Seasonal forecast

Motivations

The seasonal forecast seeks to benefit from the double following report:

-Part of atmospheric variability is explained by the variability of surface temperature and moisture, in particular in the Tropics, where the surface energy fluxes are large;

- The sea surface temperature (SST) evolves slowly, taking into account the important heat-storage capacity of the ocean and its slow dynamics.

The seasonal forecast is a probabilistic forecast of the climate on a space-time scale of the season; the essential source of predictability being contained in the initial anomaly of SST and especially in its evolution during the season.

The seasonal forecast then presents a certain number of characteristics and limits:

- The strong conditions of forcing, at the time of marked Niño episodes for example, give forecasts of better quality;

- The performances are better in the intertropical band than with the middle latitudes. On these last, the performances are better the winter than the summer, the teleconnexions tropics - extra-tropics being more intense in the winter hemisphere;

- The forecast of the temperatures is better than that of precipitations, the first field having a space-time structure of greater scale than the second;

- The forecasts are used with space-time resolutions of the season, possibly requiring an adaptation of scale in postprocessing.

Production methodology

Since the year 2000, coupled ocean-atmosphere modeling is used at CNRM for the seasonal forecast. The coupled model is composed of ARPEGE-Climat and of OPA ocean model (of the IPSL) coupled between them thanks to the coupler OASIS (of the CERFACS). A seasonal forecast consists in constructing a whole of ten coupled simulations starting of various initial states of the atmosphere and the ocean over a period of a few months. The results are typically studied by drawing aside the first month, whose analysis relates to the field of the monthly forecast.

Results

The main results were obtained within the framework of European projects: PROVOST, ELMASIFA, DEMETER, MERSEA, ENSEMBLES. Internet site of project DEMETER presents in a rather exhaustive way the results obtained. Following project DEMETER, the CNRM engaged with the Climatology Division in the group EURO-SIP whose mission is to produce multi-model seasonal forecasts (ECMWF, UKMO, Meteo-France).