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1.
Sensitivity of climate change projections to uncertainties in the estimates of observed changes in deep-ocean heat content 总被引:1,自引:0,他引:1
The MIT 2D climate model is used to make probabilistic projections for changes in global mean surface temperature and for thermosteric sea level rise under a variety of forcing scenarios. The uncertainties in climate sensitivity and rate of heat uptake by the deep ocean are quantified by using the probability distributions derived from observed twentieth century temperature changes. The impact on climate change projections of using the smallest and largest estimates of twentieth century deep ocean warming is explored. The impact is large in the case of global mean thermosteric sea level rise. In the MIT reference (“business as usual”) scenario the median rise by 2100 is 27 and 43 cm in the respective cases. The impact on increases in global mean surface air temperature is more modest, 4.9 and 3.9 C in the two respective cases, because of the correlation between climate sensitivity and ocean heat uptake required by twentieth century surface and upper air temperature changes. The results are also compared with the projections made by the IPCC AR4’s multi-model ensemble for several of the SRES scenarios. The multi-model projections are more consistent with the MIT projections based on the largest estimate of ocean warming. However, the range for the rate of heat uptake by the ocean suggested by the lowest estimate of ocean warming is more consistent with the range suggested by the twentieth century changes in surface and upper air temperatures, combined with the expert prior for climate sensitivity. 相似文献
2.
A flexible climate model for use in integrated assessments 总被引:2,自引:0,他引:2
Because of significant uncertainty in the behavior of the climate system, evaluations of the possible impact of an increase
in greenhouse gas concentrations in the atmosphere require a large number of long-term climate simulations. Studies of this
kind are impossible to carry out with coupled atmosphere ocean general circulation models (AOGCMs) because of their tremendous
computer resource requirements. Here we describe a two dimensional (zonally averaged) atmospheric model coupled with a diffusive
ocean model developed for use in the integrated framework of the Massachusetts Institute of Technology (MIT) Joint Program
on the Science and Policy of Global Change. The 2-D model has been developed from the Goddard Institute for Space Studies
(GISS) GCM and includes parametrizations of all the main physical processes. This allows it to reproduce many of the nonlinear
interactions occurring in simulations with GCMs. Comparisons of the results of present-day climate simulations with observations
show that the model reasonably reproduces the main features of the zonally averaged atmospheric structure and circulation.
The model’s sensitivity can be varied by changing the magnitude of an inserted additional cloud feedback. Equilibrium responses
of different versions of the 2-D model to an instantaneous doubling of atmospheric CO2 are compared with results of similar simulations with different AGCMs. It is shown that the additional cloud feedback does
not lead to any physically inconsistent results. On the contrary, changes in climate variables such as precipitation and evaporation,
and their dependencies on surface warming produced by different versions of the MIT 2-D model are similar to those shown by
GCMs. By choosing appropriate values of the deep ocean diffusion coefficients, the transient behavior of different AOGCMs
can be matched in simulations with the 2-D model, with a unique choice of diffusion coefficients allowing one to match the
performance of a given AOGCM for a variety of transient forcing scenarios. Both surface warming and sea level rise due to
thermal expansion of the deep ocean in response to a gradually increasing forcing are reasonably reproduced on time scales
of 100–150 y. However a wide range of diffusion coefficients is needed to match the behavior of different AOGCMs. We use results
of simulations with the 2-D model to show that the impact on climate change of the implied uncertainty in the rate of heat
penetration into the deep ocean is comparable with that of other significant uncertainties.
Received: 10 March 1997 / Accepted: 20 October 1997 相似文献
3.
C. J. C. Reason 《Meteorology and Atmospheric Physics》1996,61(1-2):1-18
Summary Parameterisations of mixing induced through shear instability, internal wave breaking, and double diffusion are investigated in simulations of ocean climate using a global ocean general circulation model (OGCM). Focus is placed on the sensitivity of the large scale circulation, water mass formation and transport of heat as measures of the model's ability to represent current climate. The model resolution is typical of OGCMs being coupled to atmospheric. GCMs in climate models and the parameterisations investigated are all computationally inexpensive enough to allow for integrations on long time scales. Under the assumption of constant vertical eddy coefficients (the control case), the model climatology displays acceptable values of North Atlantic Deep Water formation, Antarctic Circumpolar Current (ACC) transport, and Indonesian through-flow but an excessively deep and diffuse pycnocline structure with weak stratification in the deep ocean. It is found that various circulation and water mass properties are sensitive to the choice of parameterisation of vertical mixing and that determining a scheme which works satisfactorily over all regions (tropical, mid-latitude, and polar) of the domain is not straightforward. Parameterisations of internal wave breaking or upper ocean shear instability lead to some improvements in the model water mass formation. ACC and poleward heat transport when compared to the control case whereas parameterisations of double diffusive processes did not. Based on these and other results, various recommendations are made for mixing parameterisations in ocean climate models.With 8 Figures 相似文献
4.
The results from an integration of a global ocean circulation model have been condensed into an analysis of the volume, heat, and salt transports among the major ocean basins. Transports are also broken down between the model's Ekman, thermocline, and deep layers. Overall, the model does well. Horizontal exchanges of mass, heat, and salt between ocean basins have reasonable values; and the volume of North Atlantic Deep Water (NADW) transport is in general agreement with what limited observations exist. On a global basis the zonally integrated meridional heat transport is poleward at all latitudes except for the latitude band 30°S to 45°S. This anomalous transport is most likely a signature of the model's inability to form Antarctic Intermediate (AAIW) and Antarctic bottom water (AABW) properly. Eddy heat transport is strong at the equator where its convergence heats the equatorial Pacific about twice as much as it heats the equatorial Atlantic. The greater heating in the Pacific suggests that mesoscale eddies may be a vital mechanism for warming and maintaining an upwelling portion of the global conveyor-belt circulation. The model's fresh water transport compares well with observations. However, in the Atlantic there is an excessive southward transport of fresh water due to the absence of the Mediterranean outflow and weak northward flow of AAIW. Eddies in the mid-latitudes act to redistribute heat and salt down the mean gradients. Residual fluxes calculated from a sum of the computed advective (including eddies), forced, and stored fluxes of heat and salt represent transport mostly due to vertical sub-grid scale mixing processes. Perhaps the model's greatest weakness is the lack of strong AAIW and AABW circulation cells. Accurate thermohaline forcing in the North Atlantic (based on numerous hydrographic observations) helps the model adequately produce NADW. In contrast, the southern ocean is an area of sparse observation. Better thermohaline observations in this area may be needed if models such as this are to produce the deep convection that will achieve more accurate simulations of the global 3-dimensional circulation. 相似文献
5.
海洋在气候变暖过程中的重要性通常用海洋热吸收来衡量,热吸收的大小影响全球变暖的幅度。本文利用FGOALS-g2、FGOALS-s2(以下分别缩写为g2、s2)两个耦合模式的CO2浓度以每年1%速率增长(1pctCO2)试验,评估和分析海洋热吸收与气候敏感度的关系。结果表明:进入海洋净热通量(s2模式大于g2模式)会使得s2模式的海洋热吸收总体比g2模式大;更为重要的是,由于s2模式中的海洋热吸收主要集中在上层,使得耦合模式s2中的瞬态气候响应(TCR,或称气候敏感度)比g2大。当CO2浓度加倍时,在两个耦合模式中,海洋热吸收的空间分布呈现显著性的差异,s2模式中上层热吸收明显比深层大,上层热吸收主要位于太平洋和印度洋,而g2模式中上层和深层热吸收差别较小,深层主要位于大西洋和北冰洋。进一步研究表明,海洋热吸收分布特征与两个耦合模式海洋环流变化有关。在g2模式中北大西洋经圈翻转环流(AMOC)强度强且深度大,在CO2浓度加倍时,AMOC减弱小,这样AMOC可将热量带到海洋的深层,增加海洋深层热吸收。而在s2模式中,平均AMOC弱且浅,在CO2浓度加倍时,AMOC减弱明显,热量不易到达深层,主要集中在海洋上层,对气候敏感度影响更快且更强。海洋环流导致热吸收及其空间差异同时影响到气候敏感度的差异。因此,探讨海洋热吸收与气候敏感度之间的关系,利于明确气候敏感度不确定性的来源。 相似文献
6.
G. C. Hegerl K. Hasselmann U. Cubasch J. F. B. Mitchell E. Roeckner R. Voss J. Waszkewitz 《Climate Dynamics》1997,13(9):613-634
A multi-fingerprint analysis is applied to the detection and attribution of anthropogenic climate change. While a single
fingerprint is optimal for the detection of climate change, further tests of the statistical consistency of the detected climate
change signal with model predictions for different candidate forcing mechanisms require the simultaneous application of several
fingerprints. Model-predicted climate change signals are derived from three anthropogenic global warming simulations for the
period 1880 to 2049 and two simulations forced by estimated changes in solar radiation from 1700 to 1992. In the first global
warming simulation, the forcing is by greenhouse gas only, while in the remaining two simulations the direct influence of
sulfate aerosols is also included. From the climate change signals of the greenhouse gas only and the average of the two greenhouse
gas-plus-aerosol simulations, two optimized fingerprint patterns are derived by weighting the model-predicted climate change
patterns towards low-noise directions. The optimized fingerprint patterns are then applied as a filter to the observed near-surface
temperature trend patterns, yielding several detection variables. The space-time structure of natural climate variability
needed to determine the optimal fingerprint pattern and the resultant signal-to-noise ratio of the detection variable is estimated
from several multi-century control simulations with different CGCMs and from instrumental data over the last 136 y. Applying
the combined greenhouse gas-plus-aerosol fingerprint in the same way as the greenhouse gas only fingerprint in a previous
work, the recent 30-y trends (1966–1995) of annual mean near surface temperature are again found to represent a significant
climate change at the 97.5% confidence level. However, using both the greenhouse gas and the combined forcing fingerprints
in a two-pattern analysis, a substantially better agreement between observations and the climate model prediction is found
for the combined forcing simulation. Anticipating that the influence of the aerosol forcing is strongest for longer term temperature
trends in summer, application of the detection and attribution test to the latest observed 50-y trend pattern of summer temperature
yielded statistical consistency with the greenhouse gas-plus-aerosol simulation with respect to both the pattern and amplitude
of the signal. In contrast, the observations are inconsistent with the greenhouse-gas only climate change signal at a 95%
confidence level for all estimates of climate variability. The observed trend 1943–1992 is furthermore inconsistent with a
hypothesized solar radiation change alone at an estimated 90% confidence level. Thus, in contrast to the single pattern analysis,
the two pattern analysis is able to discriminate between different forcing hypotheses in the observed climate change signal.
The results are subject to uncertainties associated with the forcing history, which is poorly known for the solar and aerosol
forcing, the possible omission of other important forcings, and inevitable model errors in the computation of the response
to the forcing. Further uncertainties in the estimated significance levels arise from the use of model internal variability
simulations and relatively short instrumental observations (after subtraction of an estimated greenhouse gas signal) to estimate
the natural climate variability. The resulting confidence limits accordingly vary for different estimates using different
variability data. Despite these uncertainties, however, we consider our results sufficiently robust to have some confidence
in our finding that the observed climate change is consistent with a combined greenhouse gas and aerosol forcing, but inconsistent
with greenhouse gas or solar forcing alone.
Received: 28 April 1996 / Accepted: 27 January 1997 相似文献
7.
We describe a coupled climate model of intermediate complexity designed for use in global warming experiments. The atmospheric component is a two-dimensional (zonally averaged) statistical-dynamical model based on the Goddard Institute for Space Study's atmospheric general circulation model (GCM). In contrast with energy-balance models used in some climate models of intermediate complexity, this model includes full representation of the hydrological and momentum cycles. It also has parameterizations of the main physical processes, including a sophisticated radiation code. The ocean component is a coarse resolution ocean GCM with simplified global geometry based on the Geophysical Fluid Dynamics Laboratory modular ocean model. Because of the simplified geometry the resolution in the western boundary layers can be readily increased compared to conventional coarse resolution models, without increasing the model's computational requirements in a significant way. The ocean model's efficiency is also greatly increased by using a moderate degree of asynchronous coupling between the oceanic momentum and tracer fields. We demonstrate that this still allows an accurate simulation of transient behavior, including the seasonal cycle. A 100 years simulation with the model requires less than 8 hours on a state-of the art workstation. The main novelty of the model is therefore a combination of computational efficiency, statistical-dynamical atmosphere and 3D ocean. Long-term present-day climate simulations are carried out using the coupled model with and without flux adjustments, and with either the Gent-McWilliams (GM) parametrization scheme or horizontal diffusion (HD) in the ocean. Deep ocean temperatures systematically decrease in the runs without flux adjustment. We demonstrate that the mismatch between heat transports in the uncoupled states of two models is the main cause for the systematic drift. In addition, changes in the circulation and sea-ice formation also contribute to the drift. Flux adjustments in the freshwater fluxes are shown to have a stabilizing effect on the thermohaline circulation in the model, whereas the adjustments in the heat fluxes tend to weaken the global "conveyor". To evaluate the model's response to transient external forcing global warming simulations are also carried out with the flux-adjusted version of the coupled model. The coupled model reproduces reasonably well the behavior of more sophisticated coupled GCMs for both current climate and for the global warming scenarios. 相似文献
8.
Vertical heat transports in the ocean and their effect on time-dependent climate change 总被引:2,自引:0,他引:2
J. M. Gregory 《Climate Dynamics》2000,16(7):501-515
In response to increasing atmospheric concentrations of greenhouse gases, the rate of time-dependent climate change is determined
jointly by the strength of climate feedbacks and the efficiency of processes which remove heat from the surface into the deep
ocean. This work examines the vertical heat transport processes in the ocean of the HADCM2 atmosphere–ocean general circulation
model (AOGCM) in experiments with CO2 held constant (control) and increasing at 1 per year (anomaly). The control experiment shows that global average heat exchanges
between the upper and lower ocean are dominated by the Southern Ocean, where heat is pumped downwards by the wind-driven circulation
and diffuses upwards along sloping isopycnals. This is the reverse of the low-latitude balance used in upwelling–diffusion
ocean models, the global average upward diffusive transport being against the temperature gradient. In the anomaly experiment,
weakened convection at high latitudes leads to reduced diffusive and convective heat loss from the deep ocean, and hence to
net heat uptake, since the advective heat input is less affected. Reduction of deep water production at high latitudes results
in reduced upwelling of cold water at low latitudes, giving a further contribution to net heat uptake. On the global average,
high-latitude processes thus have a controlling influence. The important role of diffusion highlights the need to ensure that
the schemes employed in AOGCMs give an accurate representation of the relevant sub-grid-scale processes.
Received: 8 July 1999 / Accepted: 17 November 1999 相似文献
9.
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
10.
We report the analysis of two 20-year simulations performed with the low resolution version of the IPSL coupled ocean-atmosphere
model, with no flux correction at the air-sea interface. The simulated climate is characterized by a global sea surface temperature
warming of about 4 °C in 20 years, driven by a net heat gain at the top of the atmosphere. Despite this drift, the circulation
is quite realistic both in the ocean and the atmosphere. Several distinct periods are analyzed. The first corresponds to an
adjustment during which the heat gain weakens both at the top of the atmosphere and at the ocean surface, and the tropical
circulation is slightly modified. Then, the surface warming is enhanced by an increase of the greenhouse feedback. We show
that the mechanisms involved in the model share common features with sensitivity experiments to greenhouse gases or to SST
warming. At the top of the atmosphere, most of the longwave trapping in the atmosphere is driven by the tropical circulation.
At the surface, the reduction of longwave cooling is a direct response to increased temperature and moisture content at low
levels in the atmospheric model. During the last part of the simulation, a regulation occurs from evaporation at the surface
and longwave cooling at TOA. Most of the model drift is attributed to a too large heating by solar radiation in middle and
high latitudes. The reduction of the north–south temperature gradient, and the related changes in the meridional equator-to-pole
ocean heat transport lead to a warming of equatorial and subtropical regions. This is also well demonstrated by the difference
between the two simulations which differ only in the parametrization of sea-ice. When the sea-ice cover is not restored to
climatology the model does not maintain sea-ice at high latitudes. The climate warms more rapidly and the water vapor and
clouds feedback occurs earlier.
Received: 24 May 1996 / Accepted: 29 November 1996 相似文献
11.
Jeffery R. Scott Andrei P. Sokolov Peter H. Stone Mort D. Webster 《Climate Dynamics》2008,30(5):441-454
The response of the ocean’s meridional overturning circulation (MOC) to increased greenhouse gas forcing is examined using
a coupled model of intermediate complexity, including a dynamic 3-D ocean subcomponent. Parameters are the increase in CO2 forcing (with stabilization after a specified time interval) and the model’s climate sensitivity. In this model, the cessation
of deep sinking in the north “Atlantic” (hereinafter, a “collapse”), as indicated by changes in the MOC, behaves like a simple
bifurcation. The final surface air temperature (SAT) change, which is closely predicted by the product of the radiative forcing
and the climate sensitivity, determines whether a collapse occurs. The initial transient response in SAT is largely a function
of the forcing increase, with higher sensitivity runs exhibiting delayed behavior; accordingly, high CO2-low sensitivity scenarios can be assessed as a recovering or collapsing circulation shortly after stabilization, whereas
low CO2-high sensitivity scenarios require several hundred additional years to make such a determination. We also systemically examine
how the rate of forcing, for a given CO2 stabilization, affects the ocean response. In contrast with previous studies based on results using simpler ocean models,
we find that except for a narrow range of marginally stable to marginally unstable scenarios, the forcing rate has little
impact on whether the run collapses or recovers. In this narrow range, however, forcing increases on a time scale of slow
ocean advective processes results in weaker declines in overturning strength and can permit a run to recover that would otherwise
collapse. 相似文献
12.
A. Ganopolski V. Petoukhov S. Rahmstorf V. Brovkin M. Claussen A. Eliseev C. Kubatzki 《Climate Dynamics》2001,17(10):735-751
A set of sensitivity experiments with the climate system model of intermediate complexity CLIMBER-2 was performed to compare
its sensitivity to changes in different types of forcings and boundary conditions with the results of comprehensive models
(GCMs). We investigated the climate system response to changes in freshwater flux into the Northern Atlantic, CO2 concentration, solar insolation, and vegetation cover in the boreal zone and in the tropics. All these experiments were compared
with the results of corresponding experiments performed with different GCMs. Qualitative, and in many respects, quantitative
agreement between the results of CLIMBER-2 and GCMs demonstrate the ability of our climate system model of intermediate complexity
to address diverse aspects of the climate change problem. In addition, we used our model for a series of experiments to assess
the impact of some climate feedbacks and uncertainties in model parameters on the model sensitivity to different forcings.
We studied the role of freshwater feedback and vertical ocean diffusivity for the stability properties of the thermohaline
ocean circulation. We show that freshwater feedback plays a minor role, while changes of vertical diffusivity in the ocean
considerably affect the circulation stability. In global warming experiments we analysed the impact of hydrological sensitivity
and vertical diffusivity on the long-term evolution of the thermohaline circulation. In the boreal and tropical deforestation
experiments we assessed the role of an interactive ocean and showed that for both types of deforestation scenarios, an interactive
ocean leads to an additional cooling due to albedo and water vapour feedbacks.
Received: 28 May 2000 / Accepted: 9 November 2000 相似文献
13.
The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991
are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3),
that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of
up to 0.5 K, in response to radiative perturbations of up to 10 W m−2, averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response
is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude
of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity
in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of
the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative
perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale
the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature
perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found
to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence
between “volcanic sensitivity” and sensitivity to CO2 forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used
to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated
to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO2 concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out
that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced
uncertainty in the observational record, of temperature as well as forcing. 相似文献
14.
R. Bintanja 《Theoretical and Applied Climatology》1997,56(1-2):1-24
Summary In this paper a simple climate model is presented which is used to perform some sensitivity experiments. The atmospheric part is represented by a vertically and zonally averaged layer in which the surface air temperature, radiative fluxes at the surface and at the top of the atmosphere, the turbulent fluxes between atmosphere and surface and the snow cover are calculated. This atmospheric layer is coupled to a two-dimensional advection-diffusion ocean model in which the zonal overturning pattern is prescribed. The ocean model evaluates the temperature distribution, the amount of sea-ice and the meridional and vertical heat fluxes. The present-day climate simulated by the model compares reasonably well with observations of the seasonal and latitudinal distribution of temperature, radiation, surface alebdo, sea-ice and snow cover and meridional energy fluxes. Then, the sensitivity of the model-simulated present-day climate to perturbations in the incident solar radiation at the top of the atmosphere is investigated. The temperature response displays large latitudinal and seasonal variations, which is in qualitative agreement with results obtained with other climate models. It is found that the seasonal variation of sea-ice cover (and hence, the effective oceanic heat capacity) is one of the most important elements determining seasonal variations in climate sensitivity. Differences in sensitivity between the seasonal and annual mean version of the model are discussed. Finally, the equilibrium response to perturbations in some selected model variables is presented; these variables include meridional diffusion coefficients, drag coefficient, sea-ice thickness, atmospheric CO2-concentration and cloud optical thickness.With 13 Figures 相似文献
15.
S. L. Weber 《Climate Dynamics》1998,14(3):201-212
The sensitivity of a coupled model to the oceanic vertical diffusion coefficient κ
v
is examined. This is compared to the sensitivity of an ocean-only model forced by mixed boundary conditions (BC). The atmospheric
component of the coupled model is a moist energy balance model. The ocean component is a 12-level geostrophic model, defined
on a midlatitude β-plane. Atmosphere and ocean are coupled through the fluxes of heat and moisture at their interface. The
coupled model contains a number of feedback processes which are not represented in the ocean-only model. This results in a
temperature and salinity response to κ
v
which is stronger in the coupled model than in the ocean-only model. On the other hand, there is a weaker response in oceanic
processes such as meridional heat transport, deep-water formation at high latitudes, etc. Ocean-only sensitivity experiments
were also performed with modified BCs, which parametrise the feedback processes included in the coupled model. These are the
modified thermal BC of Rahmstorf and Willebrand and a modified freshwater BC proposed in the present study. Large-scale features
of the response in oceanic surface fields are well represented with modified BCs. However, the sensitivity of the deep ocean
temperature is only partly captured due to local differences in the surface response. The scaling behavior of the zonal overturning
stream function was found to depend on the surface BCs. In contrast to this, the meridional overturning stream function basically
scales with κ0.5
v
in all sensitivity experiments. Differences in the heat transport response among the experiments are thus primarily related
to differences in the temperature response.
Received: 28 February 1997/Accepted: 12 September 1997 相似文献
16.
We describe the initial bias of the climate simulated by a coupled ocean-atmosphere model. The atmospheric component is a
state-of-the-art atmospheric general circulation model, whereas the ocean component is limited to the upper ocean and includes
a mixed layer whose depth is computed by the model. As the full ocean general circulation is not computed by the model, the
heat transport within the ocean is prescribed. When modifying the prescribed heat transport we also affect the initial drift
of the model. We analyze here one of the experiments where this drift is very strong, in order to study the key processes
relating the changes in the ocean transport and the evolution of the model's climate. In this simulation, the ocean surface
temperature cools by 1.5°C in 20 y. We can distinguish two different phases. During the first period of 5 y, the sea surface
temperatures become cooler, particularly in the intertropical area, but the outgoing longwave radiation at the top-of-the-atmosphere
increases very quickly, in particular at the end of the period. An off-line version of the model radiative code enables us
to decompose this behaviour into different contributions (cloudiness, specific humidity, air and surface temperatures, surface
albedo). This partitioning shows that the longwave radiation evolution is due to a decrease of high level cirrus clouds in
the intertropical troposphere. The decrease of the cloud cover also leads to a decrease of the planetary albedo and therefore
an increase of the net short wave radiation absorbed by the system. But the dominant factor is the strong destabilization
by the longwave cooling, which is able to throw the system out of equilibrium. During the remaining of the simulation (second
phase), the cooling induced by the destabilization at the top-of-the-atmosphere is transmitted to the surface by various processes
of the climate system. Hence, we show that small variations of ocean heat transport can force the model from a stable to an
unstable state via atmospheric processes which arise wen the tropics are cooling. Even if possibly overestimated by our GCM,
this mechanism may be pertinent to the maintenance of present climatic conditions in the tropics. The simplifications inherent
in our model's design allow us to investigate the mechanism in some detail.
Received: 18 February 1998 / Accepted: 22 April 1999 相似文献
17.
A Methodology for Quantifying Uncertainty in Climate Projections 总被引:1,自引:1,他引:0
Possible climate change caused by an increase ingreenhouse gas concentrations, despite having been asubject of intensive study in recent years, is stillvery uncertain. Uncertainties in projections ofdifferent climate variables are usually described onlyby the ranges of possible values. For assessing thepossible impact of climate change, it would be moreuseful to have probability distributions for thesevariables. Obtaining such distributions is usuallyvery computationally expensive and requires knowledgeof probability distributions for characteristics ofthe climate system that affect climate projections. A fewstudies of this kind have been carried out with energybalance/upwelling diffusion models. Here wedemonstrate a methodology for performing a similarstudy with a 2 dimensional (zonally averaged) climatemodel that reproduces the behavior of coupledatmosphere/ocean general circulation models morerealistically than energy balance models. Thismethodology involves application of the DeterministicEquivalent Modeling Method to derive functionalapproximations of the model's probabilistic response.Monte Carlo analysis is then performed on theapproximations. An application of the methodology isdemonstrated by deriving the uncertainty in surfaceair temperature change and sea level rise due tothermal expansion of the ocean that result fromuncertainties in climate sensitivity and the rate ofheat uptake by the deep ocean for a prescribedincrease in atmospheric CO2 concentration. Wealso demonstrate propagation of correlateduncertainties through different models, by presentingresults that include uncertainty in projected carbonemissions. 相似文献
18.
ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model 总被引:3,自引:0,他引:3
The new version of the atmospheric general circulation model (AGCM), ECHAM4, at the Max Planck Institute for Meteorology,
Hamburg, has been coupled to the OPYC3 isopycnic global ocean general circulation and sea ice model in a multi-century present-day
climate simulation. Non-seasonal constant flux adjustment for heat and freshwater was employed to ensure a long-term annual
mean state close to present-day climatology. This study examines the simulated upper ocean seasonal cycle and interannual
variability in the tropical Pacific for the first 100 years. The coupled model’s seasonal cycle of tropical Pacific SSTs is
satisfactory with respect to both the warm pool variation and the Central and Eastern Pacific, with significant errors only
in the cold tongue around April. The cold phase cold tongue extent and strength is as observed, and for this the heat flux
adjustment does not play a decisive role. A well-established South Pacific convergence zone is characteristic for the new
AGCM version. Apart from extending the southeast trades seasonal maximum to midbasin, wind stress pattern and strength are
captured. Overall the subsurface structure is consistent with the observed, with a pronounced thermocline at about 150 m depth
in the west and rising to the surface from 160 °W to 100 °W. The current system is better resolved than in some previous global
models and, on the whole, has the expected shape. The equatorial undercurrent is correctly positioned but the core is only
half as strong as observed. The north equatorial current and counter-current also have reduced maximum speeds but the April
minimum is captured. As with the companion publication from Roeckner et al. this study finds pronounced tropical Eastern and
Central Pacific interannual variability. Simulated and observed NINO3 sea surface temperature (SST) variability is represented
by a single, rather broadband, maximum of power spectral density, centered on about 28 months for the simulation and four
years for the observations. For simulation and observations, SST, windstress, and upper ocean heat content each exhibit a
single dominant large-scale amplitude and phase pattern, suggesting that the model captures the essential dynamics. The amplitude
of the essentially standing oscillation in SST in the NINO3 region attains the observed strength, but is weaker at the eastern
boundary. Anomalies of upper ocean heat content show off-equatorial westward and equatorial eastward propagation, the latter’s
arrival in the east of the basin coinciding with the SST anomalies. Equatorial wind stress anomalies near the date line provide
the appropriate forcing and clearly form a response to the anomalous SST.
Received: 14 June 1996 / Accepted: 11 November 1997 相似文献
19.
A simple theoretical model of atmospheric radiative equilibrium is solved analytically to help understand the energetics
of maintaining Earth's tropical and subtropical climate. The model climate is constrained by energy balance between shortwave
(SW) and longwave (LW) radiative fluxes. Given a complete set of SW and LW optical properties in each atmospheric layer, the
model yields a unique equilibrium-temperature profile. In contrast, if the atmospheric temperature profile and SW properties
are prescribed, the model yields essentially two distinct LW transmissivity profiles. This bimodality is due to a nonlinear
competition between the ascending and descending energy fluxes, as well as to their local conversion to sensible heat in the
atmosphere. Idealized slab models that are often used to describe the greenhouse effect are shown to be a special case of
our model when this nonlinearity is suppressed. In this special case, only one solution for LW transmissivity is possible.
Our model's bimodality in LW transmissivity for given SW fluxes and temperature profile may help explain certain features
of Earth's climate: at low latitudes the temperature profiles are fairly homogeneous, while the humidity profiles exhibit
a bimodal distribution; one mode is associated with regions of moist-and-ascending, the other with dry-and-subsiding air.
The model's analytical results show good agreement with the European Centre for Medium-Range Weather Forecasts' reanalysis
data. Sensitivity analysis of the temperature profile with respect to LW transmissivity changes leads to an assessment of
the low-latitude climate's sensitivity to the “runaway greenhouse” effect.
Received: 7 December 1999 / Accepted: 19 February 2001 相似文献
20.
Both the magnitude and timescale of climate change in response to anthropogenic forcing are important consideration in climate change decision making. Using a familiar, yet simple global energy balance model combined with a novel method for estimating the amount of gain in the global surface temperature response to radiative forcing associated with timescales in the range 100?C103?years we show that the introduction of large-scale circulation such as meridional overturning leads to the emergence of discrete gain?Ctimescale relationships in the dynamics of this model. This same feature is found in the response of both an intermediate complexity and two atmosphere?Cocean general circulation models run to equilibrium. As a result of this emergent property of climate models, it is possible to offer credible partitioning of the full equilibrium gain of these models, and hence their equilibrium climate sensitivity, between two discrete timescales; one decadal associated with near surface ocean heat equilibration; and one centennial associated with deep ocean heat equilibration. Timescales of approximately 20 and 700?years with a 60:40 partitioning of the equilibrium gain are found for the models analysed here. A re-analysis of the emulation results of 19 AOGCMs presented by Meinshausen et al. (Atmos Chem Phys Discuss 8:6153?C6272, 2008) indicates timescales of 20 and 580?years with an approximate 50:50 partition of the equilibrium gain between the two. This suggests near equal importance of both short and long timescales in determining equilibrium climate sensitivity. 相似文献