Abstract

Arctic haze has a distinct seasonal cycle with peak concentrations in winter but pristine conditions in summer. It is demonstrated that the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation model (AM3) can reproduce the observed seasonality of Arctic black carbon (BC), an important component of Arctic haze. The model is used to study how large-scale circulation and removal drive the seasonal cycle of Arctic BC. It is found that despite large seasonal shifts in the general circulation pattern, the transport of BC into the Arctic varies little throughout the year. The seasonal cycle of Arctic BC is attributed mostly to variations in the controlling factors of wet removal, namely the hydrophilic fraction of BC and wet deposition efficiency of hydrophilic BC. Specifically, a confluence of low hydrophilic fraction and weak wet deposition, owing to slower aging process and less efficient mixed-phase cloud scavenging, respectively, is responsible for the wintertime peak of BC. The transition to low BC in summer is the consequence of a gradual increase in the wet deposition efficiency, whereas the increase of BC in late fall can be explained by a sharp decrease in the hydrophilic fraction. The results presented here suggest that future changes in the aging and wet deposition processes can potentially alter the concentrations of Arctic aerosols and their climate effects.

Abstract

The recently developed GFDL Atmospheric Model version 3 (AM3), an atmospheric general circulation model (GCM), incorporates a prognostic treatment of cloud drop number to simulate the aerosol indirect effect. Since cloud drop activation depends on cloud-scale vertical velocities, which are not reproduced in present-day GCMs, additional assumptions on the subgrid variability are required to implement a local activation parameterization into a GCM.

This paper describes the subgrid activation assumptions in AM3 and explores sensitivities by constructing alternate configurations. These alternate model configurations exhibit only small differences in their present-day climatology. However, the total anthropogenic radiative flux perturbation (RFP) between present-day and preindustrial conditions varies by ±50% from the reference, because of a large difference in the magnitude of the aerosol indirect effect. The spread in RFP does not originate directly from the subgrid assumptions but indirectly through the cloud retuning necessary to maintain a realistic radiation balance. In particular, the paper shows a linear correlation between the choice of autoconversion threshold radius and the RFP.

Climate sensitivity changes only minimally between the reference and alternate configurations. If implemented in a fully coupled model, these alternate configurations would therefore likely produce substantially different warming from preindustrial to present day.

Abstract

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.

Abstract

Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO₃, reducing ozone destruction by the NO_x catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.

Abstract

The influence of alternative ocean and atmosphere subcomponents on climate model simulation of transient sensitivities is examined by comparing three GFDL climate models used for phase 5 of the Coupled Model Intercomparison Project (CMIP5). The base model ESM2M is closely related to GFDL’s CMIP3 climate model version 2.1 (CM2.1), and makes use of a depth coordinate ocean component. The second model, ESM2G, is identical to ESM2M but makes use of an isopycnal coordinate ocean model. The authors compare the impact of this “ocean swap” with an “atmosphere swap” that produces the GFDL Climate Model version 3 (CM3) by replacing the AM2 atmospheric component with AM3 while retaining a depth coordinate ocean model. The atmosphere swap is found to have much larger influence on sensitivities of global surface temperature and Northern Hemisphere sea ice cover. The atmosphere swap also introduces a multidecadal response time scale through its indirect influence on heat uptake. Despite significant differences in their interior ocean mean states, the ESM2M and ESM2G simulations of these metrics of climate change are very similar, except for an enhanced high-latitude salinity response accompanied by temporarily advancing sea ice in ESM2G. In the ESM2G historical simulation this behavior results in the establishment of a strong halocline in the subpolar North Atlantic during the early twentieth century and an associated cooling, which are counter to observations in that region. The Atlantic meridional overturning declines comparably in all three models.

Abstract

To model aerosol–cloud interactions in general circulation models (GCMs), a prognostic cloud scheme of cloud liquid water and amount is expanded to include droplet number concentration (N_d ) in a way that allows them to be calculated using the same large-scale and convective updraft velocity field. In the scheme, the evolution of droplets fully interacts with the model meteorology. An explicit treatment of cloud condensation nuclei (CCN) activation enables the scheme to take into account the contributions to N_d of multiple aerosol species (i.e., sulfate, organic, and sea-salt aerosols) and to consider kinetic limitations of the activation process. An implementation of the prognostic scheme in the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 GCM yields a vertical distribution of N_d with a characteristic maximum in the lower troposphere; this feature differs from the profile that would be obtained if N_d is diagnosed from the sulfate mass concentration based on an often-used empirical relationship. Prognosticated N_d exhibits large variations with respect to the sulfate mass concentration. The mean values are generally consistent with the empirical relationship over ocean, but show negative biases over the Northern Hemisphere midlatitude land, perhaps owing to the neglect of subgrid variations of large-scale ascents and inadequate convective sources. The prognostic scheme leads to a substantial improvement in the agreement of model-predicted present-day liquid water path (LWP) and cloud forcing with satellite measurements compared to using the empirical relationship.

The simulations with preindustrial and present-day aerosols show that the combined first and second indirect effects of anthropogenic sulfate and organic aerosols give rise to a steady-state global annual mean flux change of −1.8 W m⁻², consisting of −2.0 W m⁻² in shortwave and 0.2 W m⁻² in longwave. The ratios of the flux changes in the Northern Hemisphere (NH) to that in Southern Hemisphere (SH) and of the flux changes over ocean to that over land are 2.9 and 0.73, respectively. These estimates are consistent with the averages of values from previous studies stated in a recent review. The model response to higher N_d alters the cloud field; LWP and total cloud amount increase by 19% and 0.6%, respectively. Largely owing to high sulfate concentrations from fossil fuel burning, the NH midlatitude land and oceans experience strong radiative cooling. So does the tropical land, which is dominated by biomass burning–derived organic aerosol. The computed annual, zonal-mean flux changes are determined to be statistically significant, exceeding the model’s natural variations in the NH low and midlatitudes and in the SH low latitudes. This study reaffirms the major role of sulfate in providing CCN for cloud formation.

Abstract

The effects of declining anthropogenic aerosols in representative concentration pathway 4.5 (RCP4.5) are assessed in four models from phase 5 the Coupled Model Intercomparison Project (CMIP5), with a focus on annual, zonal-mean atmospheric temperature structure and zonal winds. For each model, the effect of declining aerosols is diagnosed from the difference between a projection forced by RCP4.5 for 2006–2100 and another that has identical forcing, except that anthropogenic aerosols are fixed at early twenty-first-century levels. The response to declining aerosols is interpreted in terms of the meridional structure of aerosol radiative forcing, which peaks near 40°N and vanishes at the South Pole.

Increasing greenhouse gases cause amplified warming in the tropical upper troposphere and strengthening midlatitude jets in both hemispheres. However, for declining aerosols the vertically averaged tropospheric temperature response peaks near 40°N, rather than in the tropics. This implies that for declining aerosols the tropospheric meridional temperature gradient generally increases in the Southern Hemisphere (SH), but in the Northern Hemisphere (NH) it decreases in the tropics and subtropics. Consistent with thermal wind balance, the NH jet then strengthens on its poleward side and weakens on its equatorward side, whereas the SH jet strengthens more than the NH jet. The asymmetric response of the jets is thus consistent with the meridional structure of aerosol radiative forcing and the associated tropospheric warming: in the NH the latitude of maximum warming is roughly collocated with the jet, whereas in the SH warming is strongest in the tropics and weakest at high latitudes.

Abstract

This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL). The GFDL Climate Model version 3 (CM3) is formulated with effectively the same ocean and sea ice components as the earlier CM2.1 yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large-scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. The authors anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than in CM2.1, which adversely impacts the interior biases.

Abstract

The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved.

Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments.

The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic.

Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO₂. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/).

Abstract

The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for the atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol–cloud interactions, chemistry–climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical system component of earth system models and models for decadal prediction in the near-term future—for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model. Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud droplet activation by aerosols, subgrid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with ecosystem dynamics and hydrology. Its horizontal resolution is approximately 200 km, and its vertical resolution ranges approximately from 70 m near the earth’s surface to 1 to 1.5 km near the tropopause and 3 to 4 km in much of the stratosphere. Most basic circulation features in AM3 are simulated as realistically, or more so, as in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks remains problematic, as in AM2. The most intense 0.2% of precipitation rates occur less frequently in AM3 than observed. The last two decades of the twentieth century warm in CM3 by 0.32°C relative to 1881–1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of 0.56° and 0.52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol–cloud interactions, and its warming by the late twentieth century is somewhat less realistic than in CM2.1, which warmed 0.66°C but did not include aerosol–cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud–aerosol interactions to limit greenhouse gas warming.