This paper is concerned with the free oscillations in a rectangular equatorial beta-plane model of large oceans with a free surface placed symmetrically on the equator. With the depth constant and the stratification horizontally uniform, the motion may be separated into vertical modes, each having free oscillations....
Mofjeld, H. O., and Maurice Rattray. 1971. "Free oscillations in a beta-plane ocean." Journal of Marine Research 29, (3). https://elischolar.library.yale.edu/journal_of_marine_research/1219