Observation based analysis of the growth of MJO time scale from its nonlinear scale interactions with the synoptic scales

Summary The present paper addressed the issue of growth of planetary boundary-layer fluxes on the time scale of MJO based on ECMWF reanalysis daily data of 180 days covering April–September, 2001. Diagnostic analysis of this data set utilises computations of moisture and sensible heat fluxes in the frequency domain which involve nonlinear interactions phenomena of MJO time scale with the synoptic scales. Basically the whole computations performed are based on surface similarity theory and Richardson number dependent K-theory in the surface and planetary boundary layer (PBL), respectively, both invoke triple product nonlinearilies. Present observational study shows that among the totality of the triads participating in phase-locking phenomena, a prominent band of those reside in the MJO time scales (30 to 60 days) and the synoptic time scales (3 to 7 days). The study suggests that the low frequency variability on MJO time scale in moisture and sensible heat flux arises from its nonlinear interactions with synoptic time scales. The results show that the phases of the three interacting oscillations associated with specific humidity/SST, Richardson number dependent instability factor and wind shear are positive and reasonably close to each other. The amplitudes of the synoptic scale oscillations are not insignificant compared to that of MJO. These dynamical aspects regarding the phases and amplitudes of the three participating oscillations favour the nonlinear interactions of MJO to the synoptic scales and thus lead to rapid exchange of energy transfer to the former.Visiting scientist from Indian Institute of Tropical Meteorology, Pune-411008, India.