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1.
The origin of Antarctic precipitation: a modelling approach 总被引:3,自引:0,他引:3
GILLES DELAYGUE VALÉRIE MASSON JEAN JOUZEL RANDAL D. KOSTER RICHARD J. HEALY 《Tellus. Series B, Chemical and physical meteorology》2000,52(1):19-36
The contribution of different moisture sources to Antarctic precipitation for present‐day and glacial conditions is estimated with the NASA/GISS Atmospheric General Circulation Model. Despite its low horizontal resolution (8°×10°), this model simulates reasonably well the broad features of the observed present‐day hydrological cycle. Simulated present‐day Antarctic precipitation is dominated throughout the year by moisture from a subtropical/midlatitude band (30°S−60°S). The moisture supplied to a given coastal area of Antarctica originates mostly in the adjacent oceanic basin; closer to the pole, other oceanic basins can also contribute significantly. Replacing the present‐day sea surface temperatures (SSTs) and sea ice cover in the GCM with those from the CLIMAP oceanic reconstruction for the last glacial maximum (LGM), greatly increases the simulated latitudinal temperature gradient, with the consequence of slightly enhancing the contribution of low latitude moisture to Antarctic precipitation. It also changes the seasonality of the different contributions and thus their budget, particularly in coastal regions. Because the nature of LGM tropical SSTs is still under debate, we performed an additional LGM simulation in which the tropical SSTs are reduced relative to those of CLIMAP. The resulting decrease in the latitudinal gradient brings the relative contributions to Antarctic precipitation more in line with those of the present‐day simulation. 相似文献
2.
Øyvind Byrkjedal Nils Gunnar Kvamstø Marius Meland Eystein Jansen 《Climate Dynamics》2006,26(5):473-487
Published reconstructions of last glacial maximum (LGM) sea surface temperatures and sea ice extent differ significantly.
We here test the sensitivity of simulated North Atlantic climates to two different reconstructions by using these reconstructions
as boundary conditions for model experiments. An atmospheric general circulation model has been used to perform two simulations
of the (LGM) and a modern-day control simulation. Standard (CLIMAP) reconstructions of sea ice and sea surface temperatures
have been used for the first simulation, and a set of new reconstructions in the Nordic Seas/Northern Atlantic have been used
for the second experiment. The new reconstruction is based on 158 core samples, and represents ice-free conditions during
summer in the Nordic Seas, with accordingly warmer sea surface temperatures and less extensive sea ice during winter as well.
The simulated glacial climate is globally 5.7 K colder than modern day, with the largest changes at mid and high latitudes.
Due to more intense Hadley circulation, the precipitation at lower latitudes has increased in the simulations of the LGM.
Relative to the simulation with the standard CLIMAP reconstructions, reduction of the sea ice in the North Atlantic gives
positive local responses in temperature, precipitation and reduction of the sea level pressure. Only very weak signatures
of the wintertime Icelandic Low occur when the standard CLIMAP sea surface temperature reconstruction is used as the lower
boundary condition in LGM. With reduced sea ice conditions in the Nordic Seas, the Icelandic Low becomes more intense and
closer to its present structure. This indicates that thermal forcing is an important factor in determining the strength and
position of the Icelandic Low. The Arctic Oscillation is the most dominant large scale variability feature on the Northern
Hemisphere in modern day winter climate. In the simulation of the LGM with extensive sea ice this pattern is significantly
changed and represents no systematic large scale variability over the North Atlantic. Reduction of the North Atlantic sea
ice extent leads to stronger variability in monthly mean sea level pressure in winter. The synoptic variability appears at
a lower level in the simulation when standard reconstructions of the sea surface in the LGM are used. A closer inspection
of storm tracks in this model experiment shows that that the synoptic lows follow a narrow band along the ice edge during
winter. The trajectories of synoptic lows are not constrained to the sea ice edge to the same degree when the sea ice extent
is reduced. Seasonally open waters in the Nordic Seas in the new reconstruction apparently act as a moisture source, consistent
with the current understanding of the rapid growth of the Fennoscandian and Barents Ice Sheets, during the LGM. The signal
from the intensified thermal forcing in the North Atlantic in Boreal winter is carried zonally by upper tropospheric waves,
and thus generates non-local responses to the changed sea ice cover. 相似文献
3.
Tropical paleoclimates at the Last Glacial Maximum: comparison of Paleoclimate Modeling Intercomparison Project (PMIP) simulations and paleodata 总被引:12,自引:2,他引:10
S. Pinot G. Ramstein S. P. Harrison I. C. Prentice J. Guiot M. Stute S. Joussaume 《Climate Dynamics》1999,15(11):857-874
Seventeen simulations of the Last Glacial Maximum (LGM) climate have been performed using atmospheric general circulation
models (AGCM) in the framework of the Paleoclimate Modeling Intercomparison Project (PMIP). These simulations use the boundary
conditions for CO2, insolation and ice-sheets; surface temperatures (SSTs) are either (a) prescribed using CLIMAP data set (eight models) or
(b) computed by coupling the AGCM with a slab ocean (nine models). The present-day (PD) tropical climate is correctly depicted
by all the models, except the coarser resolution models, and the simulated geographical distribution of annual mean temperature
is in good agreement with climatology. Tropical cooling at the LGM is less than at middle and high latitudes, but greatly
exceeds the PD temperature variability. The LGM simulations with prescribed SSTs underestimate the observed temperature changes
except over equatorial Africa where the models produce a temperature decrease consistent with the data. Our results confirm
previous analyses showing that CLIMAP (1981) SSTs only produce a weak terrestrial cooling. When SSTs are computed, the models
depict a cooling over the Pacific and Indian oceans in contrast with CLIMAP and most models produce cooler temperatures over
land. Moreover four of the nine simulations, produce a cooling in good agreement with terrestrial data. Two of these model
results over ocean are consistent with new SST reconstructions whereas two models simulate a homogeneous cooling. Finally,
the LGM aridity inferred for most of the tropics from the data, is globally reproduced by the models with a strong underestimation
for models using computed SSTs.
Received: 9 September 1998 / Accepted: 18 March 1999 相似文献
4.
Tropical cooling and the isotopic composition of precipitation in general circulation model simulations of the ice age climate 总被引:2,自引:0,他引:2
We test the climate effects of changes in the tropical ocean by imposing three different patterns of tropical SSTs in ice
age general circulation model simulations that include water source tracers and water isotope tracers. The continental air
temperature and hydrological cycle response in these simulations is substantial and should be directly comparable to the paleoclimatic
record. With tropical cooling imposed, there is a strong temperature response in mid- to high-latitudes resulting from changes
in sea ice and disturbance of the planetary waves; the results suggest that tropical/subtropical ocean cooling leads to significant
dynamical and radiative feedbacks that might amplify ice age cycles. The isotopes in precipitation generally follow the temperature
response at higher latitudes, but regional δ18O/air temperature scaling factors differ greatly among the experiments. In low-latitudes, continental surface temperatures
decrease congruently with the adjacent SSTs in the cooling experiments. Assuming CLIMAP SSTs, 18O/16O ratios in low-latitude precipitation show no change from modern values. However, the experiments with additional cooling
of SSTs produce much lower tropical continental δ18O values, and these low values result primarily from an enhanced recycling of continental moisture (as marine evaporation
is reduced). The water isotopes are especially sensitive to continental aridity, suggesting that they represent an effective
tracer of the extent of tropical cooling and drying. Only one of the tropical cooling simulations produces generalized low-latitude
aridity. These results demonstrate that the geographic pattern of cooling is most critical for promoting much drier continents,
and they underscore the need for accurate reconstructions of SST gradients in the ice age ocean.
Received: 26 July 1999 / Accepted: 10 July 2000 相似文献
5.
Tropical precipitation, SSTs and the surface energy budget: a zonally symmetric perspective 总被引:1,自引:1,他引:0
The relative importance of sea surface temperatures (SSTs) and the surface energy budget to tropical precipitation is examined by comparing models with zonally symmetric climates, both fixed SST and coupled to a slab mixed layer ocean. Two models are considered with differing surface flux formulations and in each case solutions that are symmetric about the equator are perturbed to create interhemispheric asymmetry. When SSTs are prescribed in the two models with different flux formulations, the magnitude of tropical precipitation response to identical SST anomalies is significantly different, but the differences can be understood in terms of the altered surface fluxes. In contrast, when the net surface energy fluxes are constrained to be identical in mixed layer simulations of the two different models, the response of tropical precipitation to perturbations in the surface energy balance is very similar. Both perspectives predict qualitatively the same precipitation response, but the energy budget better predicts the magnitude of the precipitation response. Thus, we argue that the atmospheric energy budget, controlled in these experiments primarily by the surface energy budget, is more fundamental to the control of tropical precipitation than the SSTs, in these simulations with axisymmetric climates. We touch briefly on a complication in the interpretation of the model results due to the fact that fixed SST and slab-ocean versions of the model can produce different Hadley cell strengths for the same SSTs. 相似文献
6.
The response in northern hemisphere atmospheric circulation and the resulting changes in moisture sources for Greenland precipitation to glacial boundary conditions are studied in NCAR’s CCM3 atmospheric general circulation model fitted with a moisture tracking functionality. We employ both the CLIMAP SST reconstruction and a modification thereto with reconstructions of glacial ice sheets and land masks. The individual components of the boundary conditions are added first one at a time and, finally, together. These steps show the atmospheric circulation to respond approximately linearly to the boundary condition changes, and the full glacial change may thus be decomposed into contributions from SST and topography changes, respectively. We find that using the CLIMAP SST reconstruction leads to a shift from Atlantic toward Pacific source regions not found with the modified reconstruction having cooler tropics and less sea ice. The occurrence of such a shift depends chiefly on the SST reconstruction and not on the existence of the large northern hemisphere glacial ice sheets. The influence of these circulation changes on important factors for ice core interpretation such as precipitation seasonality, condensation temperatures and source temperatures are assessed. 相似文献
7.
The sensitivity of the Late Saalian (140?ka) and LGM (21?ka) Eurasian ice sheets to sea surface conditions 总被引:1,自引:1,他引:0
Florence Colleoni Johan Liakka Gerhard Krinner Martin Jakobsson Simona Masina Vincent Peyaud 《Climate Dynamics》2011,37(3-4):531-553
This work focuses on the Late Saalian (140?ka) Eurasian ice sheets?? surface mass balance (SMB) sensitivity to changes in sea surface temperatures (SST). An Atmospheric General Circulation Model (AGCM), forced with two preexisting Last Glacial Maximum (LGM, 21?ka) SST reconstructions, is used to compute climate at 140 and 21?ka (reference glaciation). Contrary to the LGM, the ablation almost stopped at 140?ka due to the climatic cooling effect from the large ice sheet topography. Late Saalian SST are simulated using an AGCM coupled with a mixed layer ocean. Compared to the LGM, these 140?ka SST show an inter-hemispheric asymmetry caused by the larger ice-albedo feedback, cooling climate. The resulting Late Saalian ice sheet SMB is smaller due to the extensive simulated sea ice reducing the precipitation. In conclusion, SST are important for the stability and growth of the Late Saalian Eurasian ice sheet. 相似文献
8.
Seong-Joong Kim Thomas J. Crowley David J. Erickson Bala Govindasamy Phillip B. Duffy Bang Yong Lee 《Climate Dynamics》2008,31(1):1-16
The climate of the last glacial maximum (LGM) is simulated with a high-resolution atmospheric general circulation model, the
NCAR CCM3 at spectral truncation of T170, corresponding to a grid cell size of roughly 75 km. The purpose of the study is
to assess whether there are significant benefits from the higher resolution simulation compared to the lower resolution simulation
associated with the role of topography. The LGM simulations were forced with modified CLIMAP sea ice distribution and sea
surface temperatures (SST) reduced by 1°C, ice sheet topography, reduced CO2, and 21,000 BP orbital parameters. The high-resolution model captures modern climate reasonably well, in particular the distribution
of heavy precipitation in the tropical Pacific. For the ice age case, surface temperature simulated by the high-resolution
model agrees better with those of proxy estimates than does the low-resolution model. Despite the fact that tropical SSTs
were only 2.1°C less than the control run, there are many lowland tropical land areas 4–6°C colder than present. Comparison
of T170 model results with the best constrained proxy temperature estimates (noble gas concentrations in groundwater) now
yield no significant differences between model and observations. There are also significant upland temperature changes in
the best resolved tropical mountain belt (the Andes). We provisionally attribute this result in part as resulting from decreased
lateral mixing between ocean and land in a model with more model grid cells. A longstanding model-data discrepancy therefore
appears to be resolved without invoking any unusual model physics. The response of the Asian summer monsoon can also be more
clearly linked to local geography in the high-resolution model than in the low-resolution model; this distinction should enable
more confident validation of climate proxy data with the high-resolution model. Elsewhere, an inferred salinity increase in
the subtropical North Atlantic may have significant implications for ocean circulation changes during the LGM. A large part
of the Amazon and Congo Basins are simulated to be substantially drier in the ice age—consistent with many (but not all) paleo
data. These results suggest that there are considerable benefits derived from high-resolution model regarding regional climate
responses, and that observationalists can now compare their results with models that resolve geography at a resolution comparable
to that which the proxy data represent. 相似文献
9.
The role of prescribing sea surface temperature in paleoclimate atmospheric simulations has been investigated by comparing
Last Glacial Maximum AGCMs experiments using different SSTs data sets as well as coupled atmosphere/oceanic mixed layer models.
Changes in the SSTs and sea-ice margin generate different patterns of zonal asymmetries in the atmospheric circulation that
are responsible for reorganisation of heat and moisture transport, leading to important variations of Northern Hemisphere
regional climates, particularly in winter. Additional sensitivity experiments have been carried out to isolate the individual
role of North Pacific and North Atlantic SSTs anomalies. We found that changes in North Pacific SSTs have a much stronger
impact over all the northern continental surfaces, including Europe and Siberia, than changes in the North Atlantic SSTs.
As these SSTs anomalies are of the order of the typical errors generated by coupled ocean-atmosphere models, this suggests
that these more complete models will likely still have problems in simulating the regional climate change at the LGM.
Received: 11 October 1999 / Accepted: 9 June 2000 相似文献
10.
Past and future polar amplification of climate change: climate model intercomparisons and ice-core constraints 总被引:2,自引:2,他引:2
V. Masson-Delmotte M. Kageyama P. Braconnot S. Charbit G. Krinner C. Ritz E. Guilyardi J. Jouzel A. Abe-Ouchi M. Crucifix R. M. Gladstone C. D. Hewitt A. Kitoh A. N. LeGrande O. Marti U. Merkel T. Motoi R. Ohgaito B. Otto-Bliesner W. R. Peltier I. Ross P. J. Valdes G. Vettoretti S. L. Weber F. Wolk Y. YU 《Climate Dynamics》2006,26(5):513-529
Climate model simulations available from the PMIP1, PMIP2 and CMIP (IPCC-AR4) intercomparison projects for past and future
climate change simulations are examined in terms of polar temperature changes in comparison to global temperature changes
and with respect to pre-industrial reference simulations. For the mid-Holocene (MH, 6,000 years ago), the models are forced
by changes in the Earth’s orbital parameters. The MH PMIP1 atmosphere-only simulations conducted with sea surface temperatures
fixed to modern conditions show no MH consistent response for the poles, whereas the new PMIP2 coupled atmosphere–ocean climate
models systematically simulate a significant MH warming both for Greenland (but smaller than ice-core based estimates) and
Antarctica (consistent with the range of ice-core based range). In both PMIP1 and PMIP2, the MH annual mean changes in global
temperature are negligible, consistent with the MH orbital forcing. The simulated last glacial maximum (LGM, 21,000 years
ago) to pre-industrial change in global mean temperature ranges between 3 and 7°C in PMIP1 and PMIP2 model runs, similar to
the range of temperature change expected from a quadrupling of atmospheric CO2 concentrations in the CMIP simulations. Both LGM and future climate simulations are associated with a polar amplification
of climate change. The range of glacial polar amplification in Greenland is strongly dependent on the ice sheet elevation
changes prescribed to the climate models. All PMIP2 simulations systematically underestimate the reconstructed glacial–interglacial
Greenland temperature change, while some of the simulations do capture the reconstructed glacial–interglacial Antarctic temperature
change. Uncertainties in the prescribed central ice cap elevation cannot account for the temperature change underestimation
by climate models. The variety of climate model sensitivities enables the exploration of the relative changes in polar temperature
with respect to changes in global temperatures. Simulated changes of polar temperatures are strongly related to changes in
simulated global temperatures for both future and LGM climates, confirming that ice-core-based reconstructions provide quantitative
insights on global climate changes.
An erratum to this article can be found at 相似文献
11.
Julián Adem 《Climate Dynamics》1991,5(3):145-160
Recent advances in the development and applications of the author's Hemispheric Thermodynamic Climate Model are presented. The model has been adapted to simulate the climates from 18 kyr BP to the present, and to study the effect of the ice sheets, the insolation anomalies and the atmospheric CO2 content on such climates. The surface ocean temperature anomaly is also simulated in the model, and comparison with values of CLIMAP (1981) for 18 kyr BP shows some agreement. A long series of numerical experiments have lead to the improvement in prediction of the monthly surface temperature anomalies. Verification of 93 predictions over the contiguous United States of America shows a useful skill in the predictions. The model is being adapted for forecasting in the Mexican Republic. Experiments to improve the skill in prediction of surface ocean temperature anomalies in the Northern Hemisphere have been carried out, and using a fine resolution grid, the model has been used to simulate the annual cycle of the normal sea surface temperatures in the Gulf of Mexico, that agrees well with observations.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil 相似文献
12.
A continuous simulation of global ice volume over the past 1 million years with 3-D ice-sheet models
B. de Boer R. S. W. van de Wal L. J. Lourens R. Bintanja T. J. Reerink 《Climate Dynamics》2013,41(5-6):1365-1384
Sea-level records show large glacial-interglacial changes over the past million years, which on these time scales are related to changes of ice volume on land. During the Pleistocene, sea-level changes induced by ice volume are largely caused by the waxing and waning of the large ice sheets in the Northern Hemisphere. However, the individual contributions of ice in the Northern and Southern Hemisphere are poorly constrained. In this study, for the first time a fully coupled system of four 3-D ice-sheet models is used, simulating glaciations on Eurasia, North America, Greenland and Antarctica. The ice-sheet models use a combination of the shallow ice and shelf approximations to determine sheet, shelf and sliding velocities. The framework consists of an inverse forward modelling approach to derive a self-consistent record of temperature and ice volume from deep-sea benthic δ18O data over the past 1 million years, a proxy for ice volume and temperature. It is shown that for both eustatic sea level and sea water δ18O changes, the Eurasian and North American ice sheets are responsible for the largest part of the variability. The combined contribution of the Antarctic and Greenland ice sheets is about 10 % for sea level and about 20 % for sea water δ18O during glacial maxima. However, changes in interglacials are mainly caused by melt of the Greenland and Antarctic ice sheets, with an average time lag of 4 kyr between melt and temperature. Furthermore, we have tested the separate response to changes in temperature and sea level for each ice sheet, indicating that ice volume can be significantly influenced by changes in eustatic sea level alone. Hence, showing the importance of a simultaneous simulation of all four ice sheets. This paper describes the first complete simulation of global ice-volume variations over the late Pleistocene with the possibility to model changes above and below present-day ice volume, constrained by observations of benthic δ18O proxy data. 相似文献
13.
Net annual mass balance was evaluated for Greenland and the Tibetan Plateau using the meteorological forcings from the NCEP reanalysis and two GCMs (FOAM1.0 and CSM1.4) for modern climate and for different time periods extending back to the beginning of the Holocene (11,000 years ago) for the climate models. The ice-sheet budget calculations, using the degree day methodology, were performed on a finer grid than the model output by interpolating monthly precipitation and surface temperature and correcting the latter to account for the GCMs smoothed topography. The computed net mass balance for Greenland in the present day is positive and it ranges between 290–300 mm water equivalent (w.e.)/year for the two models, values close to the NCEP estimate of 250 mm/year. The past climate simulations show that the Greenland mass balance has become slightly more positive since the beginning of the Holocene. The Tibetan Plateaus present-day area average net mass balance is negative and ranges between –1200 and –2000 mm w.e. /year for the two models, values bracketing the NCEP estimate of 1700 mm/year, although the balance is positive over small regions of the plateau consistent with the existence of small ice caps and glaciers. The calculated past mass balance shows an increasingly less negative value for FOAM from 11,000 years ago towards the present and expansion of the positive mass balance areas, mainly due to decreased snow ablation as the summertime insolation decreases with the changes in orbital forcing; in CSM the opposite trend occurs but changes are smaller and less systematic. The result from FOAM shows that the likelihood of ice sheets developing on the Tibetan Plateau may have increased since 11000 years ago, which is consistent with some glacial records. 相似文献
14.
Ramdane Alkama M. Kageyama G. Ramstein O. Marti P. Ribstein D. Swingedouw 《Climate Dynamics》2008,30(7-8):855-869
The presence of large ice sheets over North America and North Europe at the Last Glacial Maximum (LGM) strongly impacted Northern
hemisphere river pathways. Despite the fact that such changes may significantly alter the freshwater input to the ocean, modified
surface hydrology has never been accounted for in coupled ocean–atmosphere general circulation model simulations of the LGM
climate. To reconstruct the LGM river routing, we use the ICE-5G LGM topography. Because of the uncertainties in the extent
of the Fennoscandian ice sheet in the Eastern part of the Kara Sea, we consider two more realistic river routing scenarios.
The first scenario is characterised by the presence of an ice dammed lake south of the Fennoscandian ice sheet, and corresponds
to the ICE-5G topography. This lake is fed by the Ob and Yenisei rivers. In the second scenario, both these rivers flow directly
into the Arctic Ocean, which is more consistent with the latest QUEEN ice sheet margin reconstructions. We study the impact
of these changes on the LGM climate as simulated by the IPSL_CM4 model and focus on the overturning thermohaline circulation.
A comparison with a classical LGM simulation performed using the same model and modern river basins as designed in the PMIP2
exercise leads to the following conclusions: (1) The discharge into the North Atlantic Ocean is increased by 2,000 m3/s between 38° and 54°N in both simulations that contain LGM river routing, compared to the classical LGM experiment. (2)
The ice dammed lake is shown to have a weak impact, relative to the classical simulation, both in terms of climate and ocean
circulation. (3) In contrast, the North Atlantic deep convection and meridional overturning are weaker than during the classical
LGM run if the Ob and Yenisei rivers flow directly into the Arctic Ocean. The total discharge into the Arctic Ocean is increased
by 31,000 m3/s, relative to the classical LGM simulation. Consequentially, northward ocean heat transport is weaker, and sea ice more
extensive, in better agreement with existing proxy data. 相似文献
15.
A number of transient climate runs simulating the last 120?kyr have been carried out using FAMOUS, a fast atmosphere–ocean general circulation model (AOGCM). This is the first time such experiments have been done with a full AOGCM, providing a three-dimensional simulation of both atmosphere and ocean over this period. Our simulation thus includes internally generated temporal variability over periods from days to millennia, and physical, detailed representations of important processes such as clouds and precipitation. Although the model is fast, computational restrictions mean that the rate of change of the forcings has been increased by a factor of 10, making each experiment 12?kyr long. Atmospheric greenhouse gases (GHGs), northern hemisphere ice sheets and variations in solar radiation arising from changes in the Earth’s orbit are treated as forcing factors, and are applied either separately or combined in different experiments. The long-term temperature changes on Antarctica match well with reconstructions derived from ice-core data, as does variability on timescales longer than 10 kyr. Last Glacial Maximum (LGM) cooling on Greenland is reasonably well simulated, although our simulations, which lack ice-sheet meltwater forcing, do not reproduce the abrupt, millennial scale climate shifts seen in northern hemisphere climate proxies or their slower southern hemisphere counterparts. The spatial pattern of sea surface cooling at the LGM matches proxy reconstructions reasonably well. There is significant anti-correlated variability in the strengths of the Atlantic meridional overturning circulation (AMOC) and the Antarctic Circumpolar Current (ACC) on timescales greater than 10?kyr in our experiments. We find that GHG forcing weakens the AMOC and strengthens the ACC, whilst the presence of northern hemisphere ice-sheets strengthens the AMOC and weakens the ACC. The structure of the AMOC at the LGM is found to be sensitive to the details of the ice-sheet reconstruction used. The precessional component of the orbital forcing induces ~20?kyr oscillations in the AMOC and ACC, whose amplitude is mediated by changes in the eccentricity of the Earth’s orbit. These forcing influences combine, to first order, in a linear fashion to produce the mean climate and ocean variability seen in the run with all forcings. 相似文献
16.
Sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the Greenland
ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice. Two
types of experiments are performed: steady-state climatic conditions and simulations over the last climatic cycle with a climatic
forcing derived from the GRIP record. The experiments show that the maximum altitude of the ice sheet depends on the ice flow
parameters (deformation and sliding law coefficients, geothermal flux) and that it is low when the ice flow is fast. On the
other hand, the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is
driven by changes in accumulation rate. The ice sheet extension shows the opposite sensitivity: it is barely affected by ice
flow velocity and the ice covered area is smaller for large ablation coefficients. For colder climates, when there is no ablation,
the ice sheet extension depends on the sea level. An interesting result is that the variations with time of the altitude at
the ice divide (Summit) do not depend on the parameters we tested. The present modelled ice sheets resulting from the climatic
cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best
fit between modelled and measured geometry. It seems that, compared to the parameter set most commonly used, higher ablation
rate coefficents must be used.
Received: 19 September 1995 / Accepted: 30 May 1996 相似文献
17.
Tropical climates at the Last Glacial Maximum: a new synthesis of terrestrial palaeoclimate data. I. Vegetation, lake-levels and geochemistry 总被引:1,自引:1,他引:0
I. Farrera S. P. Harrison I. C. Prentice G. Ramstein J. Guiot P. J. Bartlein R. Bonnefille M. Bush W. Cramer U. von Grafenstein K. Holmgren H. Hooghiemstra G. Hope D. Jolly S.-E. Lauritzen Y. Ono S. Pinot M. Stute G. Yu 《Climate Dynamics》1999,15(11):823-856
Palaeodata in synthesis form are needed as benchmarks for the Palaeoclimate Modelling Intercomparison Project (PMIP). Advances
since the last synthesis of terrestrial palaeodata from the last glacial maximum (LGM) call for a new evaluation, especially
of data from the tropics. Here pollen, plant-macrofossil, lake-level, noble gas (from groundwater) and δ18O (from speleothems) data are compiled for 18±2 ka (14C), 32 °N–33 °S. The reliability of the data was evaluated using explicit criteria and some types of data were re-analysed
using consistent methods in order to derive a set of mutually consistent palaeoclimate estimates of mean temperature of the
coldest month (MTCO), mean annual temperature (MAT), plant available moisture (PAM) and runoff (P-E). Cold-month temperature
(MAT) anomalies from plant data range from −1 to −2 K near sea level in Indonesia and the S Pacific, through −6 to −8 K at
many high-elevation sites to −8 to −15 K in S China and the SE USA. MAT anomalies from groundwater or speleothems seem more
uniform (−4 to −6 K), but the data are as yet sparse; a clear divergence between MAT and cold-month estimates from the same
region is seen only in the SE USA, where cold-air advection is expected to have enhanced cooling in winter. Regression of
all cold-month anomalies against site elevation yielded an estimated average cooling of −2.5 to −3 K at modern sea level,
increasing to ≈−6 K by 3000 m. However, Neotropical sites showed larger than the average sea-level cooling (−5 to −6 K) and
a non-significant elevation effect, whereas W and S Pacific sites showed much less sea-level cooling (−1 K) and a stronger
elevation effect. These findings support the inference that tropical sea-surface temperatures (SSTs) were lower than the CLIMAP
estimates, but they limit the plausible average tropical sea-surface cooling, and they support the existence of CLIMAP-like
geographic patterns in SST anomalies. Trends of PAM and lake levels indicate wet LGM conditions in the W USA, and at the highest
elevations, with generally dry conditions elsewhere. These results suggest a colder-than-present ocean surface producing a
weaker hydrological cycle, more arid continents, and arguably steeper-than-present terrestrial lapse rates. Such linkages
are supported by recent observations on freezing-level height and tropical SSTs; moreover, simulations of “greenhouse” and
LGM climates point to several possible feedback processes by which low-level temperature anomalies might be amplified aloft.
Received: 7 September 1998 / Accepted: 18 March 1999 相似文献
18.
GCM simulations of the Last Glacial Maximum surface climate of Greenland and Antarctica 总被引:3,自引:1,他引:2
The LMDz variable grid GCM was used to simulate the Last Glacial Maximum (LGM, 21 ky Bp.) climate of Greenland and Antarctica
at a spatial resolution of about 100 km.The high spatial resolution allows to investigate the spatial variability of surface
climate change signals, and thus to address the question whether the sparse ice core data can be viewed as representative
for the regional scale climate change. This study addresses primarily surface climate parameters because these can be checked
against the, limited, ice core record. The changes are generally stronger for Greenland than for Antarctica, as the imposed
changes of the forcing boundary conditions (e.g., sea surface temperatures) are more important in the vicinity of Greenland.
Over Greenland, and to a limited extent also in Antarctica, the climate shows stronger changes in winter than in summer. The
model suggests that the linear relationship between the surface temperature and inversion strength is modified during the
LGM. The temperature dependency of the moisture holding capacity of the atmosphere alone cannot explain the strong reduction
in snowfall over central Greenland; atmospheric circulation changes also play a crucial role. Changes in the high frequency
variability of snowfall, atmospheric pressure and temperature are investigated and possible consequences for the interpretation
of ice core records are discussed. Using an objective cyclone tracking scheme, the importance of changes of the atmospheric
dynamics off the coasts of the ice sheets, especially for the high frequency variability of surface climate parameters, is
illustrated. The importance of the choice of the LGM ice sheet topography is illustrated for Greenland, where two different
topographies have been used, yielding results that differ quite strongly in certain nontrivial respects. This means that the
paleo-topography is a significant source of uncertainty for the modelled paleoclimate. The sensitivity of the Greenland LGM
climate to the prescribed sea surface conditions is examined by using two different LGM North Atlantic data sets.
Received: 23 October 1997 / Accepted: 17 March 1998 相似文献
19.
J. Oerlemans 《Climate Dynamics》2002,19(1):85-93
The total mass budget of the Antarctic ice sheet is studied with a simple axi-symmetrical model. The ice-sheet has a parabolic profile resting on a bed that slopes linearly downwards from the centre of the ice sheet into the ocean. The mean ice velocity at the grounding line is assumed to be proportional to the water depth. The accumulation rate is a linear function of the distance to the centre. Setting the total mass budget to zero yields a quadratic equation for the steady-state ice-sheet radius R. Analysis of the equilibrium states sheds light on the sensitivity of the ice-sheet radius to changes in sea level (S) and precipitation with respect to the present state (Prel). For model parameters obtained by matching the analytical model to the present state of the Antarctic ice sheet, the sensitivity values are dR/dS = -2400 and dR/dPrel = 4000 m/%. The model can also be used to study transient behaviour of the ice sheet. The characteristic relaxation time (e-folding time scale) is about 3500 years. Forcing the model with a sea-level and accumulation history over the past few hundred thousands of years yields Antarctic ice-volume curves that are similar to those obtained by comprehensive numerical modelling. The current imbalance predicted by the model corresponds to a sea-level rise of 0.25 mm yr-1. 相似文献
20.
Masa Kageyama Pascale Braconnot Laurent Bopp Véronique Mariotti Tilla Roy Marie-Noëlle Woillez Arnaud Caubel Marie-Alice Foujols Eric Guilyardi Myriam Khodri James Lloyd Fabien Lombard Olivier Marti 《Climate Dynamics》2013,40(9-10):2469-2495
The climates of the mid-Holocene (MH, 6,000 years ago) and the Last Glacial Maximum (LGM, 21,000 years ago) have been extensively documented and as such, have become targets for the evaluation of climate models for climate contexts very different from the present. In Part 1 of the present work, we have studied the MH and LGM simulations performed with the last two versions of the IPSL model: IPSL_CM4, run for the PMIP2/CMIP3 (Coupled Model Intercomparion Project) projects and IPSL_CM5A, run for the most recent PMIP3/CMIP5 projets. We have shown that not only are these models different in their simulations of the PI climate, but also in their simulations of the climatic anomalies for the MH and LGM. In the Part 2 of this paper, we first examine whether palaeo-data can help discriminate between the model performances. This is indeed the case for the African monsoon for the MH or for North America south of the Laurentide ice sheet, the South Atlantic or the southern Indian ocean for the LGM. For the LGM, off-line vegetation modelling appears to offer good opportunities to distinguish climate model results because glacial vegetation proves to be very sensitive to even small differences in LGM climate. For other cases such as the LGM North Atlantic or the LGM equatorial Pacific, the large uncertainty on the SST reconstructions, prevents model discrimination. We have examined the use of other proxy-data for model evaluation, which has become possible with the inclusion of the biogeochemistry morel PISCES in the IPSL_CM5A model. We show a broad agreement of the LGM–PI export production changes with reconstructions. These changes are related to the mixed layer depth in most regions and to sea-ice variations in the high latitudes. We have also modelled foraminifer abundances with the FORAMCLIM model and shown that the changes in foraminifer abundance in the equatorial Pacific are mainly forced by changes in SSTs, hence confirming the SST-foraminifer abundance relationship. Yet, this is not the case in all regions in the North Atlantic, where food availability can have a strong impact of foraminifer abundances. Further work will be needed to exhaustively examine the role of factors other than climate in piloting changes in palaeo-indicators. 相似文献