Westward Traveling Hemispheric Modes and Modes in a Continuous Spectrum of Atmospheric Oscillations

Atmospheric observational studies reveal large-scale westward traveling low-frequency hemispheric waves and synoptic-scale eastward traveling modes, but barotropic models of the atmosphere do not have normal-mode Rossby waves in the discrete spectrum that travel eastward or which are concentrated in one of the hemispheres. Nevertheless, we show within the linearized shallow-water model, that eastward traveling barotropic modes and westward traveling low-frequency hemispheric modes can be excited in the same way as westward traveling normal-mode Rossby waves. The study shows eastward traveling modes as famous a ubiquituos wavenumber-5 anomaly in the Southern Hemisphere and others, that agree well with observations. The study also shows that the shallow-water atmosphere supports hemispheric low-frequency modes in and out of the continuous spectrum of atmospheric oscillations and are similar with large-scale zonal components of the Branstator-Kushnir patterns and with westward traveling low-frequency waves which were observed by Madden and Speth (1989) in the Northern Hemisphere.