Description of local features associated with wet and dry spell events over Guinea Coast of West Africa

Abstract

Understanding the variability of wet and dry spells in the West African Monsoon (WAM) is essential for improving climate prediction and resource management. Using in situ daily rainfall observations from five stations located within the Guinea Coast having at least 99.5 % data record and pass quality control checks. This study examines the characteristics, atmospheric precursors, and large-scale circulation patterns associated with wet and dry spells during the summer monsoon season (1982–2020). Results reveal significant spatial variability, with Port Harcourt recording the highest wet spell frequency (>5 %), longest duration (53.38 days), and largest rainfall contribution (35.83 %). In contrast, Tabligbo experiences the highest dry spell frequency (10.92 %) and longest duration (76.23 days), contributing only 0.46 % to total monsoonal rainfall due to its location in the Dahomey Gap. Composite analysis of 841 wet and 2188 dry spell days shows a dipole-like structure, where wet spells are linked to enhanced southwesterly winds, cyclonic circulation, and increased moisture transport over the Guinea Coast, while dry spells exhibit anomalous westerlies, moisture divergence, and reduced convection. Lead-lag analysis from five days before to six days after spell onset reveals that wet spells are preceded by easterly anomalies at 850 hPa, followed by strengthening southwesterlies at onset, which peak after the onset before weakening. In contrast, dry spells develop under enhanced westerlies, inhibiting convection before onset, with intensified easterlies exporting moisture offshore from the onset. Vertical wind structures further show that wet spells weaken the African Easterly Jet (AEJ) while strengthening the Tropical Easterly Jet (TEJ), favoring deep convection, whereas dry spells is associated with deeper and stronger moisture depth that suppress convective development. These findings highlight the role of large-scale circulation in modulating regional wet and dry spells, emphasizing the need for long-term coordinated regional observations to improve monsoon forecasting, climate adaptation, and water resource management.