Circumglobal Propagation of Successive MJO Events in MERRA-2

The propagation speeds of strong circumnavigating, successive MJO events are examined in MERRA-2 reanalysis. The MJO cases are tracked by following large-scale vertical motion anomalies with zonal wavenumber of 1–2. Statistically significant signals of parameterized latent heating and adiabatic cooling are co-located with the vertical motion anomalies. They move through the equatorial Western Hemisphere at roughly 20–30 m s^-1 but much slower—about 5 m s^-1—over the tropical warm pool. The theoretically expected phase speed of the convectively coupled circumnavigating wave is also computed following theory of Neelin and Held (1997) and Emanuel et al. (1994): Reduction of effective static stability felt by the wave—caused by the climatological offset between column integrated diabatic heating and adiabatic cooling in the low-wavenumber MJO signal—makes the phase speed less than that of a corresponding dry, freely propagating wave. The offset computed using MERRA-2 output is robust from year to year and at all longitudes in the Tropics.

The above method predicts that a first baroclinic mode should propagate 20–25 m s^-1 over much of the Western Hemisphere, 20–35 m s^-1 over the eastern Atlantic and Africa, and 5–20 m s^-1 over the tropical warm pool, similar—for the Western Hemisphere—to the rates actually seen in reanalysis. The result lends support to the idea that the circumnavigating MJO is a first baroclinic convectively coupled Kelvin wave. However, in places where widespread deep convection is prevalent and the offset between diabatic heating and adiabatic cooling is large (i.e. the warm pool), the theory overestimates propagation speed. Rather, moisture wave theories are more effective at capturing the slow propagation speed of the MJO over the warm pool. Therefore, two distinct dynamic regimes—one in which gravity waves dominate and another in which moisture wave dynamics may be more applicable—govern MJO propagation depending on where its signal is located. This study motivates the need for a holistic MJO theory that contains both elements of its propagation and the transitions of the MJO from a Kelvin wave to a moisture wave, and vice versa, as it sometimes propagates around the world.