About every 10,000 years during the last glacial period, vast numbers of icebergs broke off of glaciers in Heinrich events, adding large volumes of freshwater to the North Atlantic. Their precise cause is under debate, but similar iceberg incidents known as Bond events have occurred about once every 1,000 years during the Holocene, the current epoch. New research by Liu et al. shows that the Asian summer monsoon has responded differently to the two types of events, despite their similarity.
The Asian summer monsoon is an annual wind system characterized by large amounts of rainfall. Understanding how abrupt climate changes like those accompanying Heinrich and Bond events affect the monsoons could reveal detailed climate dynamics that might aid prediction of future conditions.
To study ancient monsoons, the researchers turned to speleothems, mineral deposits in caves whose layers hold clues to past climates. They collected four stalagmites from three caves in central and southern China and analyzed oxygen isotopes captured in the layers to reconstruct ancient climate records.
The researchers found that the intensity of the Asian summer monsoon decreased rapidly at the start of each Heinrich event, reaching its weakest point within 50 years, followed by gradual intensification. The abrupt weakening implies a tight climatic link to the influx of freshwater to the North Atlantic, which would have disrupted normal ocean circulation and climate dynamics.
However, at the start of Bond events, which are smaller and briefer than Heinrich events, the Asian summer monsoon weakened more gradually, taking about 110 years to reach its weakest point. This difference suggests that the impact of freshwater input to the North Atlantic varies depending on the background state of the climate.
For instance, rapid monsoon weakening in Heinrich events suggests greater reduction in the Atlantic Meridional Overturning Circulation, a global pattern of ocean circulation, than during Bond events, which are thought to add less freshwater to the North Atlantic. The Intertropical Convergence Zone, where northern and southern winds meet near the equator, may have shifted farther southward during Heinrich events, further decreasing monsoon intensity.
Additional research is needed to clarify the precise climate dynamics underlying the observed differences in monsoon response to Heinrich and Bond events. Nonetheless, these findings provide new insights into the link between background climate and abrupt periodic changes. (Paleoceanography and Paleoclimatology, https://doi.org/10.1002/2017PA003294, 2018)
—Sarah Stanley, Freelance Writer