## 4 Drought Effects  
The El Nino, associated with the low phase of the Southern Oscillation (SO), coincides with reduced Amazon rainfall, particularly in the northern and central regions. Opposite anomalies often occur during the high phase of the SO, related to La Nina. The 1997/98 drought during an El Nino year was considered the strongest in that century [137]. Differently from the El Nino-related droughts, during 2005, large sections of southwestern Amazonia were severely affected, and this drought was considered one of the most intense droughts of the last hundred years. The 2005 drought was driven by the warming of the tropical North Atlantic, combined with the reduced intensity in northeast trade wind moisture transport into southern Amazonia and the weakened upward motion over this section of Amazonia [138]. Five years later, another strong drought hit the Amazon. The 2010 drought started during an El Nino event in early austral summer of 2010 and then became more intense during La Nina in the austral winter dry season and the following spring. These three recent droughts affect different regions of the Amazon (Figure 4). In 1998, southwest regions of the Amazon, such as Bolivia and Rondonia state in Brazil, and north-eastern regions such as Roraima state were deeply affected. In 2005, the MAP region (Madre de Dios in Peru, Acre in Brazil and Pando in Bolivia) were the most severe affect region. In 2010, both central-north and central-south Amazonia were the most affected areas, covering Roraima and Mato Grosso states in Brazil and Guiana. Figure 3: Cont. Figure 4: Drought severity can be measured by the maximum cumulative water deficit (MCWD), which corresponds to the maximum value of the accumulated water deficit (WD) that reached each area within a period of time. The WD is estimated based on the approximation that moist tropical forests transpires 100 mm\(\cdot\)month\({}^{-1}\), and therefore, when rainfall is lower than 100 mm\(\cdot\)month\({}^{-1}\), the forest is considered under water stress. Areas that experienced more severe droughts present lower MCWD values. Spatial distribution of the MCWD between January and December (**a**) 1998; (**b**) 2005; and (**c**) 2010. Data derived from TRMM (Tropical Rainfall Measuring Mission), 3B43-v6, at 0.25\({}^{\circ}\) spatial resolution. Long term measurements in central Amazonia showed that the 97/98 ENSO drought increased the tree mortality (1.91%) in comparison with 5- to 13-year measurements average (1.12%), and that trees that died due to the drought did not differ significantly in size or species composition from those that died previously [139]. After the 2005 drought extreme, the first study published was carried out by [140]. They evaluated the anomalies in the EVI generated from the MODIS c4 collection during the driest quarter of 2005 and found a significant increase in the EVI during the peak, rather than a decline. This was interpreted as a possible increase in productivity during the drought period. However, two years later a study based on field measurements evaluating the impact of the 2005 drought demonstrated that plots most affected by the rainfall suppression exhibited a decline in the rate of net aboveground biomass accumulation, and those losses were driven by occasionally large mortality increases and by widespread decline in growth [141]. More information on the effects of droughs in the Amazon can be found in the special issue of the New Phytologist, volume 187, issue 3, 2011. These contrasting results intrigued the scientific community and a re-assessment of the EVI processing was carried out by [142]. The authors reported differences with respect to the area and intensity of the 2005 increase in the EVI between results obtained from the c4 and c5 product and thus contested [140] results. [PERSON] _et al._[143] also evaluated the anomalies in the EVI c5, and found only small and scattered areas with increase in this index.