ECMWF upgraded its operational seasonal forecasts from System 2 to System 3. The change to operational status took place in March 2007, when System 2 ceased to be operational, and System 3 became the new operational forecast system.

The resolution of the atmosphere model is increased from TL95 with 40 vertical levels to TL159 with 62 vertical levels.

The resolution of the ocean model is unchanged. The horizontal zonal resolution is 1.4 degrees and the horizontal meridional resolution smoothly varies from 0.3 degrees in the equatorial region (within 10 degrees of the equator), to 1.4 degrees polewards of 30 degrees. There are 29 vertical levels.

The ensemble s are changed. The real-time forecasts will now have 41 members (previously 40). The re-forecasts will have 11 members uniformly for all start months (previously 5 members, with 40 members for May and November starts only).

The period of the re-forecasts have been extended, and covers the 25 years from 1981 to 2005. This is significantly longer than the 1987-2001 15 year period of the previous system, and should allow better calibration of the forecasts and better assessment of their skill.

The standard forecast length (for both forecasts and back integrations) is increased from 6 to 7 months. This gives a forecast range of 6.5 months from the release date of the forecasts.

Four times a year, from the Feb, May, Aug and Nov starts, 11 members (out of 41) of the forecast ensemble will run to 13 months. This will allow an "ENSO outlook" to be given. The re-forecasts for these start months have 5 ensemble members (out of 11) that extend to 13 months. (The November starts are actually 14 months long in all cases, so they extend to the end of the following calendar year.) Note that, at least initially, these extended runs will be classed as experimental rather than operational, and data from them will not be available via dissemination.

Scientifically, there have been many enhancements to the ocean analysis system used to prepare ocean initial conditions. The atmosphere model has been upgraded to IFS cycle 31r1, which is the latest version for medium-range forecasting at ECMWF (introduced on 12 September 2006). There have also been significant revisions to the perturbation methods used to generate the forecast ensemble.

Tests show that the prediction skill for Pacific equatorial SSTs (i.e. El Niño variability) is measurably improved in the new system. Changes in skill for atmospheric variables are less clear cut with the limited test sample presently available. Overall, improvements in predictive skill are expected to be more evident during the Northern Hemisphere summer season, and in tropical regions throughout the year.

Content as it was published on 07.08.2009