GCM Results Research Articles

Middle atmosphere polar warmings on Mars: Simulations and study on the validation with sub-millimeter observations

Temperature inversions and the warmings over the winter poles in the Martian atmosphere occur due to the adiabatic heating associated with the downward branch of the Hadley circulation. We present results of simulations with a recently developed GCM which suggest that the warmings are the manifestations of the global meridional transport, and are strongly related to atmospheric eddies (planetary waves and tides). To date, sets of data required for the validation of the predicted warmings and the corresponding circulation patterns do not extend far enough into the middle atmosphere of Mars. This motivated our radiative and retrieval simulations to demonstrate that sub-millimeter observations can provide the required fields to validate and constrain the GCM results.

Modeling of the northern hemisphere ice sheets during the last glacial cycle and glaciological sensitivity

We present a new three‐dimensional thermomechanically coupled ice sheet model of the northern hemisphere to reconstruct the Quaternary ice sheets during the last glacial cycle. The model includes basal sliding, internally calculated surface mass balance, glacial isostasy, and a treatment for marine calving. The time dependent forcing consists of temperature and precipitation anomalies from the United Kingdom Meteorological Office (UKMO) General Circulation Model scaled to the Greenland Ice Core Project (GRIP) ice core δ18O record. Model parameters were chosen to best match geomorphological inferences on Last Glacial Maximum extent and global eustatic sea level change. For our standard run we find a maximum ice volume of 57 × 106 km3 at 18.5 ka cal BP. This corresponds to a eustatic sea level lowering of 110 m after correction for hydro‐isostatic displacement and anomalous ice resulting from defects in the specified boundary conditions of the Paleoclimate Model Intercomparison Project (PMIP) for which the UKMO GCM results were generated. Of this 110 m, 82 m was stored in the North American ice sheet and 25 m in the Eurasian ice sheet. We determine the qualitative and quantitative response of the model from a comprehensive sensitivity study in which 11 important parameters were varied over their respective ranges of uncertainty. Model outputs comparable to the observational record were explored in detail as a linear function along the axes of parameter space of the reference model. The method reveals the dominance of climate uncertainty when modeling the Last Glacial Maximum configuration of the northern hemisphere ice sheets, but also highlights the role of ice rheology and basal processes for ice sheet thickness, and glacial isostasy and calving for the timing of maximum ice volume.

Oyashio Southward Intrusion and Cross-Gyre Transport Related to Diapycnal Upwelling in the Okhotsk Sea

Abstract The intrusion of western boundary currents (WBC) across the wind-driven gyre boundary and associated cross-gyre transport are examined using layered numerical models with realistic topography and annual mean wind stress, with special emphasis on the Oyashio southward intrusion and North Pacific Intermediate Water (NPIW) formation. Diapycnal transport due to strong tidal mixing around the Kuril Islands in the subarctic North Pacific Ocean is modeled by source/sink thickness restoring terms in the continuity equation to form the deep pycnocline observed in the Okhotsk Sea. About 3 Sv (Sv ≡ 106 m3 s−1) of the model diapycnal upwelling occurring along the Kuril Islands induces a meridional–diapycnal overturning circulation, and the transport is consistent with the estimates from recent observations with chemical tracers, GCM results, and the present study using observed density fields with enhanced diapycnal diffusivity around the Kuril Islands. The model with the diapycnal transport reproduced the r...

A coupled dynamics-microphysics model of Titan's atmosphere

We have developed a coupled general circulation model of Titan's atmosphere in which the aerosol haze is treated with a microphysical model and is advected by the winds. The radiative transfer accounts for the non uniform haze distribution and, in turn, drives the dynamics. We analyze the GCM results, especially focusing on the difference between a uniform haze layer and a haze layer coupled to the dynamics. In the coupled simulation the aerosols tend to accumulate at the poles, at latitudes higher than ±60°. During winter, aerosols strongly radiate at thermal infrared wavelengths enhancing the cooling rate near the pole. Since this tends to increase the latitudinal gradients of temperature the direct effect of this cooling excess, in contrast to the uncoupled haze case, is to increase the strength of the meridional cells as well as the strength of the zonal winds and profile. This is a positive feedback of the haze on dynamics. The coupled model reproduces observations about the state of the atmosphere better than the uniform haze model, and in addition, the northern polar hood and the detached haze are qualitatively reproduced.

A coupled dynamics-microphysics model of Titan's atmosphere

We have developed a coupled general circulation model of Titan's atmosphere in which the aerosol haze is treated with a microphysical model and is advected by the winds. The radiative transfer accounts for the non uniform haze distribution and, in turn, drives the dynamics. We analyze the GCM results, especially focusing on the difference between a uniform haze layer and a haze layer coupled to the dynamics. In the coupled simulation the aerosols tend to accumulate at the poles, at latitudes higher than ±60°. During winter, aerosols strongly radiate at thermal infrared wavelengths enhancing the cooling rate near the pole. Since this tends to increase the latitudinal gradients of temperature the direct effect of this cooling excess, in contrast to the uncoupled haze case, is to increase the strength of the meridional cells as well as the strength of the zonal winds and profile. This is a positive feedback of the haze on dynamics. The coupled model reproduces observations about the state of the atmosphere better than the uniform haze model, and in addition, the northern polar hood and the detached haze are qualitatively reproduced.

Using a GCM analogue model to investigate the potential for Amazonian forest dieback

A combined GCM analogue model and GCM land surface representation is used to investigate the influences of climatology and land surface parameterisation on modelled Amazonian vegetation change. This modelling structure (called IMOGEN) captures the main features of the changes in surface climate as estimated by a GCM with enhanced atmospheric greenhouse gas concentrations. Advantage is taken of IMOGEN’s computational speed which allows multiple simulations to be carried out to assess the robustness of the GCM results.

Roles of atmospheric waves in Venus' superrotation: Results of a Venus-like GCM for a 3D thermal forcing

Roles of atmospheric waves in Venus' superrotation: Results of a Venus-like GCM for a 3D thermal forcing

Monthly Mean Solar Radiative Fluxes and Cloud Forcing over South America in the Period of 1986–88: GCM Results and Satellite-Derived Data

Abstract The incident solar radiative fluxes, simulated by an atmospheric general circulation model over South America for the period 1986–88, are compared with the satellite-derived surface fluxes provided by the Surface Radiation Budget (SRB) datasets. The comparison shows that the model systematically overestimates both all-sky and clear-sky SRB fluxes while representing well their latitudinal variations. In order to analyze the reasons for the bias, the shortwave radiation code employed in the model is tested with more comprehensive techniques in a stand-alone mode. The results of testing demonstrate that the code underestimates solar radiation absorption in the clear-sky atmosphere due to trace gases and aerosols. The underestimation of the absorption due to aerosols contributes noticeably to the surface flux bias. The impact of clouds on the surface fluxes is estimated by calculating cloud radiative forcing, defined as the difference between the net surface fluxes in all-sky and clear-sky conditions...

Simulating climate change impacts on fire frequency and vegetation dynamics in a Mediterranean‐type ecosystem

AbstractThe impacts of climate change on Mediterranean‐type ecosystems may result from complex interactions between direct effects on water stress and subsequent modifications in flammability and fire regime leading to changes in standing biomass and plant species composition. We analysed these interrelations through a simulation approach combining scenarios of climate change developed from GCM results and a multispecies functional model for vegetation dynamics, SIERRA. A fire risk procedure based on weekly estimates of vegetation water stress has been implemented. Using climate data from 1960 to 1997, simulations of a typical maquis woodland community have been performed as baseline and compared with two climate scenarios: a change in the rainfall regime alone, and changes in both rainfall and air temperature. Climate changes are defined by an increase in temperature, particularly in summer, and a change in the rainfall pattern leading to a decrease in low rainfall events, and an increase in intense rainfall events. The results illustrate the lack of drastic changes in the succession process, but highlight modifications in the water budget and in the length of the drought periods. Water stress lower than expected regarding statistics on the current climate is simulated, emphasizing a long‐term new equilibrium of vegetation to summer drought but with a higher sensibility to rare events. Regarding fire frequency, climate changes tend to decrease the time interval between two successive fires from 20 to 16 years for the maquis shrubland and from 72 to 62 years in the forested stages. This increase in fire frequency leads to shrub‐dominated landscapes, which accentuates the yield of water by additional deep drainage and runoff.

Glacial–interglacial changes of continental weathering: estimates of the related CO 2 and HCO 3− flux variations and their uncertainties

A range of estimates for the glacial–interglacial variations in CO 2 consumption and HCO 3 − production rates by continental weathering processes were calculated with two models of continental weathering: the Gibbs and Kump Weathering Model (GKWM) [Paleoceanography 9(4) (1994) 529] and an adapted version of Amiotte Suchet and Probst's Global Erosion Model for CO 2 Consumption (GEM-CO 2) [C. R. Acad. Sci. Paris, Ser. II 317 (1993) 615; Tellus 47B (1995) 273]. Both models link CO 2 consumption and HCO 3 − production rates to the global distributions of lithology and runoff. A spectrum of 16 estimates for the runoff distribution at the Last Glacial Maximum (LGM) was constructed on the basis of two different data sets for present-day runoff and climate results from eight GCM climate simulation experiments carried out in the framework of the Paleo Modelling Intercomparison Project (PMIP). With these forcings, GKWM produced 3.55–9.0 Tmol/year higher and GEM-CO 2 4.7–13.25 Tmol/year higher global HCO 3 − (1 Tmol=10 12 mol) production rates at the LGM. Mean variations (plus/minus one standard error of the mean with 7 df) were 6.2±0.6 and 9.4±1.0 Tmol/year, respectively. The global CO 2 consumption rates obtained with GKWM were 1.05–4.5 Tmol/year (mean: 2.8±0.4 Tmol/year) higher at the LGM than at present. With GEM-CO 2, this increase was 1.95–7.15 Tmol/year (mean: 4.8±0.6 Tmol/year). The large variability in the changes obtained with each weathering model was primarily due to the variability in the GCM results. The increase in the CO 2 consumption rate due to continental shelf exposure at the LGM was always more than 60% larger than its reduction due to ice cover. For HCO 3 − production rates, the increase related to shelf exposure was always more than twice as large as the decrease due to ice cover. Flux variations in the areas exposed both now and at the LGM were, in absolute value, always more than 3.5 times lower than those in the shelf environment. The calculated CO 2 consumption rates by carbonate weathering were consistently higher at the LGM, by 2.45–4.5 Tmol/year (mean: 3.4±0.2 Tmol/year) according to GKWM and by 2.75–6.25 Tmol/year (mean: 4.6±0.4 Tmol/year) according to GEM-CO 2. For silicate weathering, GKWM produced variations ranging between a 1.9 Tmol/year decrease and a 0.4 Tmol/year increase for the LGM (mean variation: −0.7±0.2 Tmol/year); GEM-CO 2 produced variations ranging between a 0.8 Tmol/year decrease and a 1.05 Tmol/year increase (mean variation: +0.2±0.2 Tmol/year). In the mean, the calculated variations of CO 2 and HCO 3 − fluxes would contribute to reduce atmospheric p CO 2 by 5.7±1.3 ppmv (GKWM) or 12.1±1.7 ppmv (GEM-CO 2), which might thus represent a non-negligible part of the observed glacial–interglacial variation of ∼75 ppmv.

Formula for a baroclinic adjustment theory of climate

Recently, a theory relating baroclinic neutrality and midlatitudes tropopause height has beenproposed. However, GCM results have shown that the dependence of the theory on externalparameters is not consistent with that displayed by these numerical experiments. In the presentpaper we suggest an analytic formula for baroclinic adjustment to the neutrality of Eady wavesthrough tropopause modification. This formula extends considerably the abovementionedtheory by taking into account both a simple representation of the stratosphere and the topography.These modifications alter the tropopause condition for a baroclinically neutral stateand its sensitivity to the external parameters. In particular, the topography introduces a dependenceon the tropospheric vertical wind shear of the neutrality condition. This feature is notpresent in other models that assume a background state with a zero potential vorticity gradientin the troposphere.We show, furthermore, that the modified neutrality condition has sensitivitiesthat may resemble those displayed by GCM simulations, with respect to the parameters definingthe background flow.

GCM experiments on changes in atmospheric predictability associated with the PNA pattern and tropical SST anomalies

Based on results from a simple three-level quasi-geostrophic model, Lin and Derome suggestedthat atmospheric predictability is influenced by the Pacific/North American (PNA) pattern. Inthe present study, predictability experiments are conducted with the Canadian Centre forClimate Modelling and Analysis general circulation model (CCCma GCM). A 47-yr integrationof theGCM with specified sea surface temperature (SST) for the years 1948—94 is first performed.Forecasts are initiated whenever the PNA pattern is in a strong positive or strong negativephase during this simulation. For each forecast, an ensemble of six initial conditions is generatedwith small random perturbations. Forecasts initiated when the PNA is in its positive phasehave smaller growth rates of ensemble standard deviation than forecasts initiated when thePNA is in its negative phase. Regional characteristics of the prediction spread are also examined.Similar experiments are conducted to determine the relationship between atmospheric predictabilityand SST anomalies in the tropical Pacific. Forecasts initiated when tropical SST anomaliesare positive have smaller growth rates of ensemble standard deviation than forecasts initiatedwhen tropical SST anomalies are negative. However, cases with positive tropical SST anomaliesbut without a strong PNA pattern show a similar prediction spread to cases with negative SSTanomalies. The results suggest that, in comparison to the PNA pattern, the influence of tropicalSST anomalies is only secondary. A set of three-layer diagnostic equations is used to analyzethe GCM results. It is speculated that the transient eddies have a stronger influence on thecirculation anomalies (and therefore reduce the atmospheric predictability more) in the negativePNA phase than in the positive PNA phase.

Radiative cooling by stratospheric water vapor: Big differences in GCM results

The stratosphere has been cooling by about 2K/decade at 30–60 km over the past several decades and by lesser amounts toward the tropopause. Climate model calculations suggest that stratospheric water vapor is an important contributor to the observed stratospheric cooling, but there are large differences among recent GCM simulations for prescribed changes in stratospheric water vapor, which point to problems with the current GCM treatment of the absorption and emission by stratospheric water vapor. We show that the correlated k‐distribution treatment with sufficient resolution is capable of simulating accurately cooling by stratospheric water vapor. We obtain equilibrium cooling of about 0.3K that extends from 20 km to the top of the atmosphere, and adjusted radiative forcing of 0.12 Wm−2, for a stratospheric water vapor increase of 0.7 ppmv which has been estimated for the period 1979–1997.

CLIMAP SSTs re-revisited

Since the 1976 publication of the CLIMAP ice age sea surface temperature (SST) reconstruction showing a 1–2 ∘C tropical cooling a substantial debate has arisen as to whether tropical SSTs may instead have been 4–5∘ colder than present. Herein I review the arguments for large SST variations and question a number of key findings, particularly the validity of ice-age coral SST estimates and “down-projecting” tropical snowline changes to the surface. GCM results indicate that an intermediate solution requiring ∼2.5 ∘C warm pool cooling is consistent with most quantitative low elevation surface land data and is small enough to allow the persistence of tropical biota in the ocean during glacial times. The proposal reduces estimated ice-age climate sensitivity (for a doubling of CO2) from a “high-end” sensitivity of about 4.5 ∘C (for a 5–6 ∘C tropical cooling) to a “mid-range” sensitivity of about 3.0 ∘C for a 2.5 ∘C warm-pool decrease.

Validation of parametrisations for the meridional energy and moisture transport used in simple climate models

Parametrisations of meridional energy and moisture transport used in zonally averaged climate models are validated using reanalysis data and results from a doubling CO2-experiment from a general circulation model. Global meridional fluxes of moisture and sensible heat are calculated by integrating surface and top-of-the-atmosphere vertical fluxes from one pole to the other. The parametrisations include an eddy-diffusion term, representing down-gradient transport of specific humidity and temperature due to the transient atmospheric eddies at mid- and high latitudes, and simple representations of the mean meridional circulation. Qualitative and quantitative agreement between the increased hydrological cycle in the 2×CO2-run from the GCM and the parametrisation is found. The performance for the sensible heat flux shows larger differences to the GCM results, particularly at low latitudes. Seasonal variations of the moisture and sensible heat transport are well captured by parametrisations including the influence of the mean meridional circulation. Interannual variability cannot be simulated. An examination of the parametrisations on different spatial scales suggests that they should not be used for small scales. Furthermore, two closures for the zonal distribution of precipitation were examined. They are used in zonally averaged atmosphere models coupled to an ocean model with different ocean basins at one latitudinal belt. An assessment of both the reanalysis data and the GCM results shows that both closures exhibit very similar behaviour and are valid in the long-term mean and seasonal cycle. Interannual variability is not captured well. They become invalid for spatial scales smaller than 10∘.

Anthropogenic sulphate aerosol from India: estimates of burden and direct radiative forcing

A one-box chemical-meteorological model had been formulated to make preliminary estimates of sulphate aerosol formation and direct radiative forcing over India. Anthropogenic SO 2 emissions from India, from industrial fuel use and biomass burning, were estimated at 2.0 Tg S yr -1 for 1990 in the range of previous estimates of 1.54 and 2.55 Tg S yr -1 for 1987. Meteorological parameters for 1990 from 18 Indian Meteorological Department stations were used to estimate spatial average sulphate burdens through formation from SO 2 reactions in gas and aqueous phase and removal by dry and wet deposition. The hydrogen peroxide reaction was found dominating for undepleted oxidant-rich conditions. Monthly mean sulphate burdens ranged from 2–10 mg m -2 with a seasonal variation of winter–spring highs and summer lows in agreement with previous GCM studies. The sulphate burdens are dominated by sulphate removal rates by wet deposition, which are high in the monsoon period from June–November. Monthly mean direct radiative forcing from sulphate aerosols is high (−3.5 and −2.3 W m -2) in December and January, is moderate (−1.3 to −1.5 W m -2) during February to April and November and low (−0.4 to −0.6 W m -2) during May to October also in general agreement with previous GCM estimates. This model, in reasonable agreement with detailed GCM results, gives us a simple tool to make preliminary estimates of sulphate burdens and direct radiative forcing.

Validation of Vertical Mixing in an Equatorial Ocean Model Using Large Eddy Simulations and Observations

A nonlocal K-profile parameterization (KPP) of the upper-ocean boundary layer is tested for the equatorial regions. First, the short-term performance of a one-dimensional model with KPP is found to compare favorably to large eddy simulations (LES), including nonlocal countergradient heat flux. The comparison is clean because both the surface forcing and the large-scale flow are identical in the two models. The comparison is direct because the parameterized turbulent flux profiles are explicitly computed in LES. A similar comparison is less favorable when KPP is replaced by purely downgradient diffusion with Richardson-number-dependent viscosity and diffusivity because of the absence of intense convection after sunset. Sensitivity experiments are used to establish parameter values in the interior mixing of KPP. Second, the impact of the parameterization on annual means and the seasonal cycle in a general circulation model of the upper, equatorial Pacific Ocean is described. The results of GCM runs with and without KPP are compared to annual mean profiles of zonal velocity and temperature from the TOGA-TAO array. The two GCM solutions are closer to each other than to the observations, with biases in zonal velocity in the western Pacific and in subsurface temperature in the eastern Pacific. Such comparisons are never clean because neither the wind stress and the surface heat flux nor the forcing by the large-scale flow are known to sufficient accuracy. Finally, comparisons are made of the equatorial Pacific Ocean GCM results when different heat flux formulations are used. These include bulk forcing where prescribed air temperature and humidity are used, SST forcing where the use of such ocean-controlled parameters is avoided, and a fully coupled atmospheric general circulation model where there is no prescribed control over any surface fluxes. It is concluded, especially in the eastern Pacific, that the use of specified air temperature and humidity does not overly constrain the model sea surface temperature.

Application of an analog downscaling technique to the assessment of future landslide activity-a case study in the Italian Alps

Slope stability and hence landslide activity is in many cases related to climate, which influences groundwater and pore pressure fluctuations of hillslopes. An approach is presented which transforms transient GCM output by statistical downscaling to local precipitation scenarios, which together with directly derived temperature scenarios are subsequently fed into a slope hydro- logical/stability model to derive future landslide activity. This model chain is applied to a landslide in the Dolomites, Italy. Validation of the approach against independent observed records suggests its applicability for estimating future landslide activity based on GCM results. One possible way of esti- mating the quality of the approach is to determine sources of uncertainty introduced by the GCM sim- ulations and by different fitting periods of the downscaling technique. Differences between the GCM experiments are found to be more important than differences between the 2 fitting periods. The most striking result is the significant reduction of landslide activity in spring in all cases. This is attributable to the rise of winter temperature which impedes future storage of winter precipitation as snow. As one consequence, less melt water is available for the hillslope in spring, causing the decrease in activity. It can be concluded that differences between GCMs and differences between fitting periods permit quantification of part of the uncertainty inherent in climate change impact assessments. Impacts which emerge in all model combinations, such as the decrease in landslide activity in spring shown here, have a high level of confidence.

A Semiempirical Downscaling Approach for Predicting Regional Temperature Impacts Associated with Climatic Change

Abstract A statistical downscaling approach is developed for generating regional temperature change predictions from GCM results. The approach utilizes GCM free atmosphere output and surface observations in a framework conceptually similar to the model output statistics approach common in the forecasting community. The appropriateness of this approach is demonstrated through a comparison of GCM and observed free atmosphere variables. Seasonal downscaling models are presented for eight sites within four community climate model (CCM) grid cells in the United States. The majority of these models are capable of explaining more than 90% of the variance in the temperature time series. The results indicate a wide range of differences between downscaled climate change predictions and grid cell–level CCM predictions.

GCM experiments to test a proposed dynamical stabilizing mechanism in the climate system

A dynamical mechanism of sufficient strength to maintain the stability of the observed annualmean global climate against the destabilizing influence of the positive lower tropospheric watervapour/infrared radiative (WVIR) feedback on SST perturbations has recently been proposed(Bates, 1999). The mechanism consists of an evaporative negative feedback associated with theatmospheric angular momentum (AM) cycle and the surface winds which it induces. The mechanismwas found from an analysis of a simple two-zone model of the climate system. Numericalexperiments with an atmospheric GCM aimed at testing the conclusions of the simple modelare here described. Aquaplanet boundary conditions with a prescribed latitudinally-varyingSST distribution are adopted. An equinoctial distribution of solar radiation is assumed. Anequilibrium climate for the GCM is established and perturbation experiments with a uniform2 K increase in the SST are carried out. In the equilibrium and basic perturbation experiments, the GCM is run with all fields evolving freely. Further perturbation experiments are then carriedout with the cloud fields held fixed, and with both the cloud and water vapour fields held fixed, in the GCM’s radiation package. These experiments allow the relative influence of cloud andwater vapour in the infrared and solar radiative feedbacks at the surface to be estimated. Theresults of the experiments are compared with those predicted by the simple model for a similarSST perturbation. The GCM results are found to be consistent with the basic assumptions andconclusions of the simple model. In particular, they indicate that a positive WVIR feedbackexists in both the tropical and extratropical zones and confirm the existence of a negativeevaporative feedback associated with the AM cycle that is of sufficient strength to overcome it.The experiments support the conclusion based on the simple model that it is necessary toconsider both the wind factor and the humidity factor in evaluating the evaporative feedback.The cloud/infrared radiative feedback in the GCM is found to be small compared with theWVIR feedback. The experiments suggest that the feedback resulting from the absorbtion ofsolar radiation by water vapour (which was not included in the simple model) provides asignificant additional stabilizing influence on SST perturbations.