2015 Volume 93A Pages 115-137
The ensemble hindcast initialized during 12-16 October 2011 is performed using a global cloud-system-resolving model (CSRM) with a horizontal mesh size of approximately 14 km. The ensemble size is five and the duration of each simulation is 60 days. When sea surface temperature (SST) with a realistic time evolution is prescribed, not only the first but also the second Madden-Julian oscillation (MJO) event observed during the CINDY2011/DYNAMO period emerges in the ensemble mean, although the signal of the second MJO is unsatisfactory in each member. This result leads to a hypothesis that the second MJO is significantly constrained by the prescribed seasonal change of SST.
The analyses of the observational data indicate that an MJO favorable environment, in which SST of the southeastern Maritime Continent is higher than that of the Indian Ocean, is established in late November to early December. Humidity in the lower troposphere increases substantially in the southeastern Maritime Continent during the period. A pair of sensitivity tests using a global CSRM clearly shows that the eastward migration of convection during the second MJO is at least partly caused by the climatological seasonal change of SST. The results of this study indicates that it is inappropriate to treat the climatological seasonal change as the background of the MJO during this season, because its timescale is short enough to be comparable with the intraseasonal timescale of the MJO. We provide a perspective that a certain type of the MJO can be regarded as a transition process, responding to the eastward shift of the region of large-scale positive buoyancy production following the warmer SST.