Evidence for Coupled Rossby Waves in the Annual Cycle of the Indo–Pacific Ocean

Abstract

The annual cycle in TOPEX/Poseidon sea level height (SLH) and National Centers for Environmental Prediction (NCEP) sea surface temperature (SST) and meridional surface wind (MSW) from 1993 to 1998 in the trade wind zone over the Indo-Pacific Ocean from 5° to 25° latitude in both hemispheres is examined. Zonal wavenumber?frequency spectra of monthly mean SLH, SST, and MSW residuals about the annual mean find peak spectral energy density for westward-propagating waves of annual period and basinscale wavelengths in all three variables. Moreover, spectral coherence and phase between the three variables are significant, with warm SST residuals displaced 0°?90° of phase to the west of high SLH residuals, and poleward MSW residuals displaced 0°?90° of phase to the east of warm SST residuals, similar to those observed for coupled Rossby waves on interannual period scales. Removing zonal means at each latitude from the annual cycle in SLH, SST, and MSW allows westward phase propagation to be observed in each variable, together with poleward refraction stemming from the reduction in westward phase speed with latitude. The fact that the latter occurs in both oceanic and atmospheric variables indicates that annual Rossby waves in the ocean are coupled to the overlying atmosphere. The reduction in westward phase speed with latitude (and, hence, the poleward refraction) is less than expected for free Rossby waves, with the apparent coupled Rossby waves traveling faster (slower) poleward (equatorward) of ?12° latitude.