[A-AS05] Large-scale moisture and organized cloud systems

[AAS05-10] Phase-locking and early onset of climatological BSISO in the tropical Indian Ocean

*Toru Sakamoto¹, Jinro Ukita², Meiji Honda¹ (1.Niigata University, 2.Atmosphere and Ocean Research Institute, University of Tokyo)

Keywords:Intraseasonal Oscillation, BSISO, MJO, CISO

The Boreal Summer Intraseasonal Oscillation (BSISO) is a prominent sub-seasonal phenomenon during the boreal summer, characterized by convective envelopes propagating east and northward from the tropical Indian Ocean. In contrast to the wintertime MJO (Madden and Julian Oscillation), the phase of BSISO tends to be closely associated with the annual cycle, especially with the onset of the local Asian summer monsoon. BSISO also appears in the daily climatological mean field, referred to as the climatological BSISO (CBSISO). However, there remains open whether CBSISO is a statistically significant feature. Nor do we have understanding of possible mechanisms. Here we confirm the presence of CBSISO based on multiple 42-year records from satellite observations and show evidence for the early onset of CBSISO in the tropical Indian Ocean in early April.

The CBSISO is seen in both the outgoing longwave radiation and precipitation records from April to December even without band-pass filtering. We found a significant spectral peak in the CBSISO at approximately 40-day period in the tropical Indian Ocean and western Pacific. Based on the phase information of individual years, we identified a group of approximately one-third of 42 years which are closely in-phase with respect to the CBSISO’ phase while two other groups are out of phase to each other. Focusing on this in-phase composite analysis, we found that the onset of the CBSISO occurs in early April in the Indian Ocean. In the presentation, we shall discuss possible mechanisms for this early onset of the CBSISO.