Comprehensive General Circulation Model Research Articles

Mid-Latitude Jet Response to Pan-Arctic and Regional Arctic Warming in Idealized GCM

To study the dynamical mechanism by which Arctic amplification affects extreme weather events in mid-latitude, we investigated the local and remote circulation response to pan-Arctic and regional Arctic thermal forcing. A comprehensive atmospheric GCM (General Circulation Model) coupled to a slab mixed-layer ocean model is used for the experiment. With the increasing thermal forcing in the pan-Arctic configuration, the mid-latitude jet tends to shift equatorward, mainly due to the southward shift of the convergence zone of eddy-heat flux and eddy-momentum flux. From the regional Arctic forced experiments, zonal mean response is similar to the response from the pan-Arctic configuration. The non-zonal response is characterized by the 300 hPa circumpolar zonal wind of wavenumber-1 structure, which establishes an enhanced wavier mid-latitude jet. In the polar region at 300 hPa, regional thermal forcing drives a distinct east–west dipole circulation pattern, in which anticyclonic circulation is located to the west of the thermal forcing, and cyclonic circulation is located to the east. The lower-level circulation shows the opposite pattern to the upper-level circulation in the polar region. While the strength of circulation increases with gradual thermal forcing, the overall dipole pattern is unchanged. In regional warming simulation, compared to the pan-Arctic warming, increasing residual heat flux in a dipole pattern causes enhanced heat advection to mid-latitude.

Changes in Poleward Atmospheric Energy Transport over a Wide Range of Climates: Energetic and Diffusive Perspectives and A Priori Theories

Abstract The midlatitude poleward atmospheric energy transport increases in radiatively forced simulations of warmed climates across a range of models from comprehensive coupled general circulation models (GCMs) to idealized aquaplanet moist GCMs to diffusive moist energy balance models. These increases have been rationalized from two perspectives. The energetic (or radiative) perspective takes the atmospheric energy budget and decomposes energy flux changes (radiative forcing, feedbacks, or surface fluxes) to determine the energy transport changes required by the budget. The diffusive perspective takes the net effect of atmospheric macroturbulence to be a diffusive energy transport down-gradient, so transport changes can arise from changes in mean energy gradients or turbulent diffusivity. Here, we compare these perspectives in idealized moist, gray-radiation GCM simulations over a wide range of climates. The energetic perspective has a dominant role for radiative forcing in this GCM, with cancellation between the temperature feedback components that account for the GCM’s nonmonotonic energy transport changes in response to warming. Comprehensive CMIP5 simulations have similarities in the Northern Hemisphere to the idealized GCM, although a comprehensive GCM over several CO2 doublings has a distinctly different feedback evolution structure. The diffusive perspective requires a non-constant diffusivity to account for the idealized GCM-simulated changes, with important roles for the eddy velocity, dry static stability, and horizontal energy gradients. Beyond diagnostic analysis, GCM-independent a priori theories for components of the temperature feedback are presented that account for changes without knowledge of a perturbed climate state, suggesting that the energetic perspective is the more parsimonious one.

The performance of filtered leapfrog schemes in benchmark simulations

AbstractThe stabilisation of the leapfrog time‐stepping scheme provided by the Robert–Asselin filter has enabled decades of atmospheric and oceanic research and weather and climate predictions. The unfortunate concomitant reduction from second‐order accuracy to first‐order accuracy inflicted by the filter has recently ushered in a new generation of leapfrog time filters that preserve the second‐order accuracy, including the Robert–Asselin–Williams (RAW) filter and its variants. These modern filtered leapfrog schemes have previously been shown to improve numerical simulations made using both simple conceptual models and comprehensive general circulation models. However, their performance in standard benchmark experiments has not previously been assessed. Here we evaluate these filtered leapfrog schemes in four classic benchmark experiments: linear scalar advection; a nonlinear density current in the quasi‐compressible equations; a nonlinear rising warm bubble in the fully compressible equations; and the linked behaviour of nonlinear twin tropical cyclones in the rotating shallow‐water equations. For a given time‐step size, the filtered leapfrog schemes are found to compare favourably with the third‐order Runge–Kutta (RK3) scheme. They are also less computationally expensive than RK3, at roughly one‐third to one‐half the cost per time step. For a given computational expenditure, the filtered leapfrog schemes are found to produce smaller errors with respect to the analytical solution (where available) than RK3. Furthermore, the filtered leapfrog schemes are found to be numerically stable, even when the discretisation method splits the slow advection and diffusion modes from the fast acoustic and gravity‐wave modes. Given that implementing filter upgrades requires only minimally invasive changes to an existing computer code, our results provide support for the continued use of filtered leapfrog schemes in atmosphere and ocean models.

Midlatitude Error Growth in Atmospheric GCMs: The Role of Eddy Growth Rate

AbstractSeveral studies have established that atmospheric flows have a finite range of predictability, which may be reasonably considered a consequence of the underlying dynamics. In the midlatitudes, error growth is predominantly associated with baroclinic disturbances. We consider midlatitude error growth in two models: an idealized dry dynamical core and a comprehensive atmospheric general circulation model (GCM). By systematically varying equator to pole temperature gradients in the dynamical core, we show that with increasing Eady growth rates, the time elapsed before errors saturate decreases, shortening the window in which weather predictions may be useful. We also consider the limits of midlatitude predictability in the comprehensive moist GCM in a range of climates. Our results show that the times to error saturation are shorter in warmer climates than colder climates, suggesting that warmer climates are inherently less predictable.

Impact of an AMOC weakening on the stability of the southern Amazon rainforest

The Atlantic Meridional Overturning Circulation (AMOC) and the Amazon rainforest are potential tipping elements of the Earth system, i.e., they may respond with abrupt and potentially irreversible state transitions to a gradual change in forcing once a critical forcing threshold is crossed. With progressing global warming, it becomes more likely that the Amazon will reach such a critical threshold, due to projected reductions of precipitation in tropical South America, which would in turn trigger vegetation transitions from tropical forest to savanna. At the same time, global warming has likely already contributed to a weakening of the AMOC, which induces changes in tropical Atlantic sea-surface temperature (SST) patterns that in turn affect rainfall patterns in the Amazon. A large-scale decline or even dieback of the Amazon rainforest would imply the loss of the largest terrestrial carbon sink, and thereby have drastic consequences for the global climate. Here, we assess the direct impact of greenhouse gas-driven warming of the tropical Atlantic ocean on Amazon rainfall. In addition, we estimate the effect of an AMOC slowdown or collapse, e. g. induced by freshwater flux into the North Atlantic due to melting of the Greenland Ice Sheet, on Amazon rainfall. In order to provide a clear explanation of the underlying dynamics, we use a simple, but robust mathematical approach (based on the classical Stommel two-box model), ensuring consistency with a comprehensive general circulation model (HadGEM3). We find that these two processes, both caused by global warming, are likely to have competing impacts on the rainfall sum in the Amazon, and hence on the stability of the Amazon rainforest. A future AMOC decline may thus counteract direct global-warming-induced rainfall reductions. Tipping of the AMOC from the strong to the weak mode may therefore have a stabilizing effect on the Amazon rainforest.

TransEBM v. 1.0: description, tuning, and validation of a transient model of the Earth's energy balance in two dimensions

Abstract. Modeling the long-term transient evolution of climate remains a technical and scientific challenge. However, understanding and improving modeling of the long-term behavior of the climate system increases confidence in projected changes in the mid- to long-term future. Energy balance models (EBMs) provide simplified and computationally efficient descriptions of long timescales and allow large ensemble runs by parameterizing energy fluxes. In this way, they can be used to pinpoint periods and phenomena of interest. Here, we present TransEBM, an extended version of the two-dimensional energy balance model by Zhuang et al. (2017a). Transient CO2, solar insolation, orbital configuration, fixed ice coverage, and land–sea distribution are implemented as effective radiative forcings at the land surface. We show that the model is most sensitive to changes in CO2 and ice distribution, but the obliquity and land–sea mask have significant influence on modeled temperatures as well. We tune TransEBM to reproduce the 1960–1989 CE global mean temperature and the Equator-to-pole and seasonal temperature gradients of ERA-20CM reanalysis (Hersbach et al., 2015). The resulting latitudinal and seasonal temperature distributions agree well with reanalysis and the general circulation model (GCM) HadCM3 for a simulation of the past millennium (Bühler et al., 2020). TransEBM does not represent the internal variability of the ocean–atmosphere system, but non-deterministic elements and nonlinearity can be introduced through model restarts and randomized forcing. As the model facilitates long transient simulations, we envisage its use in exploratory studies of stochastic forcing and perturbed parameterizations, thus complementing studies with comprehensive GCMs.

Decomposing the Drivers of Polar Amplification with a Single-Column Model

AbstractThe precise mechanisms driving Arctic amplification are still under debate. Previous attribution methods compute the vertically uniform temperature change required to balance the top-of-atmosphere energy imbalance caused by each forcing and feedback, with any departures from vertically uniform warming collected into the lapse-rate feedback. We propose an alternative attribution method using a single-column model that accounts for the forcing dependence of high-latitude lapse-rate changes. We examine this method in an idealized general circulation model (GCM), finding that, even though the column-integrated carbon dioxide (CO2) forcing and water vapor feedback are stronger in the tropics, they contribute to polar-amplified surface warming as they produce bottom-heavy warming in high latitudes. A separation of atmospheric temperature changes into local and remote contributors shows that, in the absence of polar surface forcing (e.g., sea ice retreat), changes in energy transport are primarily responsible for the polar-amplified pattern of warming. The addition of surface forcing substantially increases polar surface warming and reduces the contribution of atmospheric dry static energy transport to the warming. This physically based attribution method can be applied to comprehensive GCMs to provide a clearer view of the mechanisms behind Arctic amplification.

Centennial Changes in the Indonesian Throughflow Connected to the Atlantic Meridional Overturning Circulation: The Ocean's Transient Conveyor Belt

AbstractClimate models consistently project a robust weakening of the Indonesian Throughflow (ITF) and the Atlantic meridional overturning circulation (AMOC) in response to greenhouse gas forcing. Previous studies of ITF variability have largely focused on local processes in the Indo‐Pacific Basin. Here, we propose that much of the centennial‐scale ITF weakening is dynamically linked to changes in the Atlantic Basin and communicated between basins via wave processes. In response to an AMOC slowdown, the Indian Ocean develops a northward surface transport anomaly that converges mass and modifies sea surface height in the Indian Ocean, which weakens the ITF. We illustrate these dynamic interbasin connections using a 1.5‐layer reduced gravity model and then validate the responses in a comprehensive general circulation model. Our results highlight the importance of transient volume exchanges between the Atlantic and Indo‐Pacific basins in regulating the global ocean circulation in a changing climate.

Forcing Dependence of Atmospheric Lapse Rate Changes Dominates Residual Polar Warming in Solar Radiation Management Climate Scenarios

AbstractSimulations of solar radiation management (SRM) geoengineering using comprehensive general circulation models show a residual surface warming at high latitudes. Previous work attributes this to the difference in forcing structure between the increase in greenhouse gases and decrease in insolation, but this neglects the role of the induced reduction in atmospheric energy transport. Here we show that the difference in vertical structure of temperature change between increasing CO2, decreasing insolation, and decreasing atmospheric energy transport is the dominant reason for the residual near‐surface warming at high latitudes. A single‐column model (SCM) is used to decompose the high‐latitude temperature change and shows the importance of the enhanced near‐surface warming from the CO2 increase in explaining the residual polar warming. This suite of models invites caution when attributing high‐latitude surface temperature changes to the lapse rate feedback, as various forcings and nonlocal processes affect the vertical structure of temperature change differently.

Ice sheet influence on atmospheric circulation explains the patterns of Pleistocene alpine glacier records in North America

We explore the hypothesis that the relative size of Pleistocene ice sheets in North America modulated regional climate and alpine glaciation. We compare Pleistocene alpine glacier chronologies across North America with a comprehensive general circulation model using reconstructed ice sheet extents at peak glacial conditions during Marine Isotope Stage (MIS) 2 and MIS 4. The effect of continent-wide ice sheets on atmospheric circulation during MIS 2 led to warming in Beringia and cooling in the western US; less expansive ice sheets during MIS 4 resulted in weaker ice sheet modulation of atmospheric circulation. This led to preservation of MIS 4 moraines in Beringia due to limited MIS 2 glaciation (resulting in a MIS 2/4 moraine sequence) and overriding of MIS 4 moraines – for sites with existing chronologies – in the western United States during MIS 2 (resulting in a MIS 2/6 moraine sequence). Our results highlight how influential ice sheets are for regional climate conditions.

Meridional Atmospheric Heat Transport Constrained by Energetics and Mediated by Large-Scale Diffusion

Abstract Meridional atmospheric heat transport (AHT) has been investigated through three broad perspectives: a dynamic perspective, linking AHT to the poleward flux of moist static energy (MSE) by atmospheric motions; an energetic perspective, linking AHT to energy input to the atmosphere by top-of-atmosphere radiation and surface heat fluxes; and a diffusive perspective, representing AHT in terms downgradient energy transport. It is shown here that the three perspectives provide complementary diagnostics of meridional AHT and its changes under greenhouse gas forcing. When combined, the energetic and diffusive perspectives offer prognostic insights: anomalous AHT is constrained to satisfy the net energetic demands of radiative forcing, radiative feedbacks, and ocean heat uptake; in turn, the meridional pattern of warming must adjust to produce those AHT changes, and does so approximately according to diffusion of anomalous MSE. The relationship between temperature and MSE exerts strong constraints on the warming pattern, favoring polar amplification. These conclusions are supported by use of a diffusive moist energy balance model (EBM) that accurately predicts zonal-mean warming and AHT changes within comprehensive general circulation models (GCMs). A dry diffusive EBM predicts similar AHT changes in order to satisfy the same energetic constraints, but does so through tropically amplified warming—at odds with the GCMs’ polar-amplified warming pattern. The results suggest that polar-amplified warming is a near-inevitable consequence of a moist, diffusive atmosphere’s response to greenhouse gas forcing. In this view, atmospheric circulations must act to satisfy net AHT as constrained by energetics.

A Simple Alternative Model for the Estimation of the Carbon Dioxide Effect on the Earth's Energy Balance

A simple model for the evaluation of the CO2 concentration effect on the radiative component of the Earth's energy balance is described. The model calculates a single parameter, an expected increase of the average temperature at the Earth surface caused by doubling the CO2 concentration in the atmosphere. The model is based on the spectrum of Planck radiation leaving the Earth's surface and on IR spectral characteristics of the air column above the surface. The principal distinction of the model with respect to comprehensive global general-circulation models is the representation of the atmosphere as a combination of narrow-range IR filters. Within the approximations used in the model, a doubling the CO2 concentration in the Earth's atmosphere would lead to an increase of the surface temperature by about +0.5 to 0.7 °C, hardly an effect calling for immediate drastic changes in the planet's energy policies. An increase in the absolute air humidity caused by doubling the CO2 concentration and the resulting decrease of the outgoing IR flux would produce a relatively small additional effect due to a strong overlap of IR spectral bands of CO2 and H2O, the two compounds primarily responsible for the greenhouse properties of the atmosphere.

Baroclinic Eddies and the Extent of the Hadley Circulation: An Idealized GCM Study

The Hadley circulation has widened over the past 30 years. This widening has been qualitatively reproduced in general circulation model (GCM) simulations of a warming climate. Comprehensive GCM studies suggest this widening may be caused by a poleward shift in baroclinic eddy activity. Yet the limited amplitude of the climate change signals analyzed so far precludes a quantitative comparison with theories.This study uses two idealized GCMs, one with and one without an active hydrologic cycle, to investigate changes in the extent of the Hadley circulation over a wide range of climates. The climates span global-mean temperatures from 243 to 385 K and equator-to-pole temperature contrasts from 12 to 100 K. Baroclinic eddies control the extent of the Hadley circulation across most of these climates. A supercriticality criterion that quantifies the depth of baroclinic eddies relative to that of the troposphere turns out to be a good indicator of where baroclinic eddies become deep enough to terminate the Hadley circulation. The supercriticality depends on meridional temperature gradients and an effective stability that accounts for the effect of convective heating on baroclinic eddies.As the equator-to-pole temperature contrast weakens or the convective static stability increases, convective heating increasingly influences the thermal stratification of the troposphere and the supercriticality. Consistent with the supercriticality criterion, the Hadley circulation contracts as meridional temperature gradients increase, and it widens as the effective static stability increases. The former occurs during El Niño and may account for the observed Hadley circulation contraction then; the latter occurs during global warming.

Climate and climate sensitivity to changing CO2 on an idealized land planet

AbstractThe comprehensive general circulation model ECHAM6 is used in a radiative‐convective equilibrium configuration. It is coupled to a perfectly conducting slab. To understand the local impact of thermodynamic surface properties on the land‐ocean warming contrast, the surface latent heat flux and surface heat capacity are reduced stepwise, aiming for a land‐like climate. Both ocean‐like and land‐like RCE simulation reproduce the tropical atmosphere over ocean and land in a satisfactory manner and lead to reasonable land‐ocean warming ratios. A small surface heat capacity induces a high diurnal surface temperature range which triggers precipitation during the day and decouples the free troposphere from the diurnal mean temperature. With increasing evaporation resistance, the net atmospheric cooling rate decreases because cloud base height rises, causing a reduction of precipitation. Climate sensitivity depends more on changes in evaporation resistance than on changes in surface heat capacity. A feedback analysis with the partial radiation perturbation method shows that amplified warming over idealized land can be associated with disproportional changes in the lapse rate versus the water vapor feedback. Cloud feedbacks, convective aggregation, and changes in global mean surface temperature confuse the picture.

Simulated radiative forcing from contrails and contrail cirrus

Abstract. A comprehensive general circulation model including ice supersaturation is used to estimate the climate impact of aviation induced contrails. The model uses a realistic aviation emissions inventory for 2006 to initiate contrails, and allows them to evolve consistently with the model hydrologic cycle. The radiative forcing from linear contrails is very sensitive to the diurnal cycle. For linear contrails, including the diurnal cycle of air traffic reduces the estimated radiative forcing by 29%, and for contrail cirrus estimates, the radiative forcing is reduced by 25%. Estimated global radiative forcing from linear contrails is 0.0031 ± 0.0005 Wm−2. The linear contrail radiative forcing is found to exhibit a strong diurnal cycle. The contrail cirrus radiative forcing is less sensitive to the diurnal cycle of flights. The estimated global radiative forcing from contrail cirrus is 0.013 ± 0.01 Wm−2. Over regions with the highest air traffic, the regional effect can be as large as 1 Wm−2.

The climate impact of aviation aerosols

A comprehensive general circulation model (GCM) is used to estimate the climate impact of aviation emissions of black carbon (BC) and sulfate (SO4) aerosols. Aviation BC is found not to exert significant radiative forcing impacts, when BC nucleating efficiencies in line with observations are used. Sulfate emissions from aircraft are found to alter liquid clouds at altitudes below emission (∼200 hPa); contributing to shortwave cloud brightening through enhanced liquid water path and drop number concentration in major flight corridors, particularly in the N. Atlantic. Global averaged sulfate direct and indirect effects on liquid clouds of 46 mWm−2are larger than the warming effect of aviation induced cloudiness of 16 mWm−2. The net result of including contrail cirrus and aerosol effects is a global averaged cooling of −21±11 mWm−2. These aerosol forcings should be considered with contrails in evaluating the total global impact of aviation on climate.

Upward Shift of the Atmospheric General Circulation under Global Warming: Theory and Simulations

Abstract Many features of the general circulation of the atmosphere shift upward in response to warming in simulations of climate change with both general circulation models (GCMs) and cloud-system-resolving models. The importance of the upward shift is well known, but its physical basis and the extent to which it occurs coherently across variables are not well understood. A transformation is derived here that shows how an upward shift of a solution to the moist primitive equations gives a new approximate solution with higher tropospheric temperatures. According to the transformation, all variables shift upward with warming but with an additional modification to the temperature and a general weakening of the pressure velocity. The applicability of the vertical-shift transformation is explored using a hierarchy of models from adiabatic parcel ascents to comprehensive GCMs. The transformation is found to capture many features of the response to climate change in simulations with an idealized GCM, including the mid- and upper-tropospheric changes in lapse rate, relative humidity, and meridional wind. The transformation is less accurate when applied to simulations with more realistic GCMs, but it nonetheless captures some important features. Deviations from the simulated response are primarily due to the surface boundary conditions, which do not necessarily conform to the transformation, especially in the case of the zonal winds. The results allow for a physical interpretation of the upward shift in terms of the governing equations and suggest that it may be thought of as a coherent response of the general circulation of the mid- and upper troposphere.

Barotropic Impacts of Surface Friction on Eddy Kinetic Energy and Momentum Fluxes: An Alternative to the Barotropic Governor

Abstract As the surface drag is increased in a comprehensive general circulation model (GCM), the upper-level zonal winds decrease and eddy momentum flux convergence into the jet core increases. Globally averaged eddy kinetic energy decreases, a response that is inconsistent with the conventional barotropic governor mechanism whereby decreased barotropic shears encourage baroclinic wave growth. As the conventional barotropic governor appears insufficient to explain the entire response in the comprehensive GCM, the nondivergent barotropic model on the sphere is used to demonstrate an additional mechanism for the effect of surface drag on eddy momentum fluxes and eddy kinetic energy. Analysis of the pseudomomentum budget shows that increased drag modifies the background meridional vorticity gradient, which allows for enhanced eddy momentum flux convergence and decreased eddy kinetic energy in the presence of a constant eddy source. This additional feedback may explain the changes in eddy momentum fluxes observed in the comprehensive GCM and was likely present in previous work on the barotropic governor.

Thermal effects of internal gravity waves in the Martian upper atmosphere

For the first time, gravity wave‐induced heating and cooling effects were fully and interactively incorporated into a Martian general circulation model (GCM). Simulations with a comprehensive GCM with an implemented spectral nonlinear gravity wave (GW) parameterization revealed significant thermal effects of GWs in the mesosphere and lower thermosphere (MLT) between 100 and 150 km. Wave‐induced heating and cooling rates are comparable with those due to near‐IR CO2 heating and IR CO2cooling, correspondingly. Accounting for thermal effects of GWs results in a colder simulated MLT, with the most of cooling taking place in middle‐ and high‐latitudes. In the winter hemisphere, the temperature decrease can exceed 45 K. The colder simulated MLT is in a good agreement with the SPICAM stellar occultation measurements and Mars Odyssey aerobraking temperature retrievals. Our experiments suggest that thermal effects of GWs are probably a key physical mechanism in the MLT missing in contemporary Martian GCMs.

Zonal Jets as Meridional Transport Barriers in the Subtropical and Polar Lower Stratosphere

Abstract Applications of recent results from dynamical systems theory to the study of transport and mixing in incompressible two-dimensional flows lead to the expectation that, independent of the background potential vorticity (PV) distribution, weakly perturbed zonal jets are associated with barriers that inhibit meridional transport. Here the authors provide evidence in support of this expectation based on the analysis of isentropic winds in the lower stratosphere as produced by the Canadian Middle Atmosphere Model (CMAM), a comprehensive general circulation model. Specifically, barriers to meridional transport are found to be associated with the (eastward) austral polar night jet, for which the meridional gradient of background PV is large, and also for the (westward) boreal summer subtropical jet, for which the background PV gradient is quite small. The identification of the meridional transport barriers is based on the computation of finite-time Lyapunov exponents (FTLEs), which characterize the amount of stretching about fluid particle trajectories. Being composed of regular fluid particle trajectories lying on invariant tori, the meridional transport barriers are identified with topologically circular, local minimizing curves or trenches of the backward-plus-forward FTLE field. Results from explicit passive tracer advection experiments and flux computations are also presented, which are consistent with results inferred using the FTLE diagnostic.