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Abstract A recently developed linear model of eastward-propagating disturbances has two separate unstable modes: convectively coupled Kelvin waves destabilized by the wind dependence of the surface enthalpy flux, and slow, MJO-like modes destabilized by cloud–radiation interaction and driven eastward by surface enthalpy fluxes. This latter mode survives the weak temperature gradient (WTG) approximation and has a time scale dictated by the time it takes for surface fluxes to moisten tropospheric columns. Here we extend that model to include higher-order modes and show that planetary-scale low-frequency waves with more complex structures can also be amplified by cloud–radiation interactions. While most of these waves survive the WTG approximation, their frequencies and growth rates are seriously compromised by that approximation. Applying instead the assumption of zonal geostrophy results in a better approximation to the full spectrum of modes. For small cloud–radiation and surface flux feedbacks, Kelvin waves and equatorial Rossby waves are destabilized, but when these feedbacks are strong enough, the frequencies do not lie close to classical equatorial dispersion curves except in the case of higher-frequency Kelvin and Yanai waves. An eastward-propagating n = 1 mode, in particular, has a structure resembling the observed structure of the MJO.
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Interpretation of the spectrum of eastward‐moving tropical convective anomalies
Abstract Wave‐number‐frequency power spectrum analysis has been used as a primary tool to detect the ranges of wave numbers and frequencies about which observed convectively coupled equatorial waves are active. Previous works have suggested that activity in these waves clusters between roughly 12 and 60 m equivalent depths because spectral peaks normalized by dividing by a smoothed spectral background follow those ranges. Through a combination of wave‐number‐frequency power spectrum analysis, filtering and linear regression, this work shows that the traditional approach generates confusion because it conflates different, sometimes conflicting, signals from around the world that contribute to the same parts of the spectrum. Results also suggest that the traditional method leads us to ignore substantial power associated with variability structurally consistent with observed Kelvin waves but that occurs at lower frequencies. Wave signals at these frequencies are stronger than but similar to Kelvin wave signals coincident with the Kelvin peak in the normalized spectrum. Results suggest that the wave signal itself has red properties, possibly because more strongly convectively coupled waves propagate more slowly. The slower, more intense wave signals outside of the standard band would impact tangible weather signals and should not be ignored in operations. Instead, results support the view that disturbances labelled 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.
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- Award ID(s):
- 1757342
- PAR ID:
- 10453425
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Quarterly Journal of the Royal Meteorological Society
- Volume:
- 146
- Issue:
- 727
- ISSN:
- 0035-9009
- Page Range / eLocation ID:
- p. 795-806
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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