Tuesday, 15 January 2002: 4:14 PM
Historical Seasonal Forecasts with a Simple GCM
A simple General Circulation Model (GCM) using a time-independent forcing is used to perform a series of seasonal predictions. The predictions are done for 51 winter seasons (DJF) from 1948 to 1998. Ensembles of 20 forecasts are produced, with initial conditions of December 1st plus small perturbations. The model uses a forcing field that is calculated empirically from the National Centers for Environmental Prediction (NCEP) / National Center for Atmospheric Research (NCAR) reanalyses. The forcing used for a given winter is the sum of the winter climatological forcing plus an anomaly. The anomalous forcing is taken to be that of the month prior to the start of the forecast (November), which is also calculated from NCEP data. The forecast system does not use any data from the winter months (DJF) being predicted.
The ensemble mean predictions for the 51 winters are verified against the NCEP reanalyses. The simple GCM is found to have statistically significant skill in forecasting the DJF mean 500 mb height field in areas of the globe that are nearly the same as those of a full GCM, albeit at somewhat reduced levels.