Contributions of the African Easterly Waves and the Northern Vortices to Atlantic Tropical Cyclone Formation

It is well-known that African Easterly Waves serve as the precursor for many of the Atlantic tropical cyclones. At least since the GATE studies, a northern vortex in the lower troposphere on the poleward side of the African Easterly Jet has been distinguished from the wave structure on the equatorward side that has maximum amplitude at 650-700 mb. This northern vortex has been detected in spectra, composite analyses, numerical model forecasts, and re-analyses. The nature or the amount of linkage between the northern vortex and the southern wave is unclear, especially as some recent analyses and forecasts suggest an origin downstream from the Hoggar Mountains. The contribution of the northern vortex to downstream tropical cyclone formation is also unclear. Whereas the maximum vorticity being in the lower troposphere would seem favorable, other factors such as vertical wind shear in a baroclinic system are unfavorable.

A new tool to study the northern vortex in relation to the southern wave and downstream tropical cyclones is an objective vortex identification and tracking technique. Analyzed and forecast vortices, defined by 850 hPa relative vorticity above a threshold value, are automatically identified, tracked, and cataloged in a database. For each tropical vortex in the database, 14 parameters such as wind shear, midlevel moisture, and midlevel warm core, etc.) are also catalogued in the vortex database. The database structure allows for detailed analysis of forecast performance with respect to these parameters.

In this study, analyses and forecasts made by the National Centers for Environmental Prediction Global Forecast System, the United States Navy Operational Global Atmospheric Prediction System, and the United Kingdom Meteorological Office Global Model are analyzed with respect to physical quantities that are relevant to African Easterly Waves and downstream tropical cyclones. Three sets of analyses and forecasts are used, and are compared with satellite imagery, to obtain the most realistic possible representation of the circulations given the sparsity of the observations over Africa. The set of 14 parameters associated with each vortex and each model will be examined to distinguish the conditions leading to realistic wave structures and track forecasts. Particular attention will be given to the evolution of the northern vortex and the southern wave as they approach and cross the west coast, and especially their relationship to Atlantic tropical cyclone formations.