Comparative study of the 1982-1983 and 1997-1998 El Nino events over different types of vegetation in South America

This work presents results which help to understand the behaviour of the Normalized Difference Vegetation Index (NDVI) anomalies over the South America continent during the two strongest El Nino events of the last century (1982-1983 and 1997-1998). The data used are parts of a long-term series ( July 1981 to December 1999) of calibrated NDVI data derived from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) datasets. Special emphasis has been given to the analysis of the response of the major Brazilian vegetation types. This paper introduces an approach that enhances NDVI anomalies relative to the long-term climatology of the region. We find a negative NDVI anomaly for most of the region during the 1982-1983 event, whereas for the 1997-1998 event positive NDVI anomalies were observed over most regions. Only the 'Nordeste' region showed a similar vegetation response for both events. We identify three possible factors that may play a role in the different NDVI responses to the two El Nino events. Firstly, poor intercalibration of sensors may account for some, but not all of the differences. Secondly the response of the vegetation may depend upon the climate conditions prior to the El Nino events. Thirdly, the difference in the onset date and the duration of the mature phase of the two El Nino events, associated with very different Atlantic surface temperatures are shown to have dynamical consequences which may impact upon the vegetation.