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1.
The conditions of development of mid-latitude depressions (synoptic eddies) in the winter Northern Hemisphere mid-latitudes
at the Last Glacial Maximum (LGM, 21 000 years ago) are very different from the present ones: this period is characterised
by a general cooling of the extra-tropics, with massive ice sheets over the Northern Hemisphere continents and sea-ice extending
very far south over the North Atlantic. The present work uses regression analysis to study the characteristics of the synoptic
eddies in present-day and LGM climate simulations by the Atmospheric General Circulation Model (AGCM) of the UK Universities'
Global Atmospheric Programme (UGAMP). In the LGM experiment, the structure of the Pacific eddies is similar to the present-day
(PD) situation, but they are weaker. On the other hand, the Atlantic eddies show an increased zonal wavelength and a much
shallower structure in the temperature and vertical wind perturbations. To understand the changes of these characteristics
from present-day to LGM, we compare them to those computed for the most unstable modes of the corresponding mean flows, determined
using a dry primitive equation model. A normal-mode stability analysis is carried both on zonally symmetric and asymmetric
flows for each of the Northern Hemisphere storm-tracks. The changes in the most unstable normal modes found by both these
analyses give a good account of changes in the structure of the perturbations as retrieved from the AGCM, suggesting that
changes in the mean state (especially the temperature gradient) is the main driver of these changes. However in the case of
the present-day Atlantic storm-track, the growth rate of these modes is found to be very low compared to the other cases.
A complementary analysis evaluates the importance of non-modal growth, in the form of downstream development of perturbations,
for each of the storm-tracks. This type of growth is found to be especially important in the case of the present-day Atlantic
storm-track.
Received: 29 September 1999 / Accepted: 17 November 1999 相似文献
2.
The Last Glacial Maximum climate over Europe and western Siberia: a PMIP comparison between models and data 总被引:3,自引:1,他引:3
M. Kageyama O. Peyron S. Pinot P. Tarasov J. Guiot S. Joussaume G. Ramstein 《Climate Dynamics》2001,17(1):23-43
Under the framework of the Palaeoclimate Modelling Intercomparison Project (PMIP), 17 climate models, 16 of which are atmospheric
general circulation models, have been run to simulate the climate of the Last Glacial Maximum (21 000 years ago) using the
same set of boundary conditions. Parallel to these numerical experiments, new, consistent, data bases have been developed
on a continental scale. The present work compares the range of the model responses to the large perturbation corresponding
to the conditions of the Last Glacial Maximum with consistently derived climate reconstructions from pollen records over Europe
and western Siberia. It accounts for the differences in the model results due to the models themselves and directly compares
this “error bar” due to the models to the uncertainties in the climate reconstructions from the pollen records. Overall the
Last Glacial Maximum climate simulated by the models over western Europe is warmer, especially in winter, and wetter than
the one depicted by the reconstructions. This is the region where the reconstructed increase in temperature, precipitation
and moisture index from the Last Glacial Maximum to the present conditions is largest. The same disagreement, but of smaller
amplitude, is found over Central Europe and the eastern Mediterranean Basin, while models and data are in broad agreement
over western Siberia. The numerous modelling results allow a study of the link between the changes in atmospheric circulation
and those in temperature, and an interpretation of the discrepancies in precipitation in terms of those in temperature.
Received: 1 February 2000 / Accepted: 9 May 2000 相似文献
3.
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
4.
The Younger Dryas (YD) stadial signified an interruption of the warming during the transition from the last glacial to the
present interglacial. The mechanism responsible for this cooling is still uncertain, so valuable information concerning climate
variability can be obtained by numerical simulation of the YD climate. We performed four experiments on the Younger Dryas
climate with the Hamburg atmospheric general circulation model. Here we use the results of these experiments, which differed
in prescribed boundary conditions, to characterize the atmospheric winter circulation during the YD stadial in the North Atlantic/European
sector. The 10 year means of the following variables are presented: sea level pressure, 500 hPa geopotential heights and 200 hPa
winds. In addition, we used daily values to calculate an index to assess the occurrence of blocking and strong zonal flow
and to compute storm tracks. Our results show that the YD cooling in Europe was present with a strong and stable westerly
circulation without blocking. This is in conflict with an earlier study suggesting frequent easterly winds over NW-Europe.
In our experiments the sea-ice cover in the North Atlantic Ocean was the crucial factor forcing this specific YD circulation.
Moreover, the jet stream over the North Atlantic was strengthened considerably, causing an enhanced cyclonic activity over
the Eurasian continent. The YD winter circulation was different from the circulation found in most simulation studies on the
Last Glacial Maximum, since no glacial anticyclones were present and no split of the jet stream occurred.
Received: 1 November 1995 / Accepted: 29 May 1996 相似文献
5.
Long-Term Global Warming Scenarios Computed with an Efficient Coupled Climate Model 总被引:13,自引:0,他引:13
We present global warming scenarios computed with an intermediate-complexity atmosphere-ocean-sea ice model which has been extensively validated for a range of past climates (e.g., the Last Glacial Maximum). Our simulations extend to the year 3000, beyond the expected peak of CO2 concentrations. The thermohaline ocean circulation declines strongly in all our scenarios over the next 50 years due to a thermal effect. Changes in the hydrological cycle determine whether the circulation recovers or collapses in the long run. Both outcomes are possible within present uncertainty limits. In case of a collapse, a substantial long-lasting cooling over the North Atlantic and a drying of Europe is simulated. 相似文献
6.
C. Casty D. Handorf C. C. Raible J. F. González-Rouco A. Weisheimer E. Xoplaki J. Luterbacher K. Dethloff H. Wanner 《Climate Dynamics》2005,24(7-8):809-822
Recurrent climate winter regimes are examined from statistically reconstructed and modelled 500 hPa geopotential height fields
over the North Atlantic/European sector for the period 1659–1990. We investigate the probability density function of the state
space spanned by the first two empirical orthogonal functions of combined winter data. Regimes are detected as patterns that
correspond to areas of the state space with an unexpected high recurrence probability using a Monte Carlo approach. The reconstruction
and the model reveal four recurrent climate regimes. They correspond to the two phases of the North Atlantic Oscillation and
two opposite blocking patterns. Complemented by the investigation of the temporal evolution of the climate regimes this leads
to the conclusion that the reconstructed and the modelled data for this geographic sector reproduce low-frequency atmospheric
variability in the form of regime-like behaviour. The overall evidence for recurrent climate regimes is higher for the model
than for the reconstruction. However, comparisons with independent data sources for the period 1659–1990 revealed a more realistic
temporal evolution of the regimes for the reconstructed data.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
7.
Last Glacial Maximum climate of the former Soviet Union and Mongolia reconstructed from pollen and plant macrofossil data 总被引:2,自引:2,他引:0
P. E. Tarasov O. Peyron J. Guiot S. Brewer V. S. Volkova L. G. Bezusko N. I. Dorofeyuk E. V. Kvavadze I. M. Osipova N. K. Panova 《Climate Dynamics》1999,15(3):227-240
An improved concept of the best analogues method was used to reconstruct the Last Glacial Maximum (LGM) climate from a set
of botanical records from the former Soviet Union and Mongolia. Terrestrial pollen and macrofossil taxa were grouped into
broad classes – plant functional types (PFTs), defined by the ecological and climatic parameters used in the BIOME1 model.
PFT scores were then calibrated in terms of modern climate using 1245 surface pollen spectra from Eurasia and North America.
In contrast to individual taxa, which exhibit great variability and may not be present in the palaeoassemblages, even in suitable
climates, PFTs are more characteristic of the vegetation types. The modified method thus allows climate reconstruction at
time intervals with partial direct analogues of modern vegetation (e.g. the LGM). At 18 kBP, mean temperatures were 20–29 °C
colder than today in winter and 5–11 °C colder in summer in European Russia and Ukraine. Sites from western Georgia show negative,
but moderate temperature anomalies compared to today: 8–11 °C in January and 5–7 °C in July. LGM winters were 7–15 °C colder
and summers were 1–7 °C colder in Siberia and Mongolia. Annual precipitation sums were 50–750 mm lower than today across northern
Eurasia, suggesting a weakening of the Atlantic and Pacific influences. Reconstructed drought index shows much drier LGM conditions
in northern and mid-latitude Russia, but similar to or slightly wetter than today around the Black Sea and in Mongolia, suggesting
compensation of precipitation losses by lower-than-present evaporation.
Received: 11 May 1998 / Accepted: 25 September 1998 相似文献
8.
Samuel Albani Natalie M. Mahowald Barbara Delmonte Valter Maggi Gisela Winckler 《Climate Dynamics》2012,38(9-10):1731-1755
Mineral dust aerosols represent an active component of the Earth’s climate system, by interacting with radiation directly, and by modifying clouds and biogeochemistry. Mineral dust from polar ice cores over the last million years can be used as paleoclimate proxy, and provide unique information about climate variability, as changes in dust deposition at the core sites can be due to changes in sources, transport and/or deposition locally. Here we present results from a study based on climate model simulations using the Community Climate System Model. The focus of this work is to analyze simulated differences in the dust concentration, size distribution and sources in current climate conditions and during the Last Glacial Maximum at specific ice core locations in Antarctica, and compare with available paleodata. Model results suggest that South America is the most important source for dust deposited in Antarctica in current climate, but Australia is also a major contributor and there is spatial variability in the relative importance of the major dust sources. During the Last Glacial Maximum the dominant source in the model was South America, because of the increased activity of glaciogenic dust sources in Southern Patagonia-Tierra del Fuego and the Southernmost Pampas regions, as well as an increase in transport efficiency southward. Dust emitted from the Southern Hemisphere dust source areas usually follow zonal patterns, but southward flow towards Antarctica is located in specific areas characterized by southward displacement of air masses. Observations and model results consistently suggest a spatially variable shift in dust particle sizes. This is due to a combination of relatively reduced en route wet removal favouring a generalized shift towards smaller particles, and on the other hand to an enhanced relative contribution of dry coarse particle deposition in the Last Glacial Maximum. 相似文献
9.
S. P. Harrison P. J. Bartlein S. Brewer I. C. Prentice M. Boyd I. Hessler K. Holmgren K. Izumi K. Willis 《Climate Dynamics》2014,43(3-4):671-688
Past climates provide a test of models’ ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others. 相似文献
10.
A statistical learning method called random forests is applied to the prediction of transitions between weather regimes of
wintertime Northern Hemisphere (NH) atmospheric low-frequency variability. A dataset composed of 55 winters of NH 700-mb geopotential
height anomalies is used in the present study. A mixture model finds that the three Gaussian components that were statistically
significant in earlier work are robust; they are the Pacific–North American (PNA) regime, its approximate reverse (the reverse
PNA, or RNA), and the blocked phase of the North Atlantic Oscillation (BNAO). The most significant and robust transitions in the Markov chain generated by these regimes are PNA → BNAO, PNA → RNA and BNAO → PNA. The break of a regime and subsequent onset of another one is forecast for these three transitions. Taking the relative costs
of false positives and false negatives into account, the random-forests method shows useful forecasting skill. The calculations
are carried out in the phase space spanned by a few leading empirical orthogonal functions of dataset variability. Plots of
estimated response functions to a given predictor confirm the crucial influence of the exit angle on a preferred transition
path. This result points to the dynamic origin of the transitions. 相似文献
11.
Christophe Cassou Marie Minvielle Laurent Terray Claire P��rigaud 《Climate Dynamics》2011,36(1-2):19-39
The links between the observed variability of the surface ocean variables estimated from reanalysis and the overlying atmosphere decomposed in classes of large-scale atmospheric circulation via clustering are investigated over the Atlantic from 1958 to 2002. Daily 500?hPa geopotential height and 1,000?hPa wind anomaly maps are classified following a weather-typing approach to describe the North Atlantic and tropical Atlantic atmospheric dynamics, respectively. The algorithm yields patterns that correspond in the extratropics to the well-known North Atlantic-Europe weather regimes (NAE-WR) accounting for the barotropic dynamics, and in the tropics to wind classes (T-WC) representing the alteration of the trades. 10-m wind and 2-m temperature (T2) anomaly composites derived from regime/wind class occurrence are indicative of strong relationships between daily large-scale atmospheric circulation and ocean surface over the entire Atlantic basin. High temporal correlation values are obtained basin-wide at low frequency between the observed fields and their reconstruction by multiple linear regressions with the frequencies of occurrence of both NAE-WR and T-WC used as sole predictors. Additional multiple linear regressions also emphasize the importance of accounting for the strength of the daily anomalous atmospheric circulation estimated by the combined distances to all regimes centroids in order to reproduce the daily to interannual variability of the Atlantic ocean. We show that for most of the North Atlantic basin the occurrence of NAE-WR generally sets the sign of the ocean surface anomaly for a given day, and that the inter-regime distances are valuable predictors for the magnitude of that anomaly. Finally, we provide evidence that a large fraction of the low-frequency trends in the Atlantic observed at the surface over the last 50?years can be traced back, except for T2, to changes in occurrence of tropical and extratropical weather classes. All together, our findings are encouraging for the prospects of basin-scale ocean dynamical downscaling using a weather-typing approach to reconstruct forcing fields for high resolution ocean models (Part II) from coarse resolution climate models. 相似文献
12.
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
13.
The simulated low-frequency variability patterns of the atmospheric circulation, ranging from interannual to interdecadal timescales, are studied in an area encompassing southern South America. The experiment is a transient simulation performed with the IPSL CCM2 coupled global model, in which the greenhouse forcing is continuously increasing. The main modes of low-frequency variability are found to remain stationary throughout the simulation, suggesting they depend more on the internal dynamics of the atmospheric flow than on its external forcing. Inspection of the circulation regimes that represent the more recurrent patterns at interannual and interdecadal timescales showed that climate change manifests itself as a change in regime population, suggesting that the negative phase of the Antarctic Oscillation-like pattern becomes more frequented in a climate change scenario. Changes of regime occurrence are superimposed to a positive trend whose spatial pattern is reminiscent of the structure of the Antarctic Oscillation-mode of variability. Moreover, it resembles the spatial patterns of those regimes that show a significant change in population. The change in regime frequencies of the circulation patterns of low-frequency variability are in opposite phase with respect to the trend, thus, the behaviour of these patterns of variability, superimposed to a changing mean state, modulates the climate change signal. The analysis of the high frequencies, in terms of recurrent patterns representing intraseasonal and synoptic-scale of variability, shows no significant changes in regime characteristics, concerning both spatial and temporal behaviour. 相似文献
14.
Pablo Ortega Didier Swingedouw Valérie Masson-Delmotte Camille Risi Bo Vinther Pascal Yiou Robert Vautard Kei Yoshimura 《Climate Dynamics》2014,43(9-10):2585-2605
Greenland ice cores offer seasonal to annual records of δ18O, a proxy for precipitation-weighted temperature, over the last few centuries to millennia. Here, we investigate the regional footprints of the North Atlantic weather regimes on Greenland isotope and climate variability, using a compilation of 22 different shallow ice-cores and the atmospheric pressure conditions from the twentieth century reanalysis (20CR). As a first step we have verified that the leading modes of winter and annual δ18O are well correlated with oceanic (Atlantic multidecadal oscillation) and atmospheric [North Atlantic oscillation (NAO)] indices respectively, and also marginally with external forcings, thus confirming earlier studies. The link between weather regimes and Greenland precipitation, precipitation-weighted temperature and δ18O is further explored by using an isotope simulation from the LMDZ-iso model, where the 3-dimensional wind fields are nudged to those of 20CR. In winter, the NAO+ and NAO? regimes in LMDZ-iso produce the largest isotopic changes over the entire Greenland region, with maximum anomalies in the South. Likewise, the Scandinavian blocking and the Atlantic ridge also show remarkable imprints on isotopic composition over the region. To assess the robustness and model dependency of our findings, a second isotope simulation from the isotopic model is also explored. The percentage of Greenland δ18O variance explained by the ensemble of weather regimes is increased by a factor near two in both LMDZ-iso and IsoGSM when compared to the contribution of the NAO index only. Similarly, weather regimes provide a net gain in the δ18O variance explained of similar magnitude for the whole set of ice core records. Greenland δ18O also appears to be locally affected by the low-frequency variations in the centres of action of the weather regimes, with clearer imprints in the LMDZ-iso simulation. This study opens the possibility for reconstructing past changes in the frequencies of occurrence of the weather regimes, which would rely on the sensitive regions identified here, and the use of additional proxies over the North Atlantic region. 相似文献
15.
A present-day climatic model is presented in which extended wet spells of near-decadal duration and dry spells of similar length are explained on the basis of surface and upper tropospheric circulation variations. Wet spells are shown to be the result of increased tropical atmospheric disturbances and tropical-temperate interaction, and to be linked to variations in the Walker Circulation. Conversely, dry spells are shown to result from diminished tropical activity over southern Africa, equatorward movement of westerly storm tracks and temperate perturbations in the westerlies.The present-day analogue is compared to preliminary spatial reconstructions of the climate of southern Africa over the last twenty-five millennia and is shown to have wide applicability in the explanation of the late-Quaternary palaeoclimates of the subcontinent. In particular, it is argued that the Last Glacial Maximum was associated with northward-displaced circulation conditions similar to those of present-day dry spells over the summer rainfall region, whereas the extensive moist conditions that prevailed for several thousand years after 9000 BP were analogous to present-day wet spell conditions with little apparent displacement of major circulation features. 相似文献
16.
Summary The existence of low-frequency variability in climatic-hydrological-oceanic variables may be useful for long-term forecasting
and climate modelling. By using long time series this paper attempts to identify large-scale quasi-cycles in the precipitation
regimes of Northern Argentina, moisture advection from the Atlantic Ocean and the streamflow of the Paraná River. This work
also shows the presence of coherent waves with long periodicity between the three series. As the three variables are estimated
over different time intervals, the presence of waves in each variable is studied separately, to show they respond to the same
process. The three variables show significant interdecadal variability at low frequency (22–26 years), which might be related
to the ENSO cycle modulation and to the intensification or weakening of the South American Low Level Jet (SALLJ) and South
Atlantic Convergence Zone (SACZ). 相似文献
17.
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
18.
Virginie Guemas David Salas-Mélia Masa Kageyama Hervé Giordani Aurore Voldoire Emilia Sanchez-Gomez 《Climate Dynamics》2010,34(4):527-546
This study aims at understanding the summer ocean-atmosphere interactions in the North Atlantic European region on intraseasonal
timescales. The CNRMOM1d ocean model is forced with ERA40 (ECMWF Re-Analysis) surface fluxes with a 1-h frequency in solar
heat flux (6 h for the other forcing fields) over the 1959–2001 period. The model has 124 vertical levels with a vertical
resolution of 1 m near the surface and 500 m at the bottom. This ocean forced experiment is used to assess the impact of the
North Atlantic weather regimes on the surface ocean. Composites of sea surface temperature (SST) anomalies associated with
each weather regime are computed and the mechanisms explaining these anomalies are investigated. Then, the SST anomalies related
to each weather regime in the ocean-forced experiment are prescribed to the ARPEGE Atmosphere General Circulation Model. We
show that the interaction with the surface ocean induces a positive feedback on the persistence of the Blocking regime, a
negative feedback on the persistence of the NAO-regime and favours the transition from the Atlantic Ridge regime to the NAO-regime
and from the Atlantic Low regime toward the Blocking regime. 相似文献
19.
Past changes in the density and momentum structure of oceanic circulation are an important aspect of changes in the Atlantic Meridional Overturning Circulation and consequently climate. However, very little is known about past changes in the vertical density structure of the ocean, even very extensively studied systems such as the North Atlantic. Here we exploit the physical controls on the settling depth of the dense Mediterranean water plume derived from the Strait of Gibraltar to obtain the first robust, observations-based, probabilistic reconstruction of the vertical density gradient in the eastern North Atlantic during the last 30,000?years. We find that this gradient was weakened by more than 50%, relative to the present, during the last Glacial Maximum, and that changes in general are associated with reductions in AMOC intensity. However, we find only a small change during Heinrich Event 1 relative to the Last Glacial Maximum, despite strong evidence that overturning was substantially altered. This implies that millennial-scale changes may not be reflected in vertical density structure of the ocean, which may be limited to responses on an ocean-overturning timescale or longer. Regardless, our novel reconstruction of Atlantic density structure can be used as the basis for a dynamical measure for validation of model-based AMOC reconstructions. In addition, our general approach is transferrable to other marginal sea outflow plumes, to provide estimates of oceanic vertical density gradients in other locations. 相似文献
20.
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