Atlantic ITCZ dynamics during millennial-scale North Atlantic cold events

The modern rainfall regime over semiarid northeastern Brazil (NEB) is primarily controlled by the seasonal migration of the Intertropical Convergence Zone (ITCZ), with the rainy season occurring during March-April, when the ITCZ reaches its southernmost position. It is well accepted that reductions in cross-equatorial northward heat transport mediated by the Atlantic Meridional Overturning Circulation (AMOC) during abrupt cold phases of Dansgaard-Oeschger (DO) cycles, namely Greenland stadials (GS) and Heinrich stadials (HS), triggered southward migrations of the ITCZ. These migrations led to enhanced precipitation over NEB, a signal that is more clearly captured in paleoclimate records during HS.

Here, we present a reconstruction of millennial-scale Atlantic ITCZ dynamics based on the longest continuous paleoprecipitation records available for NEB, spanning the last 160 thousand years (kyr) at a temporal resolution of ca. 30 years. In addition, we use numerical climate model outputs to investigate the mechanisms underlying this millennial-scale variability. The hydroclimate records are derived from a composite of iron-to-calcium (Fe/Ca) and iron-to-potassium (Fe/K) log-ratios measured in bulk sediments from marine sediment cores MD23-3670Q and MD23-3671 (1365 m water depth; 1°34.7′ S, 43°1.4′ W), retrieved offshore NEB during the AMARYLLIS-AMAGAS II cruise in 2023. High ln(Fe/Ca) and ln(Fe/K) values reflect increases in continental precipitation, which enhance chemical weathering, erosion, and terrigenous discharge to the adjacent continental margin. Our records reveal enhanced continental precipitation during cold phases (i.e., GS and HS) of the 25 DO cycles identified in the NGRIP ice core, reinforcing the strong teleconnection between tropical hydroclimate variability and high-latitude climate changes.

The records further indicate consistently higher continental precipitation over NEB during HS than during GS. We show that terrigenous input (i.e., continental precipitation) is inversely related to AMOC strength (r = 0.78, p < 0.05) and to mid- to high-latitude North Atlantic sea surface temperatures (SSTs) (r = 0.9, p < 0.05). Particularly, HS are systematically associated with the highest ln(Fe/Ca) values, the weakest AMOC conditions, and the lowest North Atlantic SSTs.

Numerical simulations performed with the Institut Pierre Simon Laplace climate model version 4 show a gradual increase in annual NEB rainfall as AMOC intensity is progressively reduced. This enhanced rainfall results from a gradual (i) lengthening of the rainy season over NEB and (ii) increase in mean monthly precipitation during the rainy season. The lengthening of the rainy season is driven by both a southward shift in the annual mean ITCZ position and an expansion of its southward seasonal migration range. Meanwhile, we propose that the increase in mean monthly precipitation is related to warmer SSTs in the tropical South Atlantic, which can enhance deep atmospheric convection and act as a direct moisture source for the adjacent continent. Together, these findings suggest that enhanced rainfall over NEB during North Atlantic cold events is not solely driven by a southward migration of the ITCZ, thereby advancing our understanding of tropical atmospheric dynamics during episodes of AMOC slowdown.