South American precipitation dipole forced by interhemispheric temperature gradient

Abstract

Tropical South American hydroclimate sustains the world’s highest biodiversity and hundreds of millions of people. Whitin this region, Amazonia and northeastern Brazil have attracted much attention due to their high biological and social vulnerabilities to climate change (i.e. considered climate change hotspots). Still, their future response to climate change remains uncertain. On precession timescale, it has been suggested that periods of decreased western Amazonian precipitation were accompanied by increased northeastern Brazilian precipitation and vice-versa, setting an east–west tropical South American precipitation dipole. However, the very existence of this precession-driven precipitation dipole remains unsettled given the scarcity of long and appropriate northeastern Brazilian records. Here we show that the precession-driven South American precipitation dipole has persisted over the last 113 ka as revealed by a northern northeastern Brazilian precipitation record obtained from quartz thermoluminescence sensitivity measured in marine sediment cores. Precession-induced austral summer insolation changes drove the precipitation dipole through the interhemispheric temperature gradient control over the regional Walker circulation and the Intertropical Convergence Zone seasonal migration range. Since modern global warming affects the interhemispheric temperature gradient, our study provides insights about possible future tropical South American hydroclimate responses.

The Zonal Patterns in Late Quaternary Tropical South American Precipitation

“…The zonal SAPD is likely driven by multiple factors but, on precession timescales, it is agreed that changes in austral summer insolation are critical (M. C. Campos et al., 2022; Cheng et al., 2013; Cruz et al., 2009; Liu & Battisti, 2015; Prado et al., 2013). Precession drives summer insolation with a ∼21 kyr beat, and this forcing carries no zonal component.…”

“…However, the opposing increase in eastern precipitation requires some zonal shift in atmospheric circulation, and the cause is debated. One theory posits that weaker subsidence over northeast Brazil must compensate for weaker convection to the west, increasing northeast Brazil rainfall (M. C. Campos et al., 2022; Cruz et al., 2009; Shimizu et al., 2020). Another argues that northeast Brazil rainfall increases as south African summer cooling shifts the subtropical rain band, the South Atlantic Convergence Zone, northward, and north African cooling shifts the tropical rain band, the Inter‐Tropical Convergence Zone (ITCZ), southward (Liu & Battisti, 2015), consistent with a broader seasonal ITCZ migration (Chiessi et al., 2021).…”

“…Low austral summer insolation (as during the mid-Holocene, ∼7 ka) weakens the South American Monsoon and decreases rainfall in western tropical South America (P. A. M. C. Campos et al, 2022;Cruz et al, 2009;Liu & Battisti, 2015).…”

“…Tropical South America spans about one‐tenth of the Earth's circumference from east to west (zonally). There is mounting evidence that rainfall across this stretch has varied in a zonal “dipole” fashion in the late Quaternary (here, the last ∼25 kyr) with rainfall increasing in northeastern Brazil at the expense of drying in western Amazonia, and vice versa (M. C. Campos et al., 2022; Cheng et al., 2013; Cruz et al., 2009; Martin et al., 1997). This zonal rainfall pattern is called the “South American Precipitation Dipole” (SAPD), a term that describes the opposing east‐west patterns of past rainfall anomalies (Figures 1a and 1b), and is distinct from the precipitation dipole studied in the modern climate between southeastern South America and the South Atlantic Convergence Zone (Boers et al., 2014; Nogués‐Paegle & Mo, 1997).…”

“…Baker, Rigsby, et al, 2001;Cruz et al, 2009;Fritz et al, 2004;Tapia et al, 2003;Wang et al, 2017). Still, conflicting model and proxy interpretations cast doubt on what drives the SAPD (M. C. Campos et al, 2022;Cruz et al, 2009;Liu & Battisti, 2015), and even whether it exists at all (Wang et al, 2017).…”