On Misinterpretation of the Background Spectrum of Tropical OLR Anomalies

Wavenumber frequency power spectrum analysis has been used as a primary tool to detect the ranges of wavenumbers and frequencies about which observed convectively coupled equatorial waves are active. Previous works have suggested that activity in these waves clusters about roughly 12-60 m equivalent depths because spectral peaks normalized by dividing by a smoothed spectral background follow those ranges. Through a combination of wavenumber frequency power spectrum analysis and filtering this work confirms that the traditional filtering approach omits substantial power associated with Kelvin waves that occurs at lower frequencies. Wave signals at these frequencies are stronger than but structurally consistent with Kelvin wave signals coincident with the Kelvin peak in the normalized spectrum. Results suggest that the wave signal itself has red properties. The slower more intense wave signals outside of the standard band would impact tangible weather signals and should not be ignored in operations. Similarly, previous works have perpetuated an illusion to theoreticians that Kelvin waves are tightly confined into phase speeds between 11 and 30 ms⁻¹. Instead, results support the emerging view that disturbances labeled as Kelvin waves form a continuum with the Madden Julian Oscillation (MJO), and suggest that the whole region of the spectrum from the broadly recognized Kelvin band to the MJO should be considered together.