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. The sensitivities of the Madden–Julian oscillation (MJO) forecasts to various different configurations of theparameterized physics are examined with the global model of ECMWF's IntegratedForecasting System (IFS). The motivation for the study was to simulatethe MJO as a nonlinear free wave under active interactions withhigher-latitude Rossby waves. To emulate free dynamics in the IFS, variousmomentum-dissipation terms (“friction”) as well as diabatic heating wereselectively turned off over the tropics for the range of the latitudes from20∘ S to 20∘ N. The reduction of friction sometimes improves the MJO forecasts,although without any systematic tendency. Contrary to the original motivation,emulating free dynamics with an operational forecast model turned out to berather difficult, because forecast performance sensitively depends on thespecific type of friction turned off. The result suggests the need fortheoretical investigations that much more closely follow the actualformulations of model physics: a naive approach with a dichotomy of with orwithout friction simply fails to elucidate the rich behaviour of complexoperational models. The paper further exposes the importance of physicalprocesses other than convection for simulating the MJO in global forecastmodels.

Sensitivities of the Madden–Julian oscillation forecasts to configurations of physics in the ECMWF global model