共查询到20条相似文献,搜索用时 687 毫秒
1.
Reinhard Calov Andrey Ganopolski Vladimir Petoukhov Martin Claussen Victor Brovkin Claudia Kubatzki 《Climate Dynamics》2005,24(6):563-576
The sensitivity of the last glacial-inception (around 115 kyr BP, 115,000 years before present) to different feedback mechanisms
has been analysed by using the Earth system model of intermediate complexity CLIMBER-2. CLIMBER-2 includes dynamic modules
of the atmosphere, ocean, terrestrial biosphere and inland ice, the last of which was added recently by utilising the three-dimensonal
polythermal ice-sheet model SICOPOLIS. We performed a set of transient experiments starting at the middle of the Eemiam interglacial
and ran the model for 26,000 years with time-dependent orbital forcing and observed changes in atmospheric CO2 concentration (CO2 forcing). The role of vegetation and ocean feedback, CO2 forcing, mineral dust, thermohaline circulation and orbital insolation were closely investigated. In our model, glacial inception,
as a bifurcation in the climate system, appears in nearly all sensitivity runs including a run with constant atmospheric CO2 concentration of 280 ppmv, a typical interglacial value, and simulations with prescribed present-day sea-surface temperatures
or vegetation cover—although the rate of the growth of ice-sheets growth is smaller than in the case of the fully interactive
model. Only if we run the fully interactive model with constant present-day insolation and apply present-day CO2 forcing does no glacial inception appear at all. This implies that, within our model, the orbital forcing alone is sufficient
to trigger the interglacial–glacial transition, while vegetation, ocean and atmospheric CO2 concentration only provide additional, although important, positive feedbacks. In addition, we found that possible reorganisations
of the thermohaline circulation influence the distribution of inland ice. 相似文献
2.
Orbital forcing of the climate system is clearly shown in the Earths record of glacial–interglacial cycles, but the mechanism underlying this forcing is poorly understood. Traditional Milankovitch theory suggests that these cycles are driven by changes in high latitude summer insolation, yet this forcing is dominated by precession, and cannot account for the importance of obliquity in the Ice Age record. Here, we investigate an alternative forcing based on the latitudinal insolation gradient (LIG), which is dominated by both obliquity (in summer) and precession (in winter). The insolation gradient acts on the climate system through differential solar heating, which creates the Earths latitudinal temperature gradient (LTG) that drives the atmospheric and ocean circulation. A new pollen-based reconstruction of the LTG during the Holocene is used to demonstrate that the LTG may be much more sensitive to changes in the LIG than previously thought. From this, it is shown how LIG forcing of the LTG may help explain the propagation of orbital signatures throughout the climate system, including the Monsoon, Arctic Oscillation and ocean circulation. These relationships are validated over the last (Eemian) Interglacial, which occurred under a different orbital configuration to the Holocene. We conclude that LIG forcing of the LTG explains many criticisms of classic Milankovitch theory, while being poorly represented in climate models. 相似文献
3.
H.-S. Liu 《Theoretical and Applied Climatology》1998,61(3-4):217-229
Summary For astronomical seasons, Rubincam insolation deviations at latitude 65° N varied from 218.50 Wm−2 to 225.75 Wm−2 (3%). The periodicity of the insolation cycles varied from 36.7 Kyr to 44.7 Kyr (20%) due to phase shift. Phase shift of
insolation variations can induce asymmetry of the insolation cycles, permitting rapid melting and prolonged glaciation of
ice sheets to occur. For instance, an abnormal decrease of the insolation frequency during the longer period of glacial interval
would prolong glaciation into deep ice age. In this study, we apply Rubincam’s insolation equations to investigate the phase
shift effect of insolation variations on climate change. Using complex transforms of the changing insolation, we have detected
a phase modulation signal in the insolation variations. As a result, an especially new and interesting series of the phase-related
insolation pulsation is established. The phase modulated insolation is then introduced as a forcing function into energy balance
climate models. Results of model computations shed new insights into the spectrum of the paleoclimatic proxy-data. It is shown
that phase modulation of the insolation may provide an appropriate and complete external forcing mechanism to which the climate
system would respond. The 100 Kyr cycle of the frequency modulation of the Rubincam’s insolation variations does seem adequate
to change the climate.
Received July 16, 1997 Revised May 18, 1998 相似文献
4.
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
5.
The dynamics of glacial cycles is studied in terms of the dynamical systems theory. We explore the dependence of the climate state on the phase of the astronomical forcing by examining five conceptual models of glacial cycles proposed in the literature. The models can be expressed as quasiperiodically forced dynamical systems. It is shown that four of them exhibit a strange nonchaotic attractor (SNA), which is an intermediate regime between quasiperiodicity and chaos. Then, the dependence of the climate state on the phase of the astronomical forcing is not given by smooth relations, but constitutes a geometrically strange set. Our result suggests that SNA is a candidate for representing the dynamics of glacial cycles, in addition to well-known quasiperiodicity and chaos. 相似文献
6.
Transient simulation of the last glacial inception. Part I: glacial inception as a bifurcation in the climate system 总被引:2,自引:2,他引:0
Reinhard Calov Andrey Ganopolski Martin Claussen Vladimir Petoukhov Ralf Greve 《Climate Dynamics》2005,24(6):545-561
We study the mechanisms of glacial inception by using the Earth system model of intermediate complexity, CLIMBER-2, which
encompasses dynamic modules of the atmosphere, ocean, biosphere and ice sheets. Ice-sheet dynamics are described by the three-dimensional
polythermal ice-sheet model SICOPOLIS. We have performed transient experiments starting at the Eemiam interglacial, at 126 ky
BP (126,000 years before present). The model runs for 26 kyr with time-dependent orbital and CO2 forcings. The model simulates a rapid expansion of the area covered by inland ice in the Northern Hemisphere, predominantly
over Northern America, starting at about 117 kyr BP. During the next 7 kyr, the ice volume grows gradually in the model at
a rate which corresponds to a change in sea level of 10 m per millennium. We have shown that the simulated glacial inception
represents a bifurcation transition in the climate system from an interglacial to a glacial state caused by the strong snow-albedo
feedback. This transition occurs when summer insolation at high latitudes of the Northern Hemisphere drops below a threshold
value, which is only slightly lower than modern summer insolation. By performing long-term equilibrium runs, we find that
for the present-day orbital parameters at least two different equilibrium states of the climate system exist—the glacial and
the interglacial; however, for the low summer insolation corresponding to 115 kyr BP, we find only one, glacial, equilibrium
state, while for the high summer insolation corresponding to 126 kyr BP only an interglacial state exists in the model.
相似文献
Reinhard CalovEmail: |
7.
The response of the LLN 2-D climate model to the insolation and CO2 forcings during the Eemian interglacial is compared to reconstructions obtained from deep-sea cores drilled in the Norwegian
Sea and in the North Atlantic. Both reconstructions and modeling results show a decrease of sea-surface temperature (SST)
in the higher latitudes (70–75 °N zonal belt for the model and the Norwegian Sea for the proxy records), associated with a
more moderate cooling at lower latitudes (50–55 °N and North Atlantic), at the middle of isotopic substage 5e, several millenia
before the beginning of continental ice-sheet growth. Such a comparison between the simulated SST and ice volume of the Northern
Hemisphere has been extended to the whole last glacial-interglacial cycle. The influence of the insolation forcing on SST
and the shortcomings of the model due to its zonal character are discussed.
Received: 6 July 1995/Accepted: 19 December 1995 相似文献
8.
J. Oerlemans 《Climatic change》1982,4(4):353-374
An attempt is made to simulate the Pleistocene glacial cycles with a numerical model of the Northern Hemisphere ice sheets.
This model treats the vertically-integrated ice flow along a meridian, including computation of bedrock adjustment and temperature
distribution in the ice. Basal melt water is traced and controls ice-mass discharge.
The model produces asymmetric glacial cycles, even when it is not forced. Model parameters can be chosen such that cycles
with a duration of about 100 000 yr occur. Due to the production of basal melt water and bedrock sinking, deglaciations are
very rapid.
The occurrence of glacial cycles in the model is a stable feature, but thephase of the cycles is very sensitive to the model parameters. The main conclusion is that ice-sheet dynamics may provide an explanation
for the Pleistocene glacial cycles. However, the ‘predictability’ of the ice-volume record appears to be small. 相似文献
9.
The notion is pervasive in the climate science community and in the public at large that the climate impacts of fossil fuel
CO2 release will only persist for a few centuries. This conclusion has no basis in theory or models of the atmosphere/ocean carbon
cycle, which we review here. The largest fraction of the CO2 recovery will take place on time scales of centuries, as CO2 invades the ocean, but a significant fraction of the fossil fuel CO2, ranging in published models in the literature from 20–60%, remains airborne for a thousand years or longer. Ultimate recovery
takes place on time scales of hundreds of thousands of years, a geologic longevity typically associated in public perceptions
with nuclear waste. The glacial/interglacial climate cycles demonstrate that ice sheets and sea level respond dramatically
to millennial-timescale changes in climate forcing. There are also potential positive feedbacks in the carbon cycle, including
methane hydrates in the ocean, and peat frozen in permafrost, that are most sensitive to the long tail of the fossil fuel
CO2 in the atmosphere. 相似文献
10.
Claudia Kubatzki Martin Claussen Reinhard Calov Andrey Ganopolski 《Climate Dynamics》2006,27(4):333-344
We investigate the sensitivity of simulations of the last glacial inception (LGI) with respect to initial (size of the Greenland ice sheet) and surface (state of ocean/vegetation) conditions and two different CO2 reconstructions. Utilizing the CLIMBER-2 Earth system model, we obtain the following results: (a) ice-sheet expansion in North America at the end of the Eemian can be reduced or even completely suppressed when pre-industrial or Eemian ocean/vegetation is prescribed. (b) A warmer surrounding ocean and, in particular, a large Laurentide ice sheet reduce the size of the Greenland ice sheet before and during the LGI. (c) A changing ocean contributes much stronger to the expansion of the Laurentide ice sheet when we apply the CO2 reconstruction according to Barnola et al. (Nature 329:408–414, 1987) instead of Petit et al. (Nature 399:429–436, 1999). (d) In the fully coupled model, the CO2 reconstruction used has only a small impact on the simulated ice sheets but it does impact the course of the climatic variables. (e) For the Greenland ice sheet, two equilibrium states exist under the insolation and CO2 forcing at 128,000 years before present (128 kyear BP); the one with an ice sheet reduced by about one quarter as compared to its simulated pre-industrial size and the other with nearly no inland ice in Greenland. (f) Even the extreme assumption of no ice sheet in Greenland at the beginning of our transient simulations does not alter the simulated expansion of northern hemispheric ice sheets at the LGI. 相似文献
11.
A general circulation model is used to examine the effects of reduced atmospheric CO2, insolation changes and an updated reconstruction of the continental ice sheets at the Last Glacial Maximum (LGM). A set
of experiments is performed to estimate the radiative forcing from each of the boundary conditions. These calculations are
used to estimate a total radiative forcing for the climate of the LGM. The response of the general circulation model to the
forcing from each of the changed boundary conditions is then investigated. About two-thirds of the simulated glacial cooling
is due to the presence of the continental ice sheets. The effect of the cloud feedback is substantially modified where there
are large changes to surface albedo. Finally, the climate sensitivity is estimated based on the global mean LGM radiative
forcing and temperature response, and is compared to the climate sensitivity calculated from equilibrium experiments with
atmospheric CO2 doubled from present day concentration. The calculations here using the model and palaeodata support a climate sensitivity
of about 1 Wm-2 K-1 which is within the conventional range.
Received: 8 February 1997 / Accepted: 4 June 1997 相似文献
12.
We assess two parametrisations of sea-ice in a coupled atmosphere–mixed layer ocean–sea-ice model. One parametrisation represents
the thermodynamic properties of sea-ice formation alone (THERM), while the other also includes advection of the ice (DYN).
The inclusion of some sea-ice dynamics improves the model's simulation of the present day sea-ice cover when compared to observations.
Two climate change scenarios are used to investigate the effect of these different parametrisations on the model's climate
sensitivity. The scenarios are the equilibrium response to a doubling of atmospheric CO2 and the response to imposed glacial boundary conditions. DYN produces a smaller temperature response to a doubling of CO2 than THERM. The temperature response of THERM is more similar to DYN in the glacial case than in the 2×CO2 case which implies that the climate sensitivity of THERM and DYN varies with the nature of the forcing. The different responses
can largely be explained by the different distribution of Southern Hemisphere sea-ice cover in the control simulations, with
the inclusion of ice dynamics playing an important part in producing the differences. This emphasises the importance of realistically
simulating the reference climatic state when attempting to simulate a climate change to a prescribed forcing. The simulated
glacial sea-ice cover is consistent with the limited palaeodata in both THERM and DYN, but DYN simulates a more realistic
present day sea-ice cover. We conclude that the inclusion of simple ice dynamics in our model increases our confidence in
the simulation of the anomaly climate.
Received: 24 May 2000 / Accepted: 25 October 2000 相似文献
13.
Olivier Arzel Matthew H. England Alain Colin de Verdière Thierry Huck 《Climate Dynamics》2012,39(1-2):259-275
The origin and bifurcation structure of abrupt millennial-scale climate transitions under steady external solar forcing and in the absence of atmospheric synoptic variability is studied by means of a global coupled model of intermediate complexity. We show that the origin of Dansgaard-Oeschger type oscillations in the model is caused by the weaker northward oceanic heat transport in the Atlantic basin. This is in agreement with previous studies realized with much simpler models, based on highly idealized geometries and simplified physics. The existence of abrupt millennial-scale climate transitions during glacial times can therefore be interpreted as a consequence of the weakening of the negative temperature-advection feedback. This is confirmed through a series of numerical experiments designed to explore the sensitivity of the bifurcation structure of the Atlantic meridional overturning circulation to increased atmospheric CO2 levels under glacial boundary conditions. Contrasting with the cold, stadial, phases of millennial oscillations, we also show the emergence of strong interdecadal variability in the North Atlantic sector during warm interstadials. The instability driving these interdecadal-interstadial oscillations is shown to be identical to that found in ocean-only models forced by fixed surface buoyancy fluxes, that is, a large-scale baroclinic instability developing in the vicinity of the western boundary current in the North Atlantic. Comparisons with modern observations further suggest a physical mechanism similar to that driving the 30–40?years time scale associated with the Atlantic multidecadal oscillation. 相似文献
14.
Time-dependent response of a zonally averaged ocean–atmosphere–sea ice model to Milankovitch forcing
An ocean–atmosphere–sea ice model is developed to explore the time-dependent response of climate to Milankovitch forcing for
the time interval 5–3 Myr BP. The ocean component is a zonally averaged model of the circulation in five basins (Arctic, Atlantic,
Indian, Pacific, and Southern Oceans). The atmospheric component is a one-dimensional (latitudinal) energy balance model,
and the sea-ice component is a thermodynamic model. Two numerical experiments are conducted. The first experiment does not
include sea ice and the Arctic Ocean; the second experiment does. Results from the two experiments are used to investigate
(1) the response of annual mean surface air and ocean temperatures to Milankovitch forcing, and (2) the role of sea ice in
this response. In both experiments, the response of air temperature is dominated by obliquity cycles at most latitudes. On
the other hand, the response of ocean temperature varies with latitude and depth. Deep water formed between 45°N and 65°N
in the Atlantic Ocean mainly responds to precession. In contrast, deep water formed south of 60°S responds to obliquity when
sea ice is not included. Sea ice acts as a time-integrator of summer insolation changes such that annual mean sea-ice conditions
mainly respond to obliquity. Thus, in the presence of sea ice, air temperature changes over the sea ice are amplified, and
temperature changes in deep water of southern origin are suppressed since water below sea ice is kept near the freezing point. 相似文献
15.
The stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean—atmosphere—sea ice coupled model. It turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates. The large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further South. The nature of the instability is of oceanic origin, identical to that found in ocean models under mixed boundary conditions. With similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates, the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters, the deep water temperatures being regulated by the temperature of freezing. Weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation. Within the unstable regime periodic millennial oscillations occur spontaneously. The climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients. Both states are unstable. The atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content. During the abrupt warming events that punctuate the course of a millennial oscillation, sea ice variations are shown respectively to damp (amplify) the amplitude of the oceanic (atmospheric) response. This sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period, the so called Dansgaard—Oeschger events, may be internal instabilities of the climate system. 相似文献
16.
Sea ice induced changes in ocean circulation during the Eemian 总被引:1,自引:1,他引:0
We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes. 相似文献
17.
A multi-model analysis of the role of the ocean on the African and Indian monsoon during the mid-Holocene 总被引:1,自引:6,他引:1
Y. Zhao P. Braconnot O. Marti S.P. Harrison C. Hewitt A. Kitoh Z. Liu U. Mikolajewicz B. Otto-Bliesner S.L. Weber 《Climate Dynamics》2005,25(7-8):777-800
We investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid-Holocene (6,000 year
BP) using results from seven coupled ocean–atmosphere general circulation models. We examine how the dipole in late summer
sea-surface temperature (SST) anomalies in the tropical Atlantic increases the length of the African monsoon, how this dipole
structure is created and maintained, and how the late summer SST warming in the northwest Indian Ocean affects the monsoon
retreat in this sector. Similar mechanisms are found in all of the models, including a strong wind evaporation feedback and
changes in the mixed layer depth that enhance the insolation forcing, as well as increased Ekman transport in the Atlantic
that sharpens the Atlantic dipole pattern. We also consider changes in interannual variability over West Africa and the Indian
Ocean. The teleconnection between variations in SST and Sahelian precipitation favor a larger impact of the Atlantic dipole
mode in this region. In the Indian Ocean, the strengthening of the Indian dipole structure in autumn has a damping effect
on the Indian dipole mode at the interannual time scale. 相似文献
18.
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
19.
Matthew C. Wyant Christopher S. Bretherton Julio T. Bacmeister Jeffrey T. Kiehl Isaac M. Held Ming Zhao Stephen A. Klein Brian J. Soden 《Climate Dynamics》2006,27(2-3):261-279
Low-latitude cloud distributions and cloud responses to climate perturbations are compared in near-current versions of three leading U.S. AGCMs, the NCAR CAM 3.0, the GFDL AM2.12b, and the NASA GMAO NSIPP-2 model. The analysis technique of Bony et al. (Clim Dyn 22:71–86, 2004) is used to sort cloud variables by dynamical regime using the monthly mean pressure velocity ω at 500 hPa from 30S to 30N. All models simulate the climatological monthly mean top-of-atmosphere longwave and shortwave cloud radiative forcing (CRF) adequately in all ω-regimes. However, they disagree with each other and with ISCCP satellite observations in regime-sorted cloud fraction, condensate amount, and cloud-top height. All models have too little cloud with tops in the middle troposphere and too much thin cirrus in ascent regimes. In subsidence regimes one model simulates cloud condensate to be too near the surface, while another generates condensate over an excessively deep layer of the lower troposphere. Standardized climate perturbation experiments of the three models are also compared, including uniform SST increase, patterned SST increase, and doubled CO2 over a mixed layer ocean. The regime-sorted cloud and CRF perturbations are very different between models, and show lesser, but still significant, differences between the same model simulating different types of imposed climate perturbation. There is a negative correlation across all general circulation models (GCMs) and climate perturbations between changes in tropical low cloud cover and changes in net CRF, suggesting a dominant role for boundary layer cloud in these changes. For some of the cases presented, upper-level clouds in deep convection regimes are also important, and changes in such regimes can either reinforce or partially cancel the net CRF response from the boundary layer cloud in subsidence regimes. This study highlights the continuing uncertainty in both low and high cloud feedbacks simulated by GCMs. 相似文献
20.
L. G. Bell 《Theoretical and Applied Climatology》2003,74(3-4):245-253
Summary ?Evidence is presented for a previously unknown climate cycle of 30,000 yr period. The cycle is deemed to be related to the
gyroscopic precession, or wobble of the Earth axis. Since it inhibits glaciation, the 30,000 yr cycle is called the summer
cycle while its counterpart, the 22,000 yr “precession” cycle, is the winter cycle. Because of the aspect presented to the
Sun, summer is effectively longer than winter. This is used to explain the difference between summer and winter cycle periods
and that of the wobble. Some of the problems encountered in interpreting oxygen ratio glaciation data are resolved by knowledge
of the existence of the summer cycle and a means is devised for determining the relative volume of glacial ice. An argument
is made that essentially eliminates the 100,000 yr-orbit cycle, by itself or in combination with other attitude cycles, as
a possible cause of the glacial/interglacial cycles.
Received August 17, 2002; accepted September 3, 2002 相似文献