Simulated Climate and Climate Change in the GFDL CM2.5 High-Resolution Coupled Climate Model

Coupled climate model simulations of volcanic eruptions and abrupt changes in CO2 concentration are compared in multiple realizations of the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1). The change in global-mean surface temperature (GMST) is analyzed to determine whether a fast component of the climate sensitivity of relevance to the transient climate response (TCR; defined with the 1% yr−1 CO2-increase scenario) can be estimated from shorter-time-scale climate changes. The fast component of the climate sensitivity estimated from the response of the climate model to volcanic forcing is similar to that of the simulations forced by abrupt CO2 changes but is 5%–15% smaller than the TCR. In addition, the partition between the top-of-atmosphere radiative restoring and ocean heat uptake is similar across radiative forcing agents. The possible asymmetry between warming and cooling climate perturbations, which may affect the utility of volcanic eruptions for estimating the TCR, is assessed by comparing simulations of abrupt CO2 doubling to abrupt CO2 halving. There is slightly less (~5%) GMST change in 0.5 × CO2 simulations than in 2 × CO2 simulations on the short (~10 yr) time scales relevant to the fast component of the volcanic signal. However, inferring the TCR from volcanic eruptions is more sensitive to uncertainties from internal climate variability and the estimation procedure. The response of the GMST to volcanic eruptions is similar in GFDL CM2.1 and GFDL Climate Model, version 3 (CM3), even though the latter has a higher TCR associated with a multidecadal time scale in its response. This is consistent with the expectation that the fast component of the climate sensitivity inferred from volcanic eruptions is a lower bound for the TCR.

Water vapour plays a key role in the Earth's energy balance. Almost 50% of the absorbed solar radiation at the surface is used to cool the surface, through evaporation, and warm the atmosphere, through release of latent heat. Latent heat is the single largest factor in warming the atmosphere and in transporting heat from low to high latitudes. Water vapour is also the dominant greenhouse gas and contributes to a warming of the climate system by some 24°C (Kondratev 1972). However, water vapour is a passive component in the troposphere as it is uniquely determined by temperature and should therefore be seen as a part of the climate feedback system. In this short overview, we will first describe the water on planet Earth and the role of the hydrological cycle: the way water vapour is transported between oceans and continents and the return of water via rivers to the oceans. Generally water vapour is well observed and analysed; however, there are considerable obstacles to observing precipitation, in particular over the oceans. The response of the hydrological cycle to global warming is far reaching. Because different physical processes control the change in water vapour and evaporation/precipitation, this leads to a more extreme distribution of precipitation making, in general, wet areas wetter and dry areas dryer. Another consequence is a transition towards more intense precipitation. It is to be expected that the changes in the hydrological cycle as a consequence of climate warming may be more severe that the temperature changes.

A novel tropical cyclone (TC) detection technique designed for coarse-resolution models is tested and evaluated. The detector, based on the Okubo–Weiss–Zeta parameter (OWZP), is applied to a selection of Coupled Model Intercomparison Project, phase 3 (CMIP3), models [Commonwealth Scientific and Industrial Research Organisation Mark, version 3.5 (CSIRO-Mk3.5); Max Planck Institute ECHAM5 (MPI-ECHAM5); and Geophysical Fluid Dynamics Laboratory Climate Model, versions 2.0 (GFDL CM2.0) and 2.1 (GFDL CM2.1)], and the combined performance of the model and detector is assessed by comparison with observed TC climatology for the period 1970–2000. Preliminary TC frequency projections are made using the three better-performing models by comparing the detected TC climatologies between the late twentieth and late twenty-first centuries. Very reasonable TC formation climatologies were detected in CSIRO-Mk3.5, MPI-ECHAM5, and GFDL CM2.1 for most basins, with the exception of the North Atlantic, where a large underdetection was present in all models. The GFDL CM2.0 model was excluded from the projection study because of a systematic underdetection in all basins. The above detection problems have been reported in other published studies, which suggests model rather than detector limitations are mostly responsible. This study demonstrates that coarse-resolution climate models do in general produce TC-like circulations with realistic geographical and seasonal distributions detectable by the OWZP TC detector. The preliminary projection results are consistent with the published literature, based on higher-resolution studies, of a global reduction of TCs between about 6% and 20%, with a much larger spread of results (about +20% to −50%) in individual basins.

This study explores the potential impact anthropogenic climate change may have upon hazardous convective weather (HCW; i.e., tornadoes, large hail, and damaging wind gusts) in the United States. Utilizing the Weather Research and Forecasting (WRF) Model, high-resolution (4 km) dynamically downscaled simulations of the Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL CM3), are produced for a historical (1971–2000) and future (2071–2100) period. Synthetic HCW day climatologies are created using upward vertical velocity (UVV) exceeding 22 m s−1 as a proxy for HCW occurrence and subsequently compared to the environmental approach of estimating changes in daily frequency of convective environments favorable for HCW (NDSEV) from the driving climate model. Results from the WRF simulations demonstrate that the proxy for HCW becomes more frequent by the end of the twenty-first century, with the greatest absolute increases in daily frequency occurring during the spring and summer. Compared to NDSEV from GFDL CM3, both approaches suggest a longer HCW season, perhaps lengthening by more than a month. The change in environmental estimates are 2–4 times larger than that gauged from WRF; further analyses show that the conditional probability of HCW given NDSEV declines during summer for much of the central United States, a result that may be attributed to both an increase in the magnitude of convective inhibition (CIN) and decreased forcing for ascent, hindering convective initiation. Such an outcome supports the motivation for continued use of dynamical downscaling to overcome the limitations of the GCM-based environmental analysis.

Over the past century, land-use has changed in southeast Queensland, and when coupled with climatic change, the risk of flooding has increased. This research aims to examine impacts of climate and land-use changes on flood runoff in southeast Queensland, Australia. A rainfall–runoff model, RORB, was calibrated and validated using observed flood hydrographs for one rural and one urbanized catchment, for 1961–1990. The validated model was then used to generate flood hydrographs using projected rainfall based on two climate models: the Geophysical Fluid Dynamics Laboratory Climate Model 2.1 (GFDL CM2.1) and the Conformal-Cubic Atmospheric Model (CCAM), for 2016–2045. Projected daily rainfall for the two contrasting periods was used to derive adjustment factors for a given frequency of occurrence. Two land-use change scenarios were used to evaluate likely impacts. Based on the projected rainfall, the results showed that, in both catchments, future flood magnitudes are unlikely to increase for large flood events. Extreme land-use change would significantly impact flooding in the rural catchment, but not the urbanized catchment.Editor Z.W. Kundzewicz; Associate editor Y. Gyasi-Agyei

The unphysical virtual salt flux (VSF) formulation widely used in the ocean component of climate models has the potential to cause systematic and significant biases in modeling the climate system and projecting its future evolution. Here a freshwater flux (FWF) and a virtual salt flux version of the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1) are used to evaluate and quantify the uncertainties induced by the VSF formulation. Both unforced and forced runs with the two model versions are performed and compared in detail. It is found that the differences between the two versions are generally small or statistically insignificant in the unforced control runs and in the runs with a small external forcing. In response to a large external forcing, however, some biases in the VSF version become significant, especially the responses of regional salinity and global sea level. However, many fundamental aspects of the responses differ only quantitatively between the two versions. An unexpected result is the distinctly different ENSO responses. Under a strong external freshwater forcing, the great enhancement of the ENSO variability simulated by the FWF version does not occur in the VSF version and is caused by the overexpansion of the top model layer. In summary, the principle assumption behind using virtual salt flux is not seriously violated and the VSF model has the ability to simulate the current climate and project near-term climate evolution. For some special studies such as a large hosing experiment, however, both the VSF formulation and the use of the FWF in the geopotential coordinate ocean model could have some deficiencies and one should be cautious to avoid them.

A regional atmosphere model [Weather Research and Forecasting model version 3 (WRF3)] and a land surface model [Community Land Model, version 3.5 (CLM3.5)] were coupled to study the interactions between the atmosphere and possible future California land-cover changes. The impact was evaluated on California’s climate of changes in natural vegetation under climate change and of intentional afforestation. The ability of WRF3 to simulate California’s climate was assessed by comparing simulations by WRF3–CLM3.5 and WRF3–Noah to observations from 1982 to 1991. Using WRF3–CLM3.5, the authors performed six 13-yr experiments using historical and future large-scale climate boundary conditions from the Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1). The land-cover scenarios included historical and future natural vegetation from the Mapped Atmosphere-Plant-Soil System-Century 1 (MC1) dynamic vegetation model, in addition to a future 8-million-ha California afforestation scenario. Natural vegetation changes alone caused summer daily-mean 2-m air temperature changes of −0.7° to +1°C in regions without persistent snow cover, depending on the location and the type of vegetation change. Vegetation temperature changes were much larger than the 2-m air temperature changes because of the finescale spatial heterogeneity of the imposed vegetation change. Up to 30% of the magnitude of the summer daily-mean 2-m air temperature increase and 70% of the magnitude of the 1600 local time (LT) vegetation temperature increase projected under future climate change were attributable to the climate-driven shift in land cover. The authors projected that afforestation could cause local 0.2°–1.2°C reductions in summer daily-mean 2-m air temperature and 2.0°–3.7°C reductions in 1600 LT vegetation temperature for snow-free regions, primarily because of increased evapotranspiration. Because some of these temperature changes are of comparable magnitude to those projected under climate change this century, projections of climate and vegetation change in this region need to consider these climate–vegetation interactions.

Recent diagnostics with the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1), coupled model’s twentieth-century simulations reveal that this particular model demonstrates skill in capturing the mean and variability associated with the South Asian summer monsoon precipitation. Motivated by this, the authors examine the future projections of the mean monsoon and synoptic systems in this model’s simulations in which quadrupling of CO2 concentrations are imposed. In a warmer climate, despite a weakened cross-equatorial flow, the time-mean precipitation over peninsular parts of India increases by about 10%–15%. This paradox is interpreted as follows: the increased precipitation over the equatorial western Pacific forces an anomalous descending circulation over the eastern equatorial Indian Ocean, the two regions being connected by an overturning mass circulation. The spatially well-organized anomalous precipitation over the eastern equatorial Indian Ocean forces twin anticyclones as a Rossby wave response in the lower troposphere. The southern component of the anticyclone opposes and weakens the climatological cross-equatorial monsoon flow. The patch of easterly anomalies centered in the southern Arabian Sea is expected to deepen the thermocline north of the equator. Both these factors limit the coastal upwelling along Somalia, resulting in local sea surface warming and eventually leading to a local maximum in evaporation over the southern Arabian Sea. It is shown that changes in SST are predominantly responsible for the increase in evaporation over the southern Arabian Sea. The diagnostics suggest that in addition to the increased CO2-induced rise in temperature, evaporation, and atmospheric moisture, local circulation changes in the monsoon region further increase SST, evaporation, and atmospheric moisture, leading to increased rainfall over peninsular parts of India. This result implies that accurate observation of SST and surface fluxes over the Indian Ocean is of urgent need to understand and monitor the response of the monsoon in a warming climate. To understand the regional features of the rainfall changes, the International Pacific Research Center (IPRC) Regional Climate Model (RegCM), with three different resolution settings (0.5° × 0.5°, 0.75° × 0.75°, and 1.0° × 1.0°), was integrated for 20 yr, with lateral and lower boundary conditions taken from the GFDL model. The RegCM solutions confirm the major results obtained from the GFDL model but also capture the orographic nature of monsoon precipitation and regional circulation changes more realistically. The hypothesis that in a warmer climate, an increase in troposphere moisture content favors more intense monsoon depressions is tested. The GFDL model does not reveal any changes, but solutions from the RegCM suggest a statistically significant increase in the number of storms that have wind speeds of 15–20 m s−1 or greater, depending on the resolution employed. Based on these regional model solutions a possible implication is that in a CO2-richer climate an increase in the number of flood days over central India can be expected. The model results obtained here, though plausible, need to be taken with caution since even in this “best” model systematic errors still exist in simulating some aspects of the tropical and monsoon climates.

Although the effect of simulated climate change on nitrous oxide (N2O) emissions and on associated microbial communities has been reported, it is not well understood if these effects are short-lived or long-lasting. Here, we conducted a field study to determine the interactive effects of simulated warmer and drier conditions on nitrifier and denitrifier communities and N2O emissions in an acidic soil and the longevity of the effects. A warmer (+2.3°C) and drier climate (-7.4% soil moisture content) was created with greenhouses. The variation of microbial population abundance and community structure of ammonia-oxidizing archaea (AOA), bacteria (AOB), and denitrifiers (nirK/S, nosZ) were determined using real-time PCR and high-throughput sequencing. The results showed that the simulated warmer and drier conditions under the greenhouse following urea application significantly increased N2O emissions. There was also a moderate legacy effect on the N2O emissions when the greenhouses were removed in the urea treatment, although this effect only lasted a short period of time (about 60days). The simulated climate change conditions changed the composition of AOA with the species affiliated to marine group 1.1a-associated lineage increasing significantly. The abundance of all the functional denitrifier genes decreased significantly under the simulated climate change conditions and the legacy effect, after the removal of greenhouses, significantly increased the abundance of AOB, AOA (mainly the species affiliated to marine group 1.1a-associated lineage), and nirK and nosZ genes in the urea-treated soil. In general, the effect of the simulated climate change was short-lived, with the denitrifier communities being able to return to ambient levels after a period of adaptation to ambient conditions. Therefore, the legacy effect of simulated short-time climate change conditions on the ammonia oxidizer and denitrifier communities and N2O emissions were temporary and once the conditions were removed, the microbial communities were able to adapt to the ambient conditions.

The motivation for this North Atlantic Treaty Organization (NATO) Advanced Study Institute (ASI) came from the fact that a hierarchy of physically based mathematical models of the earth's climate has been and is being developed to quantitatively investigate the earth's past, present, and potential future climates. These models are currently being applied, for example, to understand the causes of the ice ages that have occurred with a period of about 100,000 years during the last million years, to study the predictability of climate on the time scale of 1‐3 months (with particular attention on the worldwide El Niño phenomena), and to project the potential climatic consequences of the increasing concentration of atmospheric CO2 and other trace gases. These studies are showing great potential toward the development of a comprehensive theory of climate. However, the development, testing, and application of mathematical climate models requires the dedicated efforts of an increasing number of specially trained and highly motivated scientists. The objective of the AS1 was to provide this training and motivation through a course taught by internationally recognized experts on physically based modeling and simulation of climate and climatic change.

ABSTRACTThe response of the tropical Indian Ocean (TIO) to greenhouse gases (GHGs) and aerosols are investigated based on historical single-forcing and all-forcing simulations using the Geophysical Fluid Dynamics Laboratory Climate Model, version 3 (GFDL CM3). Results reveal a positive Indian Ocean Dipole (pIOD)-like pattern in GHG forcing but a negative Indian Ocean Dipole (nIOD)-like pattern in aerosol forcing. The GHG-induced pIOD-like pattern features less (more) sea surface temperature (SST) warming over the southeastern (western) TIO, accompanied by equatorial easterly anomalies, as well as a shallower thermocline off Sumatra. The aerosol-induced nIOD-like pattern displays the reverse features, characterized by less (more) SST cooling over the southeastern (western) TIO, anomalous equatorial westerlies, and a deeper thermocline off Sumatra. Although the aerosol-induced pattern appears to resemble a reversal of the GHG-induced pattern, there is a strong asymmetry in the SST changes over the southeastern TIO, where the cooling responding to aerosol forcing exceeds the warming in response to GHG forcing, and a negative SST residual is thus produced. A mixed-layer heat budget analysis suggests that the negative SST residual results mainly from asymmetric responses of shortwave radiation, zonal advection, and diffusion to GHGs and aerosols. For comparison, the formation processes for the negative SST skewness over the southeastern TIO between the internal pIOD and nIOD are also discussed.

Simulation of present day climate with a new version of climate model developed in INM RAS is considered. The model differs from a previous version by the change in cloudiness and condensation scheme, that leads to higher sensitivity to CO2 increase. The changes are included also in calculation of aerosol evolution, land snow, atmospheric boundary layer parameterizations and other blocks. The model is capable to reproduce near surface air temperature, precipitation, sea level pressure, cloud radiation forcing and other parameters better than previous version. The largest improvement can be seen in simulation of temperature in tropical troposphere, polar tropopause, and surface temperature in the Southern ocean. Simulation of climate changes in 1850–2021 by two model versions is considered.

This special issue of Climatic Change contains a series of research articles documenting co-ordinated work carried out within a 3-year European Union project ‘Prediction of Regional scenarios and Uncertainties for Defining European Climate change risks and Effects’ (PRUDENCE). The main objective of the PRUDENCE project was to provide high resolution climate change scenarios for Europe at the end of the twenty-first century by means of dynamical downscaling (regional climate modelling) of global climate simulations. The first part of the issue comprises seven overarching PRUDENCE papers on: (1) the design of the model simulations and analyses of climate model performance, (2 and 3) evaluation and intercomparison of simulated climate changes, (4 and 5) specialised analyses of impacts on water resources and on other sectors including agriculture, ecosystems, energy, and transport, (6) investigation of extreme weather events and (7) implications of the results for policy. A paper summarising the related MICE (Modelling the Impact of Climate Extremes) project is also included. The second part of the issue contains 12 articles that focus in more detail on some of the themes summarised in the overarching papers. The PRUDENCE results represent the first comprehensive, continental-scale intercomparison and evaluation of high resolution climate models and their applications, bringing together climate modelling, impact research and social sciences expertise on climate change.

The dynamics of the Atlantic meridional overturning circulation (AMOC) vary considerably among different climate models; for example, some models show clear peaks in their power spectra while others do not. To elucidate these model differences, transfer functions are used to estimate the frequency domain relationship between surface forcing fields, including sea surface temperature, salinity, and wind stress, and the resulting AMOC response. These are estimated from the outputs of the Coupled Model Intercomparison Project phase 5 (CMIP5) and phase 3 (CMIP3) control runs for eight different models, with a specific focus on Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (GFDL CM2.1), and the Community Climate System Model, version 4 (CCSM4), which exhibit rather different spectral behavior. The transfer functions show very little agreement among models for any of the pairs of variables considered, suggesting the existence of systematic model errors and that considerable uncertainty in the simulation of AMOC in current climate models remains. However, a robust feature of the frequency domain analysis is that models with spectral peaks in their AMOC correspond to those in which AMOC variability is more strongly excited by high-latitude surface perturbations that have periods corresponding to the frequency of the spectral peaks. This explains why different models exhibit such different AMOC variability. These differences would not be evident without using a method that explicitly computes the frequency dependence rather than a priori assuming a particular functional form. Finally, transfer functions are used to evaluate two proposed physical mechanisms for model differences in AMOC variability: differences in Labrador Sea stratification and excitation by westward-propagating subsurface Rossby waves.

The impact of enhanced atmospheric CO2 concentrations on tropical convection and sea surface temperatures (SSTs) over the global tropics is assessed using five fully coupled atmospheric–oceanic general circulation models (AOGCMs). Relationships between SST and either outgoing longwave radiation or convective precipitation rates are evaluated for three climate states: present day, a doubled-CO2 scenario, and a quadrupled-CO2 scenario. All AOGCMs capture a relationship between present-day outgoing longwave radiation (OLR) and SST and between convective precipitation rate (PRC) and SST: deep tropical convection (DTC)—signified by rapidly decreasing OLR and rapidly increasing PRC rates—occurs above an SST threshold of around 25°C. Consistent across all AOGCMs, as concentrations increase to 2 × CO2 and 4 × CO2, the threshold SSTs for DTC to occur shift to 25.5°–28°C and 26.5°–30°C, respectively. Annual PRC rates in the 20°N–20°S region increase for two AOGCMs [Meteorological Research Institute Coupled General Circulation Model, version 2.3.2 (MRI CGCM2.3.2) and ECHAM5/Max Planck Institute Ocean Model (MPI-OM)] with increasing CO2, but PRC in the other three AOGCMs [Geophysical Fluid Dynamics Laboratory Climate Model versions 2.0 and 2.1 (GFDL CM2.0 and CM2.1) and National Center for Atmospheric Research (NCAR) Parallel Climate Model (PCM)] exhibits almost no change. Within this tropical zone, increased CO2 concentrations yield up to a 6.1% increase in the number of locations with monthly averaged PRC exceeding two established DTC thresholds (12 and 14 mm day−1). These results indicate that, although the SST threshold for DTC is projected to shift with increasing atmospheric CO2 concentrations, there will not be an expansion of regions experiencing DTC. One implication of these findings is that there will be little change in regions experiencing tropical cyclogenesis in future climate states.