## Abstract

Observational evidence for an equatorial non-dispersive mode propagating at the speed of gravity waves is strong, and while the structure and dispersion relation of such a mode can be accurately described by a wave theory on the equatorial β-plane, prior theories on the sphere were unable to find such a mode except for particular asymptotic limits of gravity wave phase speeds and/or certain zonal wave numbers. Here, an ad hoc solution of the linearized rotating shallow-water equations (LRSWE) on a sphere is developed, which propagates eastward with phase speed that nearly equals the speed of gravity waves at all zonal wave numbers. The physical interpretation of this mode in the context of other modes that solve the LRSWE is clarified through numerical calculations and through eigenvalue analysis of a Schrödinger eigenvalue equation that approximates the LRSWE. By comparing the meridional amplitude structure and phase speed of the ad hoc mode with those of the lowest gravity mode on a non-rotating sphere we show that at large zonal wave number the former is a rotation-modified counterpart of the latter. We also find that the dispersion relation of the ad hoc mode is identical to the n = 0 eastward propagating inertia–gravity (EIG0) wave on a rotating sphere which is also nearly non-dispersive, so this solution could be classified as both a Kelvin wave and as the EIG0 wave. This is in contrast to Cartesian coordinates where Kelvin waves are a distinct wave solution that supplements the EIG0 mode. Furthermore, the eigenvalue equation for the meridional velocity on the β-plane can be formally derived as an asymptotic limit (for small (Lamb Number)^{-1/4}) of the corresponding second order equation on a sphere, but this expansion is invalid when the phase speed equals that of gravity waves i.e. for Kelvin waves. Various expressions found in the literature for both Kelvin waves and inertia–gravity waves and which are valid only in certain asymptotic limits (e.g. slow and fast rotation) are compared with the expressions found here for the two wave types.

Original language | American English |
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Pages (from-to) | 1554-1564 |

Number of pages | 11 |

Journal | Quarterly Journal of the Royal Meteorological Society |

Volume | 143 |

Issue number | 704 |

DOIs | |

State | Published - Apr 2017 |

### Bibliographical note

Funding Information:Financial support for this work was provided by ISF grant No. 1558/14 and by a European Research Council starting grant under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 677756) to HU (CG). NP acknowledges the discussions held with Y. De-Leon of HU that helped clarify some of the subtle numerical points addressed in this study.

Publisher Copyright:

© 2017 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley & Sons Ltd on behalf of the Royal Meteorological Society.

## Keywords

- Kelvin waves on a sphere
- equatorial Kelvin wave
- equatorial inertia–gravity waves
- shallow-water waves