Author: Dorília Cunha
The Amazon region is home to the largest contiguous rainforest on the planet, the largest river in the world, bordered by the planet’s largest floodable surface and contributes to a water discharge equal to 18% of all freshwater discharged into the oceans, it is also home to approximately 15% of all biodiversity on Earth. The Brazilian Amazon is home to over twenty-five million people. Although hydrological fluctuations in the Amazon have already affected local communities and rainforest ecosystems over the past millennia (Meggers, 1994; Cordeiro et al., 2014; Marengo & Espinoza, 2016), intense human interventions in the forest, caused by the effects of regional deforestation or the impact of forest fires, tend to increasingly prolong the dry season and delay the start of the rainy season in the Amazon.
The forest’s response to the growing environmental changes that the planet is facing has become the object of numerous studies and has awakened the interest of global society.
The Amazon Basin is a key component of the global climate system. Still, deciphering its climatic variability remains a major challenge due to the great difficulty in finding historical data. Speleothems, which have been one of the most important means of climate registration, are rare in this area, and sediments collected along the Amazon delta may not be able to demonstrate internal hydrological variability due to the abundance of Andean sediments in the sediment supply (Govin et al., 2014).
Currently, the seasonal precipitation cycle over the Amazon region is dominated mainly by variations in the intensity of the South American Summer Monsoon (Monção de Verão da América do Sul, MVAS) and by changes in the Intertropical Convergence Zone (ITCZ) (Marengo, 2004; Garreaud et al., 2009). The MVAS involves two main components: one associated with the Intertropical Convergence Zone (ITCZ) in the equatorial Atlantic and convection over Amazonia; the other subtropical associated with the South Atlantic Convergence Zone (SACZ) and related to southeast South America (see figure).
The climate seasonality cycles in northern South America are marked by the movements of the ITCZ, which are related to the dynamics of northeastern trade winds and impact the oceans and the continental mass. The migration of the ITCZ occurs due to variations in the surface temperature of the Atlantic Ocean, which cause contrasts with the temperature of the South American continent and create the characteristic seasonal rainfall cycle of the Amazon (Noguès-Peagle et al., 2002; Hastenrath & Lamb, 1977). Changes in sea surface temperatures in the Atlantic and Pacific Oceans severely affect the transport of moisture to the Amazon and partially control the discharge of the Amazon River and its tributaries (Marengo & Espinoza, 2016).
Interannual rainfall anomalies in South America are caused by the El Niño-Southern Oscillation (ENOS) cycles. These cycles are characterised by anomalous warming of the surface of the tropical Pacific Ocean and promote periods of drought in the Amazon region and Northeastern Brazil and rainy periods on the west coast of South America. On the other hand, La Niña anomalies are characterised by abnormal cooling of surface waters in the tropical Pacific Ocean and generate increased rainfall in the Amazon basin and the Southern Region of Brazil (Cheng et al., 2013; Vuille et al., 2003; Hoffmann et al., 2003; Garreaud et al., 2009; Bookhagen & Strecker, 2010). The irregular fluctuations between hot (El Niño) and cold (La Niña) phases vary from two to seven years. Rainfall and temperature anomalies associated with the occurrence of El Niño and La Niña events are the main source of inter-annual variability in much of South America (Gerreaud et al., 2009).
The consequences of the climatic imbalance in the Amazon can be devastating, the irregular supply of water, for example, drastically affects the productivity of tropical ecosystems. Unfortunately, forecasts made through recent analyses point to a higher probability of drought frequencies in the Amazon over the next 100 years mainly due to climate change, deforestation and burning (Cox et al., 2008; Malhi et al., 2009). In addition, global warming may also lead to intensified El Niño events (Hansen, Huntingfoud; Cox, 2006) besides periods of intensified drought that have the potential to reduce the above ground biomass stock (Rolim et al., 2005; Phillips et al., 2009; Lewis et al., 2011), as well as change species composition in the long term (Engelbrecht et al., 2007; Nepstand et al., 2007; Fonty et al., 2009; Phillips et al., 2010).
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