Modulation of the West African monsoon and the tropical easterly jet on Sudan rainfall by La Niña during the 2007 summer

Abstract

The summer rains (July to September) in 2007 caused massive flooding in Sudan. It is important to understand the factors that lead to heavy rainfall to improve future forecast accuracy of similar events. This will help to minimize the associated loss of lives and destruction of property. We investigate the anomalous atmospheric circulation associated with the heavy rainfall event in Sudan during the 2007 summer. The study used precipitation data from the Climatic Research Unit (CRU) TS version 4.06 for precipitation data, and atmospheric variables from the National Center for Environmental Prediction (NCEP)—Department of Energy (DOE) reanalysis II. The study area was divided into three zones, and the time series for each zone was calculated by averaging the data from all grids within that zone. The results show that the August 2007 rainfall significantly exceeded the long-term average in all zones, with zones 1 and 2 experiencing the most substantial increase of approximately 100 mm/month, while zone 3 saw an increase of about 40 mm/month. The 2007 summer rain is significantly influenced by the West African monsoon (WAM) and the tropical easterly jet (TEJ). The WAM strengthened south-to-southwesterly winds, bringing excess moisture from the Equatorial Atlantic Ocean (EAO) into Sudan. The absence of Harmattan winds allowed the moist air to penetrate further north. The convergence of moisture fluxes from the East Atlantic Ocean, the Congo Forest, and the Red Sea significantly facilitated favorable conditions for strong convection, contributing to the high rainfall in Sudan. Additionally, this study looks at how the TEJ contributes to the moist air associated with the 2007 summer rainfall. Results show that strong La Niña conditions of 2007 intensified and shifted the TEJ westward, enhancing the northward penetration of moist air and increasing rainfall over Sudan. The findings further indicate that La Niña conditions influenced the WAM flow, shifting the Intertropical Convergence Zone (ITCZ) by approximately 2° northward. This knowledge of how La Niña influences the characteristics of the WAM and TEJ can help local weather forecasters improve the accuracy of their predictions for heavy rainfall events in Sudan on a seasonal timescale.