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1.
W. May  E. Roeckner 《Climate Dynamics》2001,17(5-6):407-420
 The climate response to increasing levels of atmospheric greenhouse gases, prescribed according to the International Panel of Climate Change (IPCC) scenario IS92a, is studied in two model simulations. The reference simulation is a transient response experiment performed with a medium-resolution (T42) coupled general circulation model of the atmosphere and ocean (ECHAM4/OPYC) developed at the Max-Planck-Institute for Meteorology. For two 30-year “time slices”, representing the present-day climate and the future climate at the time of effective CO2 doubling, the annual mean climate states are compared with those obtained from the high-resolution (T106) ECHAM4 model forced with monthly sea surface temperatures and sea-ice from the coupled model. The large-scale changes in temperature, zonal wind, sea-level pressure and precipitation are broadly similar. This applies, in particular, to the respective zonal means. In general, except for precipitation, the responses in the time-slice experiments are slightly weaker than those simulated in the coupled model due to a smaller effect of the horizontal resolution on the simulations of the future (warmer) period than on the simulations of the present period. On a regional scale, the impact of horizontal resolution is smaller in the Southern than in the Northern Hemisphere, where the response differences are caused mainly by changes in the positions of the stationary waves. Although the precipitation responses are broadly similar, there are few notable exceptions such as a more pronounced maximum over the equatorial oceans in the T106 experiment but a weaker response over low-latitude land areas. Differences in precipitation response are found especially in areas with strong topographical control such as South America, for example. Received: 17 January 2000 / Accepted: 7 July 2000  相似文献   

2.
 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   

3.
The impact of different ocean models or sea surface temperature (SST) and sea-ice concentrations on cyclone tracks in the Northern Hemisphere midlatitudes is determined within a hierarchy of model simulations. A reference simulation with the coupled atmosphere ocean circulation model ECHAM/HOPE is compared with simulations using ECHAM and three simplified ocean and sea-ice representations: (1) a variable depth mixed layer (ML) ocean, (2) forcing by varying SST and sea-ice, and (3) with climatological SST and sea-ice; the latter two are from the coupled ECHAM/HOPE integration. The reference simulation reproduces the observed cyclone tracks. The cyclones are tracked automatically by a standard routine and the variability of individual cyclone trajectories within the storm tracks is determined by a cluster approach. In the forced simulation with varying SST, the geographical distribution and the statistics of the cyclones are not altered compared to the coupled reference simulation. In the ML- and the climatological simulation, deviations of the mean cyclone distribution are found which occur mainly in the North Pacific, and can partially be traced back to missing El Niño/Southern Oscillation (ENSO) variability. The climatological experiment is superior to the ML-experiment. The variability of the individual cyclone trajectories, as determined by the cluster analysis, reveals the same types and frequencies of propagation directions for all four representations of the lower boundary. The largest discrepancies for the cluster occupations are found for the climatological and the ML-simulation.  相似文献   

4.
Several studies have analysed the atmospheric response to sea-ice changes in the Arctic region, but only few have considered the Antarctic. Here, the atmospheric response to sea-ice variability in the Southern Hemisphere is investigated with the atmospheric general circulation model ECHAM5. The model is forced by the present and a projected future seasonal cycle of Antarctic sea ice. In September, the mean atmospheric response exhibits distinct similarities to the structure of the negative phase of the Southern Annular Mode, the leading mode of Southern Hemisphere variability. In the reduced Antarctic sea-ice integration, there is an equatorward shift of the Southern Hemisphere mid-latitude jet and the storm tracks. In contrast to a recent previous study, our findings indicate that a substantial impact of Southern Hemispheric future sea-ice reduction on the mid-latitude circulation cannot be ruled out.  相似文献   

5.
This study focuses on the differences between the present-day climate and the climate of the last glacial maximum (LGM) of 18 000 y BP using a zonally averaged energy balance climate model. The ocean is represented by a 2-D model with prescribed overturning pattern in which the overturning velocities can be adjusted freely. We discuss what influence the use of ice-age conditions (i.e. enhanced land-ice cover, reduced CO2-concentration and reduced oceanic overturning rate) has on the differences between ice-age and present-day climate. When compared to LGM sea-surface temperatures derived from proxy data, the model is able to simulate fairly well the important features of the meridional distribution of these temperature differences. Applying reduced ocean overturning rates during the LGM significantly decreases poleward heat transport in the oceans, thereby allowing for additional cooling of the polar regions and less cooling of the equatorial region. As a result, the agreement with CLIMAP proxy temperature differences increases, especially in the equatorial region. This mechanism can explain the slight differences in the CLIMAP proxy equatorial surface temperatures between the LGM and the present-day climate.  相似文献   

6.
Three different reconstructed wind-stress fields which take into account variations of the North Atlantic Oscillation, one general circulation model wind-stress field, and three radiative forcings (volcanic activity, insolation changes and greenhouse gas changes) are used with the UVic Earth System Climate Model to simulate the surface air temperature, the sea-ice cover, and the Atlantic meridional overturning circulation (AMOC) since 1500, a period which includes the Little Ice Age (LIA). The simulated Northern Hemisphere surface air temperature, used for model validation, agrees well with several temperature reconstructions. The simulated sea-ice cover in each hemisphere responds quite differently to the forcings. In the Northern Hemisphere, the simulated sea-ice area and volume during the LIA are larger than the present-day area and volume. The wind-driven changes in sea-ice area are about twice as large as those due to thermodynamic (i.e., radiative) forcing. For the sea-ice volume, changes due to wind forcing and thermodynamics are of similar magnitude. Before 1850, the simulations suggest that volcanic activity was mainly responsible for the thermodynamically produced area and volume changes, while after 1900 the slow greenhouse gas increase was the main driver of the sea-ice changes. Changes in insolation have a small effect on the sea ice throughout the integration period. The export of the thicker sea ice during the LIA has no significant effect on the maximum strength of the AMOC. A more important process in altering the maximum strength of the AMOC and the sea-ice thickness is the wind-driven northward ocean heat transport. In the Southern Hemisphere, there are no visible long-term trends in the simulated sea-ice area or volume since 1500. The wind-driven changes are roughly four times larger than those due to radiative forcing. Prior to 1800, all the radiative forcings could have contributed to the thermodynamically driven changes in area and volume. In the 1800s the volcanic forcing was dominant, and during the first part of the 1900s both the insolation changes and the greenhouse gas forcing are responsible for thermodynamically produced changes. Finally, in the latter part of the 1900s the greenhouse gas forcing is the dominant factor in determining the sea-ice changes in the Southern Hemisphere.
Jan SedláčekEmail:
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7.
In this study the global coupled atmosphere-ocean general circulation model ECHAM2/OPYC and its performance in simulating the present-day climate is presented. The model consists of the T21-spectral atmosphere general circulation model ECHAM2 and the ocean general circulation model OPYC with a resolution corresponding to a T42 Gaussian grid, with increasing resolution towards the equator. The sea-ice is represented by a dynamic thermodynamic sea-ice model with rheology. Both models are coupled using the flux correction technique. With the coupled model ECHAM2/OPYC a 210-year integration under present-day greenhouse gas conditions has been performed. The coupled model simulates a realistic mean climate state, which is close to the observations. The model generates several ENSO events without external forcing. Using traditional and advanced (POP-technique) methods these ENSO events have been analyzed. The results are consistent with the delayed action oscillator theory. The model simulates both a tropical and an extra-tropical response to ENSO, which are in good agreement with observations.  相似文献   

8.
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   

9.
Sea ice has been suggested, based on simple models, to play an important role in past glacial–interglacial oscillations via the so-called “sea-ice switch” mechanism. An important requirement for this mechanism is that multiple sea-ice extents exist under the same land ice configuration. This hypothesis of multiple sea-ice extents is tested with a state-of-the-art ocean general circulation model coupled to an atmospheric energy–moisture-balance model. The model includes a dynamic-thermodynamic sea-ice module, has a realistic ocean configuration and bathymetry, and is forced by annual mean forcing. Several runs with two different land ice distributions represent present-day and cold-climate conditions. In each case the ocean model is initiated with both ice-free and fully ice-covered states. We find that the present-day runs converge approximately to the same sea-ice state for the northern hemisphere while for the southern hemisphere a difference in sea-ice extent of about three degrees in latitude between the different runs is observed. The cold climate runs lead to meridional sea-ice extents that are different by up to four degrees in latitude in both hemispheres. While approaching the final states, the model exhibits abrupt transitions from extended sea-ice states and weak meridional overturning circulation, to less extended sea ice and stronger meridional overturning circulation, and vice versa. These transitions are linked to temperature changes in the North Atlantic high-latitude deep water. Such abrupt changes may be associated with Dansgaard–Oeschger events, as proposed by previous studies. Although multiple sea ice states have been observed, the difference between these states is not large enough to provide a strong support for the sea-ice-switch mechanism.  相似文献   

10.
The ice-covered Earth instability found in energy balance models is studied with a zonal mean statistical dynamical atmospheric model coupled to a global mixed layer ocean model. The response of the model to changes in solar constant is examined in two parallel studies, one with and one without a fixed meridional heat transport (a Q-flux) being included in the ocean model. The Q-flux is derived so as to make the climate with the current value of the solar constant resemble the earths current climate. In both cases the climate displays a hysteresis loop as the solar constant decreases and then increases, with two equilibrium states being possible for a range of values of the solar constant. In the case without a Q-flux, as in energy balance models, one state corresponds to an ice-covered Earth, and the other is partially covered. In the case with a Q-flux, because the poleward Q-flux is stronger in the Southern Hemisphere, one state corresponds to an ice-covered Northern Hemisphere, but a Southern Hemisphere that is only partially ice-covered; the other state has much reduced ice-cover in both hemispheres. In the case when the Q-flux is present, the sensitivity of the state with smaller ice-cover is about half as much, and the hysteresis loop extends over a smaller range of values of the solar constant. Also in this case there is a strong ice-covered Earth instability that sets in when the solar constant is about 13–14% below the current value. However in the case without a Q-flux the ice-covered Earth instability virtually disappears. The different behavior is attributed to the much lower efficiency of the meridional heat transport in the case with no Q-flux. The behavior in this case may be more realistic for cold climates. The results in both cases confirm the simple analytical relation between global mean surface temperature and global ice area found in energy balance models.  相似文献   

11.
To predict the evolution of glaciers in an enhanced greenhouse climate, results from a global climate model, a glacier melt/accumulation model, and a glacier flow model were combined. The method was applied to Storglaciären, a small well-studied glacier in northern Sweden. The difference between the present climate and a 2 × CO2 climate around the year 2050 was extracted from a model experiment with the ECHAM4-T106 high resolution climate model for time slices at present and in 2050, using prescribed boundary conditions of sea surface temperature and sea-ice distribution, which are derived from a lower resolution transient run of the ECHAM4-T42/OPIC-coupled atmosphere ocean model between present and 2050. The local climatic conditions on the glacier for 2050 were obtained by adding the modelled local climate changes to the observed local present-day climate. The combination of the comprehensive models presented offers a tool to test and calibrate simplified models which are applicable to a much larger sample of glaciers. For the region of Storglaciären, the GCM projected temperature is found to increase most strongly during the winter months, but also shows a warming during the transition from spring to summer, and again between summer and fall, thus extending the melt season by three to four weeks. Precipitation, on the other hand, decreases by approximately 5% during May to September while there is a stronger increase of approximately 14% for the rest of the year. The consequent increase in winter accumulation on Storglaciären is more than compensated by the increase in ablation during the melt season. The glacier flow model predicts a 300 m retreat of the glacier terminus by the middle of the next century, and a loss of 30% of the present ice mass.  相似文献   

12.
Response of the tropical Pacific to changes in extratropical clouds   总被引：1，自引：0，他引：1  
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13.
 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   

14.
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15.
 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   

16.
We study the mechanisms of glacial inception by using the Earth system model of intermediate complexity, CLIMBER-2, which encompasses dynamic modules of the atmosphere, ocean, biosphere and ice sheets. Ice-sheet dynamics are described by the three-dimensional polythermal ice-sheet model SICOPOLIS. We have performed transient experiments starting at the Eemiam interglacial, at 126 ky BP (126,000 years before present). The model runs for 26 kyr with time-dependent orbital and CO2 forcings. The model simulates a rapid expansion of the area covered by inland ice in the Northern Hemisphere, predominantly over Northern America, starting at about 117 kyr BP. During the next 7 kyr, the ice volume grows gradually in the model at a rate which corresponds to a change in sea level of 10 m per millennium. We have shown that the simulated glacial inception represents a bifurcation transition in the climate system from an interglacial to a glacial state caused by the strong snow-albedo feedback. This transition occurs when summer insolation at high latitudes of the Northern Hemisphere drops below a threshold value, which is only slightly lower than modern summer insolation. By performing long-term equilibrium runs, we find that for the present-day orbital parameters at least two different equilibrium states of the climate system exist—the glacial and the interglacial; however, for the low summer insolation corresponding to 115 kyr BP, we find only one, glacial, equilibrium state, while for the high summer insolation corresponding to 126 kyr BP only an interglacial state exists in the model.
Reinhard CalovEmail:
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17.
Impact of ocean model resolution on CCSM climate simulations   总被引：1，自引：1，他引：0  
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   

18.
Polar amplification of climate change in coupled models   总被引：12，自引：5，他引：12  
The Northern Hemisphere polar amplification of climate change is documented in models taking part in the Coupled Model Intercomparison Project and in the new version of the Community Climate System Model. In particular, the magnitude, spatial distribution, and seasonality of the surface warming in the Arctic is examined and compared among the models. The range of simulated polar warming in the Arctic is from 1.5 to 4.5 times the global mean warming. While ice-albedo feedback is likely to account for much of the polar amplification, the strength of the feedback depends on numerous physical processes and parametrizations which differ considerably among the models. Nonetheless, the mean sea-ice state in the control (or present) climate is found to influence both the magnitude and spatial distribution of the high-latitude warming in the models. In particular, the latitude of the maximum warming is correlated inversely and significantly with sea-ice extent in the control climate. Additionally, models with relatively thin Arctic ice cover in the control climate tend to have higher polar amplification. An intercomparison of model results also shows that increases in poleward ocean heat transport at high latitudes and increases in polar cloud cover are significantly correlated to amplified Arctic warming. This suggests that these changes in the climate state may modify polar amplification. No significant correlation is found between polar amplification and the control climate continental ice and snow cover.  相似文献   

19.
ENSO teleconnections in projections of future climate in ECHAM5/MPI-OM   总被引：1，自引：1，他引：0  
The teleconnections of the El Niño/Southern Oscillation (ENSO) in future climate projections are investigated using results of the coupled climate model ECHAM5/MPI-OM. For this, the IPCC SRES scenario A1B and a quadrupled CO2 simulation are considered. It is found that changes of the mean state in the tropical Pacific are likely to condition ENSO teleconnections in the Pacific North America (PNA) region and in the North Atlantic European (NAE) region. With increasing greenhouse gas emissions the changes of the mean states in the tropical and sub-tropical Pacific are El Niño-like in this particular model. Sea surface temperatures in the tropical Pacific are increased predominantly in its eastern part and redistribute the precipitation further eastward. The dynamical response of the atmosphere is such that the equatorial east–west (Walker) circulation and the eastern Pacific inverse Hadley circulation are decreased. Over the subtropical East Pacific and North Atlantic the 200 hPa westerly wind is substantially increased. Composite maps of different climate parameters for positive and negative ENSO events are used to reveal changes of the ENSO teleconnections. Mean sea level pressure and upper tropospheric zonal winds indicate an eastward shift of the well-known teleconnection patterns in the PNA region and an increasing North Atlantic oscillation (NAO) like response over the NAE region. Surface temperature and precipitation underline this effect, particularly over the North Pacific and the central North Atlantic. Moreover, in the NAE region the 200 hPa westerly wind is increasingly related to the stationary wave activity. Here the stationary waves appear NAO-like.  相似文献   

20.
A three-level baroclinic ocean model has been developed for studying climate and climatic change.The major large-scale features of temperature field,such as the belt of cold water,strong upwelling and the main currents in the Pacific Ocean,have been reproduced.The simulated results show that the seasonal variation of current is related to that of trade wind system.The simulated equatorial countercurrent is strong in summer and weak even vanished in winter.The south equatorial current,north of the equator is stronger in winter than in summer,but the contrary is the case with the current existing in the Southern Hemisphere.  相似文献   

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