Like many meteorologically important atmospheric waves, equatorial waves result from the influence of the restoring forces of gravity, the pressure gradient force, and the apparent Coriolis force upon displacements of parcels of air in the earth's atmosphere. Of importance for the equatorial waves is that the Coriolis force changes sign at the equator. It is this that allows these waves to exist in a region trapped near the equator, that is, with their amplitude decaying away from the equator. Thus, although the Coriolis force is small, and consequently the geostrophic balance is no longer expected, the rotation of the planet still plays a very important role for these large-scale atmospheric motions near the equator.
This chapter concentrates on the equatorial waves that occur in the troposphere, that is, those that most readily influence the weather. The chapter begins with the theory needed to give a basic understanding for their existence and appearance. In many respects, the theory is equally applicable to the equatorial waves of the ocean or middle atmosphere. The chapter continues with a simulation of equatorial wave dispersion in a numerical model of the atmosphere, given a forcing that is fairly typical of an energetic weather event. The rest of the chapter is devoted to the various observations of equatorial waves, and their significance for the weather.
