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Accurate and reliable predictions and an understanding of future changes in the stratosphere are major aspects of the subject of climate change. Simulating the interaction between chemistry and climate is of particular importance, because continued increases in greenhouse gases and a slow decrease in halogen loading are expected. These both influence the abundance of stratospheric ozone. In recent years a number of coupled chemistry–climate models (CCMs) with different levels of complexity have been developed. They produce a wide range of results concerning the timing and extent of ozone-layer recovery. Interest in reducing this range has created a need to address how the main dynamical, chemical, and physical processes that determine the long-term behavior of ozone are represented in the models and to validate these model processes through comparisons with observations and other models. A set of core validation processes structured around four major topics (transport, dynamics, radiation, and stratospheric chemistry and microphysics) has been developed. Each process is associated with one or more model diagnostics and with relevant datasets that can be used for validation. This approach provides a coherent framework for validating CCMs and can be used as a basis for future assessments. Similar efforts may benefit other modeling communities with a focus on earth science research as their models increase in complexity.

An analysis is performed on the dynamical coupling between the variability of the extratropical stratospheric and tropospheric circulations during the Northern Hemisphere winter. Obtained results provide evidence that in addition to the well-known Charney and Drazin mechanism by which vertical propagation of baroclinic Rossby waves is nonlinearly influenced by the zonal mean zonal wind, topographic forcing constitutes another important mechanism by which nonlinearity is introduced in the troposphere--stratosphere wave-driven coupled variability. On the one hand, vortex variability is forced by baroclinic Rossby wave bursts, with positive (negative) peaks of baroclinic Rossby wave energy occurring during rapid vortex decelerations (accelerations). On the other hand, barotropic Rossby waves of zonal wavenumbers s = 1 and 3 respond to the vortex state, and strong evidence is presented that such a response is mediated by changes of the topographic forcing due to zonal mean zonal wind anomalies progressing downward from the stratosphere. It is shown that wavenumbers s = I and 3 are the dominant Fourier components of the topography in the high-latitude belt where the zonal mean zonal wind anomalies are stronger; moreover, obtained results are in qualitative agreement with the analytical solution provided by the simple topographic wave model of Charney and Eliassen. Finally, evidence is provided that changes of barotropic long (s [less than or equal to] 3) Rossby waves associated with vortex variability reproduce a NAO-like dipole over the Atlantic Ocean but no dipole is formed over the Pacific Ocean. Moreover, results suggest that the nonlinear wave response to topographic forcing may explain the spatial changes of the NAO correlation patterns that have been found in previous studies.

The 3D structures of the free oscillations of an adiabatic and hydrostatic atmosphere around a basic state at rest were used as a physical filtering for atmospheric data. This filtering procedure allows for the consideration of the three primitive variables (u, v, [phi]) over the whole atmosphere simultaneously. Accordingly, the computed statistics do not simply rely on the information provided by a single variable of circulation, such as the 500-hPa geopotential field. Using this method, two classical patterns were isolated in the barotropic component of the circulation, one resembling the Pacific-North America (PNA) pattern, the other similar to the North Atlantic Oscillation (NAO) pattern in summer. Associating the barotropic and the second baroclinic components, a coupling in variability was retrieved between the strength of the winter stratospheric polar vortex and the tropospheric circulation over the North Atlantic. Until now these modes had only been recovered by means of statistical analysis. This study shows their existence in physically filtered fields. The obtained results make clear that the observed winter pattern of NAO is not a simple variability mode of the atmosphere, but results instead from mean flow wave interaction that modulates tropospheric planetary Rossby waves. The association between the NAO circulation variability patterns and the anomalies of the 850-hPa temperature field was also investigated. (Manuscript received 16 January 2001, in final form 1 October 2001)