In 2019 a team of oceanographers published the first data set of continuous observations of the Atlantic Ocean’s meridional overturning circulation (AMOC)—the ocean dynamics process that brings warm, salty, tropical water to the North Atlantic where it sinks and then travels southward as deep water (see the article by Adele Morrison, Thomas Frölicher, and Jorge Sarmiento, Physics Today, January 2015, page 27). The modern observations helped oceanographers better resolve the spatial variability of the AMOC and how it affects and is affected by rapid climate change (see Physics Today, April 2019, page 19).
To better understand the primary influences on the AMOC, Aidan Starr and Ian Hall of Cardiff University in the UK, with the other members of the Expedition 361 Science Party, have now reported on oceanographic data that span the last 1.5 million years. Their work, combined with iceberg-trajectory modeling, suggests that the farflung redistribution of fresh water from the Southern Ocean decreased the strength of the AMOC during glacial events.
The paleodata come from new and existing records of debris carried by ice and a new sediment core collected from the Agulhas Plateau in the southwestern Indian Ocean. Because the icebergs there are survivors from the Southern Ocean, Starr, Hall, and colleagues could infer their movement by collecting paleodata on the debris they carried.
The researchers then compared those data to the isotopic record from foraminifera organisms in the sediment core. The isotopic record correlated significantly with the movement of water masses from the North Atlantic. The researchers found that icebergs originating in the Southern Ocean moved equatorward at the onset of glaciation events and about 1000–2000 years before the redistribution of the northern-sourced water masses.
They augmented the observations with a simulation in which the icebergs act as passive particles that are transported according to oceanic and atmospheric conditions. The map above shows the possible trajectories of the icebergs during the preindustrial (PI, pink lines) period and the last glacial maximum (LGM, blue lines), and it shows the various surface currents (black arrows) that carried the icebergs past the subtropical frontal zone (STFZ). A gold star marks the location of the Agulhas sediment core.
Although previous observations indicate that the AMOC weakened during some glacial events over the past 1.5 million years, the picture that emerges from the new data is that of an influential Southern Ocean. If enough icebergs escaped the Southern Hemisphere, the influx of melted fresh water in the North Atlantic would have limited the strength of the AMOC. That weakening would have consequently decreased the formation of the deep water that supports Earth’s transition into an ice age. (A. Starr et al., Nature 589, 236, 2021.)