Evolution of ECMWF sub-seasonal forecast skill scores

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Abstract

Sub-seasonal forecasts have been routinely produced at ECMWF since 2002 with reforecasts produced ‘on the fly’ to calibrate the real-time sub-seasonal forecasts. In this study, the skill of the reforecasts from April 2002 to March 2012 and covering a common set of years (1995 to 2001) has been evaluated. Results indicate that the skill of the ECMWF reforecasts to predict the Madden–Julian Oscillation (MJO) has improved significantly since 2002, with an average gain of about 1 day of prediction skill per year. The amplitude of the MJO has also become more realistic, although the model still tends to produce MJOs which are weaker than in the ECMWF re-analysis. As a consequence, the ability of the ECMWF model to simulate realistic MJO teleconnections over the Northern and Southern Extratropics has improved dramatically over the 10-year period. Forecast skill scores have also improved in the Extratropics. For instance, weekly mean forecasts of the North Atlantic Oscillation Index are more skilful in recent years than 10 years ago. A large part of this improvement seems to be linked to the improvements in the representation of the MJO. Skill to predict 2 m temperature anomalies over the Northern Extratropics has also improved almost continuously since 2002. Changes in the horizontal and vertical resolutions of the atmospheric model had only a small impact on the skill scores, suggesting that most of the improvements in the ECMWF sub-seasonal forecasts were due to changes in model physics which were primarily designed to improve the model climate and medium-range forecasts. The impact of changes in the data assimilation system and in the observing data has not been considered in this study, since all the reforecasts used for this study were initialized from the same re-analysis over a common set of years.

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