Simulation of the Intraseasonal Oscillation in the ECHAM-4 Model: The Impact of Coupling with an Ocean Model

Two 15 year integrations were made with the ECHAM-4 AGCM; in the first run, the model lower boundary conditions were the observed monthly mean sea surface temperatures, and in the second run, the AGCM was coupled to an intermediate ocean model. The simulation of the intraseasonal oscillation (ISO) in these two runs is compared to observations, with particular emphasis on the boreal summer ISO.

The overall effect of coupling the AGCM to the ocean model is to improve the intraseasonal variability of the model. Upon coupling, the simulated boreal winter ISO becomes more spatially coherent and has a more realistic phase speed. In the May-June season, the coupled run shows pronounced northward propagation of convection and circulation anomalies in the Indian Ocean, as in the observations, while northward propagation is absent in the uncoupled run. The simulation of the August-October ISO is degraded upon coupling, however. By making a third run with the AGCM running with the coupled model run SST climatology serving as the lower boundary condition, we determine that the coupled run basic state is less conducive to a strong ISO than the uncoupled run basic state. However, allowing the AGCM to interact with the underlying ocean strengthens the ISO, so that the end result of coupling is a more intense and coherent ISO during most of the year. In this model, interaction between the ocean and atmosphere helps to produce a better ISO simulation, however, the effect of coupling depends critically on the new basic state of the coupled model, as is shown by the lack of improvement in the coupled model August-October ISO simulation.