On aliasing Rossby waves induced by asynchronous time stepping

The errors introduced by asynchronous time stepping are analyzed. It is shown that asynchronous time stepping distorts the structure and the speed of the lowest mode. For typical time step ratios used in climate simulations, the lowest mode is no longer a strictly barotropic mode, and the associated wave is much slower than the barotropic Rossby waves in the oceans. As a result, seasonal cycles simulated in model runs based on asynchronous time stepping are severely contaminated.