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atmospheric response to mid-latitudinal SST anomalies and the ocean adjustment to decadal wind stress variations, which are both crucial parts
of our mechanism. We describe the results of a simple coupled oceanatmosphere model which retains the essential physics of the decadal cycles
in section 4. Section 5 deals with the decadal variability in the North Atlantic, as simulated by our coupled ocean-atmosphere general circulation
model. The paper is concluded with a summary and discussion of our
major findings in section 6.
2 Coupled ocean-atmosphere general circulation
model and observational data
2.1 Coupled model
The coupled general circulation model (CGCM) and its behaviour with
respect to climate drift and interannual variability during the first twenty
years of a 125-year integration are described in Latif et al. (1994). Overall,
the model drift is relatively small and the CGCM simulates well the decadal
variability in the North Pacific, as shown in Latif and Barnett (1994) and
Latif and Barnett (1996). As will be shown below, the CGCM succesfully
reproduces also some fundamental aspects of the decadal variability in the
North Atlantic.
The atmospheric component of our coupled GCM "ECHO" is ECHAM3, the Hamburg version of the European Centre operational weather forecasting model. The model is described in detail in two reports (Roeckner et al., 1992; DKRZ, 1992). ECHAM-3 is a global low-order spectral
model with a triangular truncation at wavenumber 42 (T42). The nonlinear terms and the parameterized physical processes are calculated on a 128
x 64 Gaussian grid which yields a horizontal resolution of about 2.8 0 x 2.8 0
. There are 19 levels in the vertical which are defined on o--surfaces in the
lower troposphere and on p-surfaces in the upper troposphere and in the
stratosphere.
The ocean model is "HOPE" (Hamburg Ocean Model in Primitive Equations) which is based on primitive equations (see Latif et al., 1994 and references therein). Its domain is global and we use realistic bottom topography.
The meridional resolution is variable, with 0.5 0 within the region 10 0 N to
10 0 S. The resolution decreases poleward to match the T42-resolution of the
atmosphere model. The zonal resolution is constant and also matches the
atmospheric response to mid-latitudinal SST anomalies and the ocean adjustment to decadal wind stress variations, which are both crucial parts
of our mechanism. We describe the results of a simple coupled oceanatmosphere model which retains the essential physics of the decadal cycles
in section 4. Section 5 deals with the decadal variability in the North Atlantic, as simulated by our coupled ocean-atmosphere general circulation
model. The paper is concluded with a summary and discussion of our
major findings in section 6.
2 Coupled ocean-atmosphere general circulation
model and observational data
2.1 Coupled model
The coupled general circulation model (CGCM) and its behaviour with
respect to climate drift and interannual variability during the first twenty
years of a 125-year integration are described in Latif et al. (1994). Overall,
the model drift is relatively small and the CGCM simulates well the decadal
variability in the North Pacific, as shown in Latif and Barnett (1994) and
Latif and Barnett (1996). As will be shown below, the CGCM succesfully
reproduces also some fundamental aspects of the decadal variability in the
North Atlantic.
The atmospheric component of our coupled GCM "ECHO" is ECHAM3, the Hamburg version of the European Centre operational weather forecasting model. The model is described in detail in two reports (Roeckner et al., 1992; DKRZ, 1992). ECHAM-3 is a global low-order spectral
model with a triangular truncation at wavenumber 42 (T42). The nonlinear terms and the parameterized physical processes are calculated on a 128
x 64 Gaussian grid which yields a horizontal resolution of about 2.8 0 x 2.8 0
. There are 19 levels in the vertical which are defined on o--surfaces in the
lower troposphere and on p-surfaces in the upper troposphere and in the
stratosphere.
The ocean model is "HOPE" (Hamburg Ocean Model in Primitive Equations) which is based on primitive equations (see Latif et al., 1994 and references therein). Its domain is global and we use realistic bottom topography.
The meridional resolution is variable, with 0.5 0 within the region 10 0 N to
10 0 S. The resolution decreases poleward to match the T42-resolution of the
atmosphere model. The zonal resolution is constant and also matches the