Zonal wind vacillation and its interaction with the ocean: Implications for interannual variability and predictability

The influence of air-sea interaction on the vacillation of the southern midlatitude zonal mean zonal winds is explored by comparing a 1000-year simulation of the CSIRO Mark 2 coupled atmosphere-ocean general circulation model with 500-year simulations of the atmospheric model with either a simple mixed-layer ocean, or specified sea surface temperatures (SSTs). The zonally averaged SSTs in the interactive models respond, in a dipole pattern, to the surface heat fluxes perturbed by the vacillation, with ocean dynamics modifying the pattern, particularly around 60°S. The feedback of the ocean on the atmosphere is most prominent in the lower tropospheric temperatures, and it is significant on the interannual and longer timescales. Interdecadal variability of the 500-hPa wind index is increased 50% by the ocean. A simple stochastic model of the system is able to reproduce the index statistics over a range of timescales. The structure of the vacillation, particularly the surface pressure anomalies and near-surface winds, differs as the timescale increases. Strong relationships exist between the winds, SSTs, and precipitation anomalies in both annual and four-year means. However, the predictability of temperatures and rainfall, using either wind or SST indices, is limited by the small persistence of the wind anomalies, and the weak influence of the SSTs. Relationships between time-filtered indices and the following annual means are substantial, and comparable to results recently found for the Antarctic Circumpolar Wave. However, these are not realizable as predictions, prompting caution with regard to predictability based on Southern Ocean temperatures.