, which is however, mainly found at the lowest model levels and only represents a small fraction of the
observed variability in the upper troposphere. 相似文献
14.
Central America has high biodiversity, it harbors high-value ecosystems and it??s important to provide regional climate change information to assist in adaptation and mitigation work in the region. Here we study climate change projections for Central America and Mexico using a regional climate model. The model evaluation shows its success in simulating spatial and temporal variability of temperature and precipitation and also in capturing regional climate features such as the bimodal annual cycle of precipitation and the Caribbean low-level jet. A variety of climate regimes within the model domain are also better identified in the regional model simulation due to improved resolution of topographic features. Although, the model suffers from large precipitation biases, it shows improvements over the coarse-resolution driving model in simulating precipitation amounts. The model shows a dry bias in the wet season and a wet bias in the dry season suggesting that it??s unable to capture the full range of precipitation variability. Projected warming under the A2 scenario is higher in the wet season than that in the dry season with the Yucatan Peninsula experiencing highest warming. A large reduction in precipitation in the wet season is projected for the region, whereas parts of Central America that receive a considerable amount of moisture in the form of orographic precipitation show significant decreases in precipitation in the dry season. Projected climatic changes can have detrimental impacts on biodiversity as they are spatially similar, but far greater in magnitude, than those observed during the El Ni?o events in recent decades that adversely affected species in the region. 相似文献
15.
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. 相似文献
16.
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
17.
Aspects of the surface hydrology of high resolution (T106) versions of the ECHAM3 and ECHAM4 general circulation models are
analysed over the European region and compared with available observations. The focus is on evaporation, and surface measurements
are shown to be useful for the identification of systematic deficiencies in the regional-scale performance of climate models
on an annual and seasonal basis, such as the excessive summer dryness over continents. The annual mean evaporation at the
available European observation sites is overestimated by 4 mm/month by the ECHAM3 T106, quantitatively consistent with an
overestimated surface net radiation of 4 Wm–2 over Europe. In winter, ECHAM3 shows an overestimated evaporation which compensates for an overestimated downward sensible
heat flux. This is primarily related to a too strong zonalisation of the large-scale flow and associated overestimated warm
air advection and windspeed. Inaccurate local land surface parameters (e.g. leaf area index, roughness length) are minor contributors
to the overestimation. In early summer, the excessive solar radiation at the surface calculated with the ECHAM3 radiation
scheme generates a too large evaporation and an excessive depletion of the soil moisture reservoirs. This favours the subsequent
excessive summer dryness over Europe with too low values of evaporation, convective precipitation and soil moisture content,
leading to a too high surface temperature. In the ECHAM4 T106 simulation, the problem of the European summer dryness is largely
reduced, and the simulated evaporation as well as convective precipitation, cloud amount and soil moisture content during
summer are substantially improved. The new ECHAM4 radiation scheme appears to be an important factor for this improvement,
since it calculates smaller insolation values in better agreement with observations and subsequently may avoid an excessive
drying of the soil.
Received: 20 September 1995 / Accepted: 10 May 1996 相似文献
18.
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
19.
Recent global-scale analyses of the CMIP3 model projections for the twenty-first century indicate a strong, coherent decreased
precipitation response over Central America and the Intra-America Seas region. We explore this regional response and examine
the models’ skill in representing present-day climate over this region. For much of Central America, the annual cycle of precipitation
is characterized by a rainy season that extends from May to October with a period of reduced precipitation in July and August
called the mid-summer drought. A comparison of the climate of the twentieth century simulations (20c3m) with observations
over the period 1961–1990 shows that nearly all models underestimate precipitation over Central America, due in part to an
underestimation of sea surface temperatures over the tropical North Atlantic and an excessively smooth representation of regional
topographical features. However, many of the models capture the mid-summer drought. Differences between the A1B scenario (2061–2090)
and 20c3m (1961–1990) simulations show decreased precipitation in the future climate scenario, mostly in June and July, just
before and during the onset of the mid-summer drought. We thus hypothesize that the simulated twenty-first century drying
over Central America represents an early onset and intensification of the mid-summer drought. An analysis of circulation changes
indicates that the westward expansion and intensification of the North Atlantic subtropical high associated with the mid-summer
drought occurs earlier in the A1B simulations, along with stronger low-level easterlies. The eastern Pacific inter-tropical
convergence zone is also located further southward in the scenario simulations. There are some indications that these changes
could be forced by ENSO-like warming of the tropical eastern Pacific and increased land–ocean heating contrasts over the North
American continent. 相似文献
20.
Present-day and future Amazonian precipitation in global climate models: CMIP5 versus CMIP3 总被引：2，自引：1，他引：1
The present study aims at evaluating and comparing precipitation over the Amazon in two sets of historical and future climate simulations based on phase 3 (CMIP3) and 5 (CMIP5) of the Coupled Model Intercomparison Project. Thirteen models have been selected in order to discuss (1) potential improvements in the simulation of present-day climate and (2) the potential reduction in the uncertainties of the model response to increasing concentrations of greenhouse gases. While several features of present-day precipitation—including annual cycle, spatial distribution and co variability with tropical sea surface temperature (SST)—have been improved, strong uncertainties remain in the climate projections. A closer comparison between CMIP5 and CMIP3 highlights a weaker consensus on increased precipitation during the wet season, but a stronger consensus on a drying and lengthening of the dry season. The latter response is related to a northward shift of the boreal summer intertropical convergence zone in CMIP5, in line with a more asymmetric warming between the northern and southern hemispheres. The large uncertainties that persist in the rainfall response arise from contrasted anomalies in both moisture convergence and evapotranspiration. They might be related to the diverse response of tropical SST and ENSO (El Niño Southern Oscillation) variability, as well as to spurious behaviours among the models that show the most extreme response. Model improvements of present-day climate do not necessarily translate into more reliable projections and further efforts are needed for constraining the pattern of the SST response and the soil moisture feedback in global climate scenarios. 相似文献