On African easterly waves that impacted two tropical cyclones in 2004

Abstract

The U. S. Navy Operational Global Atmospheric Prediction System (NOGAPS) daily analysis fields are used to study the African easterly waves to show how they impacted two tropical cyclones. Danielle and Earl (2004) formed approximately at the same time in the Atlantic. A three to eight day band-pass filter is applied to the daily global analysis fields to reveal the characteristics of the African easterly waves related to the two cyclones. The time-filtered 850 mb vorticity shows that African easterly waves reaching the formation locations of Danielle and Earl on and about 14 August 2004 bear similar structures, indicating a unique situation in which one train of the easterly wave is responsible for the genesis of two tropical cyclones located one-wavelength apart. In addition, using piecewise time-filtered analysis for different latitude bands, it is demonstrated that the movements of both cyclones are significantly modulated by the propagation of the easterly waves, even during Danielle's poleward movement for more than ten-degrees latitude. Copyright 2006 by the American Geophysical Union.