Extratropical Influence on the MJO Seasonality

The Madden-Julian Oscillation (MJO) is found to be influenced by tropical internal dynamics and external forcings, such as the circumnavigating waves, produced from a 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, named the East-West Experiment (EW Exp.), 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, named the North-South Experiment (NS Exp.), model prognostic variables are relaxed in the 20-30 degree, north and south, 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 not well known.

The present study documents the MJO's horizontal and vertical structures and its propagation under suppressed external forcing. We utilize the matrix provided by the U.S. CLIVAR MJO Working Group to document the MJO structures (such as its variance, EOF, composite life cycle, eastward propagation, etc.). The results show that the MJO structure is distorted most when the influence from the extratropics (or the tropics-extratropics interactions) is suppressed compared to the suppression of circumnavigating waves Because of this significance in extratropical interactions, we further explore the relative roles of the northern and southern hemispheres on the MJO dynamics and its initiation in different seasons. Preliminary results indicate that the southern hemisphere has a dominant role in influencing the MJO structure compared to the northern hemisphere. The influences will be further quantified by a momentum budget analysis. Limitations and implications of the results will be discussed in the presentation.