Documentation of the Météo-France seasonal forecasting system 9

Météo-France has been involved in real-time seasonal forecasting activities since the late 1990s, andin operational production of seasonal forecasts for the Copernicus Climate Change Services (C3S)since 2017. This document provides the technical description of the 9th version of its seasonalforecasting system, in operations since 1 May 2025. System 9 is an updated versionbuilding on the two preceding systems 7 and 8, respectively released in 2019 and 2021, anddocumented in Batté et al. (2019) and Batté et al. (2021).Section 2 of this report provides a brief overview of the new system, while Section 3 describes indetail each model component, including the main novelty which is the NEMO 4.2 ocean model comingalong the SI3 sea-ice model. Section 4 is dedicated to the initialization strategy, for which a similarinitialization strategy as in System 7 is used. Section 5 is dedicated to the ensemble generationstrategy, which is similar to System 8. Section 6 provides an overview of the workflow procedures,from initialization to post-processing.