The Influence of the Regional Hadley and Walker Circulations on Precipitation Patterns over Africa in El Niño, La Niña, and Neutral Years

Paleo-ENSO influence on African environments and early modern humans

“…During La Niña conditions (negative ENSO phase), the entire system reverses from the El Niño scenario (see Fig. 1 in the main text) leading to the development of a negative Indian Ocean Dipole and Atlantic Niña phase (1). In addition to the outlined mechanism the interferences between WC also causes precipitation anomalies during the subsequent summer month which can regionally vary from the winter counterparts.…”

“…During El Niño (positive ENSO phase), warming of the eastern and central equatorial Pacific causes ascending motion over the central and eastern Pacific and subsidence over Indonesia (see Fig. 1 in the main text) (1). Over the Indian Ocean this atmospheric configuration leads to a weakening or even reversal of the westerlies initiating pooling of warm water in its northwestern part (positive Indian Ocean Dipole phase).…”

“…In combination with the dominant subsidence in western Africa, the reduced evaporation over the cooled eastern equatorial Atlantic increases aridity across southern Africa in boreal winter and in addition the Sahel Zone during boreal summer. In phase with the above discussed alterations of the Walker Circulation (WC), the Hadley Circulation (HC) -which transports moisture latitudinally -also weakens in both hemispheres due to reduced moisture ascension in the African tropics (1). The lack of moisture transport through the HC towards the subtropics adds to the drier conditions under El Niño conditions in south and northwestern Africa in the respective hemisphere summer.…”

“…In addition to the outlined mechanism the interferences between WC also causes precipitation anomalies during the subsequent summer month which can regionally vary from the winter counterparts. An example is ENSO interference with the Congo Air Boundary which leads during the summer months also to additional rainfall in western Africa from the Sahel to the Congo basin (1). Although ENSO variability most strongly affects the "short rain season during the winter months it nevertheless shifts the yearly precipitation budget of these regions to generally wetter or drier conditions relative to non-El Niño years (3,4).…”

El Niño Southern Oscillation (ENSO) effects on fisheries and aquaculture

“…Precipitation decreases during December and February in La Niña years in Uganda, Kenya and northern United Republic of Tanzania, but increases in coastal southern United Republic of Tanzania. Rain also increases during June to August under La Niña in southern Sudan, northwest Ethiopia, Uganda and eastern United Republic of Tanzania (de Oliveira et al, 2018). Variability in the short rains in the African Great Lakes region is driven by ENSO, with El Niño years being associated with above normal rainfall in the short rains period (Kizza et al, 2009).…”