Ocean GCM and Coupled GCM errorsin 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 1979-2001. Regressed MA SST anomaly and surface wind anomaly

33%

First EOF of the JA rainfall from GPCP 1979-2001. 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. DelecluseSubmitted to special DEMETER issue of TELLUS

Drift and offset of the 6-month forecasts

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Agcm:driftOgcm:offset

5-month Driftsof 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)