We investigated the role of Sumatra Island convection over the maritime continent during the preconditioning stage of the Madden-Julian Oscillation (MJO) using intensive observations of CINDY2011/DYNAMO and HARIMAU2011. CINDY2011/DYNAMO and HARIMAU2011 were conducted over the Indian Ocean from October 2011 to January 2012 and Sumatra Island, Indonesia in December 2011. Both observation datasets covered the preconditioning stage of the MJO in December 2011. We found that convection was activated over the Sumatra Island with diurnal cycle associated with the moist air mass, which originated from a tropical depression generated in the South China Sea. Then, two-day period disturbances that propagated westward to the central Indian Ocean were coupled with the diurnal cycle of convection over the Sumatra Island. The structure of the two-day period disturbances was consistent with that of westward propagating inertio-gravity waves. When the westward propagating disturbances arrived over the central Indian Ocean, low-level moisture advection was excited. Moistening process was promoted in Gan Island over the central Indian Ocean, which had a two-day period. After the favorable condition of large-scale convection was established, the MJO was activated in the central Indian Ocean. The two-day period westward disturbances were organized when large-scale moisture convergence became positive in Sumatra Island and continued until a strong low-level westerly wind of the active phase of the MJO was formed.
2015 by Meteorological Society of Japan