The ECMWF is a numerical system and its predictions are probabilistic. The system consists of an ocean
analysis to estimate the initial state of the ocean and a global coupled ocean-atmosphere general circulation
model to calculate the evolution of the ocean and atmosphere.
The ocean model used is based on HOPE (Hamburg Ocean Primitive Equation model) version 2 (Latif et al.
1994, Wolff et al. 1997). The model is global and has 29 vertical levels. Horizontal discretisation is on
an Arakawa E grid with a variable grid spacing: the zonal resolution is 1.4 degrees and the meridional
resolution varies from 0.3 degrees in the equatorial region (within 10 degrees of the equator), smoothly
increasing to 1.4 polewards of 30 degrees.
The atmospheric component of the coupled model is the ECMWF IFS (Integrated Forecast System) model version
23r4. Except for resolution, this is the same model as was used for NWP in early 2001. It is also the same
cycle as is used in ERA 40, except that there are 40 levels in the vertical, compared to the 60 used in
ERA40 and the horizontal resolution used for the atmospheric component is TL95. The spectral representation
is used only for the dynamical part of the model calculations. All of the model physical parameterization
(including clouds, rain and the land surface) are calculated on a Gaussian grid with about 1.875 degrees
spacing. The atmospheric model uses a two time-level semi-Lagrangian scheme for its dynamics with a 1-hour
time step. Documentation on the seasonal forecast system is available at
http://www.ecmwf.int/products/forecasts/seasonal/documentation
The real-time predictions consists of an ensemble of 40 members. The hind-cast simulations are defined for the period
198-2001 and is based on a total of 75 integrations ((a 5-member ensemble for 15 years). For May and November starts the
hind-cast ensembles have been extended to 40 members (these additional integrations have been used in the verification
to assess JJA and DJF seasons).
A large number of parameters is verified and the results are shown at the ECMWF web site. For the moment we have
submitted the 2m temperature ROC area for the: tropics, Northern and Southern extratropics.
Lead time is 1 month and the 4 standard seasons are verified (DJF MAM JJA SON)
The scores are computed for the period (1987-2002) combining the hind-cast and a real-time forecast.
Tha scores for SON and MAM are estimated by using 80 (5*16) simulations initiated respectively in August and February.
The DJF and JJA scores are estimated by using 640 (40*16) simulations initiated respectively in November and May.
Values from ERA-40 re-analysis are used to verify the 2m temperature. Verification has been computed at the standard spatial resolution.