Abstract

The upwelling branch of the Brewer Dobson circulation in the tropical lower stratosphere is projected to strengthen in the coming decades, a fact which has important implications for the rate of recovery of the ozone layer and for surface climate. While the conventional understanding is that the increase in upwelling is driven dynamically, recent work has suggested some aspects of the present day upwelling are better thought of as being determined by local radiative heating. Similar arguments suggest that the projected increases may also be radiatively determined.

Evaluating this hypothesis requires separating changes in the radiative heating of the tropical lower stratosphere into a ‘relaxational’ and an ‘external’ component. We discuss two diagnostic approaches to this decomposition and argue, based on output from integrations of the UKCA , that significant external heating is indeed expected in the lower tropical stratosphere as a result of increases in carbon dioxide. This external heating can therefore be expected to strengthen the upwelling.

Preliminary estimates of the magnitude of this change in upwelling will also be presented.