Investigating links between wave driving and tropical lower stratospheric temperatures using simple dynamical models

It is widely accepted that stratospheric wave driving plays an important role in tropical upwelling and hence in tropical lower stratospheric temperatures. The latter are, of course, important in determining entry values to the stratosphere of water vapour concentrations. Randel et al (J. Atmos. Sci. 2002), for example, have demonstrated a strong connection between extratropical wave forcing and 100mb vertical velocities in the tropics, on a range of time scales from days to a year. However, particularly when considering timescales of several months or longer, there is still uncertainty and disagreement over what parts of the wave forcing are important (extratropical versus tropical, tropospheric versus stratospheric). Aspects of the Randel et al study, e.g. the apparent phase relation between tropical upwelling and tropical temperatures on annual timescales, also seem puzzling.

The study reported here is using linearised dynamical models, including a new adjoint-based approach, to provide a quantitative link between wave forcing and tropical lower stratospheric temperatures. ERA-40 data and ECMWF operational analysis data is used in conjunction with these models to gain improved understanding of the role of wave driving, over the last two decades, in determining temperature variability on monthly to multiannual timescales in the tropical lower stratosphere. The limitations of linearised models (e.g. with respect to time scale) for this purpose are also being examined.