GCM sensitivity experiments with locally modified land surface properties over tropical South America

Ensembles of 1-year-long experiments with a relatively high-resolution ECMWF model were conducted in order to investigate the impact of modified land surface properties on local, regional and large-scale atmospheric circulations. The modifications consisted of changes to land cover and increased albedo over the northern part of South America. In many respects the experimental design resembles the setting of classical deforestation experiments. The local model response to imposed modifications, which includes a reduction in precipitation as well as in evaporation and an increase in surface temperature, was found to be stronger in dry (July–September, JAS) than in wet (January–March, JFM) season, and in the ensemble with higher albedo value. Local drying is discussed in terms of locally generated overturning that resembles a direct thermal circulation. The effects of this circulation seem to be dominant over the reduction in large-scale moisture supply from the adjacent ocean. On large scales, changes to the Pacific branch of the Walker circulation lead, through modified divergent flow, to a tropics-wide impact on precipitation. In addition to South America, the largest changes are seen in the south Pacific convergence zone in JFM, while the impact on the Atlantic inter-tropical convergence zone is stronger in JAS. In the extratropics, there is little change in precipitation. In the upper troposphere, a distinctive teleconnection wave-pattern could be seen in the Pacific/North American region during JFM. A notable feature in the upper-air model response in JAS is a wave train extending from South America, over the northern Atlantic into Europe. With regard to the interaction between the land surface response and model systematic errors, our results suggest that the erroneous shift of the downward branch of the Pacific/South American Walker circulation is likely to be a cause, rather than a consequence, of the rainfall deficit over South America in the model climatology.     

Climate sensitivity to changes in solar insolation in a simple coupled climate model

A simple coupled ocean, atmosphere and sea-ice model is presented. The idealised model consists of a zonally averaged land and ocean strip of constant angular width extending from pole to pole. The meridional energy transport in the ocean is modelled by contributions from the large scale thermohaline overturning cells and from horizontal diffusive fluxes. The atmospheric meridional energy transports are parametrised as diffusive fluxes in addition to advective transports in the Hadley domain. This parametrisation resolves the equatorward moisture transport as well as the poleward transport of potential energy in the upper branch of the Hadley circulation. The model reproduces the annual averaged meridional energy transports in the climate system with a small number of free model parameters. The basic feedbacks between the three climatic components are studied by investigating the model's sensitivity towards reductions in the solar insolation. It is found that the meridional energy transport in the ocean does not amplify the ice albedo feedback. This has important implications for modelling the climate sensitivity in atmosphere-only models, as these would exaggerate the sensitivity to changes in the solar insolation if their parametrisations of the meridional energy transport are constrained by surface temperatures. The role of the dependence of the atmospheric transports on the meridional temperature gradient is shown to have a significant influence on the sensitivity on the coupled model, and the inclusion of seasonal cycles greatly increase the models sensitivity. The Hadley circulation does significantly alter the strength of the ice-albedo feedback in the coupled model. The idealised configuration of the model makes it a useful tool for studying the feedbacks in the ocean-atmosphere-sea ice system in the context of the "Snowball Earth" hypothesis.     

Planetary boundary layer and surface layer sensitivity to land surface parameters

The sensitivity of the development of the convective planetary boundary layer (PBL) and the surface layer are examined using a coupled surface parameterization and detailed PBL model. First, the coupling is verified against observations from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE). Results of the sensitivity experiments indicate that the PBL is most sensitive to the amount of soil water content, and the proximity of the soil water content to critical soil texture values (field capacity and wilting point). While vegetation cover is not the most sensitive parameter at the surface, its influence on the surface energy and hydrologic balance is crucial. Model sensitivity to minimum stomatal resistance, type of soil parameterization and canopy height (surface roughness and displacement depth) is also discussed.     

Climate sensitivity due to increased CO2: experiments with a coupled atmosphere and ocean general circulation model

A version of the National Center for Atmospheric Research community climate model — a global, spectral (R15) general circulation model — is coupled to a coarse-grid (5° latitude-] longitude, four-layer) ocean general circulation model to study the response of the climate system to increases of atmospheric carbon dioxide (CO₂). Three simulations are run: one with an instantaneous doubling of atmospheric CO₂ (from 330 to 660 ppm), another with the CO₂ concentration starting at 330 ppm and increasing linearly at a rate of 1% per year, and a third with CO₂ held constant at 330 pm. Results at the end of 30 years of simulation indicate a globally averaged surface air temperature increase of 1.6° C for the instantaneous doubling case and 0.7°C for the transient forcing case. Inherent characteristics of the coarse-grid ocean model flow sea-surface temperatures (SSTs) in the tropics and higher-than-observed SSTs and reduced sea-ice extent at higher latitudes] produce lower sensitivity in this model after 30 years than in earlier simulations with the same atmosphere coupled to a 50-m, slab-ocean mixed layer. Within the limitations of the simulated meridional overturning, the thermohaline circulation weakens in the coupled model with doubled CO₂ as the high-latitude ocean-surface layer warms and freshens and westerly wind stress is decreased. In the transient forcing case with slowly increasing CO₂ (30% increase after 30 years), the zonal mean warming of the ocean is most evident in the surface layer near 30°–50° S. Geographical plots of surface air temperature change in the transient case show patterns of regional climate anomalies that differ from those in the instantaneous CO₂ doubling case, particularly in the North Atlantic and northern European regions. This suggests that differences in CO₂ forcing in the climate system are important in CO₂ response in regard to time-dependent climate anomaly regimes. This confirms earlier studies with simple climate models that instantaneous CO₂ doubling simulations may not be analogous in all respects to simulations with slowly increasing CO₂.A portion of this study is supported by the US Department of Energy as part of its Carbon Dioxide Research Program     

The role of surface energy balance complexity in land surface models' sensitivity to increasing carbon dioxide

Global simulations with the Bureau of Meteorology Research Centre climate model coupled to the CHAmeleon Surface Model (CHASM) are used to explore the sensitivity of simulated changes in evaporation, precipitation, air temperature and soil moisture resulting from a doubling of carbon dioxide in the atmosphere. Five simulations, using prescribed sea surface temperatures, are conducted which are identical except in the level of complexity used to represent the surface energy balance. The simulation of air temperature, precipitation, evaporation and soil moisture at 1 2 CO₂ and at 2 2 CO₂ are generally sensitive at statistically significant levels to the complexity of the surface energy balance representation (i.e. the level of complexity used to represent these processes affects the simulated climate). However, changes in mean quantities, resulting from a doubling of atmospheric CO₂, are generally insensitive to the surface energy balance complexity. Conversely, changes in the spatial and temporal variance of evaporation and soil moisture are sensitive to the surface energy balance complexity. The addition of explicit canopy interception to the simplest model examined here enables that model to capture the change in the variance of evaporation simulated by the more complex models. In order to simulate changes in the variability of soil moisture, an explicit parameterization of bare soil evaporation is required. Overall, our results increase confidence that the simulation by climate models of the mean impact of increasing CO₂ on climate are reliable. Changes in the variability resulting from increased CO₂ on air temperature, precipitation or evaporation are also likely to be reliable since climate models typically use sufficiently complex land surface schemes. However, if the impact of increased CO₂ on soil moisture is required, then a more complex surface energy balance representation may be needed in order to capture changes in variability. Overall, our results imply that the level of complexity used by most climate models to represent the surface energy balance is appropriate and does not contribute significant uncertainty in the simulation of changes resulting from increasing CO₂. Our results only relate to surface energy balance complexity, and major uncertainties remain in how to model the surface hydrology and changes in the physiology, structural characteristics and distribution of vegetation. Future developments of land surface models should therefore focus on improving the representation of these processes.     

Precipitation in the Anatolian Peninsula: sensitivity to increased SSTs in the surrounding seas

Effects of the increased sea surface temperatures (SSTs) in the surrounding seas of the Anatolian Peninsula on the precipitation it receives are investigated through sensitivity simulations using a state-of-the-art regional climate model, RegCM3. The sensitivity simulations involve 2-K increases to the SSTs of the Aegean, eastern Mediterranean and Black seas individually as well as collectively. All the simulations are integrated over a 10-year period between 1990 and 2000. The model simulations of this study indicate that the precipitation of the peninsula is sensitive to the variations of the SSTs of the surrounding seas. In general, increased SSTs lead to increases in the precipitation of the peninsula as well as that of the seas considered. The statistically significant increases at 95% confidence levels largely occur along the coastal areas of the peninsula that are in the downwind side of the seas. Significant increases do also take place in the interior areas of the peninsula, especially in the eastern Anatolia in winter. The simulations reveal that eastern Mediterranean Sea has the biggest potential to affect the precipitation in the peninsula. They also demonstrate that taking all three seas into account simultaneously enhances the effect of SSTs on the peninsula’s precipitation, and extends the areas with statistically significant increases.     

Climate sensitivity to cloud optical properties

A radiative–convective model was developed to investigate the sensitivity of climate to cloud optical properties and the related feedback processes. This model demonstrates that the Earth's surface temperature increases with cloud optical depth when the clouds are very thin but decreases with cloud optical depth when the cloud shortwave (solar) radiative forcing is larger than the cloud longwave (terrestrial) radiative forcing. When clouds are included in the model, the magnitude of the greenhouse effect due to a doubling of the CO₂ concentration varies with the cloudoptical depth: the thicker the clouds, the weaker the greenhouse warming. In addition, a small variation in the cloud droplet size has a larger impact on the equilibrium state temperature in the lower atmosphere than the warming caused by a doubling of the CO₂ concentration: a 2% increase in the average cloud droplet size per degree increase in temperature doubles the warming caused by the doubling of the CO₂ concentration. These findings suggest that physically reliable correlations between the cloud droplet size and macrophysical meteorological variables such as temperature, wind and water vapor fields are needed on a global climate scale to assess the climate impact of increases in greenhouse gases.     

Sensitivity to prescribed changes in sea surface temperature and sea ice in doubled carbon dioxide experiments

Time sclice experiments are performed with the atmospheric GCM ARPEGE, developed at Météo-France, to study the impact to increases in the atmospheric carbon dioxide. This spectral model runs at T42 horizontal resolution with 30 vertical layers including a comprehensive tropospheric and stratospheric resolution and a prognostic parameterization of the ozone mixing ratio. The model is forced in a 5-year control run by climatological SSTs and sea-ice extents in order to obtain an accurate simulation of the present-day climate. Two perturbed runs are performed using SSTs and sea-ice extents for doubled CO₂ concentration, obtained from transient runs performed by two coupled atmospheric-oceanic models run at the Max Planck Institute (MPI) in Hamburg and the Hadley Centre (HC). A global surface temperature warming of 1.6 K is obtained with the MPI SST anomalies and 1.9 K with the HC SST anomalies. The precipitation rate increases by 4.2% (and 4.7%). The features obtained in the stratosphere (a cooling increasing with the altitude and an increase in the ozone mixing ratio) are not sensitive to the oceanic forcing. On the contrary, the anomalies in the troposphere such as a warming increasing with altitude, an acceleration of westerly jets and a raised cloud height, depend on the oceanic forcing imposed in the two perturbed runs. Special attention is given to continental areas where the impact of the oceanic forcing is studied over eight regions around the globe. Regions sensitive to oceanic forcing such as Europe are identified in contrast with areas where the patterns are driven by land-surface physical processes, such as over continental Asia. Finally, the Köppen classification is applied to the climate simulated in the three experiments. Both doubled CO₂ runs show the same predominance of global warming over precipitation changes in the Kbppen analyses.     

全球变暖形势下中国陆表水分的变化

利用政府间气候变化委员会第四次评估报告（IPCCAR4）中的10个耦合模式CO：加倍试验和控制试验的模拟结果,分析了全球变暖背景下中国水分的变化。结果表明,随着全球变暖,东亚夏季风增强,冬季风减弱,使得冬夏季向中国区域输送的水汽都增强;中国区域降水,夏季除长江流域外基本都增加,冬季除华南外都增加。夏季降水蒸发差（P—E）除了在东北和南方增加外,从长江流域一直到西北有一带状减小带;冬季几乎所有模式的P—E表现为北方增加、南方减小。在全球变暖背景下,降水、蒸发和径流的综合结果以及积雪的作用使得土壤湿度在干旱区增加,且冬季干旱区土壤变湿的强度和范围大于夏季,然而在其他区域土壤湿度减少。上述结论是基于多模式集合平均结果,对未来气候的预估具有一定的参考价值,然而模式间存在较强差异性,仍具有较大不确定性。     

Evaluation of an urban land surface scheme over a tropical suburban neighborhood

A two-step statistical downscaling method has been reviewed and adapted to simulate twenty-first-century climate projections for the Gulf of Fonseca (Central America, Pacific Coast) using Coupled Model Intercomparison Project (CMIP5) climate models. The downscaling methodology is adjusted after looking for good predictor fields for this area (where the geostrophic approximation fails and the real wind fields are the most applicable). The method’s performance for daily precipitation and maximum and minimum temperature is analysed and revealed suitable results for all variables. For instance, the method is able to simulate the characteristic cycle of the wet season for this area, which includes a mid-summer drought between two peaks. Future projections show a gradual temperature increase throughout the twenty-first century and a change in the features of the wet season (the first peak and mid-summer rainfall being reduced relative to the second peak, earlier onset of the wet season and a broader second peak).     

The effect of land surface changes on Eemian climate

Transient experiments for the Eemian (128–113 ky BP) were performed with a complex, coupled earth system model, including atmosphere, ocean, terrestrial biosphere and marine biogeochemistry. In order to investigate the effect of land surface parameters (background albedo, vegetation and tree fraction and roughness length) on the simulated changes during the Eemian, simulations with interactive coupling between climate and vegetation were compared with additional experiments in which these feedbacks were suppressed. The experiments show that the influence of land surface on climate is mainly caused by changes in the albedo. For the northern hemisphere high latitudes, land surface albedo is changed partially due to the direct albedo effect of the conversion of grasses into forest, but the indirect effect of forests on snow albedo appears to be the major factor influencing the total absorption of solar radiation. The Western Sahara region experiences large changes in land surface albedo due to the appearance of vegetation between 128 and 120 ky BP. These local land surface albedo changes can be as much as 20%, thereby affecting the local as well as the global energy balance. On a global scale, latent heat loss over land increases more than 10% for 126 ky BP compared to present-day.     

Erratum to: Numerical simulation of changes in tropical cyclone intensity using a coupled air-sea model

正Erratum to:Acta Meteor Sinica DOI 10.1007/sl3351-013-0506-z The original version of this article unfortunately contained a mistake.The presentation of DOI number was incorrect.The corrected DOI number is 10.1007/sl3351-013-0503-2     

African monsoon teleconnections with tropical SSTs: validation and evolution in a set of IPCC4 simulations

A set of 12 state-of-the-art coupled ocean-atmosphere general circulation models (OAGCMs) is explored to assess their ability to simulate the main teleconnections between the West African monsoon (WAM) and the tropical sea surface temperatures (SSTs) at the interannual to multi-decadal time scales. Such teleconnections are indeed responsible for the main modes of precipitation variability observed over West Africa and represent an interesting benchmark for the models that have contributed to the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC4). The evaluation is based on a maximum covariance analysis (MCA) applied on tropical SSTs and WAM rainfall. To distinguish between interannual and multi-decadal variability, all datasets are partitioned into low-frequency (LF) and high-frequency (HF) components prior to analysis. First applied to HF observations, the MCA reveals two major teleconnections. The first mode highlights the strong influence of the El Niño Southern Oscillation (ENSO). The second mode reveals a relationship between the SST in the Gulf of Guinea and the northward migration of the monsoon rainbelt over the West African continent. When applied to HF outputs of the twentieth century IPCC4 simulations, the MCA provides heterogeneous results. Most simulations show a single dominant Pacific teleconnection, which is, however, of the wrong sign for half of the models. Only one model shows a significant second mode, emphasizing the OAGCMs’ difficulty in simulating the response of the African rainbelt to Atlantic SST anomalies that are not synchronous with Pacific anomalies. The LF modulation of these HF teleconnections is then explored through running correlations between expansion coefficients (ECs) for SSTs and precipitation. The observed time series indicate that both Pacific and Atlantic teleconnections get stronger during the twentieth century. The IPCC4 simulations of the twentieth and twenty-first centuries do not show any significant change in the pattern of the teleconnections, but the dominant ENSO teleconnection also exhibits a significant strengthening, thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing. Finally, the MCA is also applied to the LF data. The first observed mode reveals a well-known inter-hemispheric SST pattern that is strongly related to the multi-decadal variability of the WAM rainfall dominated by the severe drying trend from the 1950s to the 1980s. Whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings, only 5 among the 12 IPCC4 models capture some features of this LF coupled mode. This result suggests the need for a more detailed validation of the WAM variability, including a dynamical interpretation of the SST–rainfall relationships.     

The seasonal cycle of tropical Pacific sea surface temperature in a coupled GCM

 The mechanisms responsible for the seasonal cycle in the tropical central and eastern Pacific sea surface temperature (SST) are investigated using a coupled general circulation model. We find that the annual westward propagation of SST anomalies along the equator is explained by a two-stage process. The first stage sets the phase of the variation at the eastern boundary. The strengthening of the local Hadley Circulation in boreal summer leads to a strengthening of the northward winds that blow across the equator. These stronger winds drive enhanced evaporation and entrainment cooling of the oceanic mixed layer. The resulting change in SST is greatest in the east because the mixed layer is at its shallowest there. As the east Pacific SST cools the zonal SST gradient in the central Pacific becomes more negative. This development signals the onset of the second stage in the seasonal variation of equatorial SST. In response to the anomalous SST gradient the local westward wind stress increases. This increase drives cooling of the oceanic mixed layer in which no single mechanism dominates: enhanced evaporation, wind-driven entrainment, and westward advection all contribute. We discuss the role that equatorial upwelling plays in modulating mixed layer depth and hence the entrainment cooling, and we highlight the importance of seasonal variations in mixed layer depth. In sum these processes act to propagate the SST anomaly westward. Received: 22 February 1999 / Accepted: 20 March 2000     

Impact of land use changes on surface warming in China

Land use changes such as urbanization, agriculture, pasturing, deforestation, desertification and irrigation can change the land surface heat flux directly, and also change the atmospheric circulation indirectly, and therefore affect the local temperature. But it is difficult to separate their effects from climate trends such as greenhouse-gas effects. Comparing the decadal trends of the observation station data with those of the NCEP/NCAR Reanalysis (NNR) data provides a good method to separate the effects because the NNR is insensitive to land surface changes. The effects of urbanization and other land use changes over China are estimated by using the difference between the station and the NNR surface temperature trends. Our results show that urbanization and other land use changes may contribute to the observed 0.12℃ (10 yr)- 1 increase for daily mean surface temperature, and the 0.20℃ (10 yr)- 1 and 0.03℃ (10 yr)-1 increases for the daily minimum and maximum surface temperatures, respectively. The urban heat island effect and the effects of other land-use changes mayalso play an important role in the diurnal temperature range change. The spatial pattern of the differences in trends shows a marked heterogeneity.The land surface degradation such as deforestation and desertification due to human activities over northern China, and rapidly-developed urbanization over southern China, may have mostly contributed to the increases at stations north of about 38°N and in Southeast China, respectively. Furthermore, the vegetation cover increase due to irrigation and fertilization may have contributed to the decreasing trend of surface temperature over the lower Yellow River Basin. The study illustrates the possible impacts of land use changes on surface temperature over China.