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We present a comparison of temperature trends using different satellite and radiosonde observations and climate (GCM) and chemistry-climate model (CCM) outputs, focusing on the role of photochemical ozone depletion in the Antarctic lower stratosphere during the second half of the twentieth century. Ozone-induced stratospheric cooling peaks during November at an altitude of approximately 100 hPa in radiosonde observations, with 1969 to 1998 trends in the range of -3.8 to -4.7 K/dec. This stratospheric cooling trend is more than 50% greater than the previously estimated value of -2.4 K/dec, which suggested that the CCMs were overestimating the stratospheric cooling, and that the less complex GCMs forced by prescribed ozone were matching observations better. Corresponding ensemble mean model trends are -3.8K/dec for the CCMs, -3.5K/dec for the CMIP5 GCMs, and -2.7K/dec for the CMIP3 GCMs. Accounting for various sources of uncertainty-including sampling uncertainty, measurement error, model spread, and trend confidence intervals-observations and CCM and GCM ensembles are consistent in this new analysis. This consistency does not apply to each individual that makes up the GCM and CCM ensembles, and some do not show significant ozone-induced cooling. Nonetheless, analysis of the joint ozone and temperature trends in the CCMs suggests that the modeled cooling/ozone-depletion relationship is within the range of observations. Overall, this study emphasizes the need to use a wide range of observations for model validation as well as sufficient accounting of uncertainty in both models and measurements.

Vertical fluxes of wave activity from the troposphere to the stratosphere correlate negatively with the Northern Annular Mode (NAM) index in the stratosphere and subsequently in the troposphere. Recent studies have shown that stratospheric NAM variability is also negatively correlated with the amplitude of the wave pattern coherent with the large-scale climatological stationary wavefield; when the climatological stationary wavefield is amplified or attenuated, the stratospheric jet correspondingly weakens or strengthens. Here we quantify the importance of this linear interference effect in initiating stratosphere-troposphere interactions by performing a decomposition of the vertical wave activity flux using reanalysis data. The interannual variability in vertical wave activity flux in both the Northern and Southern Hemisphere extratropics is dominated by linear interference of quasi-stationary waves during the season of strongest stratosphere-troposphere coupling. Composite analysis of anomalous vertical wave activity flux events reveals the significant role of linear interference and shows that "linear" and "nonlinear" events are essentially independent. Linear interference is the dominant contribution to the vertical wave activity flux anomalies preceding displacement stratospheric sudden warmings (SSWs) while split SSWs are preceded by nonlinear wave activity flux anomalies. Wave activity variability controls the timing of stratospheric final warmings, and this variability is shown to be dominated by linear interference, particularly in the Southern Hemisphere. The persistence of the linear interference component of the vertical wave activity flux, corresponding to persistent constructive or destructive interference between the wave-1 component of climatological stationary wave and the wave anomaly, may help improve wintertime extratropical predictability.

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. Tw o 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 co ntrol 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 be en 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/).

The sensitivity of the tropospheric extratropical circulation to thermal perturbations of the polar stratosphere is examined in a dry primitive equation general circulation model with zonally symmetric forcing and boundary conditions. For sufficiently strong cooling of the polar winter stratosphere, the winter-hemisphere tropospheric jet shifts polewards and strengthens markedly at the surface; this is accompanied by a drop in surface pressure at high latitudes in the same hemisphere. In addition, this extratropical tropospheric response is found to be very similar to the model's leading pattern of internal variability. These results are tested for robustness at several horizontal and vertical resolutions, and the same tropospheric response is observed at all but the lowest resolution tested. The behavior of this relatively simple model is broadly consistent with recent observational and modeling studies of trends in extratropical atmospheric variability.