Tropical-Extratropical Interactions as a Dry Dynamical Pathway to the MJO

Although several previous studies have noted the existence of MJO-like variability in highly idealized (dry) general circulation models, the underlying cause of this variability has yet to be identified. Here we address this issue in the context of the Held-Suarez test case. Results show a pronounced MJO-like disturbance whose dynamical structures broadly resemble observations. To understand why, the model dynamics is altered to retain just a limited set of zonal wavenumbers. In the case where only the gravest zonal wavenumber ($k=1$) is retained, the model produces predominantly westward-moving patterns of tropical variability that are apparently forced by unstable baroclinic disturbances in midlatitudes. On the other hand, when a triad of zonal wavenumbers is retained, of the general form: $k=[1,n,n+1]$, where $n$ is an arbitrary integer greater than one, a robust MJO-like disturbance is spontaneously generated, apparently as a response to eastward-moving baroclinic wave packets in the midlatitudes. This result holds true for all values of $n$ tested between 2 and 9. The implication is that at least a portion of observed MJO variability can be attributed to a resonant forcing of the tropics by the midlatitude storm tracks.