Forced and free variability of the semi-annual wave in the ECHAM GCM

The semi-annual oscillation (SAO) of the southern hemispheric sea-level pressure (SLP) in the global atmospheric GCM ECHAM is examined. Five model runs initiated from different atmospheric states were conducted. Monthly Values of sea surface temperature (SST) and sea-ice distribution for the period 1950-1994 are specified boundary conditions for each run. The interdecadal change of the SAO as forced by the changing boundary conditions is compared to the internal model variability, represented by the five runs. The performance of the SAO in the ECHAM model is much improved compared to that of its precursor the T21 Hamburg version of the ECMWF model in 1990. Analysis of observed SLP shows that between 1973-1979 the SAO had high amplitude whereas from 1980-89 its amplitude was low. These changes go along with significant changes in SLP, i.e. the subpolar trough does not expand as far equatorward from September to December as in the years before. The model does not show this reduction of SAO strength in recent years. Multivariate Hotelling-T-2 statistic and analysis of variance techniques (ANOVA) are used to determine the boundary forced part of variance of SLP and SAO in the ECHAM simulations. The result is that only a small part of the SAO seems to be externally forced mainly from the tropics.