Projected End-of-Century Changes in the South American Monsoon in the CESM Large Ensemble

Abstract

Projected changes in the South American monsoon system by the end of the twenty-first century are analyzed using the Community Earth System Model Large Ensemble (CESM-LENS). The wet season is shorter in LENS when compared to observations, with the mean onset occurring 19 days later and the mean retreat date 21 days earlier in the season. Despite a precipitation bias, the seasonality of rainfall over South America is reproduced in LENS, as well as the main circulation features associated with the development of the South American monsoon. Both the onset and retreat of the wet season over South America are delayed in the future compared to current climate by 3 and 7 days, respectively, with a slightly longer wet season. Central and southeastern Brazil are projected to get wetter as a result of moisture convergence from the strengthening of the South Atlantic low-level jet and a weaker South Atlantic subtropical high. The Amazon is projected to get drier by the end of the century, negatively affecting rain forest productivity. During the wet season, an increase in the frequency and intensity of extreme precipitation events is found over most of South America, and especially over northeastern and southern Brazil and La Plata. Meanwhile, during the dry season an increase in the maximum number of consecutive dry days is found over northeastern Brazil and the northern Amazon.

Influence of the South American Low‐Level Jet on the Austral Summer Precipitation Trend in Southeastern South America

“…It should also be noted that the HDMF jet index calculated from ERA‐5 contains a significant positive trend (Figure S11 in Supporting Information ). While interpreting co‐occurring trends as causal requires great caution, increased specific humidity is one of the primary expected results of global warming, has been confirmed by observations as a result of enhanced evaporation from the global ocean due to increased temperatures, and has been simulated specifically in the SALLJ region by climate models (i.e., Byrne & O’Gorman, 2018; Thome Sena & Magnusdottir, 2020; Willett et al., 2007). Other studies have argued for the influence of Atlantic variability on trends in SESA precipitation and the SALLJ (Jones & Carvalho, 2018; Seager et al., 2010), yet correlations between an AMV index (Enfield et al., 2001) and HDMF yield R‐values of 0.38 and 0.45 (raw and detrended, respectively), suggesting a small influence of the AMV on the HDMF trend.…”

“…Similar to the SST analysis, the strong correlations between the HDMF index and SLPs globally are largely driven by trends in SLPs. This correlation pattern may reflect the expansion of the Hadley cell (Grise & Davis, 2020; Lu et al., 2007; Thome Sena & Magnusdottir, 2020), which is associated with the observed positive trend of the Southern Annular Mode (SAM; Figure 4b; Kang et al., 2011; Polvani et al., 2011; S. W. Son et al., 2010; S. W. Son et al., 2008; Thompson et al., 2000), but it is possible that internal variability may also play a role.…”

“…The WDMF correlations are strongest within the SALLJ region, while SESA precipitation is strongly correlated with HDMF locally, almost universally over the SESA domain and the adjacent ocean region (Figures 1c, 1d, S4c and S4d in Supporting Information ). The HDMF correlations also reflect the South Atlantic subtropical high (SASH) as a moisture source (Thome Sena & Magnusdottir, 2020); see Figure S5 in Supporting Information for maps of how SESA precipitation correlates with global SSTs and SLPs. The $${q}^{\ast }\boldsymbol{v}$$ interaction correlations reflect a combination of the WDMF and HDMF correlations (Figures 1e and S4e in Supporting Information ).…”

“…The SALLJ generally flows in a north‐northwesterly direction (i.e., Montini et al., 2019; Sasaki et al., 2022; Thome Sena & Magnusdottir, 2020) during austral summer. We therefore consider an index that represents low‐level moisture transports in that direction by weighting the zonal component by one‐third and the meridional component by negative two‐thirds (negative meridional wind speed allows a positive jet index).…”

On the Hydroclimate-Vegetation Relationship in the Southwestern Amazon During the 2000–2019 Period

“…In terms of impacted surface during extreme dry years, up to 40% (55%) of the surface in evergreen forests (savannas) shows negative NDVI anomalies higher than one standard deviation (Supplementary Figure 4). Analysing the resilience of each land cover type to drought conditions remains a key topic as future climate projections show an increase in the dry season length in southern Amazonia (Fu et al, 2013;Boisier et al, 2015;Parsons, 2020;Sena and Magnusdottir, 2020). Field-based observations of the interactions between forest and savanna root depth and hydroclimate conditions are necessary in order to a further comprehension of the resilience of land cover types to droughts in the upper Madeira Basin.…”