Tropical easterly waves originate over the African continent quite regularly and usually traverse the entire Atlantic Ocean. These disturbances quite often can be coherent in spatial structure (localized and nonlinear). Upon their journey across the ocean they encounter background environments which may be favorable or unfavorable for growth. One background effect on these disturbances is Rossby dispersion. The earth's contribution for Rossby dispersion can weaken coherent disturbances especially at lower latitudes where beta is larger. On the other hand, a group of weak sinusoidal-like disturbances may behave more like a wave-packet with the background environment providing group velocity convergence to amplify the disturbances.
In this study, the low-pass time-filtered (>20 days) absolute vorticity field will define the environmental background vorticity. This background field varies quite substantially from the earth's vorticity field in certain regions. Areas of negative absolute vorticity gradients, which can be barotropically unstable, are observed near Africa and over the Caribbean Sea. Regions of absolute vorticity gradients near zero will have small dispersion on coherent disturbances.
To understand the effects of the background flow and absolute vorticity field on coherent and wave-like disturbances non-divergent barotropic diagnostics will be employed. These methods will be applied to features observed in the ECMWF gridded analysis. In addition, a shallow water model simulating a weakly divergent barotropic system will be utilized to examine the effects of the zonally varying background on coherent and sinusoidal-like disturbances.