Timing and patterns of ENSO impacts in Africa over the last 30 years: insights from Normalized Difference Vegetation Index data.

Nathalie Philippon (1) , Aurélie Blais (1) , Nadège Martiny (1) , Pierre Camberlin (1) , Timm Hoffman (2)

1 CRC - Centre de Recherches de Climatologie [UMR Biogéosciences]
2 Plant Conservation Unit

In this study we reassess and provide a more complete picture of the timing and patterns of ENSO impacts for the whole of Africa over the three last decades. We analyse the vegetation photosynthetic activity estimated by the NOAA-AVHRR Normalized Difference Vegetation Index (NDVI) rather than rainfall itself, because NDVI allows us to document the impacts at fine space and time scales. The use of the monthly time-step adds important new insights to the findings of previous works based largely on annual or seasonal time-scales and on a regional spatial-scale: several dipolar and propagative patterns are highlighted. In addition, we show that the less-studied winter rainfall regions at the southwestern and northwestern tips of the continent are impacted as well. Starting from July (the beginning of the peak phase of ENSO), to June (the post phase of ENSO), the teleconnection patterns over the whole of Africa evolve as follows. From July to September, negative correlations between NDVI and N3.4 are observed north of the equator but they are not uniform in space and are moderate (~0.3) except towards the east. Conversely, positive correlations are recorded over the winter rainfall region of South Africa. In the period from October-November, two dipoles develop: a north/south one in West Africa signaling a latitudinal modulation of the ITCZ location, and a west/east one in East Africa signaling a longitudinal modulation of the ITCZ location. By December (end of the peak phase of ENSO), with the settlement of the ITCZ south of the equator, positive correlations over the Horn of Africa spread southward and westward while negative correlations appear over Mozambique, Zimbabwe and South Africa. This pattern strengthens and a dipole at 18°S is well established in January-March (the decay phase of ENSO). In the meantime, by ~2°N negative correlations spread northward, while at the northwestern tip of Africa, a west/east dipole occurs opposing Algeria and eastern Morocco (positive correlations) with western Morocco (negative correlations). Lastly, at the end of the ENSO decay phase and the beginning of its post phase (April 1 to June 1) a northward (to 10°S) and eastward (to the south of Tanzania) spread of the negative correlations south of 18°S occurs while the positive correlations over equatorial East Africa switch to being negative.

Mots clés: ENSO; Africa; Normalized Difference Vegetation Index; teleconnections

Domaines: Climatologie; Milieux et Changements globaux

Nathalie Philippon, Aurélie Blais, Nadège Martiny, Pierre Camberlin, Timm Hoffman. Timing and patterns of ENSO impacts in Africa over the last 30 years: insights from Normalized Difference Vegetation Index data.. 2012. ⟨hal-00668118⟩