Hai Lin, Gilbert Brunet - RPN, Environment Canada
Jacques Derome - McGill University
The Pacific/North American (PNA) pattern, and the North Atlantic Oscillation (NAO) are the two most important structures of atmospheric low-frequency variability over the Northern Hemisphere. Skillful seasonal predictions of these patterns are consequently of great value to the overall skill of the forecasts over a wide area. Part of the PNA variability is linked to the SST anomalies in the tropical Pacific, while the predictability of the NAO is less known.
In our recent studies, it was found that a skillful seasonal forecast is possible for both the PNA and NAO. Using a simple GCM and with a large ensemble size, Derome et al. (2005) found that the forecast PNA and NAO indices are significantly correlated with the observations during the 51 winters of 1948-1998. Lin et al. (2005) analyzed the seasonal forecasts of two Canadian models (GEM and GCM3), and found that the year-to-year time evolutions of the models two leading response patterns match quite well those of the observed PNA and NAO, while their spatial distributions are not identical. The differences between the observed and model structures are largest for the NAO of both models and the PNA of the GEM model, and least for the PNA of GCM3. A post-processing correction scheme was developed which is successful in improving the forecast skill of 500mb geopotential height and the PNA and NAO.
In this diagnostic subproject, we will extend our analysis to the models included in the SMIP2 and SMIP2/HFP. A model intercomparison of seasonal predictions of the PNA and NAO will be conducted. The winter season will be the focus since both the PNA and NAO have a strong amplitude in winter. The correction scheme proposed by Lin et al. (2005) will be applied to the different models, in the hope of improving the seasonal forecasts of the large scale patterns. Analyses will also be made of the association between the forecast NAO variability and changes in boundary conditions such as SST.
References
- Derome, J., H. Lin, G. Brunet, 2005: Seasonal forecasting with a simple Seasonal forecasting with a simple General Circulation Model. Journal of Climate, 18, 597-609.
- Lin, H., J. Derome and G. Brunet, 2005: Correction of atmospheric dynamical seasonal forecasts using the leading ocean-forced spatial patterns. Geophysical Research Letters, 32, L14804, doi:10.1029/2005GL023060.