A wave‐number frequency wavelet analysis of convectively coupled equatorial waves and the MJO over the Indian Ocean

Abstract

Convectively coupled equatorial waves and the Madden–Julian Oscillation (MJO) constitute the dominant coherent modes of planetary‐ to synoptic‐scale organized convection in the Tropics. This work presents a space–time wavelet analysis of outgoing long‐wave radiation (OLR) data globally at wave‐numbers 0–1 and isolated more closely about the Indian Ocean at higher wave numbers. The mean power spectrum shows broad similarity to Fourier power spectra in previous works after normalization by a red background. The seasonal cycle of the power spectrum is analysed, along with its association with the phases of an index of the MJO and background‐state zonal winds at a variety of pressure levels over the Indian Ocean. Results show substantial variability across the seasonal cycle and the MJO in the distribution of power above the background. Results also show that the spectral peaks associated with the MJO and Kelvin and equatorial Rossby waves lose 20–50% of their normal variance during periods of strong easterly wind in the upper troposphere over the Indian Ocean, while at the same time, fast westward‐moving waves increase between wave‐numbers 7 and 14 and periods of 2–6 days.