This paper describes the development and evaluation of the UK's new high resolution global coupled model, HiGEM, which is based on the latest climate configuration of the Met Office Unified Model, HadGEM1. In HiGEM, the horizontal resolution has been increased to 1.25 • x 0.83 • in longitude and latitude for the atmosphere, and 1/3 • x 1/3 • globally for the ocean. Multi-decadal integrations of HiGEM, and the lower resolution HadGEM, are used to explore the impact of resolution on the fidelity of climate simulations.Generally SST errors are reduced in HiGEM. Cold SST errors associated with the path of the North Atlantic drift improve, and warm SST errors are reduced in upwelling stratocumulus regions where the simulation of low level cloud is better at higher resolution. The ocean model in HiGEM allows ocean eddies to be partially resolved, which dramatically improves the representation of sea surface height variability. In the Southern Ocean, most of the heat transports in HiGEM is achieved by resolved eddy motions which replaces the parametrised eddy heat transport in the lower resolution model. HiGEM is also able to more realistically simulate small-scale features in the windstress curl around islands and oceanic SST fronts, which may have implications for oceanic upwelling and ocean biology.Higher resolution in both the atmosphere and the ocean allows coupling to occur on small spatial scales. In particular the small scale interaction recently seen in satellite imagery between the atmosphere and Tropical instability waves in the Tropical Pacific ocean is realistically captured in HiGEM. Tropical instability waves play a role in improving the simulation of the mean state of the Tropical Pacific which has important implications for climate variability.In particular all aspects of the simulation of ENSO (spatial patterns, the timescales at which ENSO occurs, and global teleconnections) are much improved in HiGEM.2

Abstract. The EU CANDIDOZ project investigated the chemical and dynamical influences on decadal ozone trends focusing on the Northern Hemisphere. High quality longterm ozone data sets, satellite-based as well as ground-based, and the long-term meteorological reanalyses from ECMWF and NCEP are used together with advanced multiple regression models and atmospheric models to assess the relative roles of chemistry and transport in stratospheric ozone changes. This overall synthesis of the individual analyses in CANDIDOZ shows clearly one common feature in the NH mid latitudes and in the Arctic: an almost monotonic negative trend from the late 1970s to the mid 1990s followed by an increase. In most trend studies, the Equivalent Effective Stratospheric Chlorine (EESC) which peaked in 1997 as a consequence of the Montreal Protocol was observed to describe

SUMMARYThe cause of the observed mid-latitude decline in ozone in a 20-year integration of a stratospheric chemical transport model forced by European Centre for Medium-Range Weather Forecasts analyses for the years 1979 to 1998 is investigated. A very simple chemical scheme for ozone is used which includes no interannual variation so that any modelled interannual variability, or trend, must arise from changes in the meteorology. The integration from 1979 to 1998 does show a downward trend in mid-latitude ozone in which many of the observed features are reproduced, especially between the middle 1980s and the early 1990s. A detailed statistical trend analysis shows that the quantitative comparison between modelled and observed trend is very sensitive to the choice of period considered. However, there is good qualitative agreement in terms of the latitudinal variation of the trend. For the different periods considered, the dynamically driven model trend accounts for at least half of the observed northern mid-latitude trend averaged over December to February. The vertical variation of the modelled and observed trends agree qualitatively. The modelled total ozone correlates well with the vertical winter EP-flux at 100 hPa and the North Atlantic Oscillation index, suggesting that long-term changes in stratospheric circulation are intimately connected to the observed mid-latitude ozone trend.

SUMMARYThe cause of the observed mid-latitude decline in ozone in a 20-year integration of a stratospheric chemical transport model forced by European Centre for Medium-Range Weather Forecasts analyses for the years 1979 to 1998 is investigated. A very simple chemical scheme for ozone is used which includes no interannual variation so that any modelled interannual variability, or trend, must arise from changes in the meteorology. The integration from 1979 to 1998 does show a downward trend in mid-latitude ozone in which many of the observed features are reproduced, especially between the middle 1980s and the early 1990s. A detailed statistical trend analysis shows that the quantitative comparison between modelled and observed trend is very sensitive to the choice of period considered. However, there is good qualitative agreement in terms of the latitudinal variation of the trend. For the different periods considered, the dynamically driven model trend accounts for at least half of the observed northern mid-latitude trend averaged over December to February. The vertical variation of the modelled and observed trends agree qualitatively. The modelled total ozone correlates well with the vertical winter EP-flux at 100 hPa and the North Atlantic Oscillation index, suggesting that long-term changes in stratospheric circulation are intimately connected to the observed mid-latitude ozone trend.

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