Predictability of seasonal atmospheric variations
- European Centre for Medium-Range Weather Forecasts, Berkshire (United Kingdom)
- European Centre for Medium-Range Weather Forecasts, Reading (United Kingdom)
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 used observed global sea surface temperature (SST) as a lower boundary condition. 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. the sign and magnitude of tropical Pacific SST anomalies were chosen to define an El Nino-Southern Oscillation (ENSO) index. 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, the skill of the ensemble mean tended to be highest in the spring season and the internal spread of the ensemble 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. For both strong and weak ENSO-index years, the model correctly simulated differences in the tropical stratosphere associated with the quasi-biennial oscillation (QBO). 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 affecting tropical Pacific area. 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. 23 refs., 18 figs., 2 tabs.
- OSTI ID:
- 7164201
- Journal Information:
- Journal of Climate; (United States), Journal Name: Journal of Climate; (United States) Vol. 7:2; ISSN JLCLEL; ISSN 0894-8755
- Country of Publication:
- United States
- Language:
- English
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