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1.
Experiments with the coupled climate model CLIMBER-3α, which contains an oceanic general circulation model, show deep upwelling in the Southern Ocean to be proportional to
the surface wind stress in the latitudinal band of Drake Passage. At the same time, the distribution of the Southern Ocean
upwelling onto the oceanic basins is controlled by buoyancy distribution; the inflow into each basin being proportional to
the respective meridional density difference. We observe approximately the same constant of proportionality for all basins,
and demonstrate that it can be directly related to the flow geometry. For increased wind stress in the Southern Ocean, the
overturning increases both in the Atlantic and the Indo-Pacific basin. For strongly reduced wind stress, the circulation enters
a regime where Atlantic overturning is maintained through Pacific upwelling, in order to satisfy the transports set by the
density differences. Previous results on surface buoyancy and wind stress forcing, obtained with different models, are reproduced
within one model in order to distill a consistent picture. We propose that both Southern Ocean upwelling and meridional density
differences set up a system of conditions that determine the global meridional overturning circulation. 相似文献
2.
Effects of the seasonal variation in thermohaline and wind forcing on the abyssal circulation are investigated by using an
ocean general circulation model. To isolate effects of the seasonality in the thermohaline forcing from those in the wind
forcing, we carry out three experiments with (1) annual-mean wind forcing and perpetual-winter thermohaline forcing, (2) annual-mean
wind forcing and seasonal thermohaline forcing, and (3) seasonal wind forcing and seasonal thermohaline forcing. The deep
water under the seasonal thermohaline forcing becomes warmer than under the perpetual-winter thermohaline forcing. Although
the perpetual-winter thermohaline forcing is widely used and believed to reproduce the deep water better than the annual-mean
forcing, the difference between the results of the perpetual-winter and the seasonal thermohaline forcing is significant.
The seasonal variation of the Ekman convergence and divergence produces meridional overturning cells extending to the bottom
because the period of seasonal cycle is shorter than the adjustment timescale by baroclinic Rossby waves. The heat transport
owing to those Ekman flows and temperature anomalies makes the upper water (0–200 m) colder at low to mid-latitudes (40S–40N)
and warmer at high latitudes. Also the deep water becomes warmer owing to the warming of the northern North Atlantic, the
main source region of North Atlantic Deep Water. The model is also synchronously (i.e., without acceleration) integrated with
seasonal forcing for 5400 y. A past study suggested that under seasonal forcing, a sufficient equilibrium state can be achieved
after only decades of synchronous integration following more than 10 000 y of accelerated integration. Here, the result so
obtained is compared with that of the 5400-y synchronous integration. The difference in the global average temperature is
as small as 0.12 °C, and most of the difference is confined to the Southern Ocean.
Received: 1 May 1998 / Accepted: 5 January 1999 相似文献
3.
CMIP1 evaluation and intercomparison of coupled climate models 总被引:10,自引:1,他引:10
The climates simulated by 15 coupled atmosphere/ocean climate models participating in the first phase of the Coupled Model
Intercomparison Project (CMIP1) are intercompared and evaluated. Results for global means, zonal averages, and geographical
distributions of basic climate variables are assembled and compared with observations. The current generation of climate models
reproduce the major features of the observed distribution of the basic climate parameters, but there is, nevertheless, a considerable
scatter among model results and between simulated and observed values. This is particularly true for oceanic variables. Flux
adjusted models generally produce simulated climates which are in better accord with observations than do non-flux adjusted
models; however, some non-flux adjusted model results are closer to observations than some flux adjusted model results. Other
model differences, such as resolution, do not appear to provide a clear distinction among model results in this generation
of models. Many of the systematic differences (those differences common to most models), evident in previous intercomparison
studies are exhibited also by the CMIP1 group of models although often with reduced magnitudes. As is characteristic of intercomparison
results, different climate variables are simulated with different levels of success by different models and no one model is
“best” for all variables. There is some evidence that the “mean model” result, obtained by averaging over the ensemble of
models, provides an overall best comparison to observations for climatological mean fields. The model deficiencies identified
here do not suggest immediate remedies and the overall success of the models in simulating the behaviour of the complex non-linear
climate system apparently depends on the slow improvement in the balance of approximations that characterize a coupled climate
model. Of course, the results of this and similar studies provide only an indication, at a particular time, of the current
state and the moderate but steady evolution and improvement of coupled climate models.
Received: 26 January 2000 / Accepted: 9 June 2000 相似文献
4.
A model study of the Little Ice Age and beyond: changes in ocean heat content,hydrography and circulation since 1500 总被引:1,自引:1,他引:0
The Earth System Climate Model from the University of Victoria is used to investigate changes in ocean properties such as
heat content, temperature, salinity, density and circulation during 1500 to 2000, the time period which includes the Little
Ice Age (LIA) (1500–1850) and the industrial era (1850–2000). We force the model with two different wind-stress fields which
take into account the North Atlantic Oscillation. Furthermore, temporally varying radiative forcings due to volcanic activity,
insolation changes and greenhouse gas changes are also implemented. We find that changes in the upper ocean (0–300 m) heat
content are mainly driven by changes in radiative forcing, except in the polar regions where the varying wind-stress induces
changes in ocean heat content. In the full ocean (0–3,000 m) the wind-driven effects tend to reduce, prior to 1700, the downward
trend in the ocean heat content caused by the radiative forcing. Afterwards no dynamical effect is visible. The colder ocean
temperatures in the top 600 m during the LIA are caused by changes in radiative forcing, while the cooling at the bottom is
wind-driven. The changes in salinity are small except in the Arctic Ocean. The reduced salinity content in the subsurface
Arctic Ocean during the LIA is a result from reduced wind-driven inflow of saline water from the North Atlantic. At the surface
of the Arctic Ocean the changes in salinity are caused by changes in sea–ice thickness. The changes in density are a composite
picture of the temperature and salinity changes. Furthermore, changes in the meridional overturning circulation (MOC) are
caused mainly by a varying wind-stress forcing; the additional buoyancy driven changes due to the radiative forcings are small.
The simulated MOC is reduced during the LIA as compared to the industrial era. On the other hand, the ventilation rate in
the Southern Ocean is increased during the LIA. 相似文献
5.
Yves Plancherel 《Climate Dynamics》2014,42(11-12):2983-3004
A comparative analysis of the state and response of the latitude–depth meridional overturning streamfunctions in the Climate Model Inter-comparison Project 3 (CMIP3) model set is presented. Simulated overturning strengths of the North Atlantic cell tend to converge towards observational estimates. The models whose simulations of the North Atlantic cell are closest to observational estimates indicate a 29.5 ± 13 % decrease in the maximum intensity of that cell by 2,100. In contrast, agreement with regard to the state and the response to anthropogenic radiative forcing of the global Southern Ocean abyssal cell is poor among the models. A weak relationship between the mean state and the response of the abyssal cell can be used to constrain the reduction of the Southern abyssal cell by 2,100 to 29.3 ± 20.7 %, in rough agreement with the decrease predicted in the Northern cell. The biases across the CMIP3 models in the Northern deep cell and Southern abyssal cell cannot be related dynamically by a buoyancy-based seesaw-like argument. The absence or presence of characteristic relationships between the state and evolution of different features of the overturning streamfunction indicate that the main reasons for across-model spread are how each model deals with subgrid-scale processes and viscosity. This highlights the fact that subgrid-scale parameterizations and resolution improvements should be a priority of model development. These factors are able to explain qualitatively the inter-model differences between the Northern overturning cells of the different models. Across-model differences in the winds over the Southern Ocean are responsible for much of the disparity in the overturning circulation cells of the Southern Ocean. 相似文献
6.
Ocean dynamics play a key role in the climate system, by redistributing heat and freshwater. The uncertainty of how these processes are represented in climate models, and how this uncertainty affects future climate projections can be investigated using perturbed physics ensembles of global circulation models (GCMs). Techniques such as flux adjustments should be avoided since they can impact the sensitivity of the ensemble to the imposed forcing. In this study a method for developing an coupled ensemble with a GCM that does not use flux adjustment is presented. The ensemble is constrained by using information from a prior ensemble with a mixed layer ocean coupled to an atmosphere GCM, to reduce drifts in the coupled ensemble. Constraints on parameter perturbations are derived by using observational constraints on surface temperature, and top of the atmosphere radiative fluxes. As an example of such an ensemble developed with this methodology, uncertainty in response of the meridional overturning circulation (MOC) to increased CO2 concentrations is investigated. The ensemble mean MOC strength is 17.1?Sv and decreases by 2.1?Sv when greenhouse gas concentrations are doubled. No rapid changes or shutdown of the MOC are seen in any of the ensemble members. There is a strong negative relationship between global mean temperature and MOC strength across the ensemble which is not seen in a multimodel ensemble. A positive relationship between climate sensitivity and the decrease of MOC strength is also seen. 相似文献
7.
The atmospheric general circulation model ECHAM-4 is coupled to a chemistry model to calculate sulfate mass distribution
and the radiative forcing due to sulfate aerosol particles. The model simulates the main components of the hydrological cycle
and, hence, it allows an explicit treatment of cloud transformation processes and precipitation scavenging. Two experiments
are performed, one with pre-industrial and one with present-day sulfur emissions. In the pre-industrial emission scenario
SO2 is oxidized faster to sulfate and the in-cloud oxidation via the reaction with ozone is more important than in the present-day
scenario. The atmospheric sulfate mass due to anthropogenic emissions is estimated as 0.38 Tg sulfur. The radiative forcing
due to anthropogenic sulfate aerosols is calculated diagnostically. The backscattering of shortwave radiation (direct effect)
as well as the impact of sulfate aerosols on the cloud albedo (indirect effect) is estimated. The model predicts a direct
forcing of −0.35 W m-2 and an indirect forcing of −0.76 W m-2. Over the continents of the Northern Hemisphere the direct forcing amounts to −0.64 W m-2. The geographical distribution of the direct and indirect effect is very different. Whereas the direct forcing is strongest
over highly polluted continental regions, the indirect forcing over sea exceeds that over land. It is shown that forcing estimates
based on monthly averages rather than on instantaneous sulfate pattern overestimate the indirect effect but have little effect
on the direct forcing.
Received: 16 October 1996/Accepted: 24 October 1996 相似文献
8.
Rapid transitions and ultra-low frequency behaviour in a 40 kyr integration with a coupled climate model of intermediate complexity 总被引:1,自引:0,他引:1
A 40 kyr integration with the coupled atmosphere/ocean/sea-ice model of intermediate complexity ECBilt for present boundary
conditions has been performed. The climate of ECBilt displays quasi-periodical behaviour with a period of approximately 13 kyr.
The quasi-periodical behaviour is characterized by large changes in the overturning cell in the Southern Ocean. The southern
cell fluctuates between two quasi-stationary states, with accompaning changes in the atmospheric circulation in the Southern
Hemisphere. The transition between these states is rapid and resembles the polar halocline catastrophes and flushes as observed
in ocean general circulation models under mixed boundary conditions. The sea-ice influence on both the surface heat and fresh
water flux appears to be crucial for the existence and the prolongation of the quasi-stationary states. The atmospheric circulation
of those two quasi-stationary states displays large regional differences over Antarctica, resulting in even opposite surface
air temperature trends for certain locations during the transition from one state to another.
Received: 7 October 1999 / Accepted: 28 August 2000 相似文献
9.
We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic. 相似文献
10.
Impacts of shortwave radiation forcing on ENSO: a study with a coupled tropical ocean-atmosphere model 总被引:2,自引:0,他引:2
We describe a coupled tropical ocean-atmosphere model that represents a new class of models that fill the gap between anomaly
coupled models and fully coupled general circulation models. Both the atmosphere and ocean are described by two and half layer
primitive equation models, which emphasize the physical processes in the oceanic mixed layer and atmospheric boundary layer.
Ocean and atmosphere are coupled through both momentum and heat flux exchanges without explicit flux correction. The coupled
model, driven by solar radiation, reproduces a realistic annual cycle and El Nino-Southern Oscillation (ENSO). In the presence
of annual mean shortwave radiation forcing, the model exhibits an intrinsic mode of ENSO. The oscillation period depends on
the mean forcing that determines the coupled mean state. A perpetual April (October) mean forcing prolongs (shortens) the
oscillation period through weakening (enhancing) the mean upwelling and mean vertical temperature gradients. The annual cycle
of the solar forcing is shown to have fundamental impacts on the behavior of ENSO cycles through establishing a coupled annual
cycle that interacts with the ENSO mode. Due to the annual cycle solar forcing, the single spectral peak of the intrinsic
ENSO mode becomes a double peak with a quasi-biennial and a low-frequency (4–5 years) component; the evolution of ENSO becomes
phase-locked to the annual cycle; and the amplitude and frequency of ENSO become variable on an interdecadal time scale due
to interactions of the mean state and the two ENSO components. The western Pacific monsoon (the annual shortwave radiation
forcing in the western Pacific) is primarily responsible for the generation of the two ENSO components. The annual march of
the eastern Pacific ITCZ tends to lock ENSO phases to the annual cycle. The model's deficiencies, limitations, and future
work are also discussed.
Received: 15 June 1999 / Accepted: 11 December 1999 相似文献
11.
The effect of sea-ice on the transient atmospheric eddies of the Southern Hemisphere 总被引:1,自引:1,他引:0
Two 10 y simulations with a full seasonal cycle and 96×72×19 resolution were carried out with a version of the LMD GCM to
diagnose the role of sea-ice on the extratropical climatology of the Southern Hemisphere. The control integration used the
usual observed sea-ice distribution, while the anomaly simulation imposed a scenario in which all sea-ice was entirely replaced
by open ocean. The simulated control climate was compared with available observational-based analyses. Relevant diagnostics
of the time mean and indicators of the transient eddy activity have been evaluated for both integrations. The impact was shown
throughout the troposphere and was larger and more organised in winter. We found reduced westerly flow and both falls and
rises in sea level pressure in the region from which sea-ice was removed. The removal of ice in the Southern Ocean affects
the baroclinic structure of the atmosphere. Changes in baroclinicity and eddy activity are consistent with changes in the
mean climate. In general, the meridional wind variance, the poleward transient temperature flux and the eddy flux convergence
of westerly momentum were weaker over the Southern Ocean. However, a strengthening of the variance downstream of the subtropical
jet was found. The position of the main storm track tends to be slightly displaced equatorward in the anomaly case.
Received: 24 February 1998 / Accepted: 13 March 1999 相似文献
12.
Under external heating forcing in the Southern Ocean, climate models project anomalous northward atmosphere heat transport (AHT) across the equator, accompanied by a southward shift of the intertropical convergence zone (ITCZ). Comparison between a fully coupled and a slab ocean model shows that the inclusion of active ocean dynamics tends to partition the cross-equatorial energy transport and significantly reduce the ITCZ shift response by a factor of 10, a finding which supports previous studies. To understand how ocean dynamics damps the ITCZ's response to an imposed thermal heating in the Southern Ocean, we examine the ocean heat transport (OHT) and ocean circulation responses in a set of fully coupled experiments. Results show that both the Indo-Pacific and the Atlantic contribute to transport energy across the equator mainly through its Eulerian-mean component. However, different from previous studies that linked the changes in OHT to the changes in the wind-driven subtropical cells or the Atlantic meridional overturning circulation (AMOC), our results show that the cross-equatorial OHT anomaly is due to a broad clockwise overturning circulation anomaly below the subtropical cells (approximately bounded by the 5℃ to 20℃ isotherms and 50°S to 10°N). Further elimination of the wind-driven component, conducted by prescribing the climatological wind stress in the Southern Ocean heat perturbation experiments, leads to little change in OHT, suggesting that the OHT response is predominantly thermohaline-driven by air-sea thermal interactions. 相似文献
13.
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
14.
The predictability of atmospheric responses to global sea surface temperature (SST) anomalies is evaluated using ensemble
simulations of two general circulation models (GCMs): the GENESIS version 1.5 (GEN) and the ECMWF cycle 36 (ECM). The integrations
incorporate observed SST variations but start from different initial land and atmospheric states. Five GEN 1980–1992 and six
ECM 1980–1988 realizations are compared with observations to distinguish predictable SST forced climate signals from internal
variability. To facilitate the study, correlation analysis and significance evaluation techniques are developed on the basis
of time series permutations. It is found that the annual mean global area with realistic signals is variable dependent and
ranges from 3 to 20% in GEN and 6 to 28% in ECM. More than 95% of these signal areas occur between 35 °S–35 °N. Due to the
existence of model biases, robust responses, which are independent of initial condition, are identified over broader areas.
Both GCMs demonstrate that the sensitivity to initial conditions decreases and the predictability of SST forced responses
increases, in order, from 850 hPa zonal wind, outgoing longwave radiation, 200 hPa zonal wind, sea-level pressure to 500 hPa
height. The predictable signals are concentrated in the tropical and subtropical Pacific Ocean and are identified with typical
El Ni?o/ Southern Oscillation phenomena that occur in response to SST and diabatic heating anomalies over the equatorial central
Pacific. ECM is less sensitive to initial conditions and better predicts SST forced climate changes. This results from (1)
a more realistic basic climatology, especially of the upper-level wind circulation, that produces more realistic interactions
between the mean flow, stationary waves and tropical forcing; (2) a more vigorous hydrologic cycle that amplifies the tropical
forcing signals, which can exceed internal variability and be more efficiently transported from the forcing region. Differences
between the models and observations are identified. For GEN during El Ni?o, the convection does not carry energy to a sufficiently
high altitude, while the spread of the tropospheric warming along the equator is slower and the anomaly magnitude smaller
than observed. This impacts model ability to simulate realistic responses over Eurasia and the Indian Ocean. Similar biases
exist in the ECM responses. In addition, the relationships between upper and lower tropospheric wind responses to SST forcing
are not well reproduced by either model. The identification of these model biases leads to the conclusion that improvements
in convective heat and momentum transport parametrizations and basic climate simulations could substantially increase predictive
skill.
Received: 25 April 1996 / Accepted: 9 December 1996 相似文献
15.
Olivier Geoffroy David Saint-Martin Aurore Voldoire David Salas y Mélia Stéphane Sénési 《Climate Dynamics》2014,42(7-8):1807-1818
This study provides a comprehensive global analysis of the climate radiative feedbacks and the adjusted radiative forcing for a CO2 increase perturbation in the CNRM-CM5 climate model using the partial radiative perturbations (PRP) method. Some methodological key points of the PRP are investigated, with a particular focus on the consideration of the effect of fast adjustments. First, the standard PRP method is applied by neglecting certain fast adjustments. The effect of the field decorrelation is highlighted by performing a PRP across two different periods of a control experiment and by analyzing second-order terms. Sensitivity tests to the field substitution frequency, the sampling period and the perturbed experiment used are performed. The impact of the definition of the top of the climate system (top-of-the-atmosphere or tropopause) in the feedback estimate is also discussed. Secondly, the fast adjustment processes are taken into account by combining the PRP framework with the method of linear regression of the partial net radiative flux change against the mean surface air temperature change using a step forcing experiment. This method allows us to quantify the contribution of the different constituents to the forcing adjustment and to improve the estimation of the radiative feedbacks. It is shown that such decomposition allows the retrieval of the adjusted radiative forcing, the radiative feedbacks and the climate sensitivity as estimated with the linear regression method with a high level of accuracy, validating the partial decomposition. 相似文献
16.
Climate forcing by carbonaceous and sulfate aerosols 总被引:3,自引:0,他引:3
An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing
by anthropogenic sulfate and carbonaceous aerosols. The latter aerosols result from biomass burning as well as fossil fuel
burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount of reflected radiation
at the top-of-the-atmosphere. In contrast, sulfate aerosols are reflectant and the amount of reflected radiation depends nonlinearly
on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aerosol
at high relative humidities and find that the direct forcing by anthropogenic sulfate aerosols can decrease from −0.81 W m-2 to −0.55 Wm-2 if grid box average relative humidity is not allowed to increase above 90%. The climate forcing associated with fossil fuel
emissions of carbonaceous aerosols is calculated to range from +0.16 to +0.20 Wm-2, depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of
carbonaceous aerosols associated with biomass burning is calculated to range from −0.23 to −0.16 Wm-2. The pattern of forcing by carbonaceous aerosols depends on both the surface albedo and the presence of clouds. Multiple
scattering associated with clouds and high surface albedos can change the forcing from negative to positive.
Received: 29 September 1997 / Accepted: 10 June 1998 相似文献
17.
We consider how a highly idealized double-hemisphere basin responds to a zonally constant restoring surface temperature profile
that oscillates in time, with periods ranging from 0.5 to 32,000 years. In both hemispheres, the forcing is similar but can
be either in phase or out of phase. The set-up is such that the Northern Hemisphere always produces the densest waters. The
model’s meridional overturning circulation (MOC) exhibits a strong response in both hemispheres on decadal to multi-millennial
timescales. The amplitude of the oscillations reaches up to 140% of the steady-state maximum MOC and exhibits resonance-like
behaviour, with a maximum at centennial to millennial forcing periods. When the forcing is in phase between the Northern and
Southern Hemispheres, there is a marked decrease in the amplitude of the MOC response as the forcing period is increased beyond
the resonance period. In this case the resonance-like behaviour is identical to the one we found earlier in a single-hemisphere
model and occurs for the same reasons. When the forcing is out of phase between the Northern and Southern Hemispheres, the
amplitude of the MOC response is substantially greater for long forcing periods (millennial and longer), particularly in the
Southern Hemisphere. This increased MOC amplitude occurs because for an out of phase forcing, either the northern or the southern
deep water source is always active, leading to generally colder bottom waters and thus greater stratification in the opposite
hemisphere. This increased stratification in turn stabilises the water column and thus reduces the strength of the weaker
overturning cell. The interaction of the two hemispheres leads to response timescales of the deep ocean at half the forcing
period. Our results suggest a possible explanation for the half-precessional time scale observed in the deep Atlantic Ocean
palaeo-temperature record. 相似文献
18.
Experiments with abrupt CO2 forcing allow the diagnosis of the response of global mean temperature and precipitation in terms of fast temperature independent adjustments and slow, linear temperature-dependent feedbacks. Here we compare responses, feedbacks and forcings in experiments performed as part of version 5 of the coupled model inter-comparison project (CMIP5). The experiments facilitate, for the first time, a comparison of fully coupled atmosphere-ocean general circulation models (GCM’s) under both linearly increasing and abrupt radiative forcing. In the case of a 1 % per year compounded increase in CO2 concentration, we find that the non-linear evolution of surface air temperature in time, when combined with the linear evolution of the radiative balance at the top of the atmosphere, results in a feedback parameter and effective climate sensitivity having an offset compared to values computed from abrupt 4× CO2 forcing experiments. The linear evolution of the radiative balance at the top of the atmosphere also contributes to an offset between the global mean precipitation response predicted in the 1 % experiment using linear theory and that diagnosed from the experiments themselves, and a potential error between the adjusted radiative forcing and that produced using a standard linear formula. The non-linear evolution of temperature and precipitation responses are also evident in the RCP8.5 scenario and have implications for understanding, quantifying and emulating the global response of the CMIP5 climate GCMs. 相似文献
19.
We investigate the formation process and pathways of deep water masses in a coupled ice–ocean model of the Arctic and North
Atlantic Oceans. The intent is to determine the relative roles of these water masses from the different source regions (Arctic
Ocean, Nordic Seas, and Subpolar Atlantic) in the meridional overturning circulation. The model exhibits significant decadal
variability in the deep western boundary current and the overturning circulation. We use detailed diagnostics to understand
the process of water mass formation in the model and the resulting effects on the North Atlantic overturning circulation.
Particular emphasis is given to the multiple sources of North Atlantic Deep Water, the dominant deep water masses of the world
ocean. The correct balance of Labrador Sea, Greenland Sea and Norwegian Sea sources is difficult to achieve in climate models,
owing to small-scale sinking and convection processes. The global overturning circulation is described as a function of potential
temperature and salinity, which more clearly signifies dynamical processes and clarifies resolution problems inherent to the
high latitude oceans. We find that fluxes of deep water masses through various passages in the model are higher than observed
estimates. Despite the excessive volume flux, the Nordic Seas overflow waters are diluted by strong mixing and enter the Labrador
Sea at a lighter density. Through strong subpolar convection, these waters along with other North Atlantic water masses are
converted into the densest waters [similar density to Antarctic Bottom Water (AABW)] in the North Atlantic. We describe the
diminished role of salinity in the Labrador Sea, where a shortage of buoyant surface water (or excess of high salinity water)
leads to overly strong convection. The result is that the Atlantic overturning circulation in the model is very sensitive
to the surface heat flux in the Labrador Sea and hence is correlated with the North Atlantic Oscillation. As strong subpolar
convection is found in other models, we discuss broader implications. 相似文献
20.
A transient climate change simulation with greenhouse gas and aerosol forcing: projected climate to the twenty-first century 总被引:3,自引:0,他引:3
The potential climatic consequences of increasing atmospheric greenhouse gas (GHG) concentration and sulfate aerosol loading
are investigated for the years 1900 to 2100 based on five simulations with the CCCma coupled climate model. The five simulations
comprise a control experiment without change in GHG or aerosol amount, three independent simulations with increasing GHG and
aerosol forcing, and a simulation with increasing GHG forcing only. Climate warming accelerates from the present with global
mean temperatures simulated to increase by 1.7 °C to the year 2050 and by a further 2.7 °C by the year 2100. The warming is
non-uniform as to hemisphere, season, and underlying surface. Changes in interannual variability of temperature show considerable
structure and seasonal dependence. The effect of the comparatively localized negative radiative forcing associated with the
aerosol is to retard and reduce the warming by about 0.9 °C at 2050 and 1.2 °C at 2100. Its primary effect on temperature
is to counteract the global pattern of GHG-induced warming and only secondarily to affect local temperatures suggesting that
the first order transient climate response of the system is determined by feedback processes and only secondarily by the local
pattern of radiative forcing. The warming is accompanied by a more active hydrological cycle with increases in precipitation
and evaporation rates that are delayed by comparison with temperature increases. There is an “El Nino-like” shift in precipitation
and an overall increase in the interannual variability of precipitation. The effect of the aerosol forcing is again primarily
to delay and counteract the GHG-induced increase. Decreases in soil moisture are common but regionally dependent and interannual
variability changes show considerable structure. Snow cover and sea-ice retreat. A PNA-like anomaly in mean sea-level pressure
with an enhanced Aleutian low in northern winter is associated with the tropical shift in precipitation regime. The interannual
variability of mean sea-level pressure generally decreases with largest decreases in the tropical Indian ocean region. Changes
to the ocean thermal structure are associated with a spin-down of the Atlantic thermohaline circulation together with a decrease
in its variability. The effect of aerosol forcing, although modest, differs from that for most other quantities in that it
does not act primarily to counteract the GHG forcing effect. The barotropic stream function in the ocean exhibits modest change
in the north Pacific but accelerating changes in much of the Southern Ocean and particularly in the north Atlantic where the
gyre spins down in conjunction with the decrease in the thermohaline circulation. The results differ in non-trivial ways from
earlier equilibrium 2 × CO2 results with the CCCma model as a consequence of the coupling to a fully three-dimensional ocean model and the evolving nature
of the forcing.
Received: 24 September 1998 / Accepted: 8 October 1999 相似文献