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Abstract
Over the course of the twentieth century the African Sahel experienced large variations in annual precipitation; including the wet period during the 1950s and 1960s and the long‐term drought during the 1970s and 1980s. Feedbacks between the land surface and atmosphere can affect rainfall variability at monthly, annual and decadal time scales. However, the strength of the coupling between the land surface and precipitation is still highly uncertain, with climate‐model derived estimates differing by an order of magnitude. Here a statistical model of vegetation greenness is used to estimate the vegetation‐rainfall coupling strength in the Sahel, based on monthly satellite‐derived vegetation index and meteorological data. Evidence is found for a positive feedback between vegetation and rainfall at the monthly time scale, and for a vegetation memory operating at the annual time scale. These vegetation‐rainfall interactions increase the interannual variation in Sahelian precipitation; accounting for as much as 30% of the variability in annual precipitation in some hot spot regions between 15° and 20°N.
Highlights
Suppression is associated with enhanced dust generation and subsequent radiative feedbacks in the lower atmosphere [Tegen et al, 1996; Prospero and Nees, 1986]
[4] We analyse monthly satellite-based normalized difference vegetation index (NDVI) data collected by the Advanced Very High Resolution Radiometer (AVHRR) for 1982 –1999 sampled to 0.5° Â 0.5° resolution [Hall et al, 2005; S
Uncertainty analysis of global vegetation, precipitation and temperature data sets with a statistical vegetation index simulation model, in preparation, 2006, hereinafter referred to as Los et al, in preparation, 2006] and monthly ground-based precipitation and temperature data at the same spatial resolution provided by the Climate Research Unit (CRU) for 1901 – 2000 [Mitchell and Jones, 2005]
Summary
[2] Several factors contribute to the year-to-year variability in precipitation in the Sahel [Nicholson, 2000], used here to indicate the region south of the Sahara that includes the Sudanese zone. The version of AVHRR NDVI data we use in the current study is the Fourier Adjusted, Solar and sensor zenith angle corrected, Interpolated and Reconstructed (FASIR) NDVI data (Los et al, in preparation, 2006). This data set is derived from the Pathfinder Advanced Very High Resolution Radiometer (AVHRR) Land (PAL) channel 1 and 2 data that were corrected for Rayleigh scattering and ozone absorption by the atmosphere and sensor degradation and inter-calibration differences in respective AVHRRs aboard the NOAA 7, 9, 11 and 14 satellites [James and Kalluri, 1994].
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