Observed Synoptic-Scale Variability during the Developing Phase of an ISO over the Indian Ocean during MISMO

Abstract

Abstract A case study of an intraseasonal oscillation (ISO) is investigated in the period leading up to its convectively active phase during the Mirai Indian Ocean Cruise for the Study of the MJO-Convection Onset (MISMO), which was conducted during boreal autumn 2006. Detailed observations, including apparent heat and moisture analyses, reveal that synoptic-scale variability of heat and moisture sources and sinks associated with the passage of three eastward-propagating cloud systems (EPCSs) was prominent during this period. These systems with periods of ∼6 days propagated through the MISMO domain, priming the atmosphere for a convectively active phase of the ISO. The prominent shallow heating during this period may explain the rather slow (8 m s−1) propagation speed for EPCSs. The zonal structure and sign of the frictional convergence show that these EPCSs have common characteristics to the frictional Kelvin mode studied by Ohuchi and Yamasaki. With the analyses of the period-averaged vertical profiles, the EPCSs were identified as the principal mechanism to moisten the atmosphere prior to the ISO convectively active phase.