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1.
The Canadian Centre for Climate Modelling and Analysis (CCCma) global coupled model is used to investigate the potential
climate effects of increasing greenhouse gas (GHG) concentrations and changes in sulfate aerosol loadings. The forcing scenario
adopted closely resembles that of Mitchell et al. for both the greenhouse gas and aerosol components. Its implementation in
the model and the resulting changes in forcing are described. Five simulations of 200 years in length, nominally for the years
1900 to 2100, are available for analysis. They consist of a control simulation without change in forcing, three independent
simulations with the same greenhouse gas and aerosol changes, and a single simulation with greenhouse gas only forcing. Simulations
of the evolution of temperature and precipitation from 1900 to the present are compared with available observations. Temperature
and precipitation are primary climate variables with reasonable temporal and spatial coverage in the observational record
for the period. The simulation of potential climate change from the present to the end of the twenty-first century, based
on projected GHG and aerosol forcing changes, is discussed in a companion paper. For the historical period dealt with here,
the GHG and aerosol forcing has changed relatively little compared to the forcing changes projected to the end of the twenty-first
century. Nevertheless, the forced climate signal for temperature in the model is reasonably consistent with the observed global
mean temperature from the instrumental record. This is true also for the trend in zonally averaged temperature as a function
of latitude and for some aspects of the geographical and regional distributions of temperature. Despite the modest change
in overall forcing, the difference between GHG+aerosol and GHG-only forcing is discernible in the temperature response for
this period. Changes in precipitation, on the other hand, are much less evident in both the instrumental and simulated record.
There is an apparent increasing trend in average precipitation in both the observations and the model results over that part
of the land for which observations are available. Regional and geographical changes and trends (which are less affected by
sampling considerations), if they exist, are masked by the large natural variability of precipitation in both model and observations.
Received: 24 September 1998 / Accepted: 8 October 1999 相似文献
2.
The modification of greenhouse gas warming by the direct effect of sulphate aerosols 总被引:1,自引:0,他引:1
The Canadian Centre for Climate Modelling and Analysis (CCCma) second generation climate model (GCMII) consists of an atmospheric
GCM coupled to mixed layer ocean. It is used to investigate the climate response to a doubling of the CO2 concentration together with the direct effect of scattering by sulphate aerosols. As expected, the aerosols offset some of
the greenhouse gas (GHG) warming; the global annual mean screen temperature change due to doubled CO2 is 3.4 °C in this model and this is reduced to 2.7 °C when an estimate of the direct effect of anthropogenic sulphate aerosols
is included. The pattern of climate response to the comparatively localized aerosol forcing is not itself localized, and it
bears a striking resemblance to the response pattern that arises from the globally distributed change in GHG forcing. This
“non-local” response to “localized” forcing indicates that the pattern of climate response is determined, to first order,
by the overall magnitude of the change in forcing rather than its detailed nature or structure. Feedback processes operating
in the system apparently determine this pattern by locally amplifying and suppressing the response to the magnitude of the
change in forcing. The influence of the location of the change in forcing is relatively small. These “non-local” and “local”
effects of aerosol forcing are characterized and displayed and some of their consequences discussed. Effects on the moisture
budget and on the energetics of the global climate are also examined.
Received: 10 June 1997 / Accepted: 8 January 1998 相似文献
3.
We demonstrate that a hemispherically averaged upwelling-diffusion energy-balance climate model (UD/EBM) can emulate the
surface air temperature change and sea-level rise due to thermal expansion, predicted by the HadCM2 coupled atmosphere-ocean
general circulation model, for various scenarios of anthropogenic radiative forcing over 1860–2100. A climate sensitivity
of 2.6 °C is assumed, and a representation of the effect of sea-ice retreat on surface air temperature is required. In an
extended experiment, with CO2 concentration held constant at twice the control run value, the HadCM2 effective climate sensitivity is found to increase
from about 2.0 °C at the beginning of the integration to 3.85 °C after 900 years. The sea-level rise by this time is almost
1.0 m and the rate of rise fairly steady, implying that the final equilibrium value (the `commitment') is large. The base
UD/EBM can fit the 900-year simulation of surface temperature change and thermal expansion provided that the time-dependent
climate sensitivity is specified, but the vertical profile of warming in the ocean is not well reproduced. The main discrepancy
is the relatively large mid-depth warming in the HadCM2 ocean, that can be emulated by (1) diagnosing depth-dependent diffusivities
that increase through time; (2) diagnosing depth-dependent diffusivities for a pure-diffusion (zero upwelling) model; or (3)
diagnosing higher depth-dependent diffusivities that are applied to temperature perturbations only. The latter two models can be run to equilibrium, and with a climate sensitivity of 3.85 °C, they give sea-level rise
commitments of 1.7 m and 1.3 m, respectively.
Received: 27 April 1999 / Accepted: 13 September 2000 相似文献
4.
The Canadian Centre for Climate Modelling and Analysis global coupled model and its climate 总被引:16,自引:5,他引:11
G. M. Flato G. J. Boer W. G. Lee N. A. McFarlane D. Ramsden M. C. Reader A. J. Weaver 《Climate Dynamics》2000,16(6):451-467
A global, three-dimensional climate model, developed by coupling the CCCma second-generation atmospheric general circulation
model (GCM2) to a version of the GFDL modular ocean model (MOM1), forms the basis for extended simulations of past, current
and projected future climate. The spin-up and coupling procedures are described, as is the resulting climate based on a 200 year
model simulation with constant atmospheric composition and external forcing. The simulated climate is systematically compared
to available observations in terms of mean climate quantities and their spatial patterns, temporal variability, and regional
behavior. Such comparison demonstrates a generally successful reproduction of the broad features of mean climate quantities,
albeit with local discrepancies. Variability is generally well-simulated over land, but somewhat underestimated in the tropical
ocean and the extratropical storm-track regions. The modelled climate state shows only small trends, indicating a reasonable
level of balance at the surface, which is achieved in part by the use of heat and freshwater flux adjustments. The control
simulation provides a basis against which to compare simulated climate change due to historical and projected greenhouse gas
and aerosol forcing as described in companion publications.
Received: 24 September 1998 / Accepted: 8 October 1999 相似文献
5.
Observed and projected climate change in Taiwan 总被引:1,自引:0,他引:1
Summary
This study examined the secular climate change characteristics in Taiwan over the past 100 years and the relationship with
the global climate change. Estimates for the likelihood of future climate changes in Taiwan were made based on the projection
from the IPCC climate models.
In the past 100 years, Taiwan experienced an island-wide warming trend (1.0–1.4 °C/100 years). Both the annual and daily temperature
ranges have also increased. The warming in Taiwan is closely connected to a large-scale circulation and SAT fluctuations,
such as the “cool ocean warm land” phenomenon. The water vapor pressure has increased significantly and could have resulted
in a larger temperature increase in summer. The probability for the occurrence of high temperatures has increased and the
result suggests that both the mean and variance in the SAT in Taiwan have changed significantly since the beginning of the
20th century. Although, as a whole, the precipitation in Taiwan has shown a tendency to increase in northern Taiwan and to
decrease in southern Taiwan in the past 100 years, it exhibits a more complicated spatial pattern. The changes occur mainly
in either the dry or rainy season and result in an enhanced seasonal cycle. The changes in temperature and precipitation are
consistent with the weakening of the East Asian monsoon.
Under consideration of both the warming effect from greenhouse gases and the cooling effect from aerosols, all projections
from climate models indicated a warmer climate near Taiwan in the future. The projected increase in the area-mean temperature
near Taiwan ranged from 0.9–2.7 °C relative to the 1961–1990 averaged temperature, when the CO2 concentration increased to 1.9 times the 1961–1990 level. These simulated temperature increases were statistically significant
and can be attributed to the radiative forcing associated with the increased concentration of greenhouse gases and aerosols.
The projected changes in precipitation were within the range of natural variability for all five models. There is no evidence
supporting the possibility of precipitation changes near Taiwan based on the simulations from five IPCC climate models.
Received February 5, 2001 Revised July 30, 2001 相似文献
6.
David P. Rowell 《Climate Dynamics》2005,25(7-8):837-849
A scenario of European climate change for the late twenty-first century is described, using a high-resolution state-of-the-art
model. A time-slice approach is used, whereby the atmospheric general circulation model, HadAM3P, was integrated for two periods,
1960–1990 and 2070–2100, using the SRES A2 scenario. For the first time an ensemble of such experiments was produced, along
with appropriate statistical tests for assessing significance. The focus is on changes to the statistics of seasonal means,
and includes analysis of both multi-year means and interannual variance. All four seasons are assessed, and anomalies are
mapped for surface air temperature, precipitation and snow mass. Mechanisms are proposed where these are dominated by straightforward
local processes. In winter, the largest warming occurs over eastern Europe, up to 7°C, mean snow mass is reduced by at least
80% except over Scandinavia, and precipitation increases over all but the southernmost parts of Europe. In summer, temperatures
rise by 6–9°C south of about 50°N, and mean rainfall is substantially reduced over the same area. In spring and autumn, anomalies
tend to be weaker, but often display patterns similar to the preceding season, reflecting the inertia of the land surface
component of the climate system. Changes in interannual variance are substantial in the solsticial seasons for many regions
(note that for precipitation, variance estimates are scaled by the square of the mean). In winter, interannual variability
of near-surface air temperature is considerably reduced over much of Europe, and the relative variability of precipitation
is reduced north of about 50°N. In summer, the (relative) interannual variance of both variables increases over much of the
continent. 相似文献
7.
In this study, the contributions from changes in man-made greenhouse gases (GHG), anthropogenic aerosols (AA), and land use (LU), as well as natural solar and volcanic (NAT) forcing changes, to observed changes in surface air temperature (T) and precipitation (P) over global land, especially over arid-semiarid areas, during 1946–2005 are quantified using observations and climate model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results show that the anthropogenic (ANT) forcings dominate the ubiquitous surface warming seen in observations and lead to slight increases in precipitation over most land areas, while the NAT forcing leads to small cooling over land. GHG increases are the primary factor responsible for the anthropogenic climate change, while the AA forcing offsets a large part of the GHG-induced warming and P changes. The LU forcing generally contributes little to the T and P changes from 1946 to 2005 over most land areas. Unlike the consistent temperature changes among most model simulations, precipitation changes display a large spread among the models and are incomparable with the observations in spatial distributions and magnitude, mainly due to its large internal variability that varies among individual model runs. Using an optimal fingerprinting method, we find that the observed warming over land during 1946–2005 can be largely attributed to the ANT forcings, and the combination of the ANT and NAT forcings can explain about 85~95% of the observed warming trend over global land as well as over most arid-semiarid regions such as Northern China. However, the anthropogenic influences on precipitation over the past 60 years are generally undetectable over most land areas, including most arid-semiarid regions. This indicates that internal variability is still larger than the forced change for land precipitation. 相似文献
8.
The response of the tropical Indian Ocean (TIO) to greenhouse gases (GHGs) and aerosols are investigated based on historical single-forcing and all-forcing simulations using the Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL CM3). Results reveal a positive Indian Ocean Dipole (pIOD)-like pattern in GHG forcing but a negative Indian Ocean Dipole (nIOD)-like pattern in aerosol forcing. The GHG-induced pIOD-like pattern features less (more) sea surface temperature (SST) warming over the southeastern (western) TIO, accompanied by equatorial easterly anomalies, as well as a shallower thermocline off Sumatra. The aerosol-induced nIOD-like pattern displays the reverse features, characterized by less (more) SST cooling over the southeastern (western) TIO, anomalous equatorial westerlies, and a deeper thermocline off Sumatra. Although the aerosol-induced pattern appears to resemble a reversal of the GHG-induced pattern, there is a strong asymmetry in the SST changes over the southeastern TIO, where the cooling responding to aerosol forcing exceeds the warming in response to GHG forcing, and a negative SST residual is thus produced. A mixed-layer heat budget analysis suggests that the negative SST residual results mainly from asymmetric responses of shortwave radiation, zonal advection, and diffusion to GHGs and aerosols. For comparison, the formation processes for the negative SST skewness over the southeastern TIO between the internal pIOD and nIOD are also discussed. 相似文献
9.
Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region
This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes
of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical
files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the
tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System
on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable
water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress
data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies
conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall.
It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal
well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter
periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains
considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter
frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like
Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific.
This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the
relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes
at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the
north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical
Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or
delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through
three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West
Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic
modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental
rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create
meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy
and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and
hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near
the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked
to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel
rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region
but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations:
anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of
Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level
pressure over West Africa.
Received: 14 April 1998 / Accepted: 24 December 1998 相似文献
10.
V. Barros M. Gonzalez B. Liebmann I. Camilloni 《Theoretical and Applied Climatology》2000,67(3-4):123-133
Summary In subtropical Argentina, Paraguay and southern Brazil, precipitation is most abundant during summer but its interannual
variability is large. At this time a zone of low-level convergence, upper-level divergence, and intense convection is developed
to the north of this area. This feature is known as the South Atlantic convergence zone (SACZ) and seems to be related to
the interannual variability of summer rainfall to its south. The aim of this work is to document this relationship.
Reduced (increased) precipitation in southern Brazil, most of Uruguay and northeastern Argentina is associated with a strong
(weak) SACZ and a northward (southward) displacement of it, while increased (reduced) rainfall occurs further south in subtropical
Argentina. Also, warm (cold) SST in the region 20° S–40° S and west of 30° W is likely accompanied by a southward (northward)
shift of the SACZ. Aside of this relation with the SACZ that affect on the precipitation field of Southeastern South America,
the proximate Atlantic Ocean SST seems to force the precipitation over this region by other mechanisms as well. The result
of this additional SST forcing is to enhance the signal of the SACZ in northeastern Argentina, Uruguay and southern Brazil
and to oppose the SACZ effect in southern subtropical Argentina.
Received July 24, 1999 Revised July 5, 2000 相似文献
11.
Gareth S. Jones Jonathan M. Gregory Peter A. Stott Simon F. B. Tett Robert B. Thorpe 《Climate Dynamics》2005,25(7-8):725-738
Volcanic ‘super-eruptions’ have been suggested to have significantly influenced the Earth’s climate, perhaps causing glaciations
and impacting on the human population. Climatic changes following a hypothetical ‘super-eruption’ are simulated using a coupled
atmosphere ocean general circulation model, incorporating scaled volcanic stratospheric aerosols. Assumptions are made about
the stratospheric sulphate aerosol loading, size distribution, lifetime, chemical make up and spatial distribution. As this
study is concentrating on the physical climatological impacts over long timescales, microphysics and chemical interactive
processes are not simulated. Near-surface temperatures fall by as much as 10 K globally for a few months and a considerable
deviation from normal temperatures continues for several decades. A warming pattern is evident over northern land masses during
the winter due to increased longwave forcing and a positive AO mode. The overturning rate of the North Atlantic thermohaline
circulation doubles in intensity. Snow and ice increases in extent to a maximum coverage of 35% of the Earth. Despite these
and other impacts longer term climatic changes that could lead to a transition to a glaciation do not occur, for present day
boundary conditions and one possible plausible aerosol loading. 相似文献
12.
J. Räisänen 《Theoretical and Applied Climatology》1999,64(1-2):1-13
Summary The qualitative agreement of two climate models, HADCM2 and ECHAM3, on the response of surface climate to anthropogenic climate
forcing in the period 2020 – 2049 is studied. Special attention is paid to the role of internal climate variability as a source
of intermodel disagreement. After illustrating the methods in an intermodel comparison of simulated changes in June–August
mean precipitation, some global statistics are presented. Excluding surface air temperature, the four-season mean proportion
of areas in which the two models agree on the sign of the climatic response is only 53 – 60% both for increases in CO2 alone and for increases in CO2 together with direct radiative forcing by sulphate aerosols, but somewhat larger, 59 – 70% for the separate aerosol effect.
In areas where the response is strong (at least twice the standard error associated with internal variability) in both models,
the agreement is better and the contrast between the different forcings becomes more marked. The proportion of agreement in
such areas is 57 – 75% for the response to increases in CO2 alone, 64 – 84% for the response to combined CO2 and aerosol forcing, and as high as 88 – 94% for the separate aerosol effect. The relatively good intermodel agreement for
aerosol-induced climate changes is suggested to be associated with the uneven horizontal distribution of aerosol forcing.
Received December 2, 1998 Revised May 5, 1999 相似文献
13.
Probabilistic climate change projections using neural networks 总被引:5,自引:0,他引:5
Anticipated future warming of the climate system increases the need for accurate climate projections. A central problem are the large uncertainties associated with these model projections, and that uncertainty estimates are often based on expert judgment rather than objective quantitative methods. Further, important climate model parameters are still given as poorly constrained ranges that are partly inconsistent with the observed warming during the industrial period. Here we present a neural network based climate model substitute that increases the efficiency of large climate model ensembles by at least an order of magnitude. Using the observed surface warming over the industrial period and estimates of global ocean heat uptake as constraints for the ensemble, this method estimates ranges for climate sensitivity and radiative forcing that are consistent with observations. In particular, negative values for the uncertain indirect aerosol forcing exceeding –1.2 Wm–2 can be excluded with high confidence. A parameterization to account for the uncertainty in the future carbon cycle is introduced, derived separately from a carbon cycle model. This allows us to quantify the effect of the feedback between oceanic and terrestrial carbon uptake and global warming on global temperature projections. Finally, probability density functions for the surface warming until year 2100 for two illustrative emission scenarios are calculated, taking into account uncertainties in the carbon cycle, radiative forcing, climate sensitivity, model parameters and the observed temperature records. We find that warming exceeds the surface warming range projected by IPCC for almost half of the ensemble members. Projection uncertainties are only consistent with IPCC if a model-derived upper limit of about 5 K is assumed for climate sensitivity. 相似文献
14.
Four dynamical downscaling simulations are performed with different combinations of land cover maps and greenhouse gas (GHG) levels using the Weather Research and Forecasting (WRF) model nested in the Community Earth System (CESM) model. A pseudo-global warming downscaling method is used to effectively separate the anthropogenic signals from the internal noises of climate models. Based on these simulations, we investigate the impacts of anthropogenic increase in GHG concentrations and land use and land cover change (LULCC) on mean climate and extreme events in the arid and semi-arid regions of China. The results suggest that increased GHG concentrations lead to significant increases in the surface air temperature at 2 m height (T2m) by 1–1.5 °C and greater increase in the warm day temperature (TX90p) than the cold day temperature (TX10p) in the arid and semi-arid regions. Moreover, precipitation increases by 30–50% in the arid region in cold season (November to March) due to the GHG-induced increase in moisture recycling rate and precipitation efficiency. LULCC leads to significant decreases in the T2m, TX90p, and TX10p by approximately 0.3 °C. The regional LULCC accounts for 66 and 68% decrease in T2m in warm and cold seasons, respectively. The rest changes in T2m results from the changes in lateral boundary condition induced by the global LULCC. In response to LULCC, both the warm and cold day temperatures show a significant decrease in cold seasons, which primarily results from the regional LULCC. LULCC-induced changes in precipitation are generally weak in the arid and semi-arid regions of China. 相似文献
15.
G. C. Hegerl P. A. Stott M. R. Allen J. F. B. Mitchell S. F. B. Tett U. Cubasch 《Climate Dynamics》2000,16(10-11):737-754
Fingerprint techniques for the detection of anthropogenic climate change aim to distinguish the climate response to anthropogenic forcing from responses to other external influences and from internal climate variability. All these responses and the characteristics of internal variability are typically estimated from climate model data. We evaluate the sensitivity of detection and attribution results to the use of response and variability estimates from two different coupled ocean atmosphere general circulation models (HadCM2, developed at the Hadley Centre, and ECHAM3/LSG from the MPI für Meteorologie and Deutsches Klimarechenzentrum). The models differ in their response to greenhouse gas and direct sulfate aerosol forcing and also in the structure of their internal variability. This leads to differences in the estimated amplitude and the significance level of anthropogenic signals in observed 50-year summer (June, July, August) surface temperature trends. While the detection of anthropogenic influence on climate is robust to intermodel differences, our ability to discriminate between the greenhouse gas and the sulfate aerosol signals is not. An analysis of the recent warming, and the warming that occurred in the first half of the twentieth century, suggests that simulations forced with combined changes in natural (solar and volcanic) and anthropogenic (greenhouse gas and sulfate aerosol) forcings agree best with the observations. 相似文献
16.
D. W. Pierce T. P. Barnett N. Schneider R. Saravanan D. Dommenget M. Latif 《Climate Dynamics》2001,18(1-2):51-70
Decadal time scale climate variability in the North Pacific has implications for climate both locally and over North America.
A crucial question is the degree to which this variability arises from coupled ocean/atmosphere interactions over the North
Pacific that involve ocean dynamics, as opposed to either purely thermodynamic effects of the oceanic mixed layer integrating
in situ the stochastic atmospheric forcing, or the teleconnected response to tropical variability. The part of the variability
that is coming from local coupled ocean/atmosphere interactions involving ocean dynamics is potentially predictable by an
ocean/atmosphere general circulation model (O/A GCM), and such predictions could (depending on the achievable lead time) have
distinct societal benefits. This question is examined using the results of fully coupled O/A GCMs, as well as targeted numerical
experiments with stand-alone ocean and atmosphere models individually. It is found that coupled ocean/atmosphere interactions
that involve ocean dynamics are important to determining the strength and frequency of a decadal-time scale peak in the spectra
of several oceanic variables in the Kuroshio extension region off Japan. Local stochastic atmospheric heat flux forcing, integrated
by the oceanic mixed layer into a red spectrum, provides a noise background from which the signal must be extracted. Although
teleconnected ENSO responses influence the North Pacific in the 2–7 years/cycle frequency band, it is shown that some decadal-time
scale processes in the North Pacific proceed without ENSO. Likewise, although the effects of stochastic atmospheric forcing
on ocean dynamics are discernible, a feedback path from the ocean to the atmosphere is suggested by the results.
Received: 23 January 2000 / Accepted: 10 January 2001 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.
Impact of ocean model resolution on CCSM climate simulations 总被引:1,自引:1,他引:0
Ben P. Kirtman Cecilia Bitz Frank Bryan William Collins John Dennis Nathan Hearn James L. Kinter III Richard Loft Clement Rousset Leo Siqueira Cristiana Stan Robert Tomas Mariana Vertenstein 《Climate Dynamics》2012,39(6):1303-1328
The current literature provides compelling evidence suggesting that an eddy-resolving (as opposed to eddy-permitting or eddy-parameterized) ocean component model will significantly impact the simulation of the large-scale climate, although this has not been fully tested to date in multi-decadal global coupled climate simulations. The purpose of this paper is to examine how resolved ocean fronts and eddies impact the simulation of large-scale climate. The model used for this study is the NCAR Community Climate System Model version 3.5 (CCSM3.5)—the forerunner to CCSM4. Two experiments are reported here. The control experiment is a 155-year present-day climate simulation using a 0.5° atmosphere component (zonal resolution 0.625 meridional resolution 0.5°; land surface component at the same resolution) coupled to ocean and sea-ice components with zonal resolution of 1.2° and meridional resolution varying from 0.27° at the equator to 0.54° in the mid-latitudes. The second simulation uses the same atmospheric and land-surface models coupled to eddy-resolving 0.1° ocean and sea-ice component models. The simulations are compared in terms of how the representation of smaller scale features in the time mean ocean circulation and ocean eddies impact the mean and variable climate. In terms of the global mean surface temperature, the enhanced ocean resolution leads to a ubiquitous surface warming with a global mean surface temperature increase of about 0.2?°C relative to the control. The warming is largest in the Arctic and regions of strong ocean fronts and ocean eddy activity (i.e., Southern Ocean, western boundary currents). The Arctic warming is associated with significant losses of sea-ice in the high-resolution simulation. The sea surface temperature gradients in the North Atlantic, in particular, are better resolved in the high-resolution model leading to significantly sharper temperature gradients and associated large-scale shifts in the rainfall. In the extra-tropics, the interannual temperature variability is increased with the resolved eddies, and a notable increases in the amplitude of the El Ni?o and the Southern Oscillation is also detected. Changes in global temperature anomaly teleconnections and local air-sea feedbacks are also documented and show large changes in ocean–atmosphere coupling. In particular, local air-sea feedbacks are significantly modified by the increased ocean resolution. In the high-resolution simulation in the extra-tropics there is compelling evidence of stronger forcing of the atmosphere by SST variability arising from ocean dynamics. This coupling is very weak or absent in the low-resolution model. 相似文献
19.
P. D. Williams E. Guilyardi R. T. Sutton J. M. Gregory G. Madec 《Climate Dynamics》2006,27(6):593-611
Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean–atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10–20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean’s thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases. 相似文献
20.
Bayesian multi-model projection of climate: bias assumptions and interannual variability 总被引:1,自引:0,他引:1
Current climate change projections are based on comprehensive multi-model ensembles of global and regional climate simulations.
Application of this information to impact studies requires a combined probabilistic estimate taking into account the different
models and their performance under current climatic conditions. Here we present a Bayesian statistical model for the distribution
of seasonal mean surface temperatures for control and scenario periods. The model combines observational data for the control
period with the output of regional climate models (RCMs) driven by different global climate models (GCMs). The proposed Bayesian
methodology addresses seasonal mean temperatures and considers both changes in mean temperature and interannual variability.
In addition, unlike previous studies, our methodology explicitly considers model biases that are allowed to be time-dependent
(i.e. change between control and scenario period). More specifically, the model considers additive and multiplicative model
biases for each RCM and introduces two plausible assumptions (“constant bias” and “constant relationship”) about extrapolating
the biases from the control to the scenario period. The resulting identifiability problem is resolved by using informative
priors for the bias changes. A sensitivity analysis illustrates the role of the informative prior. As an example, we present
results for Alpine winter and summer temperatures for control (1961–1990) and scenario periods (2071–2100) under the SRES
A2 greenhouse gas scenario. For winter, both bias assumptions yield a comparable mean warming of 3.5–3.6°C. For summer, the
two different assumptions have a strong influence on the probabilistic prediction of mean warming, which amounts to 5.4°C
and 3.4°C for the “constant bias” and “constant relation” assumptions, respectively. Analysis shows that the underlying reason
for this large uncertainty is due to the overestimation of summer interannual variability in all models considered. Our results
show the necessity to consider potential bias changes when projecting climate under an emission scenario. Further work is
needed to determine how bias information can be exploited for this task. 相似文献