Variability of Convectively Coupled Equatorial Waves

1. Variability of Convectively Coupled Equatorial Waves Kazuyoshi Kikuchi To understand details of the propagation and changes in convectively coupled easterly waves, we have developed the Combined Fourier-Wavelet Transform (CFWT) method, which uses Fourier transform for longitude and wavelet transform for time changes. This method provides instant spectral distributions from 2D data at any given time. The long-term average spectral distributions from CFWT agree remarkably well with those from Fast Fourier Transform (Fig. 1), indicating that an instantaneous CFWT spectrum is a reasonable snapshot. The time-varying CFWT spectra reveal that the dominant disturbance in the convection switched from mixed Rossby-gravity (MRG) waves into tropical depression (TD) waves within a short period. The CFWT method can be applied to characterize the convection in isolated, transient disturbances including tropical cyclones. Fig. 1. Estimates of the normalized zonal wavenumber-frequency power spectrum of the antisymmetric component of cloudiness (averaged between 15⁰N and 15⁰S) from July 1983 to June 2006 using (a) CFWT and (b) conventional FFT. Fig. 2. Convectively Coupled Equatorial Waves in 2001 boreal summer: (a) Time-longitude section of meridionally averaged antisymmetric infrared anomalies between 15⁰S and 15⁰N, (b) as in (a) but for MRG-TD-filtered disturbances superimposed in the foreground, (c) maximum values of the normalized spectrum of the westward propagating antisymmetric component between zonal wavenumbers –1 to – 20 as a function of frequency and time; (d) and (e) average power over the interval shown in the left and right bolded boxes in (c), respectively.