Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.3.2.4.5 South American Monsoon =====

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Since AR5, there has been improved understanding of changes in the South American monsoon (SAmerM) as evidenced from paleoclimate records, instrumental observations and climate model simulations. However, general circulation models (GCMs) still exhibit difficulties in reproducing SAmerM precipitation amount ( [[#Rojas--2016|Rojas et al., 2016]] ; [[#D’Agostino--2020b|D’Agostino et al., 2020b]] ). Paleoclimate evidence suggests a relatively stronger SAmerM during the 1400–1600 period (Bird et al. , 2011b; Vuille et al. , 2012; Ledru et al. , 2013; Apaéstegui et al. , 2014; Novello et al. , 2016; Wortham et al. , 2017). Last millennium GCM simulations are able to reproduce stronger SAmerM during the 1400–1600 period in comparison with warmer epochs such as the 900–1100 period (Rojas et al., 2016) or the current warming period (Díaz and Vera, 2018). PMIP3/CMIP5 simulations indicate a consistent weaker SAmerM during the mid-Holocene (6000 years ago; see Cross-Chapter Box 2.1) in comparison to current conditions (Bird et al., 2011a; [[#Mollier-Vogel--2013|Mollier-Vogel et al., 2013]] ; [[#Prado--2013a|Prado et al., 2013a]] ; [[#D’Agostino--2020b|D’Agostino et al., 2020b]] ), thus favouring savannah/grassland-like vegetation (Smith and Mayle, 2018), in agreement with climate reconstructions from different proxies (Prado et al., 2013b). Signals of weak and strong SAmerM during mid-Holocene and LGM, respectively, are evident also in high-resolution long-term (i.e., more than about 22,000 years) rainfall reconstructions based on oxygen isotopes in speleothems from Brazil (Novello et al. , 2017; Stríkis et al. , 2018; Campos et al., 2019).

Isotope records from caves in the central Peruvian Andes show that the late Holocene (&lt;3000 years ago) was characterized by multi-decadal and centennial-scale periods of significant decline in intensity of the SAmerM ( [[#Bird--2011a|Bird et al., 2011a]] ; [[#Vuille--2012|Vuille et al., 2012]] ). This could be partly due to a reduction in the zonal SST gradient of the Pacific Ocean, favouring El Niño-like conditions (Kanner et al., 2013). Other studies suggest increased SAmerM precipitation amount during the Late Holocene, in association with the expansion of the tropical forest (Smith and Mayle, 2018). Well-dated equilibrium lines of glaciers during the deglaciation suggest that the AMOC enhances Atlantic moisture sources and precipitation amount increase over the tropical and southern Andes ( [[#Beniston--2018|Beniston et al., 2018]] ).

Observations during 1979 – 2014 suggest that poleward shifts in the South Atlantic Convergence Zone (SACZ) noted in recent decades ( [[#Talento--2018|Talento and Barreiro, 2018]] ; [[#Zilli--2019|Zilli et al., 2019]] ), are associated with precipitation amount decrease along the equatorward margin and increase along the poleward margin of the convergenze zone ( [[#Zilli--2019|Zilli et al., 2019]] ). Several observational studies identified delayed onsets of the SAmerM after 1978 related to longer dry seasons in the southern Amazon (Fu et al. , 2013; Yin et al. , 2014; Arias et al. , 2015; Debortoli et al. , 2015; Arvor et al. , 2017; Giráldez et al. , 2020; Haghtalab et al. , 2020; Correa et al. , 2021). In contrast, other studies indicate a trend toward earlier onsets of the SAmerM ( [[#Jones--2013|Jones and Carvalho, 2013]] ). These discrepancies are explained by the methodology used and the domain considered for the SAmerM, confirming the occurrence of delayed onsets of the SAmerM since 1978 ( [[#Correa--2021|Correa et al., 2021]] ). CMIP5 simulations show trends toward delayed onsets of the SAmerM in association with anthropogenic forcing, although the simulated trends underestimate the observed trends ( [[#Fu--2013|Fu et al., 2013]] ). Total rainfall reductions are observed in the southern Amazon during September – October – November after 1978 ( [[#Fu--2013|Fu et al., 2013]] ; [[#Bonini--2014|Bonini et al., 2014]] ; [[#Debortoli--2015|Debortoli et al., 2015]] , 2016; [[#Espinoza--2019|Espinoza et al., 2019]] ), consistent with reductions in river discharge in the region (Molina-Carpio et al. , 2017; Espinoza et al. , 2019; Heerspink et al., 2020).

Significant increases in precipitation have been observed over south-eastern Brazil during 1902 – 2005 while non-significant decreases have been found over central Brazil (Vera andDíaz, 2015). In Bolivia, increases were observed during 1965 – 1984, while reductions have occurred since then ( [[#Seiler--2013|Seiler et al., 2013]] ). However, the Peruvian Amazon does not reveal significant changes in mean rainfall during 1965–2007 ( [[#Lavado--2013|Lavado et al., 2013]] ; [[#Ronchail--2018|Ronchail et al., 2018]] ). Historical simulations from CMIP5 ensembles adequately capture the observed summer precipitation amount over central and south-eastern Brazil, thereby providing ''high confidence'' in interpreting the observed variability of SAmerM for the period 1960 – 1999 ( [[#Gulizia--2015|Gulizia and Camilloni, 2015]] ; [[#Pascale--2019|Pascale et al., 2019]] ). Also, CMIP5 simulations indicate that the anthropogenic forcing associated with increased GHG emissions is necessary to explain the positive trends in upper-troposphere zonal winds observed over the South American Altiplano ( [[#Vera--2019|Vera et al., 2019]] ). However, the detection of anthropogenically-induced signals for precipitation is still ambiguous in monsoon regions, like the SAmerM ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ).

In summary, there is ''high confidence'' that the SAmerM onset has been delayed since the late 1970s. This is reproduced by CMIP5 simulations that consider anthropogenic forcing. There is also ''high confidence'' that precipitation during the dry-to-wet transition season has been reduced over the southern Amazon. Paleoclimate reconstructions and simulations suggest a weaker SAmerM during warmer epochs such as the Mid-Holocene or the 900–1100 period, and stronger monsoon during colder epochs such as the LGM or the 1400–1600 period ( ''high con'' ''fidence'' ).

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