a. CFS hindcasts description
We analyzed the output from the CFS retrospective predictions (or alternatively referred to as hindcasts) that cover all 12 calendar months from 1981 to 2005. These hindcast runs, each of which is 9-month integration, are an ensemble of 15 members starting from perturbed real-time oceanic and atmospheric initial conditions (ICs) from the NCEP Global Ocean Data Assimilation (D. Behringer et al. 2005, unpublished manuscript) and the NCEP/Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (R2) (Kanamitsu et al. 2002), respectively. For the hindcasts starting from a specific month (e.g., May IC), the ICs for the 15 members in the ensemble include 9 to 13 May, 19 to 23 May and 30 May to 3 June. Note that, in hindcast products of the 9-month integration, prediction at 6-month lead (L6) is longest lead month available for analysis of seasonal means while 8-month lead (L8) for monthly means. Variables analyzed include SST, precipitation, and wind at 850 hPa. Hindcast anomalies are computed by removing the model climatology for each grid point, each initial month, and each lead time from the original ensemble hindcasts. For the period 1981-2005, observed datasets used for verification include the CPC merged analysis of precipitation (CMAP) (Xie and Arkin 1996), and the NOAA optimally interpolated SST analysis (Reynolds et al. 2002). For verification observed data are interpolated to CFS’ horizontal resolution.