Anomalous northern hemisphere summer low frequency circulation in the midtroposphere of tropical West Africa

Low frequency (LF) circulation anomalies in the midtroposphere of tropical West Africa, represented by 500 hPa isobaric surface where maximum ascent and significant waves (troughs and ridges) are detected, are investigated using dynamic field elements such as geopotential height, vertical motion, velocity potential and streamfunction. Our main focus is to examine the consistency of the interactions of the dynamic fields with the dominant modes of wet and dry events at the surface, and propose physical mechanisms to explain the occurrence of the events. An application of a 10-year running mean filter algorithm to monthly NCEP/NCAR reanalysis and University of Delaware terrestrial precipitation datasets ensures that the high frequency (HF) stationary signals are attenuated, leaving the decadal non-stationary LF components, which are of interest to us. Using the atmospheric anomalies subsequently created from the prefiltered datasets, composite analysis is performed based on the positive and negative phases of the precipitation time series generated from empirical orthogonal function (EOF) analysis. We demonstrate that there are three dynamically (or physically) separable and consistent LF atmospheric circulation anomaly patterns each associated with below- and above-normal LF precipitation events during the Northern Hemisphere (NH) summer (June-September) of the region.