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Strong Relationship between Dry-Season Rainfall over West Africa and Extratropical Disturbance



Author(s)

Ewanlen Inegbenoise Rufus and Adeyeye Joseph

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DOI:10.17265/2159-581X/2017.04.003

The recent concerns for food security over Africa related to several climatic factors, such as the strength of the flood and drought within the growing and harvesting seasons and the long-term rainfall variability have motivated the study of identifying the extratropical causes of “anomalous” dry-season precipitation for the region. The paper examines the role of upper level disturbance in the unusual rainfall over the tropical region of West Africa in the dry-season for the month of December, 2014. The rainfall is examined using the GPCP (global precipitation climatology project) merged satellite-gauge daily precipitation estimate and station rain-gauge measurements obtained from Nigeria Meteorological Agency. While the atmospheric circulation features are determined by using the NCEP-NCAR (National Centers for Environmental Prediction–National Center for Atmospheric Research) reanalysis dataset, composites of NCEP/NCAR Reanalysis wind fields, pressure, temperature, humidity and moisture fluxes suggest that rainfall event that affected few countries (Nigeria, Ghana & Cote d’Ivoire) during the period is linked with an enhanced on-shore westerly low-level flow from the Gulf of Guinea into inland, northward displacement of ITD (Inter-Tropical Discontinuity), intensification of the weak dry-season Sahara heat-low and upper-cyclonic vortex which help the generating of convection over the region. The influence of synoptic systems was also evident in the rainfall analysis for December, 2014. During the periods of study, the observed low-level flow over West Africa is likely to be an important contributor to the observed dry-season heavy rainfall, regulated by extra-tropical synoptic scale disturbances. The results provide an important basis for further studies on several cases over the past decade years producing heavy rainfall exceeding some kind of climatological, statistically-based threshold.

ITD, upper-level disturbance, precipitation, tropical region.