Quasi‐biennial oscillations (QBOs) in thirteen atmospheric general circulation models forced with both observed and annually repeating sea surface temperatures (SSTs) are evaluated. In most models ...the QBO period is close to, but shorter than, the observed period of 28 months. Amplitudes are within ±20% of the observed QBO amplitude at 10 hPa, but typically about half of that observed at lower altitudes (50 and 70 hPa). For almost all models, the oscillation's amplitude profile shows an overall upward shift compared to reanalysis and its meridional extent is too narrow. Asymmetry in the duration of eastward and westward phases is reasonably well captured, though not all models replicate the observed slowing of the descending westward shear. Westward phases are generally too weak, and most models have an eastward time mean wind bias throughout the depth of the QBO. The intercycle period variability is realistic and in some models is enhanced in the experiment with observed SSTs compared to the experiment with repeated annual cycle SSTs. Mean periods are also sensitive to this difference between SSTs, but only when parametrized non‐orographic gravity wave (NOGW) sources are coupled to tropospheric parameters and not prescribed with a fixed value. Overall, however, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually. A portrait of the overall ensemble performance is provided by a normalized grading of QBO metrics. To simulate a QBO, all but one model used parametrized NOGWs, which provided the majority of the total wave forcing at altitudes above 70 hPa in most models. Hence the representation of NOGWs either explicitly or through parametrization is still a major uncertainty underlying QBO simulation in these present‐day experiments.
Quasi‐biennial oscillations (QBOs) in thirteen atmospheric general circulation models forced with both observed (orange) and annually repeating (grey) sea surface temperatures (SSTs) are evaluated over a range of metrics and compared against reanalysis (blue‐green). Mean periods are sensitive to this difference between SSTs, but only when parametrized non‐orographic gravity wave sources are coupled to tropospheric parameters (60LCAM5 and right there of) and not prescribed with fixed values. Overall, however, modelled QBOs are very similar whether or not the prescribed SSTs vary interannually.
The processes occurring in the tropical tropopause layer (TTL) are of great importance for stratosphere–troposphere exchanges and the variability of the Earth's climate. Previous studies demonstrated ...the increasing ability of atmospheric general circulation models (AGCMs) in simulating the TTL, depending on factors such as the horizontal and vertical resolution, with the major role for physical parametrizations. In this work we assess the mean state and variability of the tropical upper troposphere and lower stratosphere simulated by 13 AGCMs of the Stratosphere–troposphere Processes And their Role in Climate Quasi‐Biennial Oscillation initiative (QBOi) for the historical period. As these models internally generate quasi‐biennial oscillations (QBOs) of the stratospheric zonal wind, we can analyse the simulated QBO influence on the TTL on interannual time‐scales. We find that model biases in temperature near the tropopause are strongly related to water vapour concentrations in the lower stratosphere. A source of intermodel spread derives from stratospheric aerosols, as the responses to eruptions differ between those models prescribing volcanic aerosol forcing. The QBO influence on the thermal structure is generally realistic in the equatorial region, but the subtropical response is weak compared with the reanalysis. This is associated with a limited downward penetration of QBO winds, generally smaller QBO meridional widths, and weaker temperature anomalies, which disappear above the tropopause for most models. We discuss the QBO impacts on tropopause pressure and precipitation, characterized by large uncertainties due to the small signal in the observational records and sampling uncertainty. Realistic QBO connection with the troposphere in some models suggests that the underlying physical processes can be correctly simulated. Overall, we find that the QBOi models have limited ability to reproduce the observed modulation of the TTL processes, which is consistent with biases in the vertical and latitudinal extent of the simulated QBOs degrading this connection.
In this work we study how QBOi models simulate the influence of the quasi‐biennial oscillation on stratospheric and upper tropospheric temperatures, and associated processes. The figure shows the climatological seasonal cycle for (a) temperature near the tropical tropopause and (b) water vapour in the lower stratosphere for the models (coloured lines, dashed grey line is the multimodel mean) and the reanalysis reference (black line).
Byline: N. Butchart (1), A. A. Scaife (2), M. Bourqui (3,4), J. Grandpre (4), S. H. E. Hare (3), J. Kettleborough (5), U. Langematz (6), E. Manzini (7), F. Sassi (8), K. Shibata (9), D. Shindell ...(10), M. Sigmond (11) The effect of climate change on the Brewer--Dobson circulation and, in particular, the large-scale seasonal-mean transport between the troposphere and stratosphere is compared in a number of middle atmosphere general circulation models. All the models reproduce the observed upwelling across the tropical tropopause balanced by downwelling in the extra tropics, though the seasonal cycle in upwelling in some models is more semi-annual than annual. All the models also consistently predict an increase in the mass exchange rate in response to growing greenhouse gas concentrations, irrespective of whether or not the model includes interactive ozone chemistry. The mean trend is 11 kt s.sup.-1 year.sup.-1 or about 2% per decade but varies considerably between models. In all but one of the models the increase in mass exchange occurs throughout the year though, generally, the trend is larger during the boreal winter. On average, more than 60% of the mean mass fluxes can be explained by the EP-flux divergence using the downward control principle. Trends in the annual mean mass fluxes derived from the EP-flux divergence also explain about 60% of the trend in the troposphere-to-stratosphere mass exchange rate when averaged over all the models. Apart from two models the interannual variability in the downward control derived and actual mass fluxes were generally well correlated, for the annual mean. Author Affiliation: (1) Met Office, FitzRoy Road, Exeter, Devon, EX1 3PB, UK (2) Hadley Centre, Met Office, Exeter, UK (3) Department of Meteorology, University of Reading, Reading, UK (4) McGill University, Montreal, Canada (5) Rutherford Laboratory, British Atmospheric Data Centre, Didcot, UK (6) Freie Universitat of Berlin, Berlin, Germany (7) National Institute for Geophysics and Volcanology, Bologna, Italy (8) National Center for Atmospheric Research, Boulder, CO, USA (9) Meteorological Research Institute, Tsukuba, Japan (10) NASA-Goddard Institute for Space Studies, New York, USA (11) University of Toronto, Toronto, Canada Article History: Registration Date: 29/05/2006 Received Date: 15/11/2005 Accepted Date: 20/05/2006 Online Date: 16/08/2006
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.
The figure shows the climatological annual cycle of equatorial zonally averaged zonal wind for each calendar month from 11 models that participated in the Quasi‐Biennial Oscillation initiative (QBOi), compared with winds derived from SABER observations (lower right). The models simulate a realistic semiannual cycle but the time‐mean winds are more strongly westward than observed.
It has been suggested that the Sun may evolve into a period of lower activity over the 21st century. This study examines the potential climate impacts of the onset of an extreme “Maunder ...Minimum‐like” grand solar minimum using a comprehensive global climate model. Over the second half of the 21st century, the scenario assumes a decrease in total solar irradiance of 0.12% compared to a reference Representative Concentration Pathway 8.5 experiment. The decrease in solar irradiance cools the stratopause (∼1 hPa) in the annual and global mean by 1.2 K. The impact on global mean near‐surface temperature is small (∼−0.1 K), but larger changes in regional climate occur during the stratospheric dynamically active seasons. In Northern Hemisphere wintertime, there is a weakening of the stratospheric westerly jet by up to ∼3–4 m s−1, with the largest changes occurring in January–February. This is accompanied by a deepening of the Aleutian Low at the surface and an increase in blocking over Northern Europe and the North Pacific. There is also an equatorward shift in the Southern Hemisphere midlatitude eddy‐driven jet in austral spring. The occurrence of an amplified regional response during winter and spring suggests a contribution from a top‐down pathway for solar‐climate coupling; this is tested using an experiment in which ultraviolet (200–320 nm) radiation is decreased in isolation of other changes. The results show that a large decline in solar activity over the 21st century could have important impacts on the stratosphere and regional surface climate.
Key Points
A future decline in solar activity would not offset projected global warming
A future decline in solar activity could have larger regional effects in winter
Top‐down mechanism contributes to Northern Hemisphere regional response
We describe the HadGEM2 family of climate configurations of the Met Office Unified Model, MetUM. The concept of a model "family" comprises a range of specific model configurations incorporating ...different levels of complexity but with a common physical framework. The HadGEM2 family of configurations includes atmosphere and ocean components, with and without a vertical extension to include a well-resolved stratosphere, and an Earth-System (ES) component which includes dynamic vegetation, ocean biology and atmospheric chemistry. The HadGEM2 physical model includes improvements designed to address specific systematic errors encountered in the previous climate configuration, HadGEM1, namely Northern Hemisphere continental temperature biases and tropical sea surface temperature biases and poor variability. Targeting these biases was crucial in order that the ES configuration could represent important biogeochemical climate feedbacks. Detailed descriptions and evaluations of particular HadGEM2 family members are included in a number of other publications, and the discussion here is limited to a summary of the overall performance using a set of model metrics which compare the way in which the various configurations simulate present-day climate and its variability.
Major sudden stratospheric warmings (SSWs), vortex formation, and final breakdown dates are key highlight points of the stratospheric polar vortex. These phenomena are relevant for ...stratosphere‐troposphere coupling, which explains the interest in understanding their future changes. However, up to now, there is not a clear consensus on which projected changes to the polar vortex are robust, particularly in the Northern Hemisphere, possibly due to short data record or relatively moderate CO2 forcing. The new simulations performed under the Coupled Model Intercomparison Project, Phase 6, together with the long daily data requirements of the DynVarMIP project in preindustrial and quadrupled CO2 (4xCO2) forcing simulations provide a new opportunity to revisit this topic by overcoming the limitations mentioned above. In this study, we analyze this new model output to document the change, if any, in the frequency of SSWs under 4xCO2 forcing. Our analysis reveals a large disagreement across the models as to the sign of this change, even though most models show a statistically significant change. As for the near‐surface response to SSWs, the models, however, are in good agreement as to this signal over the North Atlantic: There is no indication of a change under 4xCO2 forcing. Over the Pacific, however, the change is more uncertain, with some indication that there will be a larger mean response. Finally, the models show robust changes to the seasonal cycle in the stratosphere. Specifically, we find a longer duration of the stratospheric polar vortex and thus a longer season of stratosphere‐troposphere coupling.
Key Points
The tropospheric signal of sudden stratospheric warming (SSWs) in the North Atlantic does not change under 4xCO2 forcing
There is high uncertainty in changes of SSW frequency under 4xCO2 forcing; single models show the rate to be significantly halved or doubled
The boreal polar vortex will form earlier and disappear later under increased CO2, extending the season of stratosphere‐troposphere coupling
The impact of stratospheric ozone on the tropospheric general circulation of the Southern Hemisphere (SH) is examined with a set of chemistry‐climate models participating in the Stratospheric ...Processes and their Role in Climate (SPARC)/Chemistry‐Climate Model Validation project phase 2 (CCMVal‐2). Model integrations of both the past and future climates reveal the crucial role of stratospheric ozone in driving SH circulation change: stronger ozone depletion in late spring generally leads to greater poleward displacement and intensification of the tropospheric midlatitude jet, and greater expansion of the SH Hadley cell in the summer. These circulation changes are systematic as poleward displacement of the jet is typically accompanied by intensification of the jet and expansion of the Hadley cell. Overall results are compared with coupled models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4), and possible mechanisms are discussed. While the tropospheric circulation response appears quasi‐linearly related to stratospheric ozone changes, the quantitative response to a given forcing varies considerably from one model to another. This scatter partly results from differences in model climatology. It is shown that poleward intensification of the westerly jet is generally stronger in models whose climatological jet is biased toward lower latitudes. This result is discussed in the context of quasi‐geostrophic zonal mean dynamics.
We analyze the stratospheric Kelvin and Rossby‐gravity wave packets with periods of a few days in nine high‐top (i.e., with stratosphere) models of the fifth Coupled Model Intercomparison Project ...(CMIP5). These models simulate realistic aspects of these waves and represent them better than the tropospheric convectively coupled waves analyzed in previous studies. There is nevertheless a large spread among the models, and those with a quasi‐biennial oscillation (QBO) produce larger amplitude waves than the models without a QBO. For the Rossby‐gravity waves this is explained by the fact that models without a QBO never have positive zonal mean zonal winds in the lower stratosphere, a situation that is favorable to the propagation of Rossby‐gravity waves. For the Kelvin waves, larger amplitudes in the presence of a QBO is counter intuitive because Kelvin waves are expected to have larger amplitude when the zonal mean zonal wind is negative, and this is always satisfied in models without a QBO. We attribute the larger amplitude to the fact that models tuned to have a QBO require finer vertical resolution in the stratosphere. We also find that models with large precipitation variability tend to produce larger amplitude waves. However, the effect is not as pronounced as was found in previous studies. In fact, even models with weak precipitation variability still have quite realistic stratospheric waves, indicating either that (i) other sources can be significant or that (ii) the dynamical filtering mitigates the differences in the sources between models.
Key Points
Stratospheric equatorial waves extracted for the first time in coupled models
Models produce realistic Kelvin and Rossby‐gravity waves but with large spread
Differences in resolution have more effects than differences in precipitation
The importance of using a general circulation model that includes a well-resolved stratosphere for climate simulations, and particularly the influence this has on surface climate, is investigated. ...High top model simulations are run with the Met Office Unified Model for the Coupled Model Intercomparison Project Phase 5 (CMIP5). These simulations are compared to equivalent simulations run using a low top model differing only in vertical extent and vertical resolution above 15 km. The period 1960–2002 is analyzed and compared to observations and the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis dataset. Long-term climatology, variability, and trends in surface temperature and sea ice, along with the variability of the annular mode index, are found to be insensitive to the addition of a well-resolved stratosphere. The inclusion of a well-resolved stratosphere, however, does improve the impact of atmospheric teleconnections on surface climate, in particular the response to El Niño–Southern Oscillation, the quasi-biennial oscillation, and midwinter stratospheric sudden warmings (i.e., zonal mean wind reversals in the middle stratosphere). Thus, including a well-represented stratosphere could improve climate simulation on intraseasonal to interannual time scales.
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