Quasi-decadal signals of Sahel rainfall and West African monsoon since the mid-twentieth century

Bastien Dieppois, Arona Diedhiou, Alain Durand, Matthieu Fournier, Nicolas Massei, David Sebag, Bernard Fontaine, Yongkang Xue

Research output: Contribution to journalArticle

Abstract

Sahel rainfall shows pronounced decadal variability and a negative trend between wet conditions in the 1950s–1960s and dry ones in the 1970s–1980s. Using continuous wavelet transform, the quasi-decadal variability (QDV) of rainfall reveals zonal contrasts. The highest QDV is identified in the 1950s–1960s over western Sahel and in the 1970s–1980s over eastern Sahel. The quasi-decadal atmospheric anomalies have been reconstructed using Fourier transform for the 1950s–1960s and the 1970s–1980s, respectively, and assessed by the composite analysis of the QDV phases for the periods before and after 1968. Over western Sahel, the rainfall QDV in the 1950s–1960s is related to the North Atlantic sea surface temperature (SST) variability, as highlighted by the wavelet coherence. A southward shift trend of the Intertropical Convergence Zone (ITCZ) is identified through an enhancement of northeasterly fluxes and moisture convergence over the western part of West Africa. A decrease (increase) of southern (northern) subtropical sinking motions seems to be involved. In the 1970s–1980s, a strengthening of cross-equatorial Atlantic SST and pressure gradients is related to an increase of monsoon flow from lower troposphere up to the midtroposphere and to the northward shift of the ITCZ, mainly over eastern Sahel. The Pacific SST influence is also identified, which involves changes in the global zonal circulation
Original languageEnglish
Pages (from-to)12587-12599
Number of pages13
JournalJournal of Geophysical Research: Atmospheres
Volume118
Issue number22
Early online date14 Nov 2013
DOIs
Publication statusPublished - 27 Nov 2013

Keywords

  • West African Monsoon
  • Sahel rainfall
  • Sea-Surface Temperature
  • Quasi-decadal scale
  • zonal contrast

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