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Introduction

In this paper results from the troposphere-stratosphere configuration of the Met. Office Unified Model (UM) (Cullen 1993; Butchart & Austin 1998) are used to investigate changes in the stratospheric climate and Brewer-Dobson circulation due increasing Greenhouse Gas (GHG) concentrations. The model had a latitude-longitude resolution of 2.5 by 3.75 and 49 levels extending from the surface to 0.1 hPa ( km). It contained a comprehensive set of physical parameterizations for sub-grid scale processes. Orographic gravity wave drag was represented (Gregory et al. 1998) only up to 20 hPa and above that replaced by Rayleigh friction, though the drag coefficient was negligible between 20 and 1 hPa [see dashed curve in Fig. 1 of Butchart & Austin (1998)]. Also the longwave radiative cooling included the effects of the minor GHGs--CH, NO, CFC11, and CFC12, as well as CO.

Starting from conditions representative of the early 1990s the model was integrated for the 60 years 1992-2051 (run ``A'') assuming the Intergovernmental Panel on Climate Change IS92a scenario for GHG concentrations, but no changes in stratospheric ozone or water vapor (Butchart et al. 2000). Sea surface conditions were taken from a separate coupled ocean-atmosphere experiment using the same GHG scenario (Mitchell et al. 1995). The integration was repeated (run ``B'') using different initial conditions, but also from the early 1990s, and the two integrations were used by Butchart et al. (2000) to separate chaotic variability from the underlying trend.