Implications of Lagrangian Tracer Transport for Coupled Chemistry-Climate Simulations
Today's coupled chemistry-climate models (CCM) consider a large number of trace species and feedback processes. Due to the radiative effect of some species, errors in simulated tracer distributions can feed back to model dynamics. Thus, shortcomings of the applied transport schemes can have severe implications for the overall model performance. Traditional Eulerian approaches show a satisfactory performance in case of homogeneously distributed trace species, but they can lead to severe problems when applied to highly inhomogeneous tracer distributions. In case of sharp gradients many schemes show a considerable numerical diffusion. Lagrangian approaches, on the other hand, combine a number of favourable numerical properties: They are strictly mass-conserving and do not suffer from numerical diffusion. Therefore they are able to maintain steeper gradients. A further advantage is that they allow the transport of a large number of tracers without being prohibitively expensive. A variety of benefits for stratospheric dynamics and chemistry resulting from a Lagrangian transport algorithm are demonstrated by the example of the CCM E39C. In an updated version of E39C, called E39C-A, the operational semi-Lagrangian advection scheme has been replaced with the purely Lagrangian scheme ATTILA. It will be shown that several model deficiencies can be cured by the choice of an appropriate transport algorithm. The most important advancement concerns the reduction of a pronounced wet bias in the extra- tropical lowermost stratosphere. In turn, the associated temperature error ("cold bias") is significantly reduced. Stratospheric wind variations are now in better agreement with observations, e.g. E39C-A is able to reproduce the stratospheric wind reversal in the Southern Hemisphere in summer which was not captured by the previous model version. Resulting changes in wave propagation and dissipation lead to a weakening of the simulated mean meridional circulation and therefore a more realistic representation of tropical upwelling. Simulated distributions of chemical tracers in the stratosphere are clearly improved. For example, the vertical distribution of stratospheric chlorine (Cly) is now in agreement with analyses derived from observations and other CCMs. As a consequence the model realistically covers the altitude of maximum ozone depletion in the stratosphere. Furthermore, the simulated temporal evolution of stratospheric Cly in the past agrees is realistically reproduced which is an important step towards more reliable projections of future changes, especially of stratospheric ozone.
Attribution of Global Mean Temperature Trends in the Middle Atmosphere: Model Diagnostics
Global-mean temperature trends in the middle atmosphere can be explained by changes in the radiative energy budget forced by atmospheric composition changes. Here, we analyze past and future temperature trends and changes in radiative heating agents in model simulations performed with the Canadian Middle Atmosphere Model (CMAM). We identify two periods of near-linear changes: 1975-1995, during the period of ozone depletion, and 2010-2040, during the period of ozone recovery. Using a 1D radiative-convective model it is shown that the past and future temperature trends can be understood as a combination of effects from changes in CO2, O3 and H2O. The results confirm the generally accepted view that while enhanced CO2 acts to cool the middle atmosphere, temperature trends are modulated by long-term changes in ozone concentrations: Halogen-induced ozone depletion in the past has lead to enhanced cooling whereas ozone recovery in the future will reduce the CO2 effect. It is also shown that in the upper part of the model domain the effect of ozone heating is not local in the vertical, which has important implications for multiple regression analysis, which assumes local control. For example increase of ozone near the stratopause in the future leads to an overall warming effect from ozone in the mesosphere (due to IR transfer), even though the mesospheric ozone concentration is actually decreasing (due to H2O increase). The importance of changes in the 9.6 μm O3 band cooling and in the mesospheric NIR CO2 heating is also analyzed.
Increase of the Tropical Tropopause Upwelling due to Greenhouse Gas Radiative Forcing Inferred From an Analysis of SBUV and TOMS DATA
A number of papers, including those of the author, using a variety of data sources, have contributed to growing evidence that the mean position of the subtropical and polar jets has moved poleward during the period from 1979 to the present. This movement of the jets has been attributed to direct radiative forcing from the greenhouse gases, leading to an increase in the strength of the Hadley cell circulation. If this is true then the upwelling across the tropical tropopause should also be increasing with time. This paper presents an analysis of both the SBUV profile and TOMS total ozone data to search for this increase and the implication of the results on future total ozone chnages.
Towards Modeling the Variability of Ozone Observed in the Deep Tropical Upper Troposphere and Lower Stratosphere
The tropical upper troposphere and lower stratosphere (UTLS) represents a key region for improvement in climate models, as the microphysical, chemical and transport processes that play significant roles in the budgets of ozone and water vapor in this layer are not well understood. Here we examine ozone profiles in two simulations using the Global Modeling Initiative (GMI) chemistry transport model (CTM). In the first simulation, temperatures and winds come from the GEOS-4 assimilation and in the second, the dynamical fields are saved from a long-term simulation of the GEOS GCM. We compare tropical UTLS ozone profiles between the two simulations and to observed ozonesonde profiles from Ticosonde/Southern Hemisphere Additional Ozonesonde (SHADOZ) campaigns since 2005 in Costa Rica at 10° N, 84.2° W and ozone profiles retrieved by the Microwave Limb Sounder (MLS) instrument on board the Aura satellite. During the northern hemisphere summer, Costa Rica lies within the Intertropical Convergence Zone (ITCZ), and the ozonesonde profiles show a pronounced maximum of temporal variability between 13 and 18 km. Ticosonde temperature and water vapor profiles as well as trajectory analyses show that this variability in ozone is driven in large part by horizontal and vertical transport induced by equatorial waves at time scales of ~5 days. July ozone profiles from the simulation driven by the assimilated dynamical fields show the same sharp increase in variability above 14 km and a similar dominance by waves in the temporal variability in the UTLS. In contrast, UTLS temporal variability in the second simulation driven by the GCM winds is reduced by a factor of 2 or more, suggesting that the dominant control on the variability of ozone mixing ratios in the tropical UTLS is advective, and not convective, transport.
Ozone depletion, surface climate, and the Annular Modes: A model study
High-latitude climate variability, such as the cooling over the Antarctic interior of recent decades, is often explained in terms of trends in the annular modes. Here we report on CCM experiments suggesting that a recent strengthening of the annular modes of both hemispheres is mainly caused by anthropogenic ozone depletion. A reference simulation covering 1959-2100 exhibits first a strengthening of both annular modes to ~2000, similar to observations, followed by a decline during the period of ozone recovery, before further strengthening after ~2045. A second simulation from 1960 to 2000 excluding anthropogenic ozone depletion exhibits no significant strengthening of either mode. Substantial surface temperature anomalies over the northern continents and Antarctica are associated with the variations of the modes in the reference simulation. For the 20th century they compare well with observations; for the first half of the 21st century we find a weakening or reversal of temperature trends in some large continental areas, for example, the Antarctic Plateau, Central Siberia, and Eastern Canada. Our results corroborate emerging evidence that ozone recovery will dominate climate change in Antarctica in the coming few decades. For the Northern Hemisphere, our results suggest a more dominant role of ozone in driving the Northern Annular Mode than has hitherto been appreciated. We speculate that prescribing zonal-mean ozone, i.e., ignoring zonal asymmetries and the coupled nature of ozone, is responsible for conventional climate models to underestimate the importance of ozone in driving the annular modes.
Mechanisms of Chemistry-Climate Coupling
Stratospheric ozone is a key element in both chemistry and climate and thus plays a central role in chemistry- climate coupling. While sometimes ozone is viewed as a climate forcing, stratospheric ozone is an internal property of the atmosphere (like water vapour or temperature) which is more appropriately viewed as part of the atmospheric response to anthropogenic forcings. Ozone depletion and recovery affect climate change; climate change affects ozone depletion and, especially, ozone recovery; and stratospheric climate change affects tropospheric climate through stratospheric ozone. The current state of knowledge concerning the mechanisms of chemistry-climate coupling will be reviewed, drawing extensively from simulations using state-of-the-art chemistry-climate models.