Lower Latitude Contribution to the Extratropical Ozone Budget Through Laminar Transport
The extratropical midlatitude lower stratospheric ozone budget is primarily determined by large-scale descent of ozone-rich air from the overlying stratosphere and poleward transport of ozone-poor air from the tropics through wave and mixing events. This study uses observations to quantify the relationship of the meridional laminar transport and, ultimately, the associated mixing of these lamina events to interannual variations of extratropical lower stratospheric ozone. We use the equivalent length diagnostic with MLS N2O to evaluate the interannual variability of Northern Hemisphere lower stratospheric mixing in the years 2005-2007. The relative ozone distribution in equivalent latitude is examined to quantify the overall effect of this mixing on the lower stratospheric ozone budget. We then compare these results to the number of lamina observed in vertical profiles of HIRDLS ozone over the same time period. In 2006, there was a major stratospheric warming associated with a reduced amount of isentropic mixing from the lower latitudes to the middle latitudes. However, the number of lamina observations in 2006 was significantly higher than other years investigated. This illustrates that lamina climatologies are not an ideal measure of tropical to midlatitude mixing. Increased mixing reduces the observable lifetime of the laminae. In addition, the number of lamina identified is not necessarily always in the same proportion to the number of wave and mixing events.
Seasonal Variations in Position and Depth of the Mixing Layer in the UTLS
Changes in O3 in the upper troposphere and lower stratosphere (UTLS) can influence the climate system. Furthermore, stratosphere- troposphere exchange events (STE) associated with these changes can significantly impact air quality. Consequently, studies aimed at understanding UTLS variability and the causes of this variability have become more frequent. Most recent studies in this area have focused on aircraft and satellite data. The work presented in this talk investigates the representation of mixing in the UTLS in numerical models, focusing on the GEOS-Chem and the Global Modeling Initiative (GMI) COMBO models. In particular, tracer-tracer correlations are used to examine regional and seasonal variations in UTLS mixing in the models.
On the QBO Modulation of the Stratospheric Tropical Upwelling as Evidenced by N2O Distributions
In the lower stratosphere tropical probability density functions (PDFs) of N2O have a distinct two peak structure evidencing the existence of a surf-zone around a region of upwelling. The separation of the two peaks is more pronounced in the winter season than in the summer season. The general morphology of N2O PDFs has been used for model validation purposes in the past. Here, we will focus on the summer time variability of N2O PDFs and how it is influenced by the phase of the quasi-biennial oscillation (QBO).We will construct observational evidence of the modulation stratifying MIPAS and MLS N2O observations. Recent chemistry- climate model integrations will be treated in the same way and will be confronted with the observational results. A special focus will be on the new UK chemistry and aerosol community model (UKCA) and its ability to simulate an internal QBO which in its periodicity is highly dependent on the simulated chemistry and in turn impacts the tropical upwelling during summer time. Looking ahead, we evaluate how the summertime N2O distributions might change in a future climate and discuss arising implications for the ascending branch of the Brewer-Dobson Circulation.
On the relation of Empirical Orthogonal Functions of the vertical ozone distribution with the 11-year solar cycle and the quasi biennial oscillation
Empirical Orthogonal Function (EOF) analysis is commonly used to describe the internal structure of data in a way that best explains the variance in the data. However a physical interpretation of these functions can be challenging. In this study, EOF analysis is applied to monthly mean zonal ozone profiles in the tropics (30S- 30N) between 20 and 40 km. Ozone data from the Solar Backscatter Ultraviolet (SBUV) and the Stratospheric Aerosol and Gas Experiment (SAGE) for the period from 1979 to 2005 are used. Data were deseasonalized and detrended prior to the analysis. It is shown that the first four EOF modes of vertical ozone distribution in the tropics capture about 70% of the variability. It is also found that these four EOF are linked to the 11-year solar cycle and Quasi-Biennial Oscillation (QBO) of the equatorial wind. For example, the correlation coefficient between the second ozone EOF and the QBO proxy exceeds 0.8. Results of this analysis can be used to improve existing statistical models of stratospheric ozone variability.
Modeling the Coupled Stratosphere-Troposphere Stationary Wave Response to Climate Change
Stationary wave models are simplified atmospheric models that elucidate the dynamics of the climatological mean zonally asymmetric component of the atmospheric circulation. We here apply a recently developed stationary wave model that captures both the stratospheric and tropospheric stationary wave field to the question of the stationary wave response to climate change. Past stationary wave models largely focused on tropospheric circulation, but the stationary wave field extends into the stratosphere and plays an important dynamical role there. Our model is able to represent the stratospheric stationary wave field and its possible coupling to the troposphere. We use the model to diagnose the stationary wave response to climate change in the Canadian Middle Atmosphere Model (CMAM) simulations. Our model allows us to separately diagnose the effects of changes to the zonally asymmetric component of diabatic heating and changes to the zonal mean basic state on the climate response. We find that in these simulations changes in the zonal mean basic state play a major role in explaining the changes in the stationary wave field. The statosphere-troposhpere dynamical coupling was explored by dividing the stationary wave response into four components: the response in the stratosphere / troposphere induced by the forcing in the stratosphere / troposphere. The stratospheric stationary wave field primarily results from the forcings from troposphere, while stratospheric diabatic heating has a minor impact on the tropospheric stationary waves; this impact is seen most strongly in the tropics and subtropics.
The new UKCA climate-chemistry model: Evaluation of the stratospheric performance
The UK Chemistry and Aerosols (UKCA) model is a new chemistry module coupled to the Met Office Unified Model capable of simulating composition and climate from the troposphere to the mesosphere. Here we assess its performance in the stratosphere. We present basic and derived dynamical and chemical model results and compare to ERA-40 reanalyses and satellite climatologies. Polar temperatures and the lifetime of the southern polar vortex are well captured, indicating that the model is suitable for assessing the ozone hole; this is partly a consequence of a good representation of meridional heat fluxes in the model. Ozone and long- lived tracers compare favourably to observations. Chemical-dynamical coupling, as evidenced by the anticorrelation between winter-spring northern polar ozone columns and the strength of the polar jet, is also well captured. We discuss remaining weaknesses and ways to improve the model. The simulation presented here forms part of our contribution to the CCMVal-2 model intercomparison.