Abstract
Daily rainfall in southern West Africa (4–8° N, 7° W–3° E) is analyzed with the aim of documenting the intense rainfall events which occur in coastal Ivory Coast, Ghana, Togo, and Benin. The daily 99th percentile (P99) shows that the coastline experiences higher intensity rainfall than inland areas. Using Tropical Rainfall Measuring Mission (TRMM) rainfall data for 1998–2014, a novel way of classifying the intense events is proposed. We consider their space-time structure over a window of 8° latitude-longitude and five days centered on the event. A total 39,680 events (62 at each location) are classified into three major types, mainly found over the oceanic regions south of 5° N, the Bight of Benin, and the inland regions respectively. These types display quite distinct rainfall patterns, propagation features, and seasonal occurrence. Three inland subtypes are also defined. The atmospheric circulation anomalies associated with each type are examined from ERA-interim reanalysis data. Intense rainfall events over the continent are mainly a result of westward propagating disturbances. Over the Gulf of Guinea, many intense events occur as a combination of atmospheric disturbances propagating westward (mid-tropospheric easterly waves or cyclonic vortices) and eastward (lower tropospheric zonal wind and moisture anomalies hypothesized to reflect Kelvin waves). Along the coast, there is a mixture of different types of rainfall events, often associated with interacting eastward- and westward-moving disturbances, which complicates the monitoring of heavy precipitation.
Highlights
Heavy rainfall events constitute a major risk for West African urban areas, where poor drainage facilities, deficient town planning, and rapid urban growth contribute to human and economic losses at times of flooding
A novel approach was tested to define the rainfall and atmospheric patterns associated with intense rainfall events at daily scale over the southern part of West Africa
The aims were to demonstrate the singularity of the coastal band along the Gulf of Guinea in terms of intense rainfall occurrence and rainfall types and to contribute to isolate the mechanisms which drive the heavy rains
Summary
Heavy rainfall events constitute a major risk for West African urban areas, where poor drainage facilities, deficient town planning, and rapid urban growth contribute to human and economic losses at times of flooding. Some flooding events result from the high water levels of the rivers coming down from inland areas, most coastal (especially urban) floods are generated by heavy in situ rainfall These heavy rains can occur in any season, but they are much more frequent around June, in connection with the mean rainfall regimes in Southern West Africa which display a main rainy season between May and July. Meynadier et al [19] found that the onset of strong southerlies at 2◦ N, due to an SST decrease associated with the equatorial cold tongue development, in late May to early June, corresponds to the start of intense Guinea Coast rainfall, which lasts about two weeks It is still not well understood how these mechanisms relate to easterly waves.
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