Predictability of Seasonal Atmospheric Variations

Abstract

Results from a set of 120-day ensemble integrations of a T63L19 version of the European Centre for Medium-Range Weather Forecasts (ECMWF) model are described. The integrations, started from observed initial conditions, used observed global sea surface temperature (SST) as a lower boundary condition. Each ensemble comprised three members initiated from consecutive analyses one day apart. The ensembles were analyzed over the last 90 days of the integration period, corresponding to conventional calendar seasons. Interannual variations in the atmosphere for the period 1986 to 1990 were studied in this way. The sign and magnitude of tropical Pacific SST anomalies were chosen to define an El Niño-Southern Oscillation (ENSO) index. Difference fields were formed from seasons in which this index was 1) large and of opposite sign and 2) small (and of opposite sign). The skill and spread of the ensemble simulations were determined over nine areas covering the globe. In general, the skill of the ensemble difference fields was higher for the strong ENSO-index years than for the weak ones, both in the tropics and the extratropics. In the northern extratropics, there was a tendency for the skill of the ensemble mean to be highest in the spring season. This was consistent with the fact that the internal spread of the ensemble also tended to be smallest in spring. Differences in zonally averaged zonal mean wind revealed that in the tropical and subtropical troposphere, the model simulations were quite accurate, particularly for the strong ENSO-index years. For both strong and weak ENSO-index years, the model correctly simulated differences in the tropical stratosphere associated with the quasi-biennial oscillation (QBO). Further experimentation confirmed that this was associated with a memory of initial conditions over the 120 days of the integration, and suggested some influence of the QBO in the upper tropical troposphere. From wind differences and analysis of changes to regime residence frequencies, it was concluded that while the SST anomalies associated with strong ENSO-index years had a significant influence on the extratropical circulation (including both North America and Europe), there was considerable intra-ensemble variability that affected the tropical Pacific area itself, including surface wind stress over the tropical Pacific. Intraensemble variability was also shown to be substantial in parts of the tropics associated with the summer monsoons over India and Southeast Asia. By contrast, rainfall over sub-Saharan Africa was more stable.