Timing and Patterns of the ENSO Signal in Africa over the Last 30 Years: Insights from Normalized Difference Vegetation Index Data

Abstract

A more complete picture of the timing and patterns of the ENSO signal during the seasonal cycle for the whole of Africa over the three last decades is provided using the normalized difference vegetation index (NDVI). Indeed, NDVI has a higher spatial resolution and is more frequently updated than in situ climate databases, and highlights the impact of ENSO on vegetation dynamics as a combined result of ENSO on rainfall, solar radiation, and temperature.The month-by-month NDVI-Niño-3.4 correlation patterns evolve as follows. From July to September, negative correlations are observed over the Sahel, the Gulf of Guinea coast, and regions from the northern Democratic Republic of Congo to Ethiopia. However, they are not uniform in space and are moderate (;0.3). Conversely, positive correlations are recorded over the winter rainfall region of South Africa. In October-November, negative correlations over Ethiopia, Sudan, and Uganda strengthen while positive correlations emerge in the Horn of Africa and in the southeast coast of South Africa. By December 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 188S is well established in February-March with reduced (enhanced) greenness during ENSO years south (north) of 188S. At the same time, at ;28N negative correlations spread northward. Last, from April to June negative correlations south of 188S spread to the north (to 108S) and to the east (to the south of Tanzania).

Identifying Droughts Affecting Agriculture in Africa Based on Remote Sensing Time Series between 2000–2016: Rainfall Anomalies and Vegetation Condition in the Context of ENSO

“…Hereby, infiltration to the soil reservoir is minor, since most of the water is lost due to immediate surface run-off [36]. What is more, the timing of rainfall deficits is decisive for the occurrence of vegetative stress and the associated decrease in VCI.…”

“…This was revealed from observed droughts during La Niña phases that coincide with respective growing seasons (see Figure 10). However, Philippon et al [36] suggested a remarkably weaker effect of ENSO for the secondary, compared to the first growing season, based on long-term analysis of NDVI and its sensitivity to ENSO. In the present study, drought signals were observed over central Ethiopia, Uganda and western Kenya in the ongoing stage of El Niño, prior to the phase of highest ENSO intensity (see Figures 8 and 9).…”

“…In this connection, operational approaches on global-and continental-scale drought monitoring [6,[17][18][19][20] as well as remote sensing-based studies on droughts in Africa focussing on selected regions and/or periods are to be mentioned. Here, topics of current interest are the ongoing debate on the Sahelian "greening" [21][22][23], drought during 2010/2011 in the Horn of Africa [13,[24][25][26], rainfall and vegetation-related drought dynamics in southern Africa [27][28][29][30], continental-scale drought assessment [12,31,32] as well as the relation between droughts in Africa and ENSO [9,[33][34][35][36]. However, either studies accounted for drought assessment on rather small regional scales and did not assess the continental level or little attention was paid to drought affecting agriculturally relevant areas and time periods.…”

Evaluating Water Controls on Vegetation Growth in the Semi-Arid Sahel Using Field and Earth Observation Data

“…Since these ecosystem services are essential, evaluating the environmental controls that modulate GPP dynamics is necessary for accurate accounting of primary productivity. The Sahel has a single growing season that starts in July and ends in October [9] with considerable inter-annual variability in the spatial distribution and quantity of rainfall [10,11]. The driest parts of the northern Sahel receive an annual average of 150 mm of rain, while the southern parts receive an annual average of ~750 mm [12] (Figure 1).…”

Influence of Indian Ocean zonal circulation on variability and predictability of Ethiopia highlands vegetation

“…Our basis is the bias-corrected July to October satellite vegetation fraction (NDVI) from 1982 to 2014 (Huete et al, 2002;Tucker et al, 2005;Philippon et al, 2014) averaged over the Ethiopian highlands (box in Figure 1a). Atmosphere and ocean reanalysis fields are studied for patterns both simultaneously and in precursor season.…”