No abstract

Abstract.The

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

Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well‐mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well‐mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, well‐mixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an “optimal detection” methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well‐mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near‐surface temperature from well‐mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near‐surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is ∼50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.

No abstract