Annual Rossby Wave in the Southern Indian Ocean: Why Does It “Appear” to Break Down in the Middle Ocean?

Abstract

Using both altimetry data (TOPEX/Poseidon and Geosat) and Levitus climatology and a linear reduced-gravity model, the authors studied the annual Rossby waves in the southern Indian Ocean from 19° to 9°S. The most striking feature from the data analysis is that the westward phase propagation of the annual variability appears to break up in the midocean, which results in two local maxima for annual variability in both the sea level and the depth of the 18°C isotherm, with one in the eastern basin and the other in the western basin. Separating the two maxima is a midocean minimum. Decomposition of the annual variability into Rossby waves and localized response indicates that the two local maxima of the annual variability simply result from the constructive interference between the localized response and the Rossby waves in the eastern and western basin. On the other hand, the midocean local minimum results from the destructive interference between the Rossby waves and the localized response. Modeling results suggest that the bulk of the annual variability in the study domain is driven by wind forcing, while forcing by throughflow through eastern boundary radiation is of minor importance. Dissipation is found to have a much greater impact on the Rossby waves than on the local response.