Cloud-Cluster-Scale Circulations and the Vorticity Budget of Synoptic-Scale Waves Over the Eastern Atlantic Intertropical Convergence Zone

Abstract

Objectively analyzed upper-air winds over the intertropical convergence zone of the eastern tropical Atlantic are separated into components representing the mean flow, westward moving synoptic-scale waves and cloud-cluster-scale motions during the third observational period of the GARP Atlantic Tropical Experiment. Vorticity budget analysis is performed for the three components of the flow. At the scale of cloud clusters, the vorticity budgets show variability that is the same order of magnitude as the vorticity field changes caused by the passage of the easterly waves. The variability patterns appear to be closely related to the phase of the wave and to cloud-cluster activity. It is concluded that the nonlinear contributions of cloud-cluster-scale motions to the average wave structure are small in the lower troposphere, but are as large as any of the vorticity budget terms involving wave divergence or vertical motion in the middle and upper troposphere. The implications of the results for numerical modeling of easterly waves are discussed.