Excitation of intermediate‐frequency equatorial waves at a western ocean boundary: With application to observations from the Indian Ocean

Abstract

A linear, continuously stratified model is used to study the equatorial waves that are generated at the western boundary of the ocean by wind fields oscillating at periods of 30 and 60 days. Solutions are found in ocean basins with a western boundary oriented meridionally or slanted at a 45° angle, like the one in the Indian Ocean. They are evaluated analytically when the boundary is meridional but must be found numerically when the boundary is slanted. The possible equatorial waves are Kelvin and Yanai waves, and at the 60‐day period there are a few Rossby waves. Because Yanai and Rossby waves typically have short wavelengths, they are not directly forced by the wind; however, they can be strongly excited at the western boundary and in that case are highly visible in the interior ocean. When the western boundary is slanted, zonal winds also excite antisymmetric equatorial waves, resulting in considerable meridional flow on the equator. At the 30‐day period, energy associated with Kelvin and Yanai waves propagates eastward and downward from the western boundary along ray paths, producing an equatorial shadow zone in the western ocean. At the 60‐day period the presence of Rossby waves complicates the radiation pattern. Solutions compare favorably with several aspects of the observed equatorial wave field in the western Indian Ocean.