Characterization of the Interannual and Intraseasonal Variability of West African Vegetation between 1982 and 2002 by Means of NOAA AVHRR NDVI Data

Abstract

The interannual and intraseasonal variability of West African vegetation over the period 1982-2002 is studied using the normalized difference vegetation index (NDVI) from the Advanced Very High Resolution Radiometer (AVHRR).The novel independent component analysis (ICA) technique is applied to extract the main modes of the interannual variability of the vegetation, among which two modes are worth describing. The first component (IC1) describes NDVI variability over the Sahel from August to October. A strong photosynthetic activity over the Sahel is related to above-normal convection and rainfall within the intertropical convergence zone (ITCZ) in summertime and is partly associated with colder (warmer) SST in the eastern tropical Pacific (the Mediterranean). The second component (IC2) depicts a dipole pattern between the Sahelian and Guinean regions during the northern summer followed by a southward-propagating signal from October to December. It is associated with a north-south dipole in convection and rainfall induced by variations in the latitudinal location of the ITCZ as a response to the occurrence of the tropical Atlantic dipole.The analysis of the intraseasonal variability of the Sahelian vegetation relies on the analysis of the seasonal marches and their main phenological stages. Green-up usually starts in early July and shows a very low year-to-year variability, while senescence ends by mid-November and is prone to larger interannual variability. Six types of vegetative seasonal marches are discriminated according to variations in the timing of phenological stages as well as in the greening intensity. These types appear to be strongly dependent on rainfall distribution and amount, particularly those recorded in late August. Finally, year-to-year memory effects are highlighted: NDVI recorded during the green-up phase in year j appears to be strongly related to the maximum NDVI value recorded at year j Ϫ 1.

Changes in Precipitation Seasonality in West Africa Predicted by RegCM3 and the Impact of Dynamic Vegetation Feedback

“…At the interannual time scale, this feedback can be facilitated by the root system of perennial grasses and/or seed bank of annual grasses, with the first flush of vegetation growth at the beginning of the rainy season supported by carbon storage in the roots resulting from previous year's carbon production or seeds produced in the previous year. In the real world, both mechanisms can support inter-annual land memory in the Sahel region found in the Normalized Difference Vegetation Index (NDVI) data [44]. In the model used here, vegetation feedback at the inter-annual time sale over grassland region relies on roots of perennial grass, as annual grass is not simulated in the model.…”

Assessing Land Degradation and Desertification Using Vegetation Index Data: Current Frameworks and Future Directions

“…In addition to spatial autocorrelation functions present [90,91], memory effects in dryland systems make inter-and intra-annual NDVI values strongly correlated [92][93][94]. Thus, assumption one above and, commonly, four, are unlikely to be met.…”

New perspectives on land–atmosphere feedbacks from the African Monsoon Multidisciplinary Analysis

“…During the AMMA aircraft campaign, one AEW event in particular -the short-lived case of 26-28 July 2006 -exhibited a notable soil moisture anomaly on the synoptic scale (Bain, 2008). Numerical modelling revealed some significant influences of the surface on the AEW, including a meridional shift and reduction in the intensity of the African Easterly Jet (AEJ) when Surface-PBL response to rain event Impact of vegetation on surface thermodynamics Land memory (Douville et al, 2007;Philippon et al, 2007) Mesoscale (5-1000 km) Surface-induced PBL gradients and daytime circulations (Taylor et al, 2007;Baldi et al, 2008). Impact of soil moisture on storms (Taylor and Ellis, 2006;Gaertner et al, 2010;Gantner and Kalthoff, 2010;Taylor et al, 2010) Positive feedbacks at convective scales within MCS (Clark et al, 2004;Taylor and Lebel, 1998), negative feedbacks associated with initiating convection (Taylor and Ellis, 2006) Impact of longer-lived surface features (vegetation, wetlands) on storms (Garcia-Carreras et al, 2010;Taylor, 2010) Unknown Synoptic-continental (>500 km) Sahelian heat low anomalies intensify overnight .…”

“…A key question therefore is to assess how much of that information is carried over from one season to the next. Using Normalised Difference Vegetation Index (NDVI) data, Philippon et al (2007) found evidence of an interannual land memory in the grasslands of Sahel. They found a significant increase during June in the correlation of NDVI with maximum NDVI recorded the previous year.…”