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Abstract

The Boreal Summer Intraseasonal Oscillation (BSISO), which is characterized by a northeastward move of convective systems at a 30- to 60-day cycle, is the dominant intraseasonal variability in the Indo-Pacific warm pool region during boreal summer. Previous studies have discussed a phase-locking feature of the BSISO with respect to the annual cycle and even suggested its presence in the daily climatological field, referred to as the climatological BSISO (CBSISO). However, there still remains open whether CBSISO is a statistically significant feature, let alone its mechanisms. Here we present strong evidence for the presence of CBSISO based on 42-year records from satellite observations. Even without bandpass filtering, the CBSISO is present in the outgoing longwave radiation and precipitation records for the April-to-December period. Statistically significant spectral peaks are found in both records at about a 40-day period in the tropical Indian Ocean and western Pacific. On the basis of the phase information of individual years, we identified a group of approximately one-fourth of 42 years, which are highly in-phase with the CBSISO. This group of in-phase years have shown an early onset of the dry CBSISO phase in the Indian Ocean in April. There is also a hint that ENSO plays a modulating role in the CBSISO phase. In the presentation, we shall discuss possible mechanisms of the CBSISO.