Séminaire
Many Holocene paleo-monsoon records show rainfall changes that vary with local orbitally-driven insolation, commonly interpreted to reflecting meridional shifts of the Intertropical Convergence Zone (ITCZ). In contrast to this orbital paradigm, however, some monsoon regions display rainfall evolution that differs from gradual precessional pacing, suggesting that direct radiative effects were dominated by dynamical responses driven by sea-surface temperature (SST) thresholds or inter-ocean SST gradients. Here we present a 12 kyr continuous U/Th-dated precipitation record from a Guatemalan speleothem showing that the Central American Monsoon strengthened abruptly around 9.0 ka and has remained strong thereafter. Our data demonstrate that the Holocene evolution of Central American rainfall is inconsistent with a dominating role of orbital forcing but was driven instead by exceeding an SST threshold in the nearby tropical Oceans, driven by the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) and its consequent impact on Central American atmospheric convective strength. Thus, the same dominant mechanism underlying current Central American monsoon variability has been active throughout the Holocene, implying that the sensitivity of this region to changes in radiative forcing – presumably including those of anthropogenic origin - is strongly mediated by internal dynamics.
Diana.Ruiz-Pino@locean-ipsl.upmc.fr