Ocean GCM and Coupled GCM errors in the tropical Atlantic

a coupled air-sea system of STCs Max Salinity /Subduction Max Salinity /Subduction Max Salinity /Subduction Max Salinity /Subduction

Ocean GCM errors: Upwelling problems SST VAR

First EOF of the MA rainfall from GPCP Regressed MA SST anomaly and surface wind anomaly 33% First EOF of the JA rainfall from GPCP Regressed JA SST anomaly and surface wind anomaly. (Kushnir, 2002, SACOS) 23% Coupled variability modes

CGCM problems: DEMETER example Seasonal forecast of tropical climate with coupled ocean-atmosphere GCMs: On the respective role of the atmosphere and the ocean model components in the drifting mean climate. A. Lazar, A. Vintzileos, F. Doblas-Reyes, P. Rogel, & P. Delecluse Submitted to special DEMETER issue of TELLUS

Drift and offset of the 6-month forecasts Agcm:drift Ogcm:offset

5-month Drifts of the forecasts Spatial comparison of the 5-month drifts for all models computed by substracting the first month to the last. Ifs/hope Ifs/orca Arpege/opa8 Arpege/orca 5-month Drift <= forced agcm bias

Persistence of the oceanic offset Change from month 1 and month 5 of the relative SST difference between two companion CGCM. For each hindcast season, the unity is assigned to the grid point when the percentage of change is smaller than 50% and the first and fifth month differences have the same sign. The results are summed over the four season a) IFS pair b) ARPEGE pair Ifs pair arpege pair Regional/seasonal result: persistence of the offset <= forced ogcm bias

Conclusions Drift of the simulated 6-month tropical climate divided into two phases: 1)1st or 2d month: forced ocean and atmosphere components are of comparable role in setting the characteristics of an initial deviation from the observations. Forced OGCM bias introduced 2) 4- 5 remaining months : the atmosphere controls the first order of the signal in term of phase and amplitude (= forced AGCM bias)