Seasonal phase lag between rainfall and vegetation activity in tropical Africa as revealed by NOAA satellite data

A seasonal phase lag in the response of vegetation to rainfall and its dependence on the type of vegetation in tropical Africa have been investigated, using weekly‐basis data of the NOAA/AVHRR‐derived NDVI (normalized difference vegetation index) and OLR (outgoing longwave radiation) for April 1985 to September 1987. The lag‐correlation analysis of the NDVI and OLR data demonstrates that the seasonal phase lag, by which the NDVI responds most markedly to the OLR, tends to increase from the Equator to the pole in the Northern and Southern Hemispheres. The lag ranges from 1–3 weeks in the equatorial rain forest to about 2–6 weeks in the wet savanna and about 6–9 weeks in the dry savanna. The strongest negative lag‐correlation coefficients at the above lags are characterized by a spatial pattern which has a maximum absolute value near the latitudes 7.5°–10° in the savanna areas of both hemispheres. The equatorial rain forest, especially its north‐eastern part, shows the weakest correlation. The results suggest that the response of vegetation to rainfall is more pronounced and has a longer lag in the savanna than in the equatorial rain forest. A regional analysis of the NDVI‐rainfall relationship for Cameroon confirms the above NDVI‐OLR lag‐correlation on the continental scale.