The coupled model is composed of ARPEGE-Climat version 3 (Déqué et al., 1994; Déqué, 2001) as an atmospheric GCM and OPA 8.1 as an oceanic GCM (Madec et al., 1997) coupled with OASIS.2.2 (Terray et al., 1995). ARPEGE-climat is a spectral AGCM using a linear T63 truncation, a reduced 128 by 64 Gaussian grid and 31 vertical levels. The dynamical core is the same as that of ECMWF (IFS) with semi-implicit, semi-lagrangian, two time level discretization scheme. The physical parameterizations are partly inherited from the Météo-France operational forecast model. OPA is a finite-difference based OGCM which solves the primitive equations with a non-linear equation of state on an Arakawa C-grid. The present configuration uses a rigid lid. The northern point of convergence has been replaced by two poles located on Asia and North America to overcome the singularity at the North Pole. Its space resolution is roughly equivalent to a geographical mesh of 2 by 1.5 degrees (with a meridional resolution of 0.5 degrees near the Equator). There are 31 vertical levels, with 10 levels in the top 100 metres. Sea-ice cover is calculated with a poor man scheme based on GCM and climatological atmospheric fluxes.

Ocean initial conditions have been obtained from 3 forced OPA integrations using daily ERA40 surface fluxes including daily wind stress perturbations (daily fields with 3 different factors: 1., 0. and -1.) together with SST perturbations added at each initial date, which yields 9 initial oceanic situations. The forced OPA integrations have been started at the beginning of each ERA40 stream (January 1958, January 1973, October 1986) from the same climatological oceanic state. This climatological state is obtained from an 8-year spin-up starting at rest and using ERA15 climatological monthly mean surface fluxes.

Déqué, M., C. Dreveton, A. Braun and D. Cariolle, 1994. The ARPEGE/IFS atmosphere model: a contribution to the French community climate modelling. Clim. Dyn. 10, 249-266.

Déqué M., 2001. Seasonal predictability of tropical rainfall: probabilistic formulation and validation. Tellus, 53A, 500-512.

Madec, G., P. Delecluse, M. Imbard and C. Levy, 1997. OPA, release 8, Ocean General Circulation Model reference manual. LODYC internal report. [Available from LODYC/IPSL, France].

Terray, L., E. Sevault, E. Guilyardi and O. Thual, 1995. OASIS 2.0, user's guide and reference manual. CERFACS Technical report. [Available from CERFACS, France].

06.12.2002