The structure and predictability of the "high-latitude mode" in the CSIRO9general circulation model

The CSIRO9 general circulation model shows a zonally symmetric mode of vari ability, which closely resembles the high-latitude mode (HLM) in middle and high latitudes of the Southern Hemisphere. The leading EOF of the zonal me an zonal wind between 30 degrees and 68 degrees S, whose amplitude has been taken as an index of the HLM, shows opposing variations centered near 40 d egrees and 60 degrees S accounting for 43% of the daily variance. Analysis has concentrated on composites for periods when the index changed quickly b etween significant peaks of the opposite sign or persisted with a large amp litude for an extended period. The momentum flux variations are small at th e northern and southern boundaries and the principal variations are centere d near 49 degrees S between the maxima in the zonal wind. The changes in an gular momentum content are around 30% smaller in the southern band. Eddy he at fluxes are less coherent but help in maintaining the zonal wind anomalie s against friction. A simple model of the zonal wind index with stochastic forcing and linear d amping reproduces its short period variations well but is less successful i n simulating the observed continuity over 10- to 20-day lags.