1.5.2.4 Effects of AMOC changes on the Amazon rainforest

The strength of the AMOC exerts a substantial influence on the climate of tropical South America – most importantly, on rainfall and its seasonal distribution (1.4.2.3). This in turn affects the state and stability of another potential tipping system in the Earth system: the Amazon rainforest.

The most important large-scale effect of the AMOC on Amazon rainfall works via the pattern of sea surface temperatures (SSTs) in the Atlantic, and the associated southward shifts of the Intertropical Convergence Zone (ITCZ) and the tropical rain belt. There is widespread agreement that a reduction or even collapse of the AMOC would lead to reduced SSTs in the North Atlantic and increased SSTs in the South Atlantic (Bellomo et al., 2023; Manabe and Stouffer, 1995). This southward shift would cause a substantial reduction in rainfall over northern South America, and an increase in rainfall over the southern Amazon rainforest as well as over northeastern Brazil, which is directly affected by the tropical rain belt (Jackson et al., 2015). Nevertheless, over the Amazon basin, rainfall change is uncertain and model-dependent (Ciemer et al., 2021; Swingedouw et al., 2013; Stouffer et al., 2006), resulting in a large uncertainty concerning the potential impact of AMOC weakening in the Amazon rainforest dieback.

Although different Earth system models have different biases in the location, shape and strength of the tropical rain belt, they generally agree on the AMOC collapse-induced increase in precipitation over the southern portion of the Amazon and northeastern Brazil (Bellomo et al., 2023; Nian et al., 2023; Orihuela-Pinto et al., 2022; Liu et al., 2020). Given that the forests in the southern half of the basin contribute mostly to the rainfall generation over the basin (Staal et al., 2018), one could speculate that this would lead to a stabilisation of the Amazon, given that a substantial fraction (24-70 per cent, Baudena et al., (2021) and references therein) of the rainfall of the basin is nonetheless produced by local moisture recycling. More generally, the full spectrum of rainforest stressors, including human-driven pressures such as land use changes driving deforestation, has to be taken into account when assessing AMOC effects over the Amazon rainforest (Lovejoy and Nobre, 2018).