On the Relative Roles of Circumnavigating Waves and Extratropics on the MJO

A GCM-based framework is introduced to diagnose the relative roles of the circumnavigating waves and the extratropics on the MJO. Apart from a standard GCM simulation (‘control'), two sensitivity tests are conducted for 20 years. In the first test, model prognostic variables are relaxed in the tropical Atlantic region (80°W-0°) toward the ‘controlled' climatological annual cycle to remove the influences from the circumnavigating waves. In the second test, model prognostic variables are relaxed in the 20°-30° latitude zones to remove the influences from the extratropics. The results suggest that the circumnavigating waves do not play any major role on the MJO, however, the MJO variance was substantially reduced in the absence of extratropical influences. The reasons behind this will be explored in the meeting. In particular, we will discuss the role of the mean state including the role played by the vertical shear on the simulated MJO.