MJO Structure Under Suppressed External Forcing

The Madden-Julian Oscillation (MJO) is found to be influenced by tropical internal dynamics and external forcings, such as the circumnavigating waves produced from the previous MJO event and extratropical influences. To find their relative roles on the structure of the MJO, a set of idealized atmospheric general circulation model (AGCM) experiments, using European Centre Hamburg Model 4 (ECHAM4), is designed. The “control” simulation is forced by the climatological monthly sea surface temperature. To suppress the influence from the circumnavigating waves, model prognostic variables are relaxed in the tropical Atlantic region (20S-20N, 80W-0) toward the controlled climatological annual cycle. Similarly, to suppress the extratropical forcing, model prognostic variables are relaxed in the 20-30 degree latitude zones. Previous studies have shown that extratropical forcing plays a dominant role on MJO structure when compared to the forcing generated by circumnavigating waves, although how the MJO structure is influenced exactly and to what extent is still not known.

The present study documents the MJO horizontal and vertical structures and its propagation under suppressed external forcing. We utilize the matrix provided by the U.S. CLIVAR Working Group to document the MJO structures (such as its variance, EOF, composite life cycle, eastward propagation etc.). We also document the seasonal variation of the MJO structures due to the different focings. The results clearly show in our simulations that the MJO structure is distorted most when the influence from the extratropics (or the tropics-extratropics interactions) is suppressed. The relative roles of the northern extratropics and the southern extratropics, as well as the significance of this work, will be discussed in the meeting.