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1.
This study evaluates the equilibrium response of a coupled ocean–atmosphere model to the doubling, quadrupling, and halving of CO2 concentration in the atmosphere. Special emphasis in the study is placed upon the response of the thermohaline circulation in the Atlantic Ocean to the changes in CO2 concentration of the atmosphere. The simulated intensity of the thermohaline circulation (THC) is similar among three quasi-equilibrium states with the standard, double the standard, and quadruple the standard amounts of CO2 concentration in the atmosphere. When the model atmosphere has half the standard concentration of CO2, however, the THC is very weak and shallow in the Atlantic Ocean. Below a depth of 3 km, the model oceans maintain very thick layer of cold bottom water with temperature close to –2 °C, preventing the deeper penetration of the THC in the Atlantic Ocean. In the Circumpolar Ocean of the Southern Hemisphere, sea ice extends beyond the Antarctic Polar front, almost entirely covering the regions of deepwater ventilation. In addition to the active mode of the THC, there exists another stable mode of the THC for the standard, possibly double the standard (not yet confirmed), and quadruple the standard concentration of atmospheric carbon dioxide. This second mode is characterized by the weak, reverse overturning circulation over the entire Atlantic basin, and has no ventilation of the entire subsurface water in the North Atlantic Ocean. At one half the standard CO2 concentration, however, the intensity of the first mode is so weak that it is not certain whether there are two distinct stable modes or not. The paleoceanographic implications of the results obtained here are discussed as they relate to the signatures of the Cenozoic changes in the oceans.An erratum to this article can be found at 相似文献
2.
It is investigated how abrupt changes in the North Atlantic (NA) thermohaline circulation (THC) affect the terrestrial carbon
cycle. The Lund–Potsdam–Jena Dynamic Global Vegetation Model is forced with climate perturbations from glacial freshwater
experiments with the ECBILT-CLIO ocean–atmosphere–sea ice model. A reorganisation of the marine carbon cycle is not addressed.
Modelled NA THC collapses and recovers after about a millennium in response to prescribed freshwater forcing. The initial
cooling of several Kelvin over Eurasia causes a reduction of extant boreal and temperate forests and a decrease in carbon
storage in high northern latitudes, whereas improved growing conditions and slower soil decomposition rates lead to enhanced
storage in mid-latitudes. The magnitude and evolution of global terrestrial carbon storage in response to abrupt THC changes
depends sensitively on the initial climate conditions. These were varied using results from time slice simulations with the
Hadley Centre model HadSM3 for different periods over the past 21 kyr. Changes in terrestrial storage vary between −67 and
+50 PgC for the range of experiments with different initial conditions. Simulated peak-to-peak differences in atmospheric
CO2 are 6 and 13 ppmv for glacial and late Holocene conditions. Simulated changes in δ13C are between 0.15 and 0.25‰. These simulated carbon storage anomalies during a NA THC collapse depend on their magnitude
on the CO2 fertilisation feedback mechanism. The CO2 changes simulated for glacial conditions are compatible with available evidence from marine studies and the ice core CO2 record. The latter shows multi-millennial CO2 variations of up to 20 ppmv broadly in parallel with the Antarctic warm events A1 to A4 in the South and cooling in the North. 相似文献
3.
A global, flux-corrected climate model is employed to predict the surface wind stress and associated wind-driven oceanic circulation
for climate states corresponding to a doubling and quadrupling of the atmospheric CO2 concentration in a simple 1% per year CO2 increase scenario. The model indicates that in response to CO2 increase, the position of zero wind stress curl in the mid-latitudes of the Southern Hemisphere shifts poleward. In addition,
the wind stress intensifies significantly in the mid-latitudes of the Southern Hemisphere. As a result, the rate of water
circulation in the subpolar meridional overturning cell in the Southern Ocean increases by about 6 Sv (1 Sv=106 m3 s−1) for doubled CO2 and by 12 Sv for quadrupled CO2, implying an increase of deep water upwelling south of the circumpolar flow and an increase of Ekman pumping north of it.
In addition, the changes in the wind stress and wind stress curl translate into changes in the horizontal mass transport,
leading to a poleward expansion of the subtropical gyres in both hemispheres, and to strengthening of the Antarctic Circumpolar
Current. Finally, the intensified near-surface winds over the Southern Ocean result in a substantial increase of mechanical
energy supply to the ocean general circulation. 相似文献
4.
Lisa M. Baldini Frank McDermott James U. L. Baldini Matthew J. Fischer Martin Möllhoff 《Climate Dynamics》2010,35(6):977-993
This two-year study investigates the relative influence of meteorological variables (precipitation amount and temperature),
atmospheric circulation, air mass history, and moisture source region on Irish precipitation oxygen isotopes (δ18Op) on event and monthly timescales. Single predictor correlations reveal that on the event scale, 20% of δ18Op variability is attributable to the amount effect and 7% to the temperature effect while on the monthly timescale the North
Atlantic Oscillation accounts for up to 20% of δ18Op variability and the amount and temperature effects are not significant. In comparison, multivariate linear regression reveals
that the interaction of temperature and precipitation amount explains up to 40% of δ18Op variance at event and monthly timescales. Five-day kinematic back trajectories suggest that the amount-weighted mean δ18Op value of southerly- and northerly-derived events are lower by 2‰ relative to events derived from the west. Because air mass
history and atmospheric circulation appear to influence δ18Op in Ireland, Irish paleo-δ18Op proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall. 相似文献
5.
H. Renssen 《Climate Dynamics》1997,13(7-8):587-599
Geological evidence points to a global Younger Dryas (YD) climatic oscillation during the last glacial/ present interglacial
transition phase. A convincing mechanism to explain this global YD climatic oscillation is not yet available. Nevertheless,
a profound understanding of the mechanism behind the YD climate would lead to a better understanding of climate variability.
Therefore, the Hamburg atmospheric circulation model was used to perform four numerical experiments on the YD climate. The
objective of this study is to improve the understanding of different forcings influencing climate during the last glacial/interglacial
transition and to investigate to what extent the model response agrees with global geological evidence of YD climate change.
The following boundary conditions were altered: sea surface conditions, ice sheets, insolation and atmospheric CO2 concentration. Sea surface temperatures based on foraminiferal assemblages proved to produce insufficient winter cooling
in the N Atlantic Ocean in two experiments. It is proposed that this discrepancy is caused by uncertainties in the reconstruction
method of sea surface temperatures. Therefore, a model-derived set of Atlantic surface ocean conditions was prescribed in
a subsequent simulation. However, the latter set represented an Atlantic Ocean without a thermohaline circulation, which is
not in agreement with evidence from ocean cores. The global response to the boundary conditions was analysed using three variables,
namely surface temperature, zonal wind speed and precipitation. The statistical significance of the changes was tested with
a two-tailed t-test. Moreover, the significant responses to cooled oceans were compared with geological evidence of a YD oscillation.
This comparison revealed a good match in Europe, Greenland, Atlantic Canada and the N Pacific region, explaining the YD oscillation
in these regions as a response to cooled N Atlantic and N Pacific Oceans.
However, the results leave the YD climate in other regions completely unexplained. This reflects either an insufficient set
of boundary conditions or the important role played by feedbacks within the coupled atmosphere-ocean-ice system. These feedbacks
are poorly represented in the used atmospheric model, since ice sheets and the ocean surface conditions have to be prescribed.
Received: 30 July 1996 / Accepted: 12 February 1997 相似文献
6.
应用一个嵌套了海洋生物地球化学循环的太平洋环流碳循环模式,分析了1960~2000年太平洋不同海区海气碳通量随时间的变化。模拟结果显示,赤道太平洋为大气CO2的排放区,南、北太平洋(南、北纬15°至模式计算区域南、北边界)为吸收区。3个海区海气碳通量随时间均存在显著的波动,其中赤道太平洋海气碳通量年际波动最显著。3个海区海气碳通量年际波动对气候事件的响应并不一致,在El Niño年赤道太平洋冷舌的强度和总溶解无机碳(DIC)的浓度以及输出生产力均会受到上升流减弱的影响而降低,La Niña年这些海气碳通量控制要素的分布情况则正好相反,但在南北太平洋副热带以及高纬度海区,El Niño和La Niña对这些要素带来的影响却并不一定相反,对输出生产力的影响甚至是一致的。以海表温度(SST)为例考察海气碳通量与物理场之间的关系表明,在赤道太平洋上升流对DIC的影响是控制海气碳通量变化的主要因素,而在其他海区,尤其是副热带海区,由于垂直运动的年际变化较小,且生物生产力水平较低,SST的波动对海气碳通量年际变化的影响更加重要。 相似文献
7.
Estimates of anthropogenic CO<Subscript>2</Subscript> uptake in a global ocean model 总被引:2,自引:0,他引:2
A global ocean general circulation model (L30T63) is employed to study the uptake and distribution
of anthropogenic CO2 in the ocean. A subgrid-scale mixing scheme called GM90 is used in the model. There are
two main GM90 parameters including isopycnal diffusivity and skew (thickness) diffusivity. Sensitivities of
the ocean circulation and the redistribution of dissolved anthropogenic CO2 to these two parameters are
examined. Two runs estimate the global oceanic anthropogenic CO2 uptake to be 1.64 and 1.73 Pg C yr-1
for the 1990s, and that the global ocean contained 86.8 and 92.7 Pg C of anthropogenic CO2 at the end of 1994,
respectively. Both the total inventory and uptake from our model are smaller than the data-based estimates.
In this presentation, the vertical distributions of anthropogenic CO2 at three meridional sections are
discussed and compared with the available data-based estimates. The inventory in the individual basins is
also calculated. Use of large isopycnal diffusivity can generally improve the simulated results, including
the exchange flux, the vertical distribution patterns, inventory, storage, etc. In terms of comparison of
the vertical distributions and column inventory, we find that the total inventory in the Pacific Ocean
obtained from our model is in good agreement with the data-based estimate, but a large difference exists
in the Atlantic Ocean, particularly in the South Atlantic. The main reasons are weak vertical mixing and
that our model generates small exchange fluxes of anthropogenic CO2 in the Southern Ocean. Improvement in
the simulation of the vertical transport and sea ice in the Southern Ocean is important in future work. 相似文献
8.
Till Kuhlbrodt Stefan Rahmstorf Kirsten Zickfeld Frode Bendiksen Vikebø Svein Sundby Matthias Hofmann Peter Michael Link Alberte Bondeau Wolfgang Cramer Carlo Jaeger 《Climatic change》2009,96(4):489-537
This paper discusses the risks of a shutdown of the thermohaline circulation (THC) for the climate system, for ecosystems
in and around the North Atlantic as well as for fisheries and agriculture by way of an Integrated Assessment. The climate
model simulations are based on greenhouse gas scenarios for the 21st century and beyond. A shutdown of the THC, complete by
2150, is triggered if increased freshwater input from inland ice melt or enhanced runoff is assumed. The shutdown retards
the greenhouse gas-induced atmospheric warming trend in the Northern Hemisphere, but does not lead to a persistent net cooling.
Due to the simulated THC shutdown the sea level at the North Atlantic shores rises by up to 80 cm by 2150, in addition to
the global sea level rise. This could potentially be a serious impact that requires expensive coastal protection measures.
A reduction of marine net primary productivity is associated with the impacts of warming rather than a THC shutdown. Regional
shifts in the currents in the Nordic Seas could strongly deteriorate survival chances for cod larvae and juveniles. This could
lead to cod fisheries becoming unprofitable by the end of the 21st century. While regional socioeconomic impacts might be
large, damages would be probably small in relation to the respective gross national products. Terrestrial ecosystem productivity
is affected much more by the fertilization from the increasing CO2 concentration than by a THC shutdown. In addition, the level of warming in the 22nd to 24th century favours crop production
in northern Europe a lot, no matter whether the THC shuts down or not. CO2 emissions corridors aimed at limiting the risk of a THC breakdown to 10% or less are narrow, requiring departure from business-as-usual
in the next few decades. The uncertainty about THC risks is still high. This is seen in model analyses as well as in the experts’
views that were elicited. The overview of results presented here is the outcome of the Integrated Assessment project INTEGRATION. 相似文献
9.
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
10.
Two climatic states and feedbacks on thermohaline circulation in an Earth system model of intermediate complexity 总被引:5,自引:0,他引:5
Zhaomin Wang 《Climate Dynamics》2005,25(2-3):299-314
The McGill Paleoclimate Model-2 (MPM-2) is employed to study climate–thermohaline circulation (THC) interactions in a pre
-industrial climate, with a special focus on the feedbacks on the THC from other climate system components. The MPM-2, a new
version of the MPM, has an extended model domain from 90S to 90N, active winds and no oceanic heat and freshwater flux adjustments.
In the MPM-2, there are mainly two stable modes for the Atlantic meridional overturning circulation (MOC) under the ‘present-day’
forcing (present-day solar forcing and the pre-industrial atmospheric CO2 level of 280 ppm). The ‘on’ mode has an active North Atlantic deep water formation, while the ‘off’ mode has no such deep
water formation. By comparing the ‘off’ mode climate state with its ‘on’ mode analogue, we find that there exist many large
differences between the two climate states, which originate from large changes in the oceanic meridional heat transports.
By suppressing or isolating each process associated with a continental ice sheet over North America, sea ice, the atmospheric
hydrological cycle and vegetation, feedbacks from these components on the Atlantic MOC are investigated. Sensitivity studies
investigating the role of varying continental ice growth and sea ice meridional transport in the resumption of the Atlantic
MOC are also carried out. The results show that a fast ice sheet growth and an enhanced southward sea ice transport significantly
favor the resumption of the Atlantic MOC in the MPM-2. In contrast to this, the feedback from the atmospheric hydrological
cycle is a weak positive one. The vegetation-albedo feedback could enhance continental ice sheet growth and thus could also
favor the resumption of the Atlantic MOC. However, before the shut-down of the Atlantic MOC, feedbacks from these components
on the Atlantic MOC are very weak. 相似文献
11.
We use a reduced complexity climate model with a three-dimensional ocean component and realistic topography to investigate
the effect of stratification-dependent mixing on the sensitivity of the North Atlantic subpolar gyre (SPG), and the Atlantic
meridional overturning circulation (AMOC), to idealized CO2 increase and peaking scenarios. The vertical diffusivity of the ocean interior is parameterized as κ ∼ N
−α, where N is the local buoyancy frequency. For all parameter values 0 ≤ α ≤ 3, we find the SPG, and subsequently the AMOC, to weaken
in response to increasing CO2 concentrations. The weakening is significantly stronger for α ≥ αcr ≈ 1.5. Depending on the value of α, two separate model states develop. These states remain different after the CO2 concentration is stabilized, and in some cases even after the CO2 concentration has been decreased again to the pre-industrial level. This behaviour is explained by a positive feedback between
stratification and mixing anomalies in the Nordic Seas, causing a persistent weakening of the SPG. 相似文献
12.
S. J. Kim 《Climate Dynamics》2004,22(6-7):639-651
The role of reduced atmospheric CO2 concentration and ice sheet topography plus its associated land albedo on the LGM climate is investigated using a coupled atmosphere-ocean-sea ice climate system model. The surface cooling induced by the reduced CO2 concentration is larger than that by the ice sheet topography plus other factors by about 30% for the surface air temperature and by about 100% for the sea surface temperature. A large inter-hemispheric asymmetry in surface cooling with a larger cooling in the Northern Hemisphere is found for both cases. This asymmetric inter-hemispheric temperature response is consistent in the ice sheet topography case with earlier studies using an atmospheric model coupled with a mixed-layer ocean representation, but contrasts with these results in the reduced CO2 case. The incorporation of ocean dynamics presumably leads to a larger snow and sea ice feedback as a result of the reduction in northward ocean heat transport, mainly as a consequence of the decrease in the North Atlantic overturning circulation by the substantial freshening of the North Atlantic convection regions. A reversed case is found in the Southern Ocean. Overall, the reduction in atmospheric CO2 concentration accounts for about 60% of the total LGM climate change. 相似文献
13.
参照Griffies et al.(2009)提出的海洋—海冰耦合模式参考试验(Coordinated Ocean-ice Reference Experiments,COREs),设计了一个800年积分的数值试验,对一个质量严格守恒的压力坐标海洋环流模式(Pressure Coordinate Ocean Model,PCOM1.0)的基本模拟性能进行了评估,并与观测资料和再分析资料进行了对比。结果表明,PCOM1.0模拟的温盐场和基本流场与COREs模式的模拟水平基本接近。其中,模拟的大西洋经向翻转流在45°N附近达到18 Sv(1 Sv=106 m3 s-1),与观测估计值接近;对海表面温度的模拟误差主要集中在北太平洋黑潮区和北大西洋湾流区等中高纬度急流区;模拟的热带太平洋温跃层过于深厚;模拟的经德雷克海峡的体积输送达130 Sv,比大部分COREs模式及再分析资料都更接近于观测估计值。 相似文献
14.
We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic. 相似文献
15.
The uptake and storage of anthropogenic carbon in the North Atlantic is investigated using different configurations of ocean
general circulation/carbon cycle models. We investigate how different representations of the ocean physics in the models,
which represent the range of models currently in use, affect the evolution of CO2 uptake in the North Atlantic. The buffer effect of the ocean carbon system would be expected to reduce ocean CO2 uptake as the ocean absorbs increasing amounts of CO2. We find that the strength of the buffer effect is very dependent on the model ocean state, as it affects both the magnitude
and timing of the changes in uptake. The timescale over which uptake of CO2 in the North Atlantic drops to below preindustrial levels is particularly sensitive to the ocean state which sets the degree
of buffering; it is less sensitive to the choice of atmospheric CO2 forcing scenario. Neglecting physical climate change effects, North Atlantic CO2 uptake drops below preindustrial levels between 50 and 300 years after stabilisation of atmospheric CO2 in different model configurations. Storage of anthropogenic carbon in the North Atlantic varies much less among the different
model configurations, as differences in ocean transport of dissolved inorganic carbon and uptake of CO2 compensate each other. This supports the idea that measured inventories of anthropogenic carbon in the real ocean cannot
be used to constrain the surface uptake. Including physical climate change effects reduces anthropogenic CO2 uptake and storage in the North Atlantic further, due to the combined effects of surface warming, increased freshwater input,
and a slowdown of the meridional overturning circulation. The timescale over which North Atlantic CO2 uptake drops to below preindustrial levels is reduced by about one-third, leading to an estimate of this timescale for the
real world of about 50 years after the stabilisation of atmospheric CO2. In the climate change experiment, a shallowing of the mixed layer depths in the North Atlantic results in a significant
reduction in primary production, reducing the potential role for biology in drawing down anthropogenic CO2. 相似文献
16.
Reducing the risk of Atlantic thermohaline circulation collapse: sensitivity analysis of emissions corridors 总被引:1,自引:1,他引:0
We present emissions corridors for the 21st century reducing the risk of collapse of the Atlantic thermohaline circulation (THC) while considering expectations about the socio-economically acceptable pace of emissions reductions. Emissions corridors embrace the range of CO2 emissions that are compatible with normatively defined policy goals or ‘guardrails’. They are calculated along the conceptual and methodological lines of the tolerable windows approach. We investigate the sensitivity of the emissions corridors to key uncertain physical quantities (i.e. climate sensitivity and North Atlantic hydrological sensitivity, emissions of non-CO2 greenhouse gases and sulfate aerosols) as well as to the guardrails. Results indicate a large dependency of the width of the emissions corridor on climate and hydrological sensitivity: for low values of the climate and/or hydrological sensitivity, the corridor boundaries are far from being transgressed by business-as-usual emissions scenarios for the 21st century. In contrast, for high values of both quantities already low non-intervention scenarios leave the corridor in the early decades of this century. The width of the CO2 emissions corridor is also affected by the emissions pathway of non-CO2 greenhouse gases and sulfate aerosols, but to a lesser extent. We further find that the choice of the policy goal strongly influences the shape of the emissions corridor. Pursuit of a more ambitious THC target, for instance, tightens the corridor considerably. More strict expectations concerning the socio-economically admissible pace of emissions reduction (expressed in terms of a maximum emissions reduction rate and a transition time towards a de-carbonizing economy) act in the same direction. This indicates that a trade-off between THC and socio-economic guardrails may be unavoidable in the case of very tight emissions corridors. 相似文献
17.
Glacial termination: sensitivity to orbital and CO2 forcing in a coupled climate system model 总被引:1,自引:0,他引:1
To study glacial termination and related feedback mechanisms, a continental ice dynamics model is globally and asynchronously
coupled to a physical climate (atmosphere-ocean-sea ice) model. The model performs well under present-day, 11 kaBP (thousand
years before present) and 21 kaBP perpetual forcing. To address the ice-sheet response under the effects of both perpetual
orbital and CO2 forcing, sensitivity experiments are conducted with two different orbital configurations (11 kaBP and 21 kaBP) and two different
atmospheric CO2 concentrations (200 ppmv and 280 ppmv). This study reveals that, although both orbital and CO2 forcing have an impact on ice-sheet maintenance and deglacial processes, and although neither acting alone is sufficient
to lead to complete deglaciation, orbital forcing seems to be more important. The CO2 forcing has a large impact on climate, not uniformly or zonally over the globe, but concentrated over the continents adjacent
to the North Atlantic. The effect of increased CO2 (from 200 ppmv to 280 ppmv) on surface air temperature has its peak there in winter associated with a reduction in sea-ice
extent in the northern North Atlantic. These changes are accompanied by an enhancement in the intensity of the meridional
overturning and poleward ocean heat transport in the North Atlantic. On the other hand, the effect of orbital forcing (from
21 kaBP to 11 kaBP) has its peak in summer. Since the summer temperature, rather than winter temperature, is found to be dominant
for the ice-sheet mass balance, orbital forcing has a larger effect than CO2 forcing in deglaciation. Warm winter sea surface temperature arising from increased CO2 during the deglaciation contributes to ice-sheet nourishment (negative feedback for ice-sheet retreat) through slightly enhanced
precipitation. However, the precipitation effect is totally overwhelmed by the temperature effect. Our results suggest that
the last deglaciation was initiated through increasing summer insolation with CO2 providing a powerful feedback.
Received: 22 February 2000 / Accepted: 17 September 2000 相似文献
18.
J Jouzel R Vaikmae J R Petit M Martin Y Duclos M Stievenard C Lorius M Toots M A Mélières L H Burckle N I Barkov V M Kotlyakov 《Climate Dynamics》1995,11(3):151-161
The two-step character of the last deglaciation is well recognized in Western Europe, in Greenland and in the North Atlantic. For example, in Greenland, a gradual temperature decrease started at the Bölling (B) around 14.5 ky BP, spanned through the Alleröd (A) and was followed by the cold Younger Dryas (YD) event which terminated abruptly around 11.5 ky BP. Recent results suggest that this BA/YD sequence may have extended throughout all the Northern Hemisphere but the evidence of a late transition cooling is still poor for the Southern Hemisphere. Here we present a detailed isotopic record analyzed in a new ice core drilled at Dome B in East Antarctica that fully demonstrates the existence of an Antarctic cold reversal (ACR). These results suggest that the two-step shape of the last deglaciation has a worldwide character but they also point to noticeable interhemispheric differences. Thus, the coldest part of the ACR, which shows a temperature drop about three times weaker than that recorded during the YD in Greenland, may have preceded the YD. Antarctica did not experienced abrupt changes and the two warming periods started there before they started in Greenland. The links between Southern and Northern Hemisphere climates throughout this period are discussed in the light of additional information derived from the Antarctic dust record. 相似文献
19.
Katrin J. Meissner Michael Eby Andrew J. Weaver Oleg A. Saenko 《Climate Dynamics》2008,30(2-3):161-174
We present several equilibrium runs under varying atmospheric CO2 concentrations using the University of Victoria Earth System Climate Model (UVic ESCM). The model shows two very different
responses: for CO2 concentrations of 400 ppm or lower, the system evolves into an equilibrium state. For CO2 concentrations of 440 ppm or higher, the system starts oscillating between a state with vigorous deep water formation in
the Southern Ocean and a state with no deep water formation in the Southern Ocean. The flushing events result in a rapid increase in atmospheric temperatures, degassing of CO2 and therefore an increase in atmospheric CO2 concentrations, and a reduction of sea ice cover in the Southern Ocean. They also cool the deep ocean worldwide. After the
flush, the deep ocean warms slowly again and CO2 is taken up by the ocean until the stratification becomes unstable again at high latitudes thousands of years later. The
existence of a threshold in CO2 concentration which places the UVic ESCM in either an oscillating or non-oscillating state makes our results intriguing.
If the UVic ESCM captures a mechanism that is present and important in the real climate system, the consequences would comprise
a rapid increase in atmospheric carbon dioxide concentrations of several tens of ppm, an increase in global surface temperature
of the order of 1–2°C, local temperature changes of the order of 6°C and a profound change in ocean stratification, deep water
temperature and sea ice cover. 相似文献
20.
CO2 and Northern Hemisphere ice volume variations over the middle and late Quaternary 总被引:2,自引:1,他引:1
The atmospheric CO2 concentrations have been reconstructed over the past 600 ka based on regression between the Vostok CO2 data and the SPECMAP oxygen isotope values. A lag of 4.5 ka (CO2 preceding δ18O) gives the best results. A polynomial of order 5 explains 66% of the Vostok CO2 variance over the last 220 ka. The Northern Hemisphere ice-sheet volume was simulated over the past 575 ka using the LLN
2-D model, forced by insolation and these statistically reconstructed atmospheric CO2 concentrations. The simulated ice volume fluctuations resemble the deep-sea oxygen isotope variations. CO2 of interglacial level is necessary for explaining both the interglacial at oxygen isotopic stage 11 and our present-day interglacial. 相似文献