The Three-Dimensional Structure of Synoptic-Scale Disturbances over the Tropical Atlantic

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Abstract
Twice-daily analyses of low-level and 200-mb winds over the tropical Atlantic region, archived by the National Hurricane Center, are used to diagnose the structure of synoptic-scale disturbances in the 3–5 day period band. The large-scale disturbances, extracted by a complex empirical orthogonal function technique, are found to have a preferred shift at 200 mb relative to the low-level troughs of somewhat less than one-quarter cycle. The presentation concentrates on July 1975, during which a repeated series of strong disturbances propagated through the region. The relationship between these disturbances and systems in the eastern Pacific is discussed. An analysis of the vorticity propagation characteristics for the disturbances during the month indicates a very different balance from level to level. At the lower level, advection by the mean wind plays a major rate; at 200 mb, the meridional advection of mean vorticity is more inportant. Rawinsonde data from several island stations are used to resolve the vertical structure of the disturbances. After adjustment for lower density aloft, the kinetic energy at the lower and upper levels is found to be almost equal. The systems propagate westward faster than the mean zonal wind at any level, with a zonal phase speed that is relatively constant with height. It is inferred that the disturbances most likely propagate as a coherent system due to vertical coupling by convection. Evidence is found that the influence of low-level waves on the evolution of 200-mb systems may be stronger than has been previously described.

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