ECHAM4 GCM Model Information

Background Information

The ECHAM climate model has been developed from the ECMWF atmospheric model (therefore the first part of its name: EC) and a comprehensive parameterisation package developed at Hamburg therefore the abbreviation HAM) which allows the model to be used for climate simulations. The model is a spectral transform model with 19 atmospheric layers and the results used here derive from experiments performed with spatial resolution T42 (which approximates to about 2.8º longitude/latitude resolution). The model has also been used at resolutions in the range T21 to T106.

ECHAM4 is the current generation in the line of ECHAM models (Roeckner, et al., 1992). A summary of developments regarding model physics in ECHAM4 and a description of the simulated climate obtained with the uncoupled ECHAM4 model is given in Roeckner et al. (1996). The initial sea surface temperature and sea-ice data is the COLA/CAC AMIP SST and sea-ice data set. The mean terrain heights are computed from high resolution US Navy data set. The fraction of grid area covered by vegetation based on the Wilson and Henderson-Sellers (1985) data set. The ocean albedo is a function of solar zenith angle and the land albedo from the satellite data of Geleyn and Preuss (1983). A diurnal cycle and gravity wave-drag is included. The time-step of the model is 24 minutes, except for radiation which uses two hours.

The ocean model is an updated version of the isopycnal model (OPYC3) developed by Josef Oberhuber (Oberhuber, 1993) at the Max-Planck-Institute for Meteorology, Hamburg, Germany. The name OPYC is derived from Ocean and isoPYCnal co-ordinates.

The concept to use isopycnals as the vertical co-ordinate system for an OGCM is based on the observation that the interior ocean behaves as a rather conservative fluid. Even over long distances the origin of water masses can be traced back by considering the distribution of active or passive tracers. Treating the ocean as a conservative fluid fails in areas of significant turbulence activity such as the surface boundary layer. A surface mixed-layer is therefore coupled to the interior ocean in order to represent near-surface vertical mixing and to improve the response time-scales to atmospheric forcing which is controlled by the mixed-layer thickness. Since the model is designed for studies on large scales, a sea ice model with rheology is included and serves the purpose of de-coupling the ocean from extreme high-latitude winter conditions and promotes a realistic treatment of the salinity forcing due to melting or freezing sea ice.

The experiments from which results are used here are the 1000-year unforced control simulation using the coupled ECHAM4/OPYC3 model and then two climate change simulations. The greenhouse gas only forced experiment (referred to as GGa1) used historical greenhouse gas forcing from 1860 to 1990 followed by a 1 per cent annum increase in radiative forcing from 1990 to 2099. The greenhouse gas and sulphate aerosol forced experiment (referred to as GSa1) used the GGa1 forcing, plus the negative forcing due to sulphate aerosols. This was represented by means of an increase in clear-sky surface albedo proportional to the local sulphate loading. The indirect effects of aerosols were not simulated. For 1860 to 1990 the historic sulphate aerosol forcing estimate was used and for 1990 to 2049 the aerosol forcing estimated for the IS92a emissions scenario. The GSa1 experiment did not extend beyond 2049.

Several papers describe results using this version of the model - see Bacher et al. (1998), Oberhuber et al. (1998), Zhang et al. (1998).

Global-Mean Temperature, Precipitation and CO2 Changes (w.r.t. 1961 - 1990) for the ECHAM4 Integrations

Notes

The climate sensitivity of ECHAM4 is about 2.6ºC.

* The CO2 concentration has been estimated assuming an IS92a mix of greenhouse gases and concentration / forcing relationships reported in the IPCC Second Assessment Report.

References and other reading

Bacher,A., Oberhuber,J.M. and Roeckner,E. 1998 ENSO dynamics and seasonal cycle in the tropical Pacific as simulated by the ECHAM4/OPYC3 coupled general circulation model Climate Dynamics, 14, 431-450.

Oberhuber,J.M. 1993 Simulation of the Atlantic circulation with a coupled sea-ice mixed layer-isopycnal general circulation model. Part I: model description J. Phys. Oceanogr., 13, 808-829.

Oberhuber,J.M., Roeckner,E., Christoph,M., Esch,M. and Latif,M. 1998 Predicting the '97 El Niño event with a global climate model Geophys. Res. Letts., 25, 2273-2276.

Roeckner,E., Arpe,K., Bengtsson,L., Brinkop,S., Dümenil,L., Esch,M., Kirk,E., Lunkeit,F., Ponater,M., Rockel,B., Suasen,R., Schlese,U., Schubert,S. and Windelband,M. 1992 Simulation of the present-day climate with the ECHAM4 model: impact of model physics and resolution Max-Planck Institute for Meteorology, Report No.93, Hamburg, Germany, 171pp.

Roeckner,E., Arpe,K., Bengtsson,L., Christoph,M., Claussen,M., Dümenil,L., Esch,M., Giorgetta,M., Schlese,U. and Schulzweida,U. 1996 The atmospheric general circulation model ECHAM-4: model description and simulation of present-day climate Max-Planck Institute for Meteorology, Report No.218, Hamburg, Germany, 90pp.

Zhang,X-H., Oberhuber,J.M., Bacher,A. and Roeckner,E. 1998 Interpretation of interbasin exchange in an isopycnal ocean Climate Dynamics, 14, 725-740.