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The Quasi‐Biennial Oscillation initiative (QBOi) is a model intercomparison programme that specifically targets simulation of the QBO in current global climate models. Eleven of the models or model versions that participated in a QBOi intercomparison study have upper boundaries in or above the mesosphere and therefore simulate the region where the stratopause semiannual oscillation (SAO) is the dominant mode of variability of zonal winds in the tropical upper stratosphere. Comparisons of the SAO simulations in these models are presented here. These show that the model simulations of the amplitudes and phases of the SAO in zonal‐mean zonal wind near the stratopause agree well with the information derived from available observations. However, most of the models simulate time‐average zonal winds that are more westward than determined from observations, in some cases by several tens of m·s$^{–1}$. Validation of wave activity in the models is hampered by the limited observations of tropical waves in the upper stratosphere but suggests a deficit of eastward forcing either by large‐scale waves, such as Kelvin waves, or by gravity waves.

CMAM30 is a 30-year data set extending from 1979 to 2010 that is generated using a version of the Canadian Middle Atmosphere Model (CMAM) in which the winds and temperatures are relaxed to the Interim Reanalysis product from the European Centre for Medium-Range Weather Forecasts (ERA-Interim). The data set has dynamical fields that are very close to the reanalysis below 1 hPa and chemical tracers that are self-consistent with respect to the model winds and temperature. The chemical tracers are expected to be close to actual observations. The data set is here compared to two satellite records – the Atmospheric Chemistry Experiment Fourier transform spectrometer and the Odin Optical Spectrograph and Infrared Imaging System – for the purpose of validating the temperature, ozone, water vapour and methane fields. Data from the Aura microwave limb sounder are also used for validation of the chemical processing in the polar vortex. It is found that the CMAM30 temperature is warmer by up to 5 K in the stratosphere, with a low bias in the mesosphere of ~ 5–15 K. Ozone is reasonable (±15 %), except near the tropopause globally and in the Southern Hemisphere winter polar vortex. Water vapour is consistently low by 10–20 %, with corresponding high methane of 10–20 %, except in the Southern Hemisphere polar vortex. Discrepancies in this region are shown to stem from the treatment of polar stratospheric cloud formation in the model.

The stratospheric climate and variability from simulations of sixteen chemistry-climate models is evaluated. On average the polar night jet is well reproduced though its variability is less well reproduced with a large spread between models. Polar temperature biases are less than 5 K except in the Southern Hemisphere (SH) lower stratosphere in spring. The accumulated area of low temperatures responsible for polar stratospheric cloud formation is accurately reproduced for the Antarctic but underestimated for the Arctic. The shape and position of the polar vortex is well simulated, as is the tropical upwelling in the lower stratosphere. There is a wide model spread in the frequency of major sudden stratospheric warnings (SSWs), late biases in the breakup of the SH vortex, and a weak annual cycle in the zonal wind in the tropical upper stratosphere. Quantitatively, "metrics" indicate a wide spread in model performance for most diagnostics with systematic biases in many, and poorer performance in the SH than in the Northern Hemisphere (NH). Correlations were found in the SH between errors in the final warming, polar temperatures, the leading mode of variability, and jet strength, and in the NH between errors in polar temperatures, frequency of major SSWs, and jet strength. Models with a stronger QBO have stronger tropical upwelling and a colder NH vortex. Both the qualitative and quantitative analysis indicate a number of common and long-standing model problems, particularly related to the simulation of the SH and stratospheric variability. Copyright 2011 by the American Geophysical Union.

The present study investigates the sensitivity of the frequency distribution of precipitation rates to the closure employed in the penetrative mass flux cumulus parameterization of Zhang and McFarlane in the Canadian regional climate model (CRCM) and in the Canadian Centre for Climate Modelling and Analysis third generation global atmospheric general circulation model (AGCM3). The effects of an alternative prognostic closure for mass flux cumulus parameterization in place of the original diagnostic closure are investigated. A set of experiments is performed in which changes in the frequency distribution of precipitation rates and cloud base mass-flux are examined as a function of the parameters that define each closure scheme. The relationship between the frequency distribution of precipitation and cloud base mass flux is examined and a self-consistent relation is found when the depth of convection is taken into account. Experiments performed with the prognostic closure favor relatively strong cloud base mass-flux and deep penetrative convection with relatively more intense convective precipitation. The mean of the frequency distribution of convective precipitation is larger and the heavier events become more intense. Also, experiments performed with the prognostic closure favor less frequent convective activity. However these changes in the distribution of convective component of precipitation are generally offset by opposite changes in the distribution of the resolved large-scale component of precipitation, resulting in relatively smaller changes in total precipitation. The altered partition of precipitation between convective and large-scale components is found to alter the energy balance and the thermodynamic equilibrium structure of the troposphere. The robustness found in the CRCM results regarding the sensitivity of the frequency distribution of precipitation to changes in the closure of the deep convection parameterization is investigated by performing a similar analysis of AGCM3 simulations. A remarkable similarity of AGCM3 and CRCM results is found suggesting that the closure sensitivity identified in this study is robust.

A comparison is undertaken of the response of a general circulation model (GCM) to the nonorographic gravity wave drag parameterizations of Hines, Warner and McIntyre, and Alexander and Dunkerton. The analysis is restricted to a comparison of each parameterization’s nonlinear dissipation mechanism since, in principle, this is the only component that differs between the schemes. This is achieved by developing a new, more general parameterization that can represent each of these dissipation mechanisms, while keeping all other aspects of the problem identical. The GCM simulations reveal differences in the climatological response to the three dissipation mechanisms. These differences are documented for both tropopause and surface launch elevations of the parameterized waves. The simulations also reveal systematic differences in the height at which momentum is deposited. This behavior is investigated further in a set of experiments designed to reduce these systematic differences, while leaving the details of the dissipation mechanisms unaltered. These sensitivity experiments demonstrate that it is possible to obtain nearly identical responses from all three mechanisms, which indicates that the GCM response is largely insensitive to the precise details of the dissipation mechanisms. This finding is supported by an additional experiment in which the nonlinear dissipation mechanisms are turned off and critical-level filtering is left to act as the only source of dissipation. In this experiment, critical-level filtering effectively replaces the nonlinear dissipation mechanism, producing a nearly identical response. The results of this study suggest that climate modeling efforts would potentially benefit more from the refinement of other aspects of the parameterization problem, such as the properties of the launch spectrum, than they have benefited from the refinement of dissipation mechanisms.

This paper investigates the temporal properties of tropical precipitation in the Canadian Centre for Climate Modelling and Analysis (CCCma) third-generation atmospheric general circulation model (AGCM3). AGCM3 employs the penetrative mass-flux (PMF) scheme of Zhang and McFarlane (ZM) for the parameterization of deep cumulus convection. It is found that the temporal variability of the ZM scheme is sensitive to a number of its internal parameters as well to the use of a prognostic, rather than diagnostic, closure condition for the cloud-base mass flux. Sensitivity experiments suggest that the ZM scheme can produce realistic amounts of variability when compared to direct radar observations of deep cumulus convection in the Tropics. A central finding of this study is that the resolved large-scale stratiform precipitation (LSP) in the model can participate in the modeling of deep latent heating and so compete with the ZM scheme in the Tropics. In modeling deep latent heating the LSP is found to mimic the behavior of a moist-convective adjustment scheme. In AGCM3 it is found that typical parameter settings of the ZM scheme place it in a regime in which the temporal variability of tropical precipitation is dominated by this behavior of the LSP, while the temporal mean is dominated by the ZM scheme. In such circumstances it is the LSP, and not the ZM scheme, that provides the primary source of resolved tropical Kelvin and mixed Rossby‐gravity waves in the GCM. Such competition between LSP and the parameterization of deep convection appears to be active in other modeling studies. Consequently, it has the potential to complicate efforts to understand the nature of resolved tropical waves in GCMs and their role in the forcing of the quasi-biennial and semiannual oscillations.

An accurate method of solution is developed for the Warner and McIntyre parameterization of nonorographic gravity wave drag for the case of hydrostatic dynamics in the absence of rotation. The new scheme is sufficiently fast that it is suitable for operational use in a general circulation model. Multiyear climate runs of the Canadian Middle Atmosphere Model are performed using the new scheme, and these are compared against equivalent climate runs using the full Warner and McIntyre scheme, which includes nonhydrostatic and rotational wave dynamics. The results indicate that the new operational scheme closely reproduces the seasonal distribution of time- and zonal-mean zonal winds, and the seasonal evolution of lower stratospheric temperatures that are obtained when the full Warner and McIntyre scheme is used. In addition, these quantities are shown to compare favorably with observations.

An efficient method of solution is developed for the Warner and McIntyre parameterization of the drag associated with nonhydrostatic non-orographic inertia-gravity waves. The scheme is sufficiently fast as to enable, for the first time, fully interactive multi-year climate simulations that include the effects of rotation and nonhydrostatic wave dynamics in the parameterization of non-orographic gravity wave drag. It is found that the new scheme alleviates much of the middle-atmosphere wind biases that occur in the Canadian Middle Atmosphere Model when either of its two operational hydrostatic non-orographic gravity wave drag parameterizations are used. The addition of the nonhydrostatic process of back-reflection has an important impact on the amount momentum flux launched into the stratosphere by the parameterization scheme. This quantity is a free parameter in the problem and it is specified here to be independent of time and geographic location. However, due to back-reflection, the net momentum flux that actually enters the stratosphere undergoes a systematic seasonal and latitudinal variation which is a consequence of the seasonal and latitudinal variation of the winds and temperatures in the middle atmosphere. This strong influence results in a characteristic latitudinal distribution for the net momentum flux entering the stratosphere in winter and summer. During these seasons, mid- to high-latitude launch momentum flux that is directed oppositely to the mesospheric jet can be reduced by as much as 75% due to back-reflection. The momentum flux launched into the stratosphere at tropical latitudes, however, is relatively unaffected by back-reflection. This has important implications for the forcing of tropical oscillations such as the semi-annual oscillation and the quasi-biennial oscillation in general circulation models.

A new parametrization of drag arising from the flow over unresolved topography (UT) in a general-circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low-level wave breaking, and a parametrization of low-level drag associated with upstream blocking and lee-vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low-level flow so that it is more parallel to unresolved two-dimensional topographic ridges. The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five-year seasonal means of present-day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity-wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity-wave field with two waves rather than just one allows 30-50% more gravity-wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere.

The evolution of stratospheric ozone from 1960 to 2100 is examined in simulations from 14 chemistry-climate models, driven by prescribed levels of halogens and greenhouse gases. There is general agreement among the models that total column ozone reached a minimum around year 2000 at all latitudes, projected to be followed by an increase over the first half of the 21st century. In the second half of the 21st century, ozone is projected to continue increasing, level off, or even decrease depending on the latitude. Separation into partial columns above and below 20 hPa reveals that these latitudinal differences are almost completely caused by differences in the model projections of ozone in the lower stratosphere. At all latitudes, upper stratospheric ozone increases throughout the 21st century and is projected to return to 1960 levels well before the end of the century, although there is a spread among models in the dates that ozone returns to specific historical values. We find decreasing halogens and declining upper atmospheric temperatures, driven by increasing greenhouse gases, contribute almost equally to increases in upper stratospheric ozone. In the tropical lower stratosphere, an increase in upwelling causes a steady decrease in ozone through the 21st century, and total column ozone does not return to 1960 levels in most of the models. In contrast, lower stratospheric and total column ozone in middle and high latitudes increases during the 21st century, returning to 1960 levels well before the end of the century in most models.

A version of the Canadian Middle Atmosphere Model that is coupled to an ocean is used to investigate the separate effects of climate change and ozone depletion on the dynamics of the Southern Hemisphere (SH) stratosphere. This is achieved by performing three sets of simulations extending from 1960 to 2099: 1) greenhouse gases (GHGs) fixed at 1960 levels and ozone depleting substances (ODSs) varying in time, 2) ODSs fixed at 1960 levels and GHGs varying in time, and 3) both GHGs and ODSs varying in time. The response of various dynamical quantities to the GHG and ODS forcings is shown to be additive; that is, trends computed from the sum of the first two simulations are equal to trends from the third. Additivity is shown to hold for the zonal mean zonal wind and temperature, the mass flux into and out of the stratosphere, and the latitudinally averaged wave drag in SH spring and summer, as well as for final warming dates. Ozone depletion and recovery causes seasonal changes in lower-stratosphere mass flux, with reduced polar downwelling in the past followed by increased downwelling in the future in SH spring, and the reverse in SH summer. These seasonal changes are attributed to changes in wave drag caused by ozone-induced changes in the zonal mean zonal winds. Climate change, on the other hand, causes a steady decrease in wave drag during SH spring, which delays the breakdown of the vortex, resulting in increased wave drag in summer.

The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 ± 0.07 K decade−1 at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade−1 at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twenty-first century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson circulation throughout the depth of the stratosphere, which reduces the mean age of air nearly everywhere at a rate of about 0.05 yr decade−1 in those models with this diagnostic. On average, the annual mean tropical upwelling in the lower stratosphere (∼70 hPa) increases by almost 2% decade−1, with 59% of this trend forced by the parameterized orographic gravity wave drag in the models. This is a consequence of the eastward acceleration of the subtropical jets, which increases the upward flux of (parameterized) momentum reaching the lower stratosphere in these latitudes.

[1] The robustness of the atmospheric circulation response to global warming in a set of atmospheric general circulation models (AGCMs) is investigated. The global-warmed climate is forced by a global pattern of warmed ocean surface temperatures that is extracted from a multi-model ensemble of coupled ocean-atmosphere climate model greenhouse warming simulations. The robustness of the warming response is evaluated by a hierarchical set of comparisons. The warming response is compared first between two independently developed AGCMs, then as a function of horizontal resolution in one model, and finally as a function of a single tuning parameter, related to orographic gravity wave drag. Across these levels of comparison, the tropical and subtropical response is generally robust in zonal wind and temperature, but the extratropical response is non-robust. On regional scales, almost every aspect of the response is non-robust, even to the variation of a single tuning parameter. Some evidence is presented that the non-robustness of the simulated response to global warming might be predicted from the (non global-warmed) control simulation.

Simulations from eleven coupled chemistry-climate models (CCMs) employing nearly identical forcings have been used to project the evolution of stratospheric ozone throughout the 21st century. The model-to-model agreement in projected temperature trends is good, and all CCMs predict continued, global mean cooling of the stratosphere over the next 5 decades, increasing from around 0.25 K/decade at 50 hPa to around 1 K/ decade at 1 hPa under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario. In general, the simulated ozone evolution is mainly determined by decreases in halogen concentrations and continued cooling of the global stratosphere due to increases in greenhouse gases (GHGs). Column ozone is projected to increase as stratospheric halogen concentrations return to 1980s levels. Because of ozone increases in the middle and upper stratosphere due to GHGinduced cooling, total ozone averaged over midlatitudes, outside the polar regions, and globally, is projected to increase to 1980 values between 2035 and 2050 and before lower stratospheric halogen amounts decrease to 1980 values. In the polar regions the CCMs simulate small temperature trends in the first and second half of the 21st century in midwinter. Differences in stratospheric inorganic chlorine (Cly) among the CCMs are key to diagnosing the intermodel differences in simulated ozone recovery, in particular in the Antarctic. It is found that there are substantial quantitative differences in the simulated Cly, with the October mean Antarctic Cly peak value varying from less than 2 ppb to over 3.5 ppb in the CCMs, and the date at which the Cly returns to 1980 values varying from before 2030 to after 2050. There is a similar variation in the timing of recovery of Antarctic springtime column ozone back to 1980 values. As most models underestimate peak Cly near 2000, ozone recovery in the Antarctic could occur even later, between 2060 and 2070. In the Arctic the column ozone increase in spring does not follow halogen decreases as closely as in the Antarctic, reaching 1980 values before Arctic halogen amounts decrease to 1980 values and before the Antarctic. None of the CCMs predict future large decreases in the Arctic column ozone. By 2100, total column ozone is projected to be substantially above 1980 values in all regions except in the tropics.

[1] The climatic consequences of relaxing the uniform greenhouse gas (GHG) assumption in the Canadian Centre for Climate Modelling and Analysis atmospheric general circulation model are examined. A simple chemical loss parameterization for nitrous oxide, methane, CFC-11, and CFC-12 is employed that includes stratospheric water vapor production from methane oxidation. Multidecadal mean distributions of these species are obtained that compare reasonably well with UARS satellite observations of the stratosphere. The radiative impact of these changes is a widespread cooling of the stratosphere (with a spatially averaged, annual mean value of 0.6 K), compared to the model with specified uniform GHG distributions. This cooling results from an approximate doubling of the amount of middle to upper stratospheric moisture (as a result of methane oxidation) and exceeds the radiatively induced warming due to decreases in the other GHGs. Annual mean temperature changes of up to +8 K in the upper winter polar stratosphere, by contrast, are dynamically induced because of increases in the residual mean circulation and associated heating.