The slowly eastward-moving equatorial convection and circulation anomaly is a major mode of the tropical intraseasonal oscillations. The dynamics of this mode may be understood in terms of moist equatorial wave dynamics. A linear semi-geostrophic model on equatorial Beta-plane is used to study the behavior of equatorial low-frequency motions.

The unstable interaction of boundary layer frictional moisture convergence with condensational heating could generate efficiently eddy available potential energy for moist Kelvin mode but not for long Rossby modes. The growing mode is rooted in a moisted Kelvin wave but modified through coupling with a long Rossby wave of the lowest meridional index.

The horizontal mode coupling makes the growing mode have a horizontal structure bearing similarity to both Kelvin and Rossby modes. It favors the amplification of long planetary scales and slows down the eastward movement. It also suppresses unrealistically fast growth of the uncoupled Kelvin waves by creating substantial meridional flows which induce kinetic energy destruction.

The model results also demonstrate that when maximum SST moves from the equator to 7.5°N, the growth rate of the unstable wave is significantly reduced, suggesting that the annual march of the "thermal equator" and associated convective heating is likely responsible for annual variations of the equatorial intraseasonal wave activities.

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