Skillful Seasonal Prediction of the Southern Annular Mode and Antarctic Ozone

Using a set of seasonal hindcast simulations produced by the Met Office Global Seasonal Forecast System 5 (GloSea5), significant predictability of the Southern Annular Mode (SAM) is demonstrated during the austral spring. The correlation of the ensemble mean 1-month lead time forecast SAM with observed values is 0.64, which is statistically significant at the 95% confidence level, and is similar to that found recently for the North Atlantic Oscillation in the same system. Significant skill is also found in the prediction of the strength of the Antarctic stratospheric polar vortex at 1-month average lead times. Due to the observed strong correlation between interannual variability in the strength of the Antarctic stratospheric circulation and ozone concentrations, it is possible to make skillful predictions of Antarctic column ozone amounts. By studying the variation of forecast skill with time and height, it is shown that skillful predictions of the SAM are significantly influenced by stratospheric anomalies which descend with time and are coupled with the troposphere. This effect allows skillful statistical forecasts of the October mean SAM to be produced based only on mid-stratosphere anomalies on 1st August. Together, these results both demonstrate a significant advance in the skill of seasonal forecasts of the Southern Hemisphere and highlight the importance of accurate modelling and observation of the stratosphere in producing long-range forecasts.

Reference: Seviour, WJM, SC Hardiman, LJ Gray, N Butchart, C MacLachlan and AA Scaife (2014). Skillful seasonal prediction of the Southern Annular Mode and Antarctic ozone, J. Climate. DOI: 10.1175/JCLI-D-14-00264.1