AGS-PRF: A Hierarchical Modeling Approach to Quantifying the Effects of Changes in Ozone and Solar Variability on the Brewer-Dobson Circulation and Tropospheric Climate

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Under this AGS-PRF proposal, the researcher will use a hierarchy of models to examine the effects of changes in ozone and solar variability on the Brewer-Dobson circulation (BDC). The BDC is a meridional-vertical circulation in the stratosphere with rising motion at the equator and sinking near the poles, and it plays a key role in determining the meridional distribution of stratospheric ozone and the concentration of ozone in the polar stratosphere. Recent work suggests that the BDC has two branches, a shallow one confined to the lower stratosphere and an upper one which extends as high as the lower mesosphere. Results indicate that the tropical upwelling in the lower branch will increase as a consequence of global warming, because of changes in subtropical and extratropical wave forcing which are not well understood. A likely contributing factor is the subtropical jet streams accelerate, and so too the critical layer for wave breaking that determines the height of orographic gravity and Rossby wave forcing. This results in a rising of the height of the critical layer and hence an increase in the penetration of gravity waves into the stratosphere. The PI also notes that while global warming is expected to cause poleward shifts of the jet streams, ozone recovery is expected to have the opposite effect, and both have consequences for the driving of the BDC by midlatitude gravity waves. In addition, the deep branch of the BDC is expected to respond to changes in ozone, greenhouse gases (GHGs), and the 11-year solar cycle. Newly derived solar variability data sets suggest that solar variability in the key UV wavelength range may be 4-6 times larger than previously understood. Thus there is a need to determine the response to the 11-year solar cycle. The PI proposes to study BDC change using a hierarchy of models which includes the full-physics, full-chemistry Whole Atmosphere Community Climate Model (WACCM), with prognostic ozone concentration, and the Specified Chemistry WACCM (SC-WACCM), in which ozone and other trace gases are prescribed. In addition, the PI will develop and use a simpler Idealized General Circulation Model (IGCM), a dry dynamical core model with thermal relaxation to a radaitive-photochemical equilibrium and perturbation heating (I believe) to represent changes in ozone specified from either observations or future climate scenario integrations of WACCM. Thus the PI will accomplish three goals: (1) examine how changes in the height, latitude, and hemispheric structure of ozone loss and recovery combine to produce changes in the BDC; (2) determine how the newly revised measurements of the solar cycle will affect the BDC and how these changes compare to previous BDC-solar cycle analysis; and (3) determine whether SC-WACCM and the idealized IGCM can reproduce the results of the full version of WACCM with enough fidelity as to make them viable tools for studying climate change at a fraction of the computational expense.