Variability of Convectively Coupled Equatorial Waves

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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 15N and 15S) 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 15S and 15N, (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.