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Abstract

A 1260-yr integration generated by the ECHAM1/LSG (Large Scale Geostrophic) coupled atmosphere-ocean general circulation model is analyzed in this paper. The analysis focuses on the climate drift and on the variations of the coupled atmosphere-ocean system after the initial climate drift has essentially died out.
The initial drift is induced, to a large extent, by the applied heat flux correction, which has very large spatially fixed values of upward heat flux in the polar regions, in particular along the Antarctic coast. The globally integrated freshwater flux becomes unbalanced during the integration, due to the changes in the snow accumulation rate over Greenland and Antarctica. The resulting net upward freshwater flux induces a linear trend in the salinity of the upper ocean. The drift of temperature and salinity in the deep ocean, which is essentially independent of the boundary condition variations during the coupled integration, is presumably related to the spinup of the deep ocean prior to the coupling.
The analysis of the last 810 yr of the integration, which is free from the strong initial drift, suggests that the tropospheric variations are white on timescales longer than I yr. The dominant Northern Hemispheric mode resembles the western Atlantic pattern. The dominant tropical and Southern Hemispheric modes are essentially zonally symmetric. All these modes can be found on both short (1 yr) and long (15 yr) timescales. For the oceanic variations, the spatial distribution of the total variance and the dominant modes and the relationships between these modes are studied. For the horizontal barotropic streamfunction, the most dominant mode describes an anomalous westward (eastward) circumpolar flow together with clockwise (anticlockwise) circulation in the Southern Atlantic and southeast of South Africa and in the Southern Pacific. For the zonally averaged meridional circulations the most dominant modes of variability describe essentially recirculations within each basin.