Greenland sediment record links Atlantic circulation to wetter, colder periods

A Greenland sediment study highlighted by the University of Bremen’s MARUM research centre on 8 October links phases of the North Atlantic Oscillation to wetter, colder conditions in southwestern Greenland over thousands of years. The finding matters because precipitation, as well as temperature, can affect glaciers and the Greenland Ice Sheet. The study appeared in Nature Communications on 21 August.

The researchers examined a sediment core recovered in 2022 from Narsaq Sound during the MSM111 BAFFDEEP expedition. They used material deposited in the fjord to build a record extending through much of the Holocene, the current warm period of roughly 12,000 years. That longer view gives them a way to examine conditions beyond the short span of direct weather measurements.

How Narsaq Sound mud records climate

The team measured niobium in the core using X-ray fluorescence. Rocks around Narsaq Sound contain the element, while its concentration in the ocean is generally low. Rivers and local glaciers can carry material from those rocks into the fjord. Changes in the amount of niobium in deposited sediment can therefore provide a clue to changes in material arriving from land.

Niobium is an indirect indicator, rather than a measurement of ancient rain or snow. Its meaning depends on the area’s geology and on how erosion, runoff and glaciers moved sediment. The authors present its use as a precipitation-related tracer for this particular setting; the same concentration change elsewhere would need its own explanation of where the sediment came from and how it arrived.

What the North Atlantic Oscillation comparison shows

The North Atlantic Oscillation, or NAO, describes a large-scale pattern of atmospheric circulation over the region. Comparing the Narsaq Sound core with other records carrying an NAO signal, the authors found an association between positive NAO phases and higher precipitation alongside lower temperatures in southwestern Greenland. MARUM says the cooler, wetter relationship was consistent over at least roughly the past 5,000 years.

Lead author Johan Faust said the persistence of that signal surprised the team. The finding extends the period over which researchers can examine the relationship, but it does not mean the core directly records each ancient storm. The interpretation joins evidence from the fjord with other climate records and the authors’ analysis of how sediment reached the site.

A separate 2017 study provides a check on how simply that relationship can be described. Using JRA-55 reanalysis for 1958 to 2013, its authors found that precipitation varied differently across southwestern and southeastern Greenland. They reported weak annual correlations between precipitation and the NAO index itself: 0.12 in the southwest and 0.25 in the southeast.

That earlier analysis found stronger relationships with the Icelandic Low and near-surface winds. Its authors described a mix of pressure systems, winds, temperature and blocking patterns behind southern Greenland’s precipitation. The sediment study adds evidence of a long-running circulation link, while the modern analysis shows why a single regional index cannot account for every local change in rain and snow.

What the record cannot predict about Greenland’s ice

Precipitation matters to glacier and ice-sheet mass balance because snowfall can add ice even as warming drives other changes. The new paper suggests that precipitation variability influenced ice mass balance during parts of the late Holocene. It does not calculate a future balance for the Greenland Ice Sheet or show that greater precipitation in one area would outweigh warming across the ice sheet.

MARUM says some climate models project more persistent positive NAO phases as the climate warms. If that happens and the historical southwestern Greenland relationship continues, precipitation there could increase and help local glaciers grow. Both conditions remain uncertain. The study itself says sensitivity to precipitation may change in a warmer climate, so its ancient record is a guide to a process that future assessments must consider, rather than a forecast of glacier growth.