Effects of Zonal Flows on Equatorially Trapped Waves

Abstract

Linear shallow water equations are employed to study the effects of basic zonal flows on equatorially trapped waves. Analytical solutions are obtained for constant basic zonal flows. It is shown that changes in the wave structures due to the non-Doppler effect of the basic zonal flow are considerable for the Rossby wave, moderate for the westward propagating mixed Rossby–gravity wave, but negligible for the other equatorial waves. The Rossby wave and the westward propagating mixed Rossby–gravity wave possess greater eigenfrequencies and are less trapped in westerlies than in easterlies. The dependence of the Rossby wave structure upon the basic zonal flow is interpreted in terms of potential vorticity conservation. In basic zonal flows with meridional shears, the eigenfrequencies are found to be larger in equatorial easterlies than in equatorial westerlies for the westward propagating waves but smaller for the eastward propagating waves. While the meridional structures of the eastward propagating waves show little sensitivity to the basic zonal flow, the Rossby wave is less trapped in equatorial westerlies but the westward propagating inertia–gravity wave is less trapped in equatorial easterlies. It is suggested, therefore, that the equatorial transient Rossby waves interact with midlatitudes more readily at the longitudes associated with tropical westerlies. Furthermore, at these same longitudes, it is possible that extratropical forcing may project onto the equatorial modes and produce equatorially trapped responses if the forcing lies within their turning latitudes, which may extend to beyond 40° latitude at these locations. The conclusion underlines the upper troposphere of the tropical eastern Pacific Ocean and possibly the tropical Atlantic Ocean as critical regions of latitudinal interaction in both directions over a wide range of time scales.